EstuariesEstuariesShipley Marine BiologyShipley Marine Biology

Drowned river valleys

Most common type of estuary They were formed by the “drowning” of low

land around the mouths of rivers when sea level rose after the last ice age

Ex: Chesapeake Bay

Types of Estuaries

Types of Estuaries Bar built estuary-

Built by the accumulation of sediments into sand bars or barrier islands

Ex: North Carolina (seen in lower part of the photo to the right near Cape Hatteras)

Tectonic estuaries –

Formed as a result of land sinking due to movements of the Earth’s crust

Ex: San Francisco Bay

Types of Estuaries

Types of Estuaries Fjords –

Deep channels cut in the coastal zone as a result of retreating glaciers

Ex: Alaska, Norway

Estuaries are the best developed in areas

where the coastal plain is flat and the continental shelf is wide.

The opposite is true in areas with steep, narrow continental shelves and coastal plains

Development of Estuaries

Physical Characteristics of Estuaries Salinity-

Can vary from 5 – 30 ppt

Salinity varies according to distance from saltwater (tides) or freshwater (river) input

Can also vary as a result of storms

Depth also contributes to salinity

The diagram to the right illustrates that the salinity is not uniform (saltwater is heavier and sinks below freshwater) – this is known as a salt wedge

Organisms in the estuaries are normally euryhaline (can tolerate a wide variety of salinities)

Some are osmoregulators that have mechanisms for keeping their internal concentration stable

Others are osmoconformers with internal concentrations that vary with their surroundings

Dealing with Salinity Changes

Flowering plants in the estuarine community

must either expel excess solutes (such as Spartina and margrove trees using salt glands) or concentrating solutes in specific tissues (such as in Salicornia, a.k.a. “pickleweed” or glasswort

Dealing with Salinity Changes

Mostly composed of sand or mud Mud is difficult to move through Mud or sand can also shift unlike hard

substrate which is a challenge for organisms as well

Particle sizes are so small that most areas are actually anoxic (devoid of oxygen)

Substrate

Like in the intertidal community (and some

areas of estuaries are intertidal), temperature can vary greatly.

Temperatures can vary especially at times when water levels are low.

Water Temperature

Water clarity is very poor in estuaries This is due to suspended sediments and particles

from rivers in the water This reduced clarity makes it more difficult for

photoautotrophs in the water Most primary production is the result of flowering

plants in the salt marsh as opposed to algae or plankton

Water Clarity

Communities within an Estuary Open water in bays

and tidal creeks Predominant

organisms include plankton, floating algae, fish and many larval forms (crabs, shrimp, molluscs, fish, etc)

At the right, this diagram shows the EXTREME IMPORTANCE of estuaries as a “nursery” area for larval species

Many marine species spend at least a portion of their life in an estuary, mainly as larvae.

Communities within an Estuary Saltmarsh

The vegetated area of an estuary

Spartina species predominate near the water in most estuaries (Juncus or needlerush can be found near the water in low salinity estuaries < 10 ppt)

Representative Inhabitants in a

Mudflat of an Estuary

Mangroves or mangals Found in tropical and subtropical

coastal areas Replace saltmarshes in these areas

normally

Communities within an Estuary

Worldwide Distribution of

Saltmarshes and Mangrove Forests

Generalized Food Webs in Estuarine

Ecosystems

Aerial View of a Mangrove

Communities within an Estuary

Some mangrove tree species possess pneumatophores, or vertical root extensions, that assist the plant with the exchange of gases (pictured at right)

Seagrass beds can be found in some

estuaries) if the water clarity permits Water clarity must be better than the

average estuary to support this growth

Communities within an Estuary

Communities within an Estuary

“Oyster reefs” are found in many estuaries

These congregations of oysters are the platform on which dozens of other species grow and thrive

The oysters provide the only hard substrate in many estuaries