II. Phytoplankton and primary production

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 188: 80 -89 . 1989

Biological oceanography in the Canadian High Arctic

A. R. Longhurst, T. Platt, W. G. Harrison, E . J . H . Head, A . W . Herman, E. Horne, R. J. Conover, W. K. W. Li, D. V. Subba Rao, D. Sameoto, J. C. Smith, and R . E . H . Smith

Longhurst, A. R., Platt, T ., Harrison, W .G ., Head, E. J .H . , Herman, A. W., Horne, E ., Conover, R .J . , Li, W. K. W., Subba Rao, D. V., Sameoto, D., Smith, J .C . , and Smith, R .E .H . 1989. Biological oceanography in the Canadian High Arctic. - Rapp. P.-v. Réun. Cons. int. Explor. Mer, 188: 80-89 .

As part of the Canadian response to concern for the consequences of development in its Arctic territories, a major programme of biological oceanography was undertaken with ice-breaking ships and shore bases. The research, based at the Bedford Institute of Oceanography, was designed to quantify basic biological parameters of microbial and algal growth, and of feeding, respiration, and growth of herbivorous plankton. Thus, rather than with narrower, site-specific investigations, it was hoped that the general principles controlling the flow of nutrients and energy through the High Arctic marine ecosystem could be understood sufficiently well that questions address­ing the well-being of valuable components of the ecosystem (mammals, birds) could be answered with insight into the performance of their energy base. The physiological ecology of bacteria, water column, and epontic algae and of the major herbivorous zooplankters has now been so well described, as have spatial and temporal relation­ships between producers and consumers, as to allow predictions to be made for performance of the pelagic ecosystem in seasons and regions not directly investi­gated. It is only by means of a concentrated group effort in fundamental research of this kind that progress can be made in the demanding environment of the High Arctic.

A. R. Longhurst, T. Platt, W. G. Harrison, E. J. H. Head, E. Horne, R. J. Conover, W. K. W. Li, D. V. Subba Rao, D. Sameoto, J. C. Smith, and R. E. H. Smith: Department o f Fisheries and Oceans, Biological Oceanography Division, Bedford Institute o f Oceanography, Dartmouth, Nova Scotia, Canada B 2Y 4A2. A . W. Her­man: Department o f Fisheries and Oceans. Metrology Division, Bedford Institute o f Oceanography, Dartmouth, Nova Scotia, Canada B 2Y 4A2.

Introduction

The Canadian Arctic seas are neither so large nor so ecologically fragile as suggested by Gerardus Mercator’s misleading projection and by current mythology, but they are still an important and diverse region relatively neglected by oceanographers.

During the last decade, the Canadian government has undertaken major scientific programmes in the Beau­fort Sea, the Arctic Ocean, the Canadian archipelago, Baffin Bay, and the Labrador Sea. The distribution, ecology, and stress-sensitivity of the marine mammals and birds of interest in the High Arctic have been stud­ied by several federal agencies, including the Depart­ment of Fisheries and Oceans (DFO). Studies of biolog­ical oceanographic processes in the same region, per­

formed mainly by the DFO Biological Oceanography Division at the Bedford Institute of Oceanography, are reported in this paper; much of this work was supported by contracts to DFO from other Canadian federal agen­cies.

We have made eight Arctic voyages on the scientific ships “Hudson” and “Baffin” and several voyages on other Canadian ice-breakers, and we have occupied an ice station at Resolute on the Northwest Passage for several winters. With the availability of the large Cana­dian ships, and the support of the Polar Continental Shelf project, we have been able to deploy special “through-the-ice” scientific equipment, as well as equip­ment normally used by biological oceanographers in open seas, so that our ship-borne Arctic studies are an extension of our North Atlantic research.

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During the same period, other DFO biologists from Bedford Institute have worked from ice stations on the Arctic Ocean, nearly to 90°N. The principal thrust of their research has been to understand the dynamics of contaminant transfer in a relatively pristine, high-lat- itude oceanic water column having very low levels of productivity. Farther south, the variable influence of Arctic water masses on the fisheries of the Grand Banks of Newfoundland has been studied.

We undertook this research in 1978 to increase our understanding of the ecological processes which control the production of Arctic plankton, which is the base of the food chain supporting char, seals, walrus, beluga, bears, and birds, themselves the mainstay of traditional Inuit life. It is intended that this research shall lead to better interpretation of the wildlife surveys, and site- specific environmental impact studies being performed by other agencies in the Canadian Arctic.

Our biological oceanography programme was there­fore planned to quantify the consequences of the Arctic environment for the physiology and growth rates of planktonic algae, and for the consumption of algae and their transformation into animal material by planktonic crustaceans. Research has concentrated principally on the ecological consequences of the following:

(a) extremely low sea-water temperatures which change only slightly during the year, or with depth;

(b) the Arctic light regime, with several months of total darkness exacerbated by snow and ice cover, a variable amount of which remains each summer; and

(c) the shallow, brackish surface layer derived from ice-melt each spring.

Biological oceanographic processes have been studied in a variety of environments during this programme: in Baffin Bay, with its ocean depths of more than 2000 m, in many of the passages within the archipelago standing on the Canadian continental shelf, and in the macrotidal Foxe Basin, where complete mixing of the water column is frequent.

Activity of Arctic bacterioplanktonThe number of bacteria in eastern Canadian Arctic waters is consistently of the order of 0.1 - 1 .0 x 106 cells m r 1. When a subsurface peak of bacterial numbers is present it occurs at the same depth as the chlorophyll a peak. Indices of heterotrophic activity correlate with the distribution of bacterial cells. In the upper 100 m, turnover rates for glucose and amino acids are between 1 and 50% per day, while below 100 m the rates are between 0.1 and 1.0% per day. On one occasion, when net growth of Arctic bacterioplankton was measured, a doubling time of 4.4 days was recorded.

Bacterial populations of High Arctic seas are found to comprise not only psychrophilic biota, but also a range of thermal types. There is active and opportunistic growth at mesobiotic (~30°C) temperatures of some strains present in the natural Arctic bacterioplankton assemblage. The same response was not obtained from natural assemblages of Arctic algae, which were much more uniform in their temperature response (Li and Dickie, 1984).

If bacterioplankton assemblages contain biota func­tional at a wide range of temperatures, this may explain why we have regularly measured significant rates of bacterial activity at close to 0°C also at lower latitudes. There is active heterotrophic utilization of organic sub­strates during the late winter algal bloom off Nova Scotia at water temperatures below 1 °C. These results are at odds with the conclusion recently reached by others that microbial utilization of algal metabolites is suppressed at very low temperatures. Rather, our re­sults support much earlier suggestions that temperature alone should not limit heterotrophic activity in bacterial populations: we think that wherever there is production of dissolved organic substrate by autotrophs, there will also be active bacterial metabolism, whatever the tem­perature (Li and Dickie, 1987).

Phytoplankton populationsDiatom blooms are generally assumed to dominate Arc­tic phytoplankton activity, and large cells (<90 % Bacil- lariophycae) are indeed abundant at <750 x 103 cells r 1, though we have recorded coccolithophores at even higher densities of <925 x 103 cells L ‘ (Trotte, 1985). However, examination of the smaller autotrophic frac­tions showed that substantial (but variable) proportions of chlorophyll a, cell numbers, and RuBPC activity are actually in a picoplankton fraction, of < 3 (xm cell diam­eter. We found that 1 0 -7 0 % of chlorophyll, and 10-25 % of photosynthesis, were in the picoplankton fraction in samples from the macrotidal Foxe Basin, and up to —60 % in a wider study of Jones and Lancaster Sounds, northern Baffin Bay, and the Labrador Sea (Trotte, 1985). Overall, probably 10—25% of Arctic primary production is due to picoplankton, compared with 20—30 % in mid-latitudes and more than 50 % in tropical oceans. Moreover, —10% of enzyme activity may derive from ultramicroplankton cells <0 .2 |xm, ap­parently capable of autotrophy and the synthesis of DNA and RNA; these minute organisms of the “fem- toplankton” are beyond the reach of optical micros­copy, and have yet to be visualized (Smith et al., 1985). There are differences in the relative temporal and spa­tial distribution of the picoplankton fraction in the Arc­tic; for example, in the well-stratified water of Jones Sound in late summer, the contribution of picoplankton is rather low. This probably reflects seasonal succession

6 Rapports et Procès-Verbaux81

of size fractions of the phytoplankton (Smith and Horne, in prep.).

Other aspects of the size structure of the phytoplank­ton can be inferred from metabolic rate measurements. Most (60% ) of the photosynthetic oxygen production and algal biomass is associated with particles >35 um whereas most respiration (> 5 0 % ) is associated with particles <1 |im, presumably bacteria and other mi- croheterotrophs. Integrated oxygen fluxes (0 -100 m) are approximately in balance, but a substantial portion of the respiration occurs below the photic zone (Harri­son, 1986).

General ecology of High Arctic algaeThe general features of the High Arctic that are rele­vant to phytoplankton production, and how they differ from the Antarctic, have been discussed many times (Nemoto and Harrison, 1981). Though Arctic and A nt­arctic seas are similar in their temperature and light regimes, they differ significantly in seasonal variability of sea-ice cover and nutrient chemistry. There are two extreme conditions: the perpetually ice-covered Arctic Ocean, and the surface layers of the open Antarctic Ocean which are continuously supplied during the ice- free summer with nutrients by strong wind-driven tur­bulence and upwelling. Like some of the more enclosed Antarctic sea areas, nutrients in the enclosed seas and passages of the Arctic archipelago are very rapidly de­pleted in the shallow, low-salinity surface layer which forms as the seasonal ice cover disperses.

Earlier investigators inferred that algal production must be low and nutrient-limited in the Arctic, com­pared with higher production, unlimited by nutrient supply, in the Antarctic. It has also been suggested that the growth of algae in very cold water depends on different responses of photosynthesis, photorespiration, and respiration from those of warmer-water species. Our findings suggest rather different interpretations.

Phytoplankton profiles in northern Baffin Bay during the open-water period characterize a “typical Arctic structure” uninfluenced by coastal topography or tidal mixing. Chlorophyll a maxima consistently occur be­tween the 10 % light level and the bottom of the photic zone, which is usually deeper than the mixed layer. These chlorophyll a maxima usually have about six times the surface values, which average 1.25 mg Chi m Below the photic zone, there are indications of enhanced microbial metabolism, associated with aggre­gations of decaying algal cells, especially at depths greater than 500 m. The magnitude and the vertical distribution of organic biomass and production are not different from what has been observed at high southerly latitudes, and are much higher than values typical of the ice-covered Polar Ocean of the Arctic (Harrison et al., 1987).

Nutrient (NO,. P 0 4. SiO,) concentrations are typ­

ically low in the mixed layer, and NO, values are some­times below the analytical limit of detection. A nutri- cline occurs, coincident with the pycnocline. Reduced forms of nitrogen (NH4, urea), on the other hand, are relatively available throughout the summer and consti­tute an important source of nitrogenous material uti­lized by planktonic algae at that time, a fact that ap­pears to have been missed by previous investigators (H arrisone /a /., 1985).

Integral (euphotic zone) primary production rate nor­malized to chlorophyll (PI, the photosynthetic index) is an indicator of phytoplankton turnover rates. Review­ing other modern data sets, we find an average PI of 6.7 (as g C(g C h i ) '1 d“1) for all Arctic waters except the Polar Ocean, which has a value of —0.2; our own data for Baffin Bay open waters indicate a PI of 4.0. Recent Antarctic data translate to an overall value of 7.7, a value perhaps not significantly higher than the average for the Arctic (Harrison et al., 1982). Using primary production and particulate carbon/chlorophyll ratios (C/Chl), we found algal growth rates to average —0.3 doublings per day (range: < 0 .1 -0 .8 ) , or about half the expected rates for prevailing water temperatures and optimal conditions of light and nutrients (Eppley, 1972) but within the range of values found in the Antarctic (Holm-Hansen et al., 1977). Our C/Chl ratios (g/g) fell within a relatively narrow range of 30—40 (W. G. Harri­son, unpubl. data).

We have measured carbon fixation and oxygen evolu­tion simultaneously in many incubations in the eastern Arctic. There was excellent agreement between the two rates, and a photosynthetic quotient (PQ = molar ratio of oxygen evolved to carbon fixed) of 1.3—1.8 was generally indicated, which is suggestive of some nitrate metabolism, even though nitrate levels are generally low (Platt et al., 1988). This, and our earlier work, suggest that as much as 50% of algal production is supported by nitrate in High Arctic seas even when nutrient concentrations in the water column are low (Harrison et al., 1985).

It has been timely to make new calculations of re­gional primary production (phytoplankton plus epontic algae) for the Arctic seas based on modern data from high northern latitudes, and on our own research. It is still extremely difficult to integrate the very high pro­duction regimes of the Arctic archipelago and coastal areas and the extremely low production of the ice-cov- ered Polar Ocean into a single estimate: this is the single largest source of uncertainty in our estimate. Our calcu­lations indicate a total Arctic production of 206 x 106 1 C yr_l, which is higher by more than an order of magni­tude than previous calculations based on generalization from very few data, and is much closer to regional assessments for the Antarctic. On a global basis, howev­er, this upward revision is relatively insignificant since the whole sea area north of the Arctic Circle accounts for less than 4 % of the world’s oceans (Subba Rao and Platt, 1984).

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In addition to under-ice and open-water-column pri­mary production, we have studied the algal blooms (often an order of magnitude denser than elsewhere in the region) that occur at glacier fronts in some Arctic fjords. Increased nutrient levels at submerged vertical ice cliffs can be explained by the upwelling of deep nutrient-rich water from below the pycnocline by melt- water-driven convection cells at the face of the glacier. It is not necessary to invoke the nutrient content of meltwater itself, as had been previously assumed, to balance the nutrient budget of the algal blooms that occur in such places.

Upwelling is restricted to a boundary layer conform­ing to theoretical expectation, and is accompanied by downwelling driven by cabbeling (mixing of waters of equal density, but different T and S to produce water of greater density because of the non-linearity of the equa­tion of state) about 0.5 km from the ice cliff (Horne,1985). We find that this process occurs wherever we have looked for it alongside a sufficiently deep, vertical ice face, and we have identified similar convection cells alongside large, free-floating icebergs.

Physiology of Arctic algaeIt might be expected that high-latitude algae would show special physiological adaptations to extremely low light, exhibiting higher light efficiencies at low irradian­ces than their low-latitude counterparts. On the other hand, low temperatures might be expected to result in lowered maximum productivity and longer doubling times, though it has been suggested that this may be overcome by adaptive adjustments in the Q 1() temper­ature response.

In fact, our incubation experiments from the Carib­bean to the Canadian Arctic archipelago have demon­strated a gradient in photosynthetic production rate for natural phytoplankton assemblages which covaried with temperature and solar radiation. These incubations showed that Tmin, Topl, and Tm;lx for photosynthesis (and change in rate per unit temperature) decrease from low to high latitudes. Our data show that algal assemblages from high latitudes are not so sensitive to relatively slight increases (AT < 2 °C ) of ambient temperatures as are low-latitude assemblages, whose in situ and optimal temperatures are relatively similar. The strong corre­spondence that we found in our experiments between(i) normalized primary production rates and light and(ii) photosynthesis and incubation temperatures con­firms that conventional nutrients are less limiting in Arctic primary production than previously thought (Harrison et al., 1982).

Arctic algae appear to allocate carbon into photosyn­thetic end-products in a manner much the same as that of other algae. Though the incorporation of labelled photosynthate into lipid by Arctic phytoplankton and ice algae is somewhat smaller than reported for the

Antarctic phytoplankton (usually <30 % compared with 5 0 -8 0 % ) , lipid incorporation is not enhanced by low temperatures over the range —1 to +6°C , or by lack of light. Actual rates of net lipid synthesis can be much higher than simultaneous rates of labelled photosyn­thate incorporation, however, and measurements of biochemical composition have suggested that microal­gae of cold waters may indeed synthesize lipid at unusu­ally high rates (4 0 -6 0 % of total C fixation), but in some way discriminate against recent photosynthate as the substrate. Species-specificity in metabolism, rather than light or temperature conditions, appears to ac­count for the variable patterns of biosynthesis in Arctic and Antarctic microalgae (Smith et al., 1988a; Smith et al., 1986; Li and Platt, 1982).

Polar ectothermal biota have been thought to possess enzymes that are more efficient catalysts than those of low latitudes, so we have measured the activation en­ergy (Ea) of the key photosynthetic carboxylating en­zyme, RuBPC, in different Arctic areas. Ea is a quanti­fication of thermal activation and we have shown this quantity to be the same for RuBPC and photosynthetic capacity in Arctic phytoplankton (Li et al., 1984). Cana­dian Arctic phytoplankton assemblages, compared with those from the Azores and the Costa Rica Dome, had Ea values that indicated lower catalytic efficiency, not higher. This is to be interpreted as the consequence of environmental differences other than absolute temper­atures; the Arctic environment may be better exploited by light-dependent metabolic processes that are temper­ature sensitive, while tropical assemblages should be more sensitive to nutrient level changes. We think that these observations demonstrate the basic difference be­tween the tropical nutrient-limited environment and the Arctic, where algae are principally light and temper­ature limited, and have light-dependent metabolic proc­esses that are temperature sensitive (Platt et al., 1983; Smith and Platt, 1985).

Comparison between results of phytoplankton pro­duction experiments in tidally mixed (Foxe Basin, Un- gava Bay) and stratified (Baffin Bay) Arctic environ­ments shows that while chlorophyll a biomass is lower in well-mixed regions, assimilation numbers are higher, resulting in equal or greater production rates per unit area. This is apparently not due to nutrient limitation in stratified areas, but to a mixing-induced photoadaptive response - or, possibly, to floristic differences (Smith et al., unpublished).

Thus, we find that the physiology and biochemistry of Arctic phytoplankton are to be interpreted as responses to one extreme of an environmental continuum from low to high latitudes; no novel physiological mecha­nisms need be invoked to explain the success of the Arctic plankton and - in fact - it would have been possible to predict the performance of Arctic algal cells under different conditions by extrapolation from what is known of the physiology of temperate-zone phytoplank­ton (Li and Dickie, 1984).

6*83

105* 65' 49*

THULE’ooSMITH

SOUND

DUNDAS

MELVILLEBAY

GREENLAND

BAFFIN

BAYBAFFIN

oISLAND65'

65°

LABRADORSEA

65° 55°75°

Figure 1. Part of the eastern Canadian Arctic archipelago and the western coast of Greenland showing the regions where the research was performed.

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Ecology of epontic algaeThe dynamics of the algae that develop in the interstitial water on the underside of sea ice with the return of sunlight to the High Arctic have also been studied. Experimental data suggest that these algae contribute less than 5 % to the total Arctic regional primary pro­duction, though a model for the theoretical upper bounds to ice-algal production suggests that they could contribute —20 % of algal production on an annual ba­sis, assuming that optimal snow and ice conditions oc­curred everywhere (Smith et al., 1988b).

As soon as sunlight returns to the High Arctic, epon­tic algae (usually pennate diatoms) develop on the un­derside of ice which is as much as 1—2 m thick. Light conditions below the ice resemble the bottom of the euphotic zone of the open sea, with usually less than 2 % of incident illumination. The photosynthetic effi­ciencies of epontic algae in the High Arctic, normalized to chlorophyll a content of cells, are unusually high so that photosynthesis can occur down to 0.01 % light (Cota, 1985). Cells from areas of heavy snow cover are easily photoinhibited in short-term photosynthesis as­says. The shade-adaptation of High Arctic epontic al­gae, compared with more variable light adaptation in Low Arctic ice floras (Legendre et al., 1988), enables them to develop very large standing crops, sometimes equalling the theoretical maximum for the available light. If in situ irradiance is artificially increased by keeping the ice surface totally snow-free, the High Arc­tic algae cannot fully exploit the increased light with increased crops even though photosynthesis is itself not photoinhibited. Increased losses from the epontic com­munity, perhaps due to excessive heating and ablation, may restrict crop size at unusually high irradiances.

Substantial fluxes of nutrients are required to support the bloom of epontic algae. Contributions from both heterotrophic regeneration and brine exclusion from the ice sheet are small in relation to demand, while there is 3 - 1 0 times more nutrient available in the mixed layer than is required to sustain the bloom. Nutrient flux depends on the state of the lunar tide and the consequent microscale turbulence at the lower ice sur­face as measured by hotwire thermisters. At neaps, there may be some nutrient limitation of algal growth (Cota et al., 1986). Consistent with this, epontic algae display marked shifts in the allocation of photosynthate to different intracellular polymer classes over the lunar tidal cycle. Spring tides (larger nutrient supply) are associated with increased allocation to protein. Neap tides result in lower photosynthetic rates and increased allocation to polysaccharides and lipids, potential car­bon storage pools which are also synthesized prefer­entially at higher irradiances (Smith et al., 1988a).

The salinity optimum of epontic algae occurs at about 30 ppt, a value typical of the salinity field of High Arctic seas, but much lower salinities occur as a result of meltwater in the spring, when a collapse of the ice-algal

bloom occurs. At that time, light penetration through the remaining ice also increases strongly, and algae respond with increased allocation to carbon storage pools; ablation of ice algae cells from the under-surface of the ice also occurs at this time (Bates and Cota, 1986; Smith et al., 1986).

Life strategies and ecology of Arctic zooplanktonThe paradigmatic high-latitude zooplankton is that of the Antarctic, commonly thought to be dominated by swarm-forming krill (Euphausia) and by large copepods (Calanus, Rhincalanus). This popular view is not consis­tent with the facts: krill do not dominate plankton sur­vey data from the Antarctic, and are only 0.5—4.1% numerically of net-caught zooplankton from regional surveys not biased towards areas with abundant krill swarms (Longhurst, 1985). Arctic plankton generally have a similar composition, for euphausiids (Meganyc- tiphanes, Thysanopoda) form 0.1—7.5% of individuals in regional surveys from West Greenland to the Norwe­gian Sea. However, in northern Baffin Bay and throughout the Canadian archipelago, there are ex­tremely low numbers of euphausiids, and here copepods reach a greater numerical dominance than in any other ocean region.

In both the Arctic and Antarctic, copepods form more than 85% numerically of all mesoplankton, and by biomass (as organic carbon) the Arctic plankton from the surface to 250 m comprises —70 % copepods, — 11% pteropods, and —10% amphipods, with lesser quantities of ostracods, coelenterates, and appendic- ularians among the major taxa (Longhurst, 1985). The Arctic zooplankton thus has the smallest proportion of predatory forms of anywhere in the ocean; these make up only 20% of zooplankton biomass compared with 33 % in the mid-latitude North Atlantic and 43 % in the tropical Atlantic (Longhurst, 1985).

That growth rates of zooplankton in the Canadian Arctic are extremely slow and generation times of the largest copepods are as long as three years is well known from the work of earlier investigators, especially Bogo- rov, Cairns, Grainger, and Heinrich, as is the fact that seasonal vertical migration dominates their distribution patterns. Our research has principally been directed at understanding how these slow-growing crustaceans uti­lize Arctic algal production in the water column and under the ice. To do this we have investigated the verti­cal distribution of the grazers within the upper 250 m in relation to the physical structure of the environment and compared it with that of their food, and to their feeding behaviour and biochemistry.

As one example, in 1984 near the end of winter off Resolute Bay, we found high concentrations of small copepods, mostly Pseudocalanus, Acartia, Oithona, and

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the nauplii of Calanus hyperboreus, in the layers imme­diately beneath the ice, using a pumping system with an articulated arm that could sample a 1-m radius around a 25-cm auger hole. Up to 106 m ' 1 older copepodids and adult Pseudocalanus were observed immediately below the ice, and these animals could fill their guts in as little as 5 minutes in the presence of sufficient suspended ice-algal cells. Both the abundance of their food, which was probably eroded from the under-surface of the ice by strong currents, and that of the copepods were influ­enced by tidal mixing (Conover et al., 1986). In the winter of 1985/1986, the water of the Northwest Passage adjacent to our sampling site remained open so that, during the spring of 1986, relatively large populations of phytoplankton were advected under the ice. Under these conditions, we found no clear relation between copepod abundance and the tidal cycle, but instead Pseudocalanus and the other small copepods aggregated near the ice during the period of declining light pene­tration in the evening and showed increased chloro- phyll-derived pigment in their guts (Conover et al.,1986). This is an unexpected observation, for diel mi­gration is not usual in high latitudes during the period of 24-hour daylight. Whatever its pattern of utilization, the epontic algal production clearly extends the growth sea­son for some Arctic zooplankton and. in the case of Pseudocalanus, enables it to complete its life cycle in a single year.

Using equipment (BIONESS, LHPR) capable of de­livering open-water plankton profiles with fine vertical resolution, we found that several species of herbivorous copepods (Calanus, and about ten times as many indi­viduals of smaller copepods) approach the surface to utilize the near-surface algal bloom that occurs shortly after ice break-up in the eastern Canadian Arctic. There were consistent minor differences in the depths selected by species, and indeed by growth stages of species. Calanus glacialis and Pseudocalanus minutus occur very close to the surface at this time, while C. finmarchicus (an expatriate Atlantic species) and Oikopleura do not. Nauplii of benthic invertebrates and of the larger cala- noids occur just below the surface (Longhurst et al., 1984).

Later in the summer, as a subsurface chlorophyll maximum develops, a separation occurs between spe­cies associated with the photic zone and deeper species such as Metriclia longa. Euchaeta norvegica, and Micro- calanus pusillus. In those (e.g., Calanus glacialis, C. hyperboreus) that have near-surface concentrations dur­ing the open-water period, these much-studied layers of abundant individuals represent only a small part of the total species population down to 2000 m (Longhurst et al., 1984). Near-surface layers of copepods closely fol­low the depth of the pycnocline and chlorophyll layer as these respond to the dynamics of circulation in ocean­ographically complex regions like Lancaster Sound (Sa­meoto et al., 1986).

With electronic sensing devices carried by a BAT-

FISH, giving values for zooplankton, chlorophyll, and CTD with a vertical resolution of ~1 m, we investigated the relationship between layers of herbivorous cope­pods and chlorophyll, and the depth of maximum algal growth rate (Herman, 1983). In the upper 100 m of northern Baffin Bay, Calanus glacialis and C. finmar­chicus occurred most abundantly at depths correspond­ing closely to the depth of maximum algal growth rate, while C. hyperboreus occurred consistently slightly deeper and at the depth of the chlorophyll maximum, which, at least in this region, probably represents a biomass maximum, since the observed range of the C:CHLA ratio is relatively low (see above).

Physiology of Arctic zooplankton

We have also carried out many experimental studies on the physiology, nutrition, and biochemistry of copepods in the Arctic, in order to understand the performance of these organisms living under conditions of low temper­atures and a short feeding season.

The physiological balance in copepods has been in­vestigated with a series of simultaneous measurements of grazing, respiration, and excretion rates in northern Baffin Bay. Weight-specific metabolic rates (6 and 30 |ii O, mg dry wt~' d~' for large and small copepods, re­spectively) and excretion rates were low, and filtration rates ranged from 5 to 25 ml mg dry wt“ 1 d~'. Growth was positive in only about 25 % of cases, suggesting that diapause had already begun in many of these copepods (Conover and Cota, 1985).

Copepods in the High Arctic exhibited strong diel feeding rhythms even when they did not show vertical migration and when diel variation of light intensity was relatively weak. All growth stages of Calanus hyperbo­reus and C. glacialis in Jones Sound in August and September began feeding synchronously in the evening, when light intensity fell below 4 W m “2, and apparently ceased in response to satiation rather than to an in­crease in illumination (Head and Harris, 1985).

Time-course grazing experiments over long periods (33-64 hours) suggested that these diel feeding rhythms were not endogenous, but more likely to be cued by changes in ambient light levels. In addition it was shown in these experiments that “containment effects” , re­ported by others from lower latitudes, such as bacterial growth and algal nutrient enrichment, were not impor­tant, perhaps because of the low experimental temper­atures (Head et al., 1986).

Some of the biochemical and physiological changes which occur in C5 and adult Calanus hyperboreus as their populations change from summer feeding to win­ter diapause conditions have been examined. This was done by comparing near-surface, feeding individuals captured at 200—500 m during the summer near-surface bloom, and also one month later when most copepods had descended. For a given stage, deep animals had

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greater lipid stores and were larger than surface ani­mals, while ammonia excretion rates were dramatically different: surface-feeding animals had high rates, while deep animals had undetectably low rates (Head and Harris, 1985).

The feeding rate of Arctic copepods has been quanti­fied by measurements of the rates of filling and clear­ance of food in their guts. A t stations off Ellesmere Island, ingestion rates calculated from experimentally determined gut filling and clearance rates were the same as in vivo rates calculated from in situ gut pigment levels, provided that we observed appropriate experi­mental conditions with respect to light intensity and time of day. These data also showed that copepods in vitro could ingest at higher rates than their natural noc­turnal rates, and also that extrapolation of daily in vivo ingestion rates from experimental data can only be satis­factory if the characteristics of the diurnal feeding curve for each situation is fully described (Head, 1986).

A simple but important assumption has been con­firmed: gut clearance time in filtered sea water is in fact a direct measure of gut turnover time in actively feeding copepods. Further experimental results raise cautions for all grazing studies, however. Copepods collected with full guts from situations where food is abundant do not feed during the first hour after capture and in the laboratory do not achieve normal in vivo gut pigment levels unless previously starved for 24 hours (Head and Harris, 1987; Head, 1988). We also found that the ratios of pigment:biogenic silica in natural epontic algae were higher than in faeces from copepods fed on such algae, suggesting the destruction of algal pigment during graz­ing, and there were also significant differences in gut clearance time when ice algae or planktonic algae were grazed as the experimental material (Head, 1988).

In late summer, we also found a correlation between the amount of gut pigment in Calanus hyperboreus in the 0 -50-m layer, and the concentration of chlorophyll at the algal maximum, provided we made our observa­tions at the appropriate time of day, i.e., between 0200 and 0400. In time-course experiments with mixed cope- pod populations, ammonia excretion was described by the sum of a straight line and a saturating curve; the latter was correlated with an estimate of community defaecation, suggesting the two processes were closely associated. The ratio of initial to basal ammonia excre­tion varied from 2 to 20. so that “defaecation-associ- ated” ammonia release may be important in the in vivo estimation of nitrogen excretion or regeneration rates. Defaecation and "defaecation-associated" ammonia ex­cretion were both correlated with the ambient chlo­rophyll concentration at the chlorophyll maximum (Head et al., 1988).

Using experimentally determined filtration rates for Arctic copepods, we have attempted to balance herb­ivore demand and algal production during the open- water season in Jones Sound. Profiles of herbivores, and their ingestion rates calculated in terms of POC,

were compared with profiles of POC, chlorophyll a , and production rate. At depths below the chlorophyll maxi­mum, potential grazing deficits were very high, and the copepods were probably not feeding; in two areas in Jones Sound, daily algal production exceeded copepod demand. At a third station, the daily ingestion exceeded primary production because copepods were very abun­dant (>2000 m ’) in the near-surface layers. Even so, however, the algal substrate was essentially unlimiting, as its standing stock was being cleared at less than 1 % per day by the grazing of the near-surface layers of Calanus. We suggest that algal cells in the near-surface layers which have high levels of easily digestible soluble carbohydrate form the preferred food of Arctic cope­pods after the main pulse of algal production has ceased (Longhurst and Head, unpublished).

Although the copepod plankton probably dominates Arctic algal consumption, we have also measured the grazing of microzooplankton at single depths in Jones Sound and Baffin Bay by the dilution method. The ciliates Lohmaniella and Strombidium and the tintinnids Leprotintinnopsis. Parafavella. and Tintinnopsis (total about 2500 cells l“1) were found to consume 8—15 % of the algal growth rate daily. This percentage is approxi­mately in line with measurements made in temperate seas, so we should not expect the protozoan plankton to perform significantly differently at high latitudes, in relation to metazoan plankton, from what is known of lower latitudes (Paranjape, in press).

Ecology of fishWe have been able to do no original work in fish ecol­ogy beyond collection of samples of larvae of polar cod with our high-resolution vertical sampling gear, and an investigation of its larval feeding behaviour in Lancaster Sound, but the importance of Boreogadus saida is such that we have interpreted what is known of its life history in terms of environmental concerns in the Canadian sector of the Arctic (Sameoto, MS; Sameoto and Lewis, MS).

Boreogadus saida is a key species in the transfer of material from planktonic production in Arctic seas to marine mammals and birds, for which it forms a vital source of food. It is consumed by fish (char, plaice), birds (murres, guillemots, kittiwakes), seals (harp, bearded), and whales (narwhal, beluga). It is a small (<32 cm) largely pelagic gadoid restricted to cold cir- cumpolar waters. Winter-spawning concentrations simi­lar to those which occur in shallow water elsewhere have not yet been found in the Canadian Arctic, with the possible exception of one region adjacent to Lancas­ter Sound, though schooling has been recorded all the way from the southern Labrador Sea to the High Arctic. Polar cod occur from the surface, where they may occur in cavities in sea ice, to as deep as 300 m. Up to one year of age they are entirely nektonic, thereafter adopting a

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more demersal habitat, though their diet always seems to be dominated by planktonic crustaceans. We have found that pelagic polar cod in Lancaster Sound are restricted to the warmer mixed layer, somewhat above the main layer of copepod biomass. We also found that larvae of polar cod dominate the fish larval component of the epiplankton. Larvae were generally found at depths where their potential copepod prey were rela­tively sparse, and this is likely to represent predator avoidance on the part of the prey organisms, as has been demonstrated previously as occurring between planktonic crustaceans and fish.

Environmental concern for this key species in Arctic ecology naturally rests on the vulnerability of the indi­vidual unit stocks, which must exist, as in other gadoids. However, we have calculated that during the very long period spent as a planktonic egg (this stage alone is 45—90 d), and based on baroclinic flow through the Canadian archipelago, an individual polar cod larva could be transported as much as —2500 km eastwards between spawning and hatching. Larvae hatched in Lancaster Sound could quite easily have originated in the Beaufort Sea. This renders it unlikely that a local pollution event could have a major regional effect on polar cod stocks, even assuming that it could actually contaminate the adults or their spawning products di­rectly.

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