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Lecture 31: ESTUARIES

What are they?
Origin
Circulation and mixing
Types

ESTUARIES

... semi-enclosed coastal embayments that are diluted by freshwater discharge; most are at mouths of rivers.
Often include wetlands -- saltwater and freshwater marshes and swamps that are continguous with estuaries.

Both estuaries and wetlands are important environments in marine ecosystems: 1. Very high primary biological productivity (highest of any marine environment) due to high nutrient supply and recycling. 2. Habitat for numerous marine organisms (especially juveniles)

Origin of coastal embayments / estuaries:

Drowned river valleys . . rise in sea level (glacial melting). Major estuaries along east coast of USA, e.g., Chesapeake Bay.
Fjords . . deep glaciated coastal valleys. Mountainous coasts of Scandanavia, Alaska, New Zealand.
Geologically active coasts . . downwarping or uplift due to tectonic forces. San Francisco Bay.
Barrier coasts . . shallow coastal regions bordered by offshore barrier islands and sand spits. Carolina and Georgia coasts of eastern USA, Texas Gulf Coast (Laguna Madre and Padre Islands), Netherlands.

General circulation in estuaries
Inflow:

Extent of SW - FW mixing -- controlling factors: (1) Tidal range -- most important. (2) Winds and waves (3) Bottom topography (4) Coriolis deflection (large estuaries)
High biological productivity -- nutrients are trapped by circulation Organic matter produced; some sinks to deep waters. Decomposition at depth (waters, sediments) Nutrients released Trapped and recycled by inward-flowing sea water!

Types of Estuaries -- degree of SW - FW mixing, overall circulation

1. Salt-wedge -- mouths of large rivers (e.g., Mississippi)
Large FW influx "holds back" SW at depth ---> salt wedge
Little FW-SW mixing, only at upper boundary of wedge

2. Well-mixed (e.g., Chesapeake Bay)
Typically shallow
Strong tidal influx of SW --> extensive mixing
Salinity ...
... unifrom from top to bottom
... increases from head to mouth of estuary
Very good FW-SW mixing
Net seaward flow >> river influx

3. Partially mixed (e.g., San Francisco Bay)
Typically deeper
River influx ª Tidal influx
Salinity ...
... defines dilute surface waters and deep marine waters
... changes both horizontally and vertically
Good FW-SW mixing
Net seaward flow >> river influx

4. Fjord-type (all fjords, also the Black Sea)
Deep; shallow sill at mouth
Highly stratified
FW flows out at surface
SW flows in at depth
Little vertical mixing of FW & SW
Decomp. of organic matter + no O2 from surface
---> anoxic conditions at depth

"Inverse estuaries" (saline lagoons [Laguna Madre]; Mediterranean Sea and the Red Sea)
Subtropical regions of high evaporation and little river input
Water leaves ...
... evaporation
... deep saline outflow
Water enters ...
... influx of "normal" sea water at surface
Just the opposite of normal estuaries!

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