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Chapter 8

Estuaries and Salt Marshes

Definition of Estuaries

Define: a partially enclosed coastal embayment where fresh water and seawater meet and mix.

Geological history

Various climate conditions

Different chemical and physical conditions

Salinity gradients

Types of Estuaries

Salt – Wedge

Well – Mixed

Partially – Mixed


1. Coastal Plain Estuary


Chesapeake Bay

mouth of Delaware and Hudson rivers

Great Britian: Cornwall and Devon
The Carmarthen Bay and Estuaries


2. Tectonic Estuary

Fig 8.1 cont.

San Francisco bay

San Francisco Bay’s History

The Parts of San Francisco Bay and its Watershed


3. Semiclosed Bay or lagoon

Major Lagoons

Pamlico Sound

North Carolina Coast

Lying behind barrier beach

Outer Banks



Classification by Salinity gradient

Positive estuary or salt wedge estuary

River-dominated or stratified

Homogeneous estuaries

Marine-dominated estuaries

Neutral estuaries:

Galveston Bay, Texas and Alligator Harbor, Florida

Negative or evaporite estuaries

Seasonal or intermittent estuary

Positive and Negative estuaries

(a) Positive or salt wedge

Seawater enters along the bottom and gradually mixes with our-flowing fresh water

(b) Negative estuary

Fresh water flow is diminished or absent during part of the year

Seawater enters along the surface. Evaporation is greater than runoff, salinity increases as one move up the estuary. Hypersaline water sinks and flows out below the incoming seawater.

Change of salinity

Changes by tide level

Changes by river discharge

Point A is on the bottom and B is intertidal

B is covered by high tide, is inundated only with high-salinity water

Unaffected by low-salinity water at low tide

Point A is covered by water of different salinities at different tides

Physical Characteristics of Estuaries

Stressful environment for organisms

yes or no

Salinity fluctuation

major feature

varies because of

seasons, topography, tides, influx of fresh water

figure 8.3

time frame 6-12 hours

Coriolis effect in Northern Hemisphere towards right

figure 8.4

Eastern shore of Chesapeake Bay

opposite in Southern Hemisphere

Results of Coriolis effect

North – South orientation of CB

Deflection of the isohalines to the right in this Northern Hemisphere estuary

Circled numbers refer to salinity values in ppt

Seasonal Changes


Due to evaporation, Freshwater flow or Both

High concentrations in upper Bay

Decrease freshwater flow

High concentration downstream, lower Bay

Increased freshwater flow

Comparison of salinity fluctuations in the water column with that interstitially in the bottom mud.


Sand and mud

Water held between particles

Change in salinity slower


Organisms subject to less drastic salinity changes

Most effective in lower intertidal

Soil at higher level

Dilution during rains

Evaporation in dry weather

Estuary Substrates

Most are soft and muddy

Sediments carried from

Seawater and freshwater


Coastal lagoons

Silt suspension

Seawater ions

Particles flocculate

Muddy bottom

Sandy bottom


Control by current

Particles size

Mouth of estuary

Catastrophic events

Massive storms


Organism mortality

East Coast

Andrews 1973

Hurricane Agnes 1972

Chesapeake Bay

Salinity reduction

Loss of organisms

Organic matter

Settled out terrestrial and marine

Rich accumulation

Habitat for bacteria


1. Variable horizontal columns

2. Depth

3. Temperate zone rivers

4. Estuarine rivers

5. Mouth of estuary

6. Variable vertical columns

Wave Action and Currents

1. Minimal wind

2. Fetch

3. Depth of water

4. Bay’s mouth narrowness limits size of wave that enters from the ocean / sea

5. Sediment deposition

6. Currents

7. Flushing Time


1. Particle suspension

2. High turbidities

3. Major ecological effect

4. Severe turbidity


1. Ample supply of O2

2. Solubility of oxygen

3. Salt wedge estuary

4. Estuarine sediments

anoxic level ( without O2 )

The Biota of Estuaries

Three major faunal components




Some typical Estuarine
Brackish water animals with 5 – 18 psu

Typical estuarine algae

A. Estuarine Vegetation

Intertidal mud flats are inhabited by a limited number of green algae

Common genera listed on left

Often seasonally abundant

Disappear during certain times of year

B. Estuarine Plankton

* Diatoms dominate

Warmer months

High turbidity

Rapid flushing


Adaptations of Estuarine Organisms

Morphological adaptations

Physiological adaptations

Behavioral adaptations

Nereis diversicolor with change in salinity

Change of salinity

Blood of the crab

Australoplax trientata

With change in salinity of the external medium

Life span of Blue Crab

Callinectes sapidus

estuaries of the Atlantic coast of United States

Ecology of Estuaries

Productivity, Organic Matter, and Food Sources

Food Webs

Plankton Cycles

Bill length of Shorebirds

Relationship to the depth of some common estuarine invertebrates

A generalized estuarine food web

The food web of a typical estuary showing some of the major aquatic trophic groupings

The distribution of salt marshes in the world

Salt Marshes

Definition and Characteristics

Environmental Characteristics

Composition and Distribution

Causes of Zonation


Interactions and Food Webs

Areal extent of salt marshes on the Atlantic coast of the U. S.

Environmental factors influencing salt marsh vegetation.

Some dominant salt marsh emergent plants

Characteristics animals present in a salt marsh at low and high tides on the Atlantic coast of North America


Zonation in a New England Marsh

Zonation in a Southern Marsh of Atlantic coast of U. S.

Generalization zonation of a Pacific coast salt marsh based on marshes of San Francisco Bay

Alternative succession patterns in bare patches of the high marsh as a result of differing physical conditions

Change in competitive relations among marsh plants under high and low nitrogen levels.

The migration of the marsh periwinkle Littoraria irrorata up the cordgrass blades to avoid predation by blue crabs

The mutual interaction between cordgrass and mussels in a salt marsh.

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