1

Coastal Oceanography

Coastal Oceanography

• 95% of ocean life is in coastal waters (320 km from shore)

• Estuaries and wetlands are among most productive ecosystems on Earth

• Major shipping routes, oil and gas exploration, recreational activities take place in these waters

• Oceanography of coastal waters is markedly different than in open ocean.

Coastal Waters

• Generally considered to be marine waters over the continental shelf– Adjoining continents and island– Area dependent on margin type

• Subduction or active margin, coastal waters are a narrow band

• Passive margins, coastal waters cover a larger area

2

Salinity and Temperature

• Shallower waters can be mixed rather thoroughly

• The Ekman layer can extend to the bottom seasonally and episodically

• Salinity – isohaline• Temperature – isothermal

Salinity

• River input decreases the salinity– Isohaline at lower salinity than adjacent open

ocean– Conversely, strong haloclines persist where

mixing is weaker– Evaporative latitudes can causecoastal hypersalinity

Coastal Salinity

3

Temperature

• In low latitudes, coastal waters can become hyperthermal– No cooler water below to be upwelled– Reflection of the sun off of the bottom

sediments• In high latitudes and in winter, waters do

not get as cold as open ocean water before freezing

Coastal Temperature

Coastal Geostrophy

• Geostrophic currents move around the gyre centers

4

Coastal Geostrophy

• Coastal geostrophic currents are created by wind and runoff

Coastal Geostrophy

• Winds can push fresh water runoff against the coast

• As fresh water continues to run off, water flows “downhill” on sloped surface

• Coriolis deflects this flow

Coastal Geostrophic Currents

• Ephemeral –– Depend on wind and amount of runoff

• They can disappear periodically, seasonally

5

Estuaries

• Estuary – partially enclosed body of water in which freshwater runoff dilutes salty ocean water– River mouths– Bays, inlets, gulfs– Lagoons

Origin of Estuaries

• Generally, result of sea level rise– Coastal plain estuary

• Rising sea fills existing river valley – “drowned river valley”

Origin of Estuaries

• Fjord estuary– Drowned glaciated valley– Deeper, steeper due to glacial erosion– Moraine deposits at mouth

6

Origin of Estuaries

• Bar-built estuary– Created by formation of sandbar deposit– Shallow, barrier island lagoons/bays

Origin of Estuaries

• Tectonic estuary– Formed by tectonic processes

• Faulting• Folding

Estuarine Mixing of Water Masses

• Stratified– Little to no mixing of freshwater into saltwater

• Partially mixed– Moderate mixing of salt and fresh water

• Fully mixed– Isohaline zones

• Salt-wedge– In regions of high runoff

• Inverse– In hypersaline estuaries

7

Estuarine Salinity

U.S. Estuaries

• Columbia River mouth – Washington Oregon border

• Chesapeake Bay estuary – drowned Susquehanna River mouth

• Mississippi River – drowned river mouth• Florida Bay – modified bar mouth

Columbia River Estuary

• Salt wedge estuary• Balance between strong tides and high

flow, with limited mixing

8

Columbia River Estuary

• Extensive dams provide irrigation and drinking water, electricity, and flood control

• Dams also have caused fairly extensive environmental changes– Salmon run– Nutrient delivery to coastal waters– Sparsely populated watershed

Chesapeake Bay Estuary• Stratified drowned river mouth estuary• Densely populated surroundings• Highly seasonal stratification• Asymmetric freshwater sources

Chesapeake Bay Estuary

• Coriolis effect of water masses

9

Chesapeake Bay Estuary

• Strong stratification during spring freshwater flow prevents oxygen from getting to deep water

Chesapeake Bay Estuary

• Coverage of anoxic waters and length of time of anoxic events has increased since the 1950’s. Why?

Lagoons

• Bar-built estuaries– Typical zonation

• Freshwater zone• Brackish zone• Saltwater zone

– Seasonality• Evaporative seasons – seawater flows in, salinity

increases• Rainy seasons – water flows out, runoff decreases

salinity

10

Laguna Madre

• North of Rio Grande River, between Texas and Mexico

• Semi-arid, hypersaline• Little tidal exchange• Inverse estuary

Local Estuaries• Lake Pontchartrain

– Rigolet’s Pass attachment to ocean– Bogue Falaya River– Mississippi River

• Mississippi River– Salt-wedge– Bottom below sea level

• 250 feet

– Oak Street submarine coffer dam

Coastal Wetlands

• Coastal wetlands– Salt marsh – Mangrove wetlands

11

Mangrove Wetlands

• Important nurseries for larval species• Buffer to coastal erosion• Use of nitrogen and phosphorous

compounds that cause eutrophication

http://academic.evergreen.edu/g/grossmaz/PANKRAGT/

Semi-enclosed Seas

• Mediterranean Sea – sea half-encircled (medi-) by land (-terra)

Mediterranean Sea

12

Mediterranean Desiccation• Sea level decreased 150m

during the last glacial maximum

• Only a low flow of Atlantic water entered the Mediterranean

• Other periods, evaporation has far exceeded inflow, drying out the Mediterranean completely and leaving thick salt deposits

http://www2.uca.es/huesped/anasim_gibraltar/english.html

Black Sea Flood

• Noah’s flood?

http://content.answers.com/main/content/wp/en/thumb/1/1b/300px-Black-sea-hist.png

Red Sea• Silled basin like the Mediterranean Sea

13

Key Concepts

• Salinity and temperature controls in coastal waters

• Geostrophic currents in coastal waters• Estuaries

– Origins– Types

• Semi-enclosed seas and regional oceanography