Download - T6.1 OCEAN CURRENTS - Dalhousie University

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Topics Natural History of Nova Scotia, Volume I

P A G E

106▼

© Nova Scotia Museum of Natural History

T6.1 OCEAN CURRENTS

The ocean is a major component in the hydrologicalcycle which exchanges water between land, sea andair (see Figure T8.1.1). In Nova Scotia the marineinfluence is apparent in the character of the physicallandscapes and biotic communities inland, at thecoast and offshore.

The ocean imposes a set of conditions on biologi-cal organisms that is different from those they en-counter on land and in most cases significantly dif-ferent from those occurring in freshwater environ-ments. On land, the atmosphere plays a role in trans-ferring heat, in providing essential gases and mois-ture, and for moving and dispersing species. Soil actsas a foundation for plant and animal life, and thegroundwater brings nutrients. In the ocean, water isthe principal medium for doing all these things.Although the sea bottom is important in supportingmarine plants and various forms of animal life, mostbiological activity takes place near the top of thewater column. Over millions of years, biological lifein the ocean has adapted to the physical processesand features — tides, currents, waves, upwellings,etc. — of the ocean, much as biological communitieshave adapted to the physical and biological land-scape of terrestrial environments.

The sea is constantly in motion. Much of themotion seems at first chaotic—the turbulence ofwaves on a rocky shore and the changing pattern andintensity of waves on the sea surface. Underneaththis exterior, there is an order that begins to becomeapparent only in special instances to the shore-basedobserver. These include the rhythmic vertical and

Figure T6.1.15: The small arrows on the earth represent the net force dueto the imbalance between the gravitational pull of the moon and thecentripetal force which leads to “tidal bulges” on two sides of the earth.The moon is shown at two positions one day apart, to illustrate the delayof the tides by ± 50 minutes each day as seen by the observer at 0.

Equator

NorthPole

Tomorrow

Today

Moon

Ocean Surface

Earth

0

Moon

Earth

00

North

Equator

horizontal movements of the tides, such as the strongnearshore tidal currents observed off Cape Split inthe Bay of Fundy (Unit 912).

The sea has a range of orderly movements knowncollectively as currents. Tidal currents are a featureof every coastal area and consist of a regular in-and-out flow strong enough to influence the movementsof boats and the activities of marine operations.Several hundred kilometres from shore, the GulfStream flows at up to 2.5 m/s. Less striking but equallysignificant currents are present in, and influence,Nova Scotian waters. These range from the steadyflow of water originating in the Gulf of St. Lawrenceand flowing southwestward along the Nova ScotiaAtlantic coast (the Nova Scotia Current), to subtlecurrents circling some of the fishing banks, whichare caused when the back-and-forth tidal move-ments do not completely erase each other, giving aresidual flow in one direction.

Ocean currents benefit marine organisms bydispersing eggs and larvae, and they serve as roadmaps and routes for migratory species. Under spe-cial circumstances, currents lead to transfers of nutri-ents between water masses, which can enhanceproductivity.

Ocean currents are important precursors to theocean environment (or climate) and biological pro-ductivity. The forces which produce ocean currentsare introduced first. These forces reappear in vari-ous combinations in estuarine, continental-shelf andopen-ocean settings.

Figure T6.1.25: Viewed from the side (north up) and with the moon northof the equator, the tidal bulges are asymmetric about the equator,resulting in the diurnal inequality in the heights of the high and low tides.

T6.1OceanCurrents

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Natural History of Nova Scotia, Volume I Topics

P A G E

107▼


Neap Tide

New Moon

Spring Tide

Neap Tide

Full Moon

Spring Tide

Pole Sun

Figure T6.1.35: Positions of the sun and moon at spring and neap tides.

Feet

5

0

Metres

1.5

0

Spring Tide Neap Tide Spring TideDays

Pictou(Semi-diurnal)

Feet MetresYarmouth(Diurnal)

Spring Tide Neap Tide Spring TideDays

0

1.5

3.0

4.54.5

0

15

10

5

Figure T6.1.4a: Typical tidal curves showing tidal patterns in Pictou and Yarmouth.

DRIVING FORCES

Tides, winds, buoyancy (density differences), wavesand remote forcing (factors that act at a distance)supply the forces that drive ocean currents. Eachdriving force is introduced here and described againin particular settings where it dominates.

TidesIn Atlantic Canada there are two tides per day andthey are usually unequal. The rise (flood) and fall(ebb) of tides are a consequence of a combination offorces: the separate gravitational forces of the moonand sun on the earth, and the centripetal forcesresulting from the revolution of the moon about theearth and the revolution of the earth about the sun1

(see Figure T6.1.1).The gravitational attraction between the moon

and earth is directly proportional to their massesand inversely proportional to the squared distanceapart. Centripetal force is the force required to holda rotating mass in orbit, e.g., the force on the rope ifyou were spinning a bucket of water around in acircle. On the side of the earth nearest the moon, thegravitational force of the moon exceeds the centrip-etal force and pulls ocean water into a tidal bulge. Onthe side of the earth farthest from the moon, centrip-etal force exceeds gravitational force and pulls thewater into a tidal bulge (see Figure T6.1.2).

Two tides per day arise because the solid earthrevolves under these two bulges once in approxi-mately twenty-five hours. The extra hour is requiredfor the earth to regain its original position with re-spect to the moon, which itself revolves around theearth. Unequal tides arise when the moon is, forexample, north of the earth’s equator.

The interaction of the earth and sun sets up apattern of forces similar to that of the earth andmoon, but the influence of the moon is about twicethat of the sun (due to its distance away), and themoon and sun patterns are not synchronized. Whenthe sun, earth and moon are aligned (in a straightline)—which occurs at new moon and full moon—the tides are larger and are called spring tides. Whenthe sun, earth and moon make a right angle—whichoccurs at the moon’s first and last quarters—thetides are reduced and are called neap tides. This isillustrated in Figure T6.1.3.2

The configuration of ocean bottom topographyand coastlines provides another layer of complexity.Thus the tides may exhibit quite complicated re-sponses. For example, Figure T6.1.4a shows exam-

Average Tides

Spring Tides

Neap Tides

EHWS

MHWS

MHWN

LHWN

MTL

HLWN

MLWN

MLWS

ELWS

UPPERSHORE

AHTL

MIDDLESHORE

ALTL

LOWERSHORE

SUB TIDALHOURS

SPLASHZONE

Figure T6.1.4b: Another tidal chart for spring, average and neap tides,showing relationship to zones on the shore.

AHTL: Average High Tide Level. ALTL: Average Low Tide Level. EHWS: Extreme High Water Spring Tides. MHWS: Mean High Water SpringTides. MHWN: Mean High Water Neap Tides. LHWN: Lowest High Water Neap Tides. MTL: Mean Tide Level. HLWN: Highest Low waterNeap Tides. MLWN: Mean Low Water Neap Tides. MLWS: Mean Low Water Spring Tides. ELWS: Extreme Low Water Spring Tides.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

108▼


intervals from 24 to 27 hours. Superimposed uponthe astronomical tides are meteorological tides re-sulting from winds and atmospheric pressures. Thus,when storms are forecast, warnings of higher-than-usual tides may be issued.

The tide is really a long wave that travels at a speedproportional to the square root of the depth, i.e.,faster in deeper water. Figure T6.1.5 shows the pro-gression of tides around the Atlantic coast of NovaScotia, with the time of high water shifting up the Bayof Fundy at equivalent speeds of 200 kilometres per

NBPEI

NS

Figure T6.1.5: Sequence of tidal heights and times for South Shore of Nova Scotia and Bay of Fundy.

ples of tidal patterns experienced in Nova Scotia.Tidal ranges vary from extremely large in the inner Bayof Fundy (Unit 913) to rather small in the Northum-berland Strait (Unit 914).3

The observed tidal response can be analysed intocomponents. The largest of these is the principallunar semi-diurnal component: high water occurstwice daily, with intervals averaging 12.4 hours. Thereare also solar semi-diurnal components, and lunarand solar diurnal components. Diurnal tides arethose where high water occurs but once daily, at

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

109▼


Figure T6.1.6: Major rivers with tidal bores in Nova Scotia.

hour. Where strong tidal currents enter a shallow,constricting estuary, the speed of the advancing waveis limited by the depth relationship. Water accumu-lates and deepens faster than the wave speed in-creases. This leads to a steep-fronted surge called atidal bore.

Tides have a significant effect on biological proc-esses in the ocean. The conspicuous vertical move-ments create a distinctive zone of shore exposed toboth seawater and the air twice a day. Tidal currentscarry nutrients and sediments, and erode and changethe shape of the seabed. Life cycles of many organ-isms are keyed to the seasonal occurrence of highesttides. The mudflats at the head of the Bay of Fundyhave the largest tides in Nova Scotia. This habitat isan important feeding area for migratory shorebirds

and other birds and for small mammals along thecoast.

Strong tidal currents concentrate dissolved nutri-ents and create conditions which favour the develop-ment of dense growths of algae or of animals, such asmussels, which feed on particles in suspension. Inoffshore waters, the hidden or internal tides lead tomixing of nutrient-rich water below into the surfacewaters, where it can be used by phytoplankton.

Atlantic Ocean

0 50km

Major rivers with tidal bores

St. CroixRiver

KennetcookRiver

ShubenacadieRiverBay of Fundy

MaccanRiver

Salmon River

RiverHebert

N

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

110▼


WindsWind-driven circulation in the oceans occurs onlyin the upper few hundred metres4. In open areasthe total transport of water is to the right of thewind direction. This is because the rotation of theearth creates a force on moving bodies called theCoriolis force.

When the wind, blowing over the surface, dragswater along, the Coriolis force acts to the right of thedirection of motion, and the water-drag force acts inthe direction opposite to the direction of motion(Figure T6.1.7A). These forces are not in balance; thisimbalance causes a change in velocity (FigureT6.1.7B). When the water velocity has rotated to theright, these forces do approach a state of balance.Then (Figure T6.1.7C) the Coriolis force and thewater drag together balance the wind stress. Thus,wind-driven currents move at an angle rotated to theright of the direction of the wind and are associatedwith the name of Walfrid V. Ekman (1905), a famousNorwegian oceanographer; hence, Ekman drift (seeFigure T6.1.9).

The interaction of winds with the ocean createconditions which impact on biological organisms.Offshore or alongshore winds push surface watersaway from the coast, leading to an inflow into coastalareas of nutrient-rich water from deeper layers, and

enhancing productivity in near coastal waters. Thisprocess of upwelling is the same one, though on areduced scale, that drives the famous upwellings offPeru and off the west coast of Africa (El Niño is achange in surface-water mass patterns off Peru thatstifles and temporarily curtails the high productivityof the upwelling).

Further at sea, patterns of winds in the equatorialzone lead to a massive northward-moving oceancurrent—the Gulf Stream—which transfers manywarm-water organisms to waters off Nova Scotia.

On a smaller scale, windrows and lines of smoothsea surface (slicks) result from steady wind whichcauses the water to circulate in a cylindrical pattern,converging along a line and bringing floating mate-

A

B

C

Forces

y

xWater drag Wind drag

Coriolis

y

x

Water dragWind drag

Coriolis

y

x

Water drag

Wind drag

Coriolis

y

xv

Water velocity

y

x

v

y

x

v

45°

The Coriolis EffectIn the Northern Hemisphere, the Coriolis force causes theaverage motion of the water to be turned to the right.

The Coriolis force is an apparent force which is usedto bring our senses into correspondence with reality.While our senses suggest that an observer standing at onepoint on the surface of the earth is stationary, actuallythis observer is rotating with the earth. Ocean waters tendto flow in the direction of the forces causing them tomove, but this direction changes from an earth observer’spoint of view, since the earth is rotating. The Coriolisforce accounts for this change and brings our rotatingframe of reference into correspondence with an absoluteframe of reference. For example, imagine an elf standingin a groove on a long-play record as it spins counter-clockwise on a turntable. Suppose, when she is just at thenorth point of her circle she bends down and rolls a tinyball toward the needle, which is at the west side of the circle(i.e., 90 degrees rotated). The ball rolls straight toward theneedle, but to the elf, moving on her curved path to the left,the ball appears to deviate to the right. This deviation isattributed to the apparent force—the Coriolis force—and isan artifact of the elf’s rotating frame of reference.

Figure T6.1.75: The beginning of a wind-driven surface current in threestages, showing the forces on the left and the water velocity on the right.In stage A the wind drag creates a flow of water which gives rise to thewater drag and the Coriolis force. In stage B the Coriolis force causes thecurrent in the water to rotate around to the right (in the northernhemisphere). The force due to the water drag and the Coriolis force rotatewith the current in the water. In the final stage (C), the current has rotatedthe amount required to have the force due to the wind drag balanced by thecombined effects of the Coriolis force and the drag of the water.

T6.1OceanCurrents

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

111▼


∆h

hp1

A B

Surfacelevel

p2

Figure T6.1.85: A pool of light water, p1, lies on top of and beside water ofgreater density, p2. If the pressure at A and B are the same, the height ofthe sea surface above A must be higher than above B.

WavesWhy are there sometimes huge wavesbreaking on the rocks at Peggy’s Cove on aday when winds are light ?

Wind waves travel at different speedsdepending on their wavelength; the longerwaves travel faster. A storm will generatewaves over a range of wavelengths. Thus,if a storm is hundreds of kilometres away(e.g., off the Carolina coast), the longerwaves can arrive here before the stormdoes, if it does.

Ekman Drift

Upwellingp2

W I N D

FrontNorth

WarmCool

East

Coast

p1

rial and organisms to pile up there. Windrows are thefocus of fish and seabird feeding and, in rare cases,can lead to accumulations of poisonous algae suffi-cient to cause a health risk.

Storms often blow unusual species of seabirdsinto Nova Scotia waters, and terrestrial birds arefrequently found great distances at sea after majorstorms.

BuoyancySome ocean currents arise where the density of thewater is changed in a significant portion of its bulk.For example, convection is driven by buoyancy. If along tank of water is cooled at the surface at one end,the water there will contract, become more denseand start to sink. Warmer water will flow horizontallytowards the cool end, to fill the void left by thesinking, colder water.

If this is seawater, and if ice is formed, the densityis increased further. This is because the salt is largelyexcluded from the ice and adds to the density of theremaining water. As this dense water sinks, water atthe other end of the tank will rise—a consequence ofthe continuity of volume—and flow toward the coolside of the tank (see Figure T6.1.8).

Since, in general, fresh water is less dense thanseawater, freshwater runoff at the coasts or precipita-tion will reduce the density. Where fresh water runs offthe land and overlies a denser layer of seawater, itlikewise sets up a horizontal pressure gradient.

In this case, the fresh water, being less dense, causesthe surface at the coast to be slightly elevated; the flowis down the gradient toward the sea. Thus, densitychanges in the ocean occur by heating or cooling, by iceformation and by the addition of fresh water.

WavesWaves can drive currents in two ways: either throughtheir particle motions, which is how tide waves leadto tidal currents, or through actually pumping en-ergy into currents, which appears to occur on thecontinental shelf off Nova Scotia (Scotian Shelf).

The tremendous energy of waves influences bio-logical organisms. The high speeds combined withthe density of seawater expose some species in thesubtidal area to forces equivalent to 1500-kph windson land. Plants have evolved firm anchor systems toattach them to the substrate, and many animalshave streamlined shapes to resist being lifted offsurfaces. Wave energy also influences the formseaweeds take during growth, and members of spe-cies in exposed environments are more elongate andblade-like than representatives in sheltered envi-ronments.5 The washing of the waves also bringsnutrients and carbon dioxide and takes away wasteproducts.

FigureT6.1.95: A perspective drawing through an upwelling region,illustrating the offshore Ekman drift in the upper layer beingreplaced near the coast by upward-moving water from the lowerlayer. The upwelling water, usually cool, is separated from theoffshore warm water by a surface front parallel to the coast. Thewind blows from the north to the south.

T6.1Ocean

Currents

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

112▼


50° W60° W70° W80° W

Cold corering

Meander

Gulf Stre

am

Warm corering

200m

30° N

40° N

50° N

the Atlantic Ocean. The complex patterns that resultare large in scale but vary significantly in size andpattern.

As a result of these patterns, Nova Scotia’s marineorganisms include species which can be found innorthern areas, such as Greenland, as well as speciestypically found in warmer waters to the south. Occa-sionally, exotic species such as the Portuguese Man-of-war and Leatherback Sea Turtle “hitchhike” toNova Scotian shores.

The interaction of water masses leads to an en-hanced mixing and productivity. (The North Atlan-tic, of which Nova Scotian waters are a part, is one ofthe most productive zones in the world’s oceans.) Asa result, many species of organisms (seabirds, mi-gratory fishes, marine mammals) feed here duringperiods of high productivity. Global warming andcooling trends, however, can have significant andunpredictable effects on marine ecosystems, becauseof the links to areas far away.

SPECIAL FEATURES

Estuarine Circulation (see T6.4)In estuaries, the addition of fresh water sets up adifferential movement of water. As the upper, less-saline layer flows seaward, it mixes with the lowerlayer. This modifies the distribution of buoyancyforces in the estuary, so that the lower, more-saltylayer flows landward, wells up near the head of theestuary and provides a portion of the upper water.This ongoing sequence of upwelling and stratifica-tion provides nutrients to the surface waters.5

Coastal UpwellingCoastal upwelling is driven by winds blowing princi-pally alongshore (as well as offshore). The surfacewater velocity is at an angle to the right of the winddirection and therefore has an on-offshore compo-nent. If the wind is alongshore in the direction suchthat a rotation of 90 degrees to the right is offshore(i.e., southwest winds along the Atlantic coast ofNova Scotia, Region 800 and Unit 911), then thesurface water velocity—the Ekman drift—is directedoffshore. The water drifting offshore is replaced byupwelling from lower layers, hence coastal upwelling(see Figure T6.1.9).

Coastal upwelling occurring along the Atlanticcoast of Nova Scotia is conducive to the growth ofphytoplankton and is a key factor in the productivityof the coastal zone. Other factors include closenessto a supply of nutrients from the seabed, estuarinecirculation in bays and inlets and the availability ofplant material in the form of detritus.

Figure T6.1.105: Diagram offrontal structure and circulation,

modified after Simpson7.Note strong along-front mean flow,

convergence and downwelling at the front,upwelling on the well-mixed side, and frontal eddies,

some of which close on themselves to form isolated patches.

Figure T6.1.11: A schematic illustration ofmeanders in the Gulf Stream and the formation ofrings, based on Richardson et al9 and Parker10.

Remote forcingRemote (non-local) forcing means that currents at aparticular place arise because of a force that has actedin another, remote area. For example, the Gulf Streamout beyond the edge of the Scotian Shelf is forcedindirectly by the northeast equatorial trade winds.

Weather and ocean patterns off Nova Scotia areinfluenced by events taking place in far-removedlocations, and these effects influence the biologicalorganisms found here. Nova Scotia is particularlyprone to remote effects, because it is positioned inmid-latitudes, that is, it lies between the influencesof cold-water masses originating in the Arctic andwarm-water masses from the south, and near a ma-jor freshwater flow of the St. Lawrence River entering

T6.1OceanCurrents

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

113▼


Figure T6.1.12: Chart showing minimum, median and maximum position of ice edge in the Gulf of St. Lawrence and on the Atlantic Coastfor the years 1963–73; late MarchExtreme Maximum Limit — defined by outer edge of rippleMedian Limit — black lineExtreme Minimum Limit — defined by grey stippleBetween the maximum and median positions, the number of years ice was present (out of 11) is shown by digits in lower right of each square.Between the median and minimum lines, the median total concentration is by digits in the upper left of each square, and the number of years that ice waspresent is shown by a smaller digit in the lower right. For example, 68 = a median concentration, 6 tenths for ice present 8 years out of 11. Within theminimum line, the years that ice was present is always equal to the number of years of data.11

Tidal FrontsA front is a sharp boundary between water masses ofdifferent properties. Simpson and Hunter6 theorizedthat a front would be found where the intensity ofturbulent mixing was just sufficient to continuouslyovercome the barrier to mixing presented by thestratification7 (see Figure T6.1.10). They derive a tidalmixing index which reflects this balance at a frontand which is mapped for the whole Gulf of Maine.8,9,10

Energy from the tides can result in significantmixing of nutrient-rich water to the surface, where itcan contribute to water-column productivity undercertain conditions. In cases where the tidally mixedwater lies adjacent to a more stable surface layer (afront), the nutrients can feed into marine-plantpopulations in the surface layer and develop highplant concentrations. A major tidal front occurs offsouthwest Nova Scotia, and herring spawn in the

area. Young herring use the production in the watercolumn as they grow. The resulting elevatedpopulations of juvenile herring at the mouth of theBay of Fundy form the basis for a herring fishery onthe New Brunswick side of the Bay.

Shelf-break FrontsThe shelf break is the region where the continentalshelf ends and the continental slope down to theabyss begins (see T3.5). The present understandingof shelf-break fronts is that as the tide impinges onthe shelf at the shelf break, tidally generated internalwaves radiate away and add energy to the mixedlayer. (Internal waves occur on the interface be-tween the upper and lower layers of the water col-umn.) The mixed layer is deepened and incorpo-rates nutrient-rich water from below. This processvaries in intensity with the semi-diurnal tide, so that

T6.1Ocean

Currents

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

114▼


Warm-core RingsNorth of Cape Hatteras, North Carolina, the GulfStream moves away from the coast, and warm-core rings form on the shoreward side of the cur-rent. After the Gulf Stream leaves the coast, itcontinues eastward as a strong, narrow stream,but at about 65 degrees west longitude it becomesunstable and begins to develop large north-southoscillations, or meanders. These meanders causethe cooler water found to the north of the GulfStream to be brought further south than usualand the warmer southern water to be transportedfurther north. Some of these meanders grow toolarge, and the ends separate into isolated rings ofwater, as illustrated in Figure T6.1.11.9,10

The clockwise-rotating rings to the north ofthe Stream are known as warm-core rings, be-cause they contain warmer water surrounded bythe cooler waters found northwest of the Stream. An

IceSometimes ferries, even though they have icebreaking capability, become stuck in the ice for hoursor even days. Before this happens, there would have been a period of noisy, bumpy crashing intothe ice. There could also have been a period when the ferry repeatedly backed up and steamed hardinto the ice to try to force its way through. Then, once stuck, the ship becomes more quiet. Onemight see ice ridges where pieces of ice are forced by pressure (windstress) up into mounds ofbroken ice, higher than the normal level of the pack ice. Finally, a large Coast Guard icebreakerappears, to free the ferry.

it produces a twice-daily augmentation of nutrients,resulting in a fairly constant elevation of rates ofgrowth of marine plants.

Tidal GyresBanks are broad, raised plateaus on the continentalshelf which, in some cases (e.g., Georges Bank, Unit931), can reach to within 30 m of the surface. Oscil-lating tidal currents washing back and forth overthe banks sometimes generate mean (steady) cur-rents at the edge of the bank where the depth isincreasing rapidly. These mean currents form a gyrearound the bank and can provide a “larval reten-tion” area where, except in extreme conditions, fisheggs and larvae are kept in place over the bank. Theprocess is a factor in the distribution of species andin maintaining the integrity of stocks for fishing,but its importance in relation to other factors hasnot been determined.

Plate T6.1.1: Gulf of St. Lawrence ice fills Halifax Harbour in early 1987. Photo: L. Morris.

T6.1OceanCurrents

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

115▼


200m

200m

Gulf Stream

Bay of Fundy

St. Lawrence Estuary

Newfoundland Shelf

Gulf ofSt. Lawrence

200m

Newfoundland Grand Banks

Strait ofBelle Isle

ScotianShelfGulf of

Maine

GeorgesBank

FlemishCap

200mAtlantic Current

Atlantic Current

Quebec

Nova Scotia

New BrunswickPEI

Labrador

Newfoundland

Figure T6.1.13: Major currents offshore Nova Scotia.

average of five warm-core rings is formed each year,but the production rate is quite variable. The diam-eter of a warm-core ring is approximately 100 km. Aring lasts for about a year before being re-absorbedinto the Stream or dispersed.5

Geostrophic BalanceThe initial tendency of water is to flow down a pres-sure gradient. For large-scale flows, the Coriolisforce exerts its influence and turns the flow to theright. At equilibrium, the pressure gradient to theleft is just balanced by the Coriolis force to theright. This geostrophic balance—between pres-sure gradient and the Coriolis force—is an every-day occurrence in the atmosphere and in theocean. In the atmosphere, it is evident in the dailyweather charts which display the distribution ofisobars. The flow of air around the high- and low-pressure systems is parallel to the isobars, notacross them, because the air is close to being ingeostrophic balance. In the ocean, the flow of

water is often parallel to the ocean isobars for thesame reason. On the Scotian Shelf, the Nova ScotiaCurrent flowing southwestward along the coast isin approximate geostrophic equilibrium.

IceSeawater freezes at between –1 and –2°C, dependingon the salinity. Mean winter temperatures at theNova Scotian coastline do not fall very far below thefreezing point, and as a result, cold and mild wintershave a very significant effect on the extent and sever-ity of the ice cover. Some coastal areas experience iceannually; in other areas ice is unusual. Sea ice regu-larly covers large areas of water in the Gulf of St.Lawrence.

Winter winds from the west to north are typicallycold and dry, whereas those from the west throughsouth to northeast are typically mild and moist. Thewinds have a decided effect on the location of areasof ice dispersal and congestion once it has formed.Prevailing winds, currents and Coriolis force all con-tribute to a residual west-to-east flow pattern. Forexample, northwest winds with accompanying coldtemperatures tend to move ice floes from west to

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

116▼


east along the Northumberland Strait. Winds blow-ing onshore produce thick, congested ice fieldsagainst windward shores.

Figure T6.1.12 shows the median, minimum andmaximum limits of ice occurrence in late March forthe years 1963–73.11 Shore ice begins to form inDecember and by late January covers much of theNorthumberland Strait. This ice breaks up and meltsas early as late March or as late as late May. ByFebruary, ice may be present along the eastern shoreof mainland Nova Scotia, depending on the severityof the winter.

Sea ice can extend into the Sydney Bight (Unit915) and in rare cases has been carried down thecoast of Nova Scotia, even entering Halifax Harbourand Bedford Basin (see Plate T6.1.1).

Ice is more often a barrier to the establishment ofmarine organisms; however, some species do takeadvantage of its features. Harp and Hooded seals inthe Gulf of St. Lawrence use the ice for giving birth(whelping). Microscopic algae can develop as film-like coatings under ice floes and live in crevices wherethey cannot be washed off. Sea ice can enable themigration of land animals; e.g., the introduction of theeastern coyote into Prince Edward Island and New-foundland across the ice of the Gulf of St. Lawrence.

CIRCULATION IN NORTHWEST ATLANTIC

The dominant driving forces for North Atlantic cir-culation are winds and buoyancy (density differ-ences). The ocean moves both horizontally and ver-tically under these influences. Density differencesare generated by heating and cooling, evaporation,precipitation and runoff.

The principal feature is a great clockwise gyre orcircular current pattern driven by the trade winds,westward in the tropical Atlantic and eastward in theNorth Atlantic, first as the Gulf Stream and then asthe North Atlantic Current (Figure T6.1.13).12

East of the Grand Banks of Newfoundland, theNorth Atlantic Current divides, part flowingnortheastward between Scotland and Iceland and

contributing to the circulation of the Norwegian,Greenland and Arctic seas. The remainder turnssouth, past Spain and North Africa, to complete theNorth Atlantic gyre and to feed into the North Equa-torial Current.4

In the tropical Atlantic, solar heating and evapo-ration in excess of precipitation and runoff create anupper layer of relatively warm, saline water.13 As thiswater flows north it gives up heat to the atmosphere,particularly in winter. Because winds in the highermid-latitudes are generally eastward, this heat iscarried over Europe, producing its relatively mildwinters. So much heat is withdrawn by the time itreaches the Greenland Sea that the water tempera-ture drops close to the freezing point. This waterremains relatively saline, and the combination oflow temperature and high salinity makes the watermore dense than deeper water below it. The watersinks to deeper levels, occasionally right to the bot-tom. There it slides under and mixes with othernear-bottom, dense water. It spreads out and flowssouthward, deep and cold. This pattern, known asthermo-haline circulation (surface warm water flow-ing north, cooling, sinking and then flowing south),provides an enormous northward movement or fluxof heat, fully comparable with that transportednorthward by the atmosphere.

LABRADOR CURRENT

The most important “drivers” for the Labrador Cur-rent appear to be the large-scale current gyres of thenorthern ocean and buoyancy fluxes (addition oflow-salinity water) associated with outflow fromHudson Strait.

The Labrador Current is formed by the conflu-ence of the West Greenland and Baffin Island cur-rents, supplemented by flows out through HudsonStrait. It flows southeastward along the coasts ofLabrador and Newfoundland. Off Newfoundland,the Labrador Current branches, the main portionflowing around the Grand Banks along the slope ofthe continental shelf and the weaker part flowinginshore over the shelf. This inshore part divides toprovide some flow through the Strait of Belle Isle; theremainder flows along the eastern and southerncoasts of Newfoundland.

Losing some flow to the North Atlantic Current,the stronger branch, consisting of relatively moretemperate waters derived from the West GreenlandCurrent, continues southwestward, even to the slopeof the Scotian Shelf.

The weaker, inshore branch, consisting mostly ofcold, low-salinity water, continues along the south-

Why does the tide sometimes stay high all day on the beachnear Pugwash?

In the western Northumberland Strait, tides are mixed butmainly diurnal, i.e., with a period of approximately 25 hours,as distinct from the more usual situation in Nova Scotia,where the semi-diurnal tides are predominant. Thus, you canexperience days when the tide stays high (within a relativelynarrow range) continuously for 12 hours!

T6.1OceanCurrents

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

117▼


ern coast of Newfoundland and through Cabot Straitto join the Gulf of St. Lawrence circulation.

CIRCULATION IN THE GULF OF ST. LAWRENCE

The Gulf of St. Lawrence is a highly stratified, “mar-ginal” (semi-enclosed) sea. (“Highly stratified”means that there are distinct layers which differ intemperature and density.) Its circulation is drivenmainly by freshwater runoff originating from the St.Lawrence River and North Shore rivers, winds, tides,and changes in temperature and salinity at the edgeof the continental shelf.14

The surface circulation in the Gulf of St. Lawrenceis essentially counterclockwise (see Figure T6.1.13).The Gaspé Current, driven by the St. Lawrence Riveroutflow, results in an eastward flow of water whichextends to the Magdalen Shallows and Northumber-land Strait. Over most of the Gulf, the mean drift iseastward. In Jacques Cartier Passage, north ofAnticosti Island, the mean flow is westward, com-pleting the counterclockwise pattern. The NorthShore rivers contribute “new” water to the westwardflow. Of course, a particular parcel of water is notlikely to flow completely around this circuit.

Most of the southeastward flow at the surfaceleaves the Gulf via Cabot Strait between Cape Bretonand Newfoundland, and is balanced mainly by thedeeper flow of more-saline water entering from thecontinental shelf. This surface flow, eastwardthrough Cabot Strait then turning southwestwardalong the Scotian Shelf, has a mean annual flow ofwater estimated at 340 000 m3/s, peaking at ap-proximately 700 000 m3/s in August, apparently inresponse to peak freshwater runoff of approximately31 000 m3/s in May. The peak outflow from CabotStrait amounts to 20 times peak freshwater runoff.Although not driven entirely by runoff buoyancy, itdoes show how the river flow in an estuary can beamplified by the estuarine processes.

The outward flow of surface waters through CabotStrait forms the Nova Scotia Current, which flowssouthwesterly along the Nova Scotian Atlantic coast(see Circulation on the Scotian Shelf).

In addition to the horizontal currents, verticalcurrents, such as upwelling flows, are importantbiologically because they bring nutrient-rich waterinto the surface, euphotic or lighted, zone. In the St.Lawrence, wind-driven upwelling occurs along theGaspé Coast and buoyancy-driven and tidally drivenupwelling occurs in the estuary.

Several Nova Scotia estuaries connect with theNorthumberland Strait and the Gulf of St. Lawrence.Currents in these estuaries are driven mainly by

For swimmers on Atlantic Coast beaches, warmest watertemperatures will occur after a period of easterly to southeast-erly winds. These winds, also associated with rain, drag warmsurface water shoreward. Winds from the northwest tosouthwest sector drag surface water offshore, causing deepercold water to upwell along the coast.

T6.1Ocean

Currents

freshwater runoff and tides. The tides are a mixtureof semi-diurnal and diurnal constituents (FigureT6.1.4a), with ranges from 1.1 to 2.9 metres. Thefreshwater runoff and relatively low tidal ranges inmany of these Nova Scotian estuaries leads to buoy-ancy effects (pressure distributions) which causelower-layer water to flow in from the mouth of theestuary, to upwell into the surface layer and then toflow seaward in a residual current which transportson the order of ten times the flow of the freshwateritself—the two-layer estuarine circulation.

CIRCULATION ON THE SCOTIAN SHELF

The continental shelf under the sea on the Atlanticcoast of Nova Scotia from Misaine Bank to North-east Channel is known as the Scotian Shelf. Impor-tant driving forces for the circulation on the ScotianShelf include buoyancy fluxes, wind stress, tidalforces, and the large-scale circulation associatedwith adjacent deep-ocean gyres.15

The general circulation on the Scotian Shelf issouthwestward, with a predominant southwesterlydrift along the coast (the Nova Scotian Current) anda minor southwesterly drift along the outer edge ofthe Shelf (see Figure T6.1.13). The mean and sea-sonal strength of the Nova Scotia Current appears tobe related to buoyancy fluxes associated with runofffrom the St. Lawrence River. Figure T6.1.15 summa-rizes actual current measurements in three depthranges, averaged over all months.16

The Nova Scotia Current flows at an annual meantransport of 340 000 m3/s, with a speed of 0.05–0.10 m/s. The maximum transport of the Nova ScotiaCurrent is approximately 700 000 m3/s. This transportcan be compared to the Gulf Stream, which transportsup to 150 million m3/s, and the Labrador Current, at5.6 million m3/s 17.

The Nova Scotia Current is dominant over thenearshore region of the Scotian Shelf. The large-scale current response on the Scotian Shelf isgeostrophic. That is, the pressure gradient is slopeddownward to the southeast and the flow is relativelysteady to the southwest, even though prevailingwinds are to the northeast.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

118▼


Recent studies suggest that, although the meanwind stress opposes the mean current, the energy inthe fluctuations of the wind is transformed into adriving force for mean current—southwestward,against the mean wind.15

Satellite images demonstrate that persistentsouthwesterly summer winds produce a band ofcold, upwelled water near the coast that subsequentlyforms eddies through instability of the upwellingfront.17 Periods of upwelling alternating with periodsof light winds and stratification can provide the con-ditions for phytoplankton blooms. Upwelling alsooccurs off Cape Sable (Unit 911), driven not by windsbut apparently by alongshore density variationsmaintained by tidal mixing.18 Upwelling offLouisbourg (Unit 911) may be similarly driven.

At the shelf “break” and slope, there are shelf-break processes and dynamic interactions with GulfStream derivatives. In the deeper layers of the slopewater, there is a Labrador component from the GrandBanks. There are responses at the shelf break to a)the tide, b) the wind field and c) the fluctuatingoffshore currents associated with the Gulf Stream.The semi-diurnal internal tide over the shelf edge attimes develops into an internal undular bore, or atother times into internal solitary waves. These wavescan bring nutrient-rich lower-layer waters into theeuphotic zone and provide good conditions for thegrowth of phytoplankton.

Offshore eddies and rings have an important in-fluence on the circulation over the slope, althoughnot, it appears, over the shelf itself. When a GulfStream ring suddenly encounters a sloping bottom,it radiates low-frequency energy in the form of “con-tinental shelf waves” with characteristic periods of10 to 30 days. On approaching the shelf, however,the energy is strongly refracted by the continentalslope and scattered into waves trapped to the shelfedge so that little energy penetrates onto the shelf.

Nevertheless, eddies do affect near-bottom currents,stress and sediment transport near the shelf break.15

CIRCULATION IN THE BAY OF FUNDY ANDTHE GULF OF MAINE

At the head of the Bay of Fundy, the tidal drivingforce is predominant and establishes a macro-tidalenvironment. Tide ranges here are among the high-est experienced anywhere in the world. (The tidegauge at Saint John Harbour has functioned almostcontinuously since 1894, forming an excellent tiderecord.) Toward the mouth of the Bay, freshwaterrunoff (buoyancy) from the Saint John River alsoplays a major role. In the Gulf of Maine, the circula-tion is driven by wind stress and non-local forcing, aswell as the tides.

The general residual circulation is counter-clockwise in the Bay of Fundy and the Gulf of Maine(see Figure T6.1.13). In the inner Bay of Fundy,gyres on either side of Minas Channel (one clock-wise and one counterclockwise) occur due to thebathymetry which induces tidal rectification. Forexample, the very strong flood-tidal stream entersMinas Basin at the northern edge as a “jet,” whilethe ebb-tidal stream is more widespread and slower.Thus, the tidal currents do not average to zero; thepattern of average currents shows a gyre, reflectingthe strong influence of the flood jet.

In the Bay of Fundy, especially in Chignecto Bayand Minas Basin, tidal amplitudes and currents areextremely high. The reason for this is that the 12.5-hour period of the lunar semi-diurnal tidal drivingforce coincides fairly closely with the travel time of along wave into the Gulf of Maine/Bay of Fundy sys-tem, which is determined by the depth and length ofthe system. The large tides give rise to unusual ef-fects, such as tidal bores and reversing falls. At themouth of the Bay of Fundy, there is a counter-clockwise gyre which seems to be driven by thedensity gradients arising from the freshwater inputof the Saint John River19 (see Figure T6.1.13).

The seasonal circulation in the Gulf of Maine ispartly determined by the distribution of dense slopewater that enters intermittently from the continen-tal slope and spreads over sills into the deep basins ofthe Gulf. Warm core rings from the Gulf Streamoccasionally approach the mouth of the NortheastChannel; at times, ring water modifies the slopewater.20 Slope water accumulates in Georges Basin.There is a counterclockwise surface circulation,which brings Scotian Shelf water westward into theGulf and contributes to the eastward jet (speeds of 30cm/s) along the inner edge of Georges Bank. The

Computer model studies of the Bay of Fundy tides suggest thatif a tidal barrage (i.e., a dam for tidal power) were placedacross Minas Basin at Economy Point, the tidal amplitudewould be increased by about 15 cm from Saint John to Boston.The barrage would shorten the Bay and bring the naturalperiod of the Bay of Fundy/Gulf of Maine system closer to theforcing tidal period. In simpler terms, consider the Bay ofFundy as a bathtub, with its natural period of sloshing, end-to-end. Suppose your child is making waves at a frequencyslightly too rapid to cause the maximum waves. If you were toinsert a barrier and shorten the tub, your child’s waves wouldthen slosh perfectly—onto the floor.

T6.1OceanCurrents

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

119▼


7 Simpson, J.H. (1982) “The shelf-sea fronts:implications of their existence and behaviour.”Phil. Trans. R. Soc. Lond. A 302.

8 Loder, J.W., and D.A. Greenberg (1986) “Pre-dicted positions of tidal fronts in the Gulf ofMaine region.” Continental Shelf Research 6.

9 Richardson, P.L., R.E. Cheneys, L.V.Worthington (1978) “A census of Gulf Streamrings, spring 1975.” Journal of GeophysicalResearch 83 (c12).

10 Parker, C.E. (1971) “Gulf Stream rings in theSargasso Sea.” Deep Sea Research 18.

11 Markham, W.E. (1980) Ice Atlas Eastern Cana-dian Seaboard. Atmospheric EnvironmentService, Environment Canada, Toronto.

12 Department of Fisheries and Oceans. SurfaceOceanography Northwest Atlantic. (SciencePoster Series. Poster #1).

13 Stewart, R.W. (1990) “The ocean and climate.”Nature and Resources 26 (4).

14 Koutitonsky, V.G., and G.L. Bugden (1991) “Thephysical oceanography of the Gulf of St.Lawrence: A review with emphasis on thesynoptic variability of the motion.” In The Gulfof St. Lawrence: Small Ocean or Big Estuary?”(Can. Spec. Publ. Fish. Aquat. Sci. 113).

15 Smith, P.C., and F.B. Schwing (1991) “Meancirculation and variability on the easternCanadian continental shelf.” Continental ShelfResearch 11 (8–10).

16 Gregory, D.N., and P.C. Smith (1988) CurrentStatistics of the Scotian Shelf and Slope.Fisheries and Oceans Canada. (Cdn. Tech.Report of Hydrog., and Oc. Sci. No. 106).

17 Petrie, B. (1987) “Undulations of the NovaScotia Current.” Atmosphere-Ocean 25 (1).

18 Smith, Peter C. (1983) “The mean and seasonalcirculation off southwest Nova Scotia.” J. Phys.Ocean 13 (6).

19 Godin, G. (1968) The 1965 Current Survey of theBay of Fundy—A New Analysis of the Data andan Interpretation of the Results. Marine Sci-ences Branch, Department of Energy, Mines &Resources, Ottawa. (MS Rept. Ser. No. 7).

20 Brooks, D.A. (1987) “The influence of warm-core rings on slope water entering the Gulf ofMaine.” J. Geophys. Res. (C Oceans) 92 (c8).

Additional Reading• Bleakney, J.S. (1992) All About Tides.

Elderhostel, Acadia University, Wolfville, N.S.(Course notes).

circulation around Georges Bank is clockwise, drivenin large part by the tides. The slope water also crossesa sill and enters Jordan Basin, where it may enhancea counterclockwise gyre partly driven by nearshorebuoyancy sources and the wind (see Figure T6.1.13).

Clockwise tidal gyres over Georges Bank andBrowns Bank are permanent features. The tidal fronton the northern and northwestern side of GeorgesBank makes a twice-daily excursion of 10 to 15 km asa result of changes in tidal-current velocity. Its posi-tion is in rough agreement with the prediction madeby Loder and Greenberg.6 Along the southern flank ofGeorges Bank, the alongshelf current is associatedwith the alongshelf wind.

Vertical currents can be inferred from water prop-erties. The Gulf of Maine bottom water originatesfrom slope water that enters the Gulf through theNortheast Channel; it is modified within the Gulf byvertical mixing with the near-surface waters of ScotianShelf origin. Wind-driven coastal upwelling is alsoobserved, associated at times with dense planktonblooms.

○ ○ ○ ○ ○ ○ ○ ○

Associated TopicsT2.7 Offshore Geology, T3.5 Offshore Bottom Char-acteristics, T5.1 The Dynamics of Nova Scotia’s Cli-mate, T5.2 Nova Scotia’s Climate, T6.2 Oceanic Envi-ronments, T6.3 Coastal Aquatic Environments, T6.4Estuaries, T7.1 Modifying Forces, T8.1 FreshwaterHydrology

Associated HabitatsH1 Offshore

References1 Pond, Stephen and George L. Pickard (1978)

Introductory Dynamic Oceanography. PergamonPress, Oxford.

2 Ingmanson, D.E., and W.J. Wallace (1973)Oceanology: An Introduction. WadsworthPublishing Co. Inc.

3 Canadian Tide and Current Tables, Vols. 1 & 2,Atlantic Coast and Bay of Fundy. Department ofFisheries and Oceans, Ottawa.

4 Pickard, George L. (1979) Descriptive PhysicalOceanography, 3rd ed. Pergamon Press, Oxford.

5 Mann, K.H., and J.R.N. Lazier (1991) Dynamicsof Marine Ecosystems. Blackwell ScientificPublications, Oxford.

6 Simpson J.H., and J.R. Hunter (1974) “Fronts inthe Irish Sea.” Nature 250.

T6.1Ocean

Currents

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

120▼


T6.2 OCEANIC ENVIRONMENTS

Temperature and salinity are important componentsof the oceanic environment influencing biologicalproductivity. Tables T6.2.1a to T6.2.1d1 show chartsof February and August temperatures and salinitiesin the shallow nearshore waters of Nova Scotia. TheFebruary upper-layer temperatures are at or close tofreezing; the south shore of Nova Scotia and Bay ofFundy show the highest values in the region. August

temperatures are highest in Northumberland Straitand become cooler southwestward from Cape Bretonalong the Atlantic Coast.

Traditionally there are three distinct oceanic en-vironments that influence Nova Scotia: the Gulf ofSt. Lawrence, the Scotian Shelf and the Bay of Fundy–Gulf of Maine. The Nova Scotian portions of theselarger areas are described in Theme Regions (Vol-

Table T6.2.1a: Average February and August temperatures and salinities at 0-, 30- and 100-m depths for Inner Scotian Shelf (District 910).1

N/A = not available. Salinity reported in particle salinity units (psu), or parts per thousand.

Table T6.2.1b: Average February and August temperatures and salinities at 0-, 30- and 100-m depths for Middle Scotian Shelf (District 920).1

N/A = not available

T6.2OceanicEnvironments

emaNtcirtsiD

tinU/

(erutarepmeT ° )C )‰(ytinilaS

m0 m03 m001 m0 m03 m001

beF guA beF guA beF guA beF guA beF guA beF guA

osnaC 119 8.0- 4.71 6.0- 7.6 9.0 0.1 2.13 7.92 2.13 2.13 1.23 7.23

erohSnretsaE 119 7.0 4.51 0.1 8.4 8.3 6.3 2.13 7.03 3.13 0.23 9.23 1.33

erohShtuoS 119 6.0 8.41 5.0 1.4 8.2 8.3 3.13 0.13 4.13 0.23 5.23 2.33

enrublehS 119 6.1 6.01 6.1 2.6 A/N 6.4 2.13 9.13 2.13 1.23 A/N 1.33

laohSrehcruL 119 9.2 7.9 2.3 8.8 A/N 2.7 5.13 1.23 6.13 5.23 A/N 2.33

thgiByendyS 519 5.1- 1.71 1.1- 8.7 2.1 2.1 4.03 4.92 2.13 7.03 4.23 7.23

emaNtcirtsiD

tinU/


m0 m03 m001 m0 m03 m001


knaByawesoR b129 0.2 6.51 1.2 8.5 0.3 6.4 6.13 3.13 6.13 9.l3 2.23 1.33

knaBelddiM e129 1.0- 3.71 4.0- 8.6 1.2 1.1 4.13 6.03 4.13 7.13 5.23 6.23

knaBeniasiM g129 8.0 7.61 1.1 6.5 5.2 0.1 7.13 3.03 8.13 4.13 4.23 6.23

uaerauqnaB h129 2.2 1.61 1.2 9.6 2.2 5.4 2.23 1.13 2.23 0.23 0.33 7.33

nisaBsegroeG b229 3.4 7.71 7.4 4.21 3.6 5.6 0.33 1.23 1.33 5.23 8.33 6.33

nisaBevaHaL c229 1.1 7.51 5.1 9.4 8.3 6.4 2.13 0.13 4.13 3.23 1.33 5.33

nisaBdlaremE d229 4.2 7.71 9.2 3.5 1.7 3.5 1.23 9.03 3.23 4.23 2.43 7.33

eniaMfofluGtsaE 329 5.3 6.21 6.3 4.9 9.4 1.7 8.13 4.23 7.13 8.23 7.23 6.33

lennahCyawesoR 329 A/N A/N A/N 3.7 A/N A/N A/N A/N A/N 6.23 A/N A/N

nisaByawesoR 329 9.1 9.01 8.1 7.4 8.3 1.3 2.13 8.13 2.13 3.23 6.23 8.23

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

121▼


Table T6.2.1c: Average February and August temperatures and salinities at 0-, 30- and 100-m depths in the Outer Scotian Shelf Region (District 930).1

N/A = not available

Table T6.2.1d: Average February and August temperatures and salinities at 0-, 30- and 100-m depths for the Continental Slope (District 940).1

N/A = not available

ume 2) and the corresponding Districts and Unitsare indicated below.

ENVIRONMENT OF THEGULF OF ST. LAWRENCE

The Gulf of St. Lawrence (Unit 914) is an inlandsea as well as a large estuary. It has been com-pared to the Baltic Sea in northern Europe be-cause both hold similar quantities of water, bothdrain fresh water from comparable watersheds, andboth have similar biological productivity. However,the Gulf of St. Lawrence exchanges ten times moreocean water than the Baltic Sea,2 for reasons related

to the larger runoff in the Gulf of St. Lawrence. Thus,while the residence time for water in the Baltic isapproximately twenty years (7000 days), in the Gulfof St. Lawrence it is 200 to 500 days.

The environment in the Gulf of St. Lawrence isinfluenced by freshwater flow from the St. LawrenceRiver, by winds and air temperatures, and by flows inand out of the Strait of Belle Isle and the Cabot Strait.In the winter a two-layer water column structureexists, with a thick, cold, relatively fresh layer overly-ing deep, warmer water from Cabot Strait (FigureT6.2.2). The cold, upper layer, formed by coolingrather than by influxes of polar water, has tempera-tures of –1 to 2°C and associated salinities of 32 to 33

T6.2Oceanic

Environments

emaNtcirtsiD

tinU/


m0 m03 m001 m0 m03 m001


knaBsegroeGE a139 9.3 1.51 5.4 7.01 7.5 2.7 7.23 3.23 0.33 6.23 1.33 4.33

laohSsegroeG a139 4.4 9.31 9.4 4.21 A/N A/N 1.33 A/N 4.33 5.23 A/N A/N

knaBsnworB b139 5.3 3.41 5.3 3.7 0.8 3.8 0.23 8.13 1.23 5.23 1.43 2.43

knaBoraccaB b139 3.2 4.41 1.3 8.7 5.4 1.4 9.13 6.13 2.23 1.23 5.23 1.33

knaBevaHaL c139 5.2 6.61 3.2 5.6 1.5 6.5 6.13 3.13 7.13 1.23 3.33 6.33

knaBdlaremE d139 4.3 1.91 7.3 1.8 1.8 9.8 4.23 2.13 5.23 9.23 4.43 7.43

knaBnretseW e139 4.3 9.81 5.3 6.8 5.7 0.7 3.23 7.13 4.23 6.23 0.43 2.43

knaBdnalsIelbaS e139 5.2 1.71 4.2 1.21 A/N 3.7 1.13 2.13 4.13 8.13 A/N 0.43

uaereuqnaB f139 2.2 1.61 1.2 9.6 2.2 5.4 2.23 1.13 2.23 0.23 0.33 7.33

lennahCEN 239 2.3 8.51 7.1 0.21 6.6 7.8 1.23 4.23 8.23 9.23 7.33 2.43

elddaS 239 A/N 1.81 A/N 9.6 A/N 1.7 A/N 1.13 A/N 8.23 A/N 0.43

ylluGehT 239 A/N 8.61 3.2 0.7 6.3 6.3 A/N 8.03 1.23 7.13 0.33 3.33

emaNtcirtsiD

tinU/


m0 m03 m001 m0 m03 m001


epolSnrehtuoS 049 8.5 7.71 3.9 5.11 1.4 2.4 2.33 3.23 4.43 0.53 9.43 9.43

epolSlartneC 049 8.4 2.91 8.8 4.9 A/N 1.4 9.23 8.13 4.43 7.43 A/N 0.53

epolSnrehtroN 049 0.4 2.81 9.6 7.7 4.3 1.4 7.23 9.13 1.43 3.43 0.53 0.53

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

122▼


In summer, the low tides that occur during theday, expose beaches and sandbars to the sun. Theincoming water is warmed as it moves over thesesand flats, retaining the high water temperature.

In the open Gulf, surface salinities range from 26to 32 psu, but may be lower in bays and estuaries. Asthe summer progresses, the surface layer in the south-western area of the Gulf becomes thinner and lesssalty, due to the arrival and influence of the springpeak of the St. Lawrence River freshwater runoff,which has been two to three months in transit. Ofcourse, changes in wind direction and strength mayalter the relative positions of the water layers, result-ing in changes in temperature and salinity.

Environmental variations linked with river flowmay be an important determinant of commercial fishproduction, especially in the spring. There is anupwelling effect of fresh water; nutrients are broughtto the surface.5 The arrival of the spring dischargefrom the St. Lawrence estuary significantly affects thehydrology of the area and, through the formation offronts, may result in increased aggregation of foodorganisms and fish larvae.6

In addition to natural effects, there are significantchanges in seasonal flow patterns resulting from thecontrol of the freshwater supply by hydroelectric

particle salinity units (psu), or parts per thousand.The deep “warm” layer has temperatures as high as6°C and salinities over 34 psu. The temperature ofthis deep, warm layer has increased by up to 2°Cbetween the mid-1960s and the late 1970s. Analysisindicates that the primary factor determining thevariation of the deep-water properties is the varia-tion of the oceanic waters at the mouth of theLaurentian Channel, rather than variation of fresh-water runoff, as previously suggested.3

In the spring, a third layer—a warm, surfacelayer—develops on top of the cold layer. The coldlayer persists at middle depths. The surface layer isapproximately 20 m thick, with temperature typi-cally around 15°C and salinity 30 psu.

In the Magdalen shallows and the Northumber-land Strait, the difference between summer and win-ter temperatures is even more pronounced, due tosummer heating of the relatively shallow water col-umn. Summer temperatures in the Northumber-land Strait are higher than on the Atlantic Coast (seeFigure T6.2.2). These warm temperatures result fromthe combination of shallow water, stratification andrelatively weak currents. The tidal range is 2 m in theNorthumberland Strait, and at certain times, onlyone tide occurs (see Figure T6.1.4a).

Figure T6.2.2: Summer stratification of ocean waters:Scotian Shelf (left) and Gulf of St. Lawrence (right).4


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

123▼


development, principally in Quebec. The effect is toreduce greatly the biologically important spring dis-charge. Total productivity of the Gulf system hasprobably already been seriously reduced, and fur-ther hydroelectric development could further re-duce productivity.6

ENVIRONMENT OF THE SCOTIAN SHELF

The environment of the Scotian Shelf (Units 911, 915and Districts 920, 930) is determined largely by thecirculation. Moving outward from the Atlantic Coastof Nova Scotia, four distinct bands of water arecrossed, as shown in Figure T6.2.37. The ScotianShelf is normally covered by a surface layer of rela-tively fresh and low-density coastal water, the NovaScotia Current. Beyond this, in the vicinity of theedge of the continental shelf and separated by asharp front, is a band of slope water consisting ofGulf Stream water diluted approximately 20 per centby coastal water. Beyond the slope water lies the GulfStream and beyond that is central Atlantic water.

On the Scotian Shelf in summer, three verticallayers of water are evident:

l. an upper, warm, “fresh” layer, typically 30 mthick, with temperatures of 5 to 20°C andsalinities of less than 32 psu

2. an intermediate, cold layer, typically 100 m inthickness, with temperatures of 0 to 4°C andwith salinities from 32 to 34 psu

3. a warm, saline, bottom layer, located at depthsbetween 90 and 200 m, with temperaturesbetween 5 and 8°C, and salinities up to 35 psu.Temperatures in the warm bottom layer may riseas high as 12°C due to incursions of slope water.

A long-term, large-scale overview of shelf sea-sur-face temperatures from Newfoundland to Marylandshows coherent trends for the Grand Banks, ScotianShelf, Gulf of Maine, and the Sydney Bight. Annualaverages vary coherently over a range of approxi-mately four Celsius degrees. Increased sea-surfacetemperature is related to increased onshore winds.8

Against this background, there are many areas onthe Scotian Shelf where the mixing/stratification se-quence (and consequent high productivity) occurs.There are coastal upwelling areas, estuarine upwelling,banks with tidal gyres, and shelf-break fronts.

Figure T6.2.3: Classification of ocean water off the Atlantic Coast.7

T6.2Oceanic

Environments

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

124▼


ENVIRONMENT OF BAY OF FUNDYAND GULF OF MAINE

The dominant influence on the marine environ-ment of the Bay of Fundy (Units 912, 913) is thatexerted by the phenomenally large tides. The watersof the Bay of Fundy come from the Scotian Shelf aswell as from the Gulf of Maine. The tidal mixing ofwaters with different characteristics tends to modifyseasonal temperature variations and to create a wa-ter column that is fairly uniform in temperature andsalinity. Ice occurs in the upper reaches of the Bay ofFundy from December to April, and ice conditionsare influenced by the macro-tidal characteristics ofthe area. Tides influence patterns of erosion andsediment deposition in coastal areas. Tidal mixing,coupled with ample sediment sources, makes thewater quite opaque, an important factor in limitingphytoplankton productivity.

The mud-flat habitat at the head of the Bay ofFundy is influenced by the large tidal extreme andseasonal occurrence of highest tides (see T6.1).

The Gulf of Maine ocean environment (parts ofUnit 911 and Districts 920, 930) is determined by theenvironmental condition of Scotian Shelf watersand by tidal and mean currents, slope-water incur-sions, and atmospheric influences. In contrast to theBay of Fundy, the tides are not so very dominant.

The intermittent incursions of relatively warm,saline slope water occurs in surface and bottomlayers. The Gulf of Maine bottom water originatesfrom slope water that enters the Gulf through theNortheast Channel. It is modified within the Gulfby vertical mixing with the near-surface waters ofScotian Shelf origin.

Analyses suggest that as much as 44 per cent ofthe new nitrate which enters the Gulf of Maine atdepth through the Northeast Channel upwells in theeastern Gulf. This feature appears to be very impor-tant to the general biological productivity of theinner Gulf of Maine.

○ ○ ○ ○ ○ ○ ○ ○

Associated TopicsT3.5 Offshore Bottom Characteristics, T5.2 NovaScotia’s Climate, T6.1 Ocean Currents, T6.3 CoastalAquatic Environments, T6.4 Estuaries, T7.1 Modi-fying Forces, T10.9 Algae, T11.7 Seabirds and Birdsof Marine Habitats, T11.12 Marine Mammals, T11.14Marine Fishes, T11.17 Marine Invertebrates

Associated HabitatsH1 Offshore, H2.4 Mud Flat

References1 Petrie, B., and F. Jordan (1992) Nearshore,

Shallow-water Temperature Atlas for NovaScotia. Cdn. Tech. Rept. Hydrog. & Ocean Sci.,Department of Fisheries and Oceans, BedfordInstitute of Oceanography.

2 Pocklington, R. (1983) “The Gulf of St. Lawrenceand the Baltic Sea: A comparison of two organicsystems.” Mar. Chem. 12 (2–3) 235.

3 Koutitonsky, V.G., and G.L. Bugden (1991) “Thephysical oceanography of the Gulf of St.Lawrence: A review with emphasis on thesynoptic variability of the motion.” In The Gulfof St. Lawrence: Small Ocean or Big Estuary?(Can. Spec. Publ. Fish. Aquat. Sci. 113).

4 Brookes, I. (1972) “The physical geography ofthe Atlantic Provinces.” In The Atlantic Prov-inces, edited by A. MacPherson. University ofToronto Press, Toronto.

5 Dunbar, M.J. (1980) “The Gulf of St. Lawrence:Physical constraints on biological production,Canada and the sea. I. Resources of the marineenvironment: East and West Coast.” Assoc. forCan. Studies 3 (1): 7–14.

6 Cote B., M. El-Sabh and R. Durantaye (1986)“Biological and physical characteristics of afrontal region associated with the arrival ofspring freshwater discharge in the southwest-ern Gulf of St. Lawrence.” The Role of Freshwa-ter Outflow in Coastal Marine Ecosystems, vol. 7.

7 Hachey, H.B. (1961) “Oceanography andCanadian Atlantic waters.” Fisheries ResearchBoard Canada. (Bulletin No. 134).

8 Thompson, K.R., R.H. Loucks and R.W. Trites(1988) “Sea surface temperature variability inthe shelf-slope region of the Northwest Atlan-tic.” Atmosphere-Ocean 26 (2).

Additional Reading• Petrie, B., and K. Drinkwater (1992) “Long-term

temperature and salinity variability on theScotian Shelf and in the Gulf of Maine.” In TheClimate of Nova Scotia; Proceedings from aSymposium, November 19, 1991. AtmosphericEnvironment Service, Environment Canada,Dartmouth, N.S.


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

125▼


T6.3 COASTAL AQUATIC ENVIRONMENTS

PROCESSES

Land obstructions prevent tidal currents movingfreely around the earth. Instead, the water movesbackwards and forwards in semi-enclosed basins.Every bay or basin has a natural period of oscillationdepending on its length, depth and shoreline. If thisperiod coincides with the period of the tides, the twowill augment each other, resulting in tides with highamplitude. When the two periods differ substan-tially, tides will tend to be low. The Bay of Fundy istuned to semi-diurnal tidal frequencies, resulting inever-increasing amplitudes towards the upperreaches.

The water column is not mixed significantly bytidal action in areas of southern Nova Scotia, and thewater may be stratified (density increasing withdepth). These areas typically receive waters whichare high in organic material, or “brown water,” whichcan block the penetration of light to the seawaterbelow and reduce productivity during periods ofhigh freshwater flow.

Wave exposure is generally lower in inlets andbays, resulting in sediment accumulation and lowerseaweed populations. Coastal oceanographic proc-esses often pile up more sediments along the coastthan can be carried laterally by currents, leading tothe formation of bars or barriers, which block themouths of some river systems (see T8.3).

LAGOONS AND BARACHOIS

The entrance to harbours can be blocked by thedevelopment of coastal spits, barrier islands andbarrier beaches, depending on local conditions.These protected areas develop an estuarine charac-ter. In the Northumberland Strait, coastal lagoonsare protected from ice scour. Species of seaweedpreferring warm water may develop there.

BRAS D’OR LAKE

The two channels—the Great and Little Bras d’OrChannels—leading into the Bras d’Or Lake systemare very narrow, restricting the volume of water en-tering and leaving with each tide. For this reason, thetidal range is small, approximately 0.08 m nearBaddeck, compared to 0.9 m in the Cabot Strait. In

T6.3Coastal Aquatic

Environments

The coastline of Nova Scotia encompasses nu-merous bays, inlets and lagoons. The largest in-clude St. Georges Bay (Units 521b/914), separat-ing Antigonish and Inverness counties, and St.Margarets Bay (District 460b, Unit 911); the small-est, coastal inlets, such as Herring Cove (Unit851), near Halifax. Bras d’Or Lake (District 560/Unit 916) in Cape Breton Island is a unique inlandwaterbody.

The majority of inlets and harbours were formedas the result of submergence of river valleys. Physicalconditions in coastal waterbodies tend to be warmer,more estuarine (see T6.4), and more sheltered thanexposed sections of the ocean coastline. Conse-quently, they have animal and plant communitiesthat differ from those found on the open coast.

TOPOGRAPHY

Coastal aquatic environments are largely determinedby the type of underlying rock. Rivers on less-resist-ant bedrock in northern Nova Scotia erode widelowlands and result in shallow bays and branchingestuaries (e.g., Pictou Harbour and East, West andMiddle rivers in Unit 521a).1

Rivers and glacial meltwaters have eroded nar-row valleys in more-resistant bedrock in southernNova Scotia and, consequently, formed deeper estu-aries and excellent harbours. The larger bays andfeatures such as St. Georges Bay reflect their positionbetween major geological formations. Fjords, whichare common in Newfoundland and along theLabrador coast, are uncommon on the Nova Scotiacoastline. They are typically deep, have a U-shapedcross section and become shallower at the outwardend. (This is called a “sill.”) Examples can be foundon Cape Breton Island at Ingonish Harbour (Unit552c) and parts of Bras d’Or Lake.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

126▼


smaller basins, such as Whycocomagh Bay, the tidalrange is almost nonexistent. Tidal currents in thetwo channels can nevertheless be strong, with greatvariations in strength and direction at differentdepths. Barometric pressures can cause water levelsin the confined lake to rise or fall by up to 0.3 m whilethe adjacent ocean is, of course, unaffected. Theocean water brought in through this process main-tains the relatively high salt content in the system.Therefore, although they occur less frequently, thebarometric tides have much greater impact on theBras d’Or Lake than do the normal tides. Strong tidalcurrents of 3 m/s in the Great Bras d’Or Channel and1.5 m/s in the Barra Strait combine with the baro-metric tides to mix the fresh and salt waters in thelakes.

LARGE BAYS

St. Georges Bay (Units 521b/914), Mahone Bay (Dis-trict 460a/Unit 911) and St. Margarets Bay (District460b/Unit 911) have marine environments similarto the conditions typical of the open ocean offshore.Oceanic organisms, including jellyfish and pelagicfish (such as tuna), often enter the bays, and seaweedpopulations can be extensive. The size of the bayspermits significant wind fetch and moderate waveexposure. These areas may be affected by influxes offresh water from coastal streams, but the bays as awhole are not estuarine. Events such as storms andtides in offshore waters have a greater impact onthese bodies than local factors.2

CULTURAL FACTORS

Coastal bays and inlets are used increasingly forculture of fish and molluscs, principally mussels.Human development tends to cluster in areas af-fording suitable harbours and availability of waterfor disposal of wastes. As a result, some of these areashave been extensively polluted (see T12.12).

○ ○ ○ ○ ○ ○ ○ ○

Associated TopicsT2.7 Offshore Geology, T3.5 Offshore Bottom Char-acteristics, T5.2 Nova Scotia’s Climate, T6.1 OceanCurrents, T6.2 Oceanic Environments, T6.4 Estuar-ies, T8.1 Freshwater Hydrology, T7.1 ModifyingForces, T8.2 Freshwater Environments, T7.2 Coast-line Environments, T7.3 Coastal Landforms, T10.6Trees, T11.6 Shorebirds and Other Birds of CoastalWetlands, T11.7 Seabirds and Birds of Marine Habi-tats, T11.12 Marine Mammals, T11.14 Marine Fishes,T11.17 Marine Invertebrates, T12.7 The Coast andResources

Associated HabitatsH1 Offshore, H2.5 Tidal Marsh, H3.1 Open WaterLotic (Rivers and Streams), H3.3 Bottom Lotic (Riv-ers and Streams)

References1 Roland, A.E. (1982) Geological Background and

Physiography of Nova Scotia. Nova ScotiaInstitute of Science, Halifax.

2 Platt, T, A. Prakash, and B. Irwin (1972)“Phytoplankton nutrients and flushing of inletson the coast of Nova Scotia.” Le NaturalisteCanadien 99.

Additional Reading• Environment Canada (1988) A Profile of Impor-

tant Estuaries in Atlantic Canada. Environmen-tal Quality Division, Conservation and Protec-tion Branch.

T6.3Coastal AquaticEnvironments

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

127▼


T6.4 ESTUARIES

FRESHWATER INFLUENCE

When river flow is much greater than tidal move-ments, the fresh water tends to remain on top of theseawater because it is less dense, forming a freshwa-ter layer above a distinct salt wedge. Such an estuaryis said to be stratified (see Figure T6.4.1). Underthese circumstances, many of the dissolved andparticulate materials brought by the river move sea-ward in the estuary and may in fact be flushed rightthrough it if the flow is very high. As the water travelsfurther into the estuary, its velocity usually decreases,allowing heavier particles to settle toward the bot-tom and resulting in an accumulation of sedimentsand other detritus in deeper water. The outward flowof the river also mixes with and carries along some ofthe underlying seawater. This phenomenon has theeffect of causing water in the deeper seawater layerto move upstream as a reaction current or counter-current (see T6.1). The reaction current can carryfree-floating objects in the water just above the bot-tom (suspended sediments, planktonic larvae, frag-ments of detritus, etc.), bringing them upstreamtoward the head of tidal influence. Because of arelative lack of mixing in the salt wedge, water re-mains at about the same salinity as that of the nearbysea. It can be inhabited by marine plants and ani-mals that in some cases move from the ocean all theway to the head of the salt wedge. The surface layer,being fresh, may be inhabited by plankton and fishoriginating in fresh water.

Few of Nova Scotia’s rivers have such a domi-nating flow all year, but during periods of snowmelt or following heavy rainstorms, many estuar-ies may become temporarily stratified. Where the

Estuaries occur at the mouths of rivers whereseawater becomes diluted by fresh water drainingfrom the land. They are among the most productiveecosystems—comparable to rainforests and coralreefs—partly because they tend to be shallow, re-ceive a continuing supply of nutrients from the riverand are mixed by the tidal movements of the sea.1

They are not easy environments for organisms toinhabit, due to variations in salinity and tempera-ture, periodic exposure to the atmosphere and thegreat influences exerted by human beings (see T12).

Nova Scotia has many estuaries (almost as manyas there are rivers) and they vary regionally.

PHYSICAL FEATURES

Important physical features which determine estua-rine conditions include the morphology of the rivermouth and availability of soft sediments (both re-lated to the geology of the area), and the relativestrengths of tidal movements and river outflow.

Tides vary over several different time scales (seeT6.1). The sea level is also influenced by wind andchanges in barometric pressure; these may eitherincrease or decrease the natural tidal movementnear a coastline. River flow, in turn, tends to beseasonal—higher following major storms or snowmelt in spring, and lower during mid-winter and indry periods such as commonly occur in summer.

The relative rate of sea-level rise can also contri-bute to estuarine conditions. Studies in ChezzetcookInlet indicate that rapid sea-level rise (3.8 mm peryear between 1920 and 1970) has resulted in wide-spread coastal erosion and subsequent infilling ofthe estuary.2

Figure T6.4.1: Diagramaticlongitudinal section and profile of astratified estuary.

Seawater

Fresh water

Profile at ARiverRiver A

Salinity

T6.4Estuaries

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

128▼


Seawater

Profile at ARiverRiver A

Salinity

Figure T6.4.2: Diagramaticlongitudinal section and profile of avertically mixed estuary.

influence of the tide has been reduced by buildingof dams or causeways (e.g., the Annapolis River atAnnapolis Royal, District 610), such stratificationmay become a permanent feature.

TIDAL INFLUENCE

Many Nova Scotia estuaries are more strongly influ-enced by tidal and wind-generated movements ofthe sea than by the river flow. The strong bottomcurrents tend to mix the fresh water and seawatertogether, so that the salinity at any point in theestuary is approximately the same from the surfaceto the bottom of the water column. Such an estuaryis said to be vertically mixed (see Figure T6.4.2).

In these estuaries, salinity declines progressivelyas one moves upstream. In vertically mixed estuar-ies, materials brought down the river from the water-shed are also mixed from top to bottom. They mayaccumulate in the upper part of the estuary, forminga zone of distinctly turbid water, known as the tur-bidity maximum. Marine organisms, having littletolerance for reduced salinity, will be confined to theouter parts of the estuary. Freshwater species will berestricted to the end where the river enters. Mostparts of such estuaries are inhabited by species thathave relatively wide tolerance of salinity and tem-perature. Furthermore, animals living attached tothe bottom or in bottom sediments may be sub-jected to widely varying salinity as the tide ebbs andfloods.

Most of the estuaries along Nova Scotia’s coast(Atlantic Coast and Gulf of St. Lawrence) have tidalranges of about 1 to 2 metres. The smallest tidalrange (less than 20 cm) is found in Bras d’Or Lake(Unit 916). In the Bay of Fundy (Units 912, 913), onthe other hand, average tidal range increases fromabout 4 m near Yarmouth to almost 12 m in MinasBasin. Estuaries with tidal ranges smaller than 2 mare classed as microtidal, those from 2 to 4 m asmesotidal and those above 4 m as macrotidal.

Estuaries with large tidal ranges naturally have

extensive areas of tidal flats that are successivelycovered and exposed as the tide floods and ebbs (seeH2.4 and H2.5). This is a difficult environment formany organisms to inhabit, owing to the interactionof physical and chemical factors: salinity and tem-perature variations, the nature of the substrate, al-ternations in wetting and drying, and so on. Conse-quently, the diversity of species is often rather lim-ited, certainly by comparison with rivers, streamsand rocky shores. Where there are few species, how-ever, those present may be extremely abundant.

SEDIMENTS

Many estuaries have extensive areas of softsediments, such as deposits of sand, silt or clay,which are exposed at low tide. Their distribution isdetermined by the strength of tidal and river cur-rents, or exposure to waves, but they each repre-sent significantly different habitats for organisms.

Sands occur where water movements are quitestrong. They are coarse in texture and drain quicklyas the tide ebbs. The larger clams, polychaetes thatconstruct tubes from sand grains, burrowing isopodsand other crustaceans are common inhabitants ofthis zone. In nutrient-rich sand flats, common inestuaries, the sand may also be colonized by benthicdiatoms, which provide the base of the sand-flatfood chain. This food source is often augmentedwith detritus from surrounding salt marshes andsubmerged Eel Grass beds, or by organic matterbrought down the river. Silts tend to be richer inorganic matter, do not drain as readily at low tideand are more likely to become anaerobic. These arealso inhabited by polychaetes, species of clams, mudsnails and small crustaceans. Clay-dominated “mudflats” occur in relatively calm water, appear “sticky,”tend to retain the water at low tide and are oftenanaerobic just below the surface because of the highcontent of plant-derived material. The surface of amud flat may sometimes be held together by carbo-hydrates exuded by microscopic diatoms living in

T6.4Estuaries

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


P A G E

129▼


CULTURAL FACTORS

There is a long tradition of use related to estuaries inNova Scotia. Most recently, aquaculture has focusedattention on maintaining the quality of estuarineenvironments (see T12.7 and T12.11).

○ ○ ○ ○ ○ ○ ○ ○

Associated TopicsT6.1 Ocean Currents, T6.2 Oceanic Environments,T6.3 Coastal Aquatic Environments, T8.2 Freshwa-ter Environments, T10.5 Seed-bearing Plants, T10.6Trees, T11.6 Shorebirds and Other Birds of CoastalWetlands, T11.7 Seabirds and Birds of Marine Habi-tats, T11.12 Marine Mammals, T11.13 FreshwaterFishes, T11.14 Marine Fishes, T11.17 Marine Inver-tebrates, T12.7 The Coast and Resources,T12.11 Animals and Resources

Associated HabitatsH2 Coastal

References1 Mann, K.H. (1982) Ecology of Coastal Waters. A

Systems Approach. Blackwell Scientific Publica-tions, Oxford. (Studies in Ecology 8).

2 Carter, R.W.G., J.D. Orford, S.C. Jennings, J.Shaw and J.P. Smith (1992) “Recent evolution ofa paraglacial estuary under conditions of rapidsea-level rise: Chezzetcook Inlet, Nova Scotia.”Proc. Geologists’ Association 103.

3. Postma, H. (1967) “Sediment transport andsedimentation in the estuarine environment.”In Estuaries, edited by G.H. Lauff. AmericanAssociation for the Advancement of Science,Washington, D.C. (Publication 83).

the upper millimetre of sediments and migrating tothe surface to photosynthesize as the tide ebbs. Eachtype of deposit has its characteristic fauna and floraand exhibits very strong interactions between thephysical and biological world.3

PRODUCTIVITY

Estuaries and other coastal environments are im-portant for biological organisms in a variety of ways.They tend to act as traps for sediment and nutrientsbrought down the river. The nutrients supportphytoplankton growth and extensive developmentof tidal marshes and frequently Eelgrass beds, whichare important to the Brant and Canada Goose. Thestems and leaves of Eelgrass decay in coastal watersand form detritus. Several estuarine species, includ-ing crustaceans, molluscs and worms, consume de-tritus either for its inherent food value or for the richmicroflora of bacteria and fungi that may be decom-posing it. Many Nova Scotia estuaries, particularly inthe inner Bay of Fundy (Unit 913a), support ecosys-tems which are almost entirely based on detritus.

The estuary is open to the sea, allowing mobilespecies, such as fish and crustaceans, to migrate intothe estuary for feeding, as well as to find appropriatespawning grounds. Estuarine circulation, and thenutrients it provides, offers an added benefit to ma-rine organisms over the already-important coastalregimes where upwelling, light and temperature arefavourable to growth. Estuaries have high produc-tivity by organisms in the water column and in sus-pension-feeding marine organisms (such as mus-sels) that feed on them, because of the physicalinteraction of the fresh and salt water and the nutri-ent supply it provides. In estuaries in which thesurface freshwater layer is well defined, a tongue ofsalt water can extend significant distances upriverand provide habitat for marine organisms.

The warm temperatures, rich food supply andrelative absence of predators make estuaries impor-tant nursery grounds (areas where young stages areable to feed, find shelter and grow rapidly). A numberof fish species (e.g., salmon, flounder, herring, shadand Striped Bass) are linked closely to estuarineenvironments at important stages in their life cycle.The fertile estuarine regions of the Bay of Fundy alsoattract migratory fish (e.g., pollock, dogfish), seabirds,shorebirds and marine mammals, which sometimescome from great distances. These estuaries are there-fore important for Nova Scotia and much of theNorthern Hemisphere.

T6.4Estuaries