Chapter 12 Estuaries

Types of Estuaries Drowned river valleys 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 Bar built estuary- Bar built estuary- –Built by the accumulation of sediments into sand bars or barrier islands –Ex: North Carolina barrier islands (seen in lower part of the photo to the right near Cape Hatteras)

Types of Estuaries Tectonic estuaries – Tectonic estuaries – –Formed as a result of land sinking due to movements of the Earth’s crust –Ex: San Francisco Bay

Types of Estuaries Fjords – Fjords – –Deep channels cut in the coastal zone as a result of retreating glaciers –Ex: Alaska, Norway

Physical Characteristics of Estuaries Salinity- 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

Dealing with Salinity Changes Organisms in the estuaries are normally ________ (can tolerate a wide variety of salinities) Organisms in the estuaries are normally ________ (can tolerate a wide variety of salinities) Some are osmoregulators that have mechanisms for keeping their internal concentration stable Some are osmoregulators that have mechanisms for keeping their internal concentration stable Others are osmoconformers with internal concentrations that vary with their surroundings Others are osmoconformers with internal concentrations that vary with their surroundings

Types of species living in estuaries in relation to salinity

Substrate Mostly composed of sand or mud Mostly composed of sand or mud Mud is difficult to move through Mud is difficult to move through Mud or sand can also shift unlike hard substrate which is a challenge for organisms as well 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) Particle sizes are so small that most areas are actually anoxic (devoid of oxygen)

Communities within an Estuary Open water in bays and tidal creeks Open water in bays and tidal creeks Predominant organisms include phyto- and zooplankton, floating macroalgae, fish and many larval forms (crabs, shrimp, molluscs, and fish) Predominant organisms include phyto- and zooplankton, floating macroalgae, fish and many larval forms (crabs, shrimp, molluscs, and fish) At the right, this diagram shows the extreme importance of estuaries as a “nursery” area for larval species 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. Many marine species spend at least a portion of their life in an estuary, mainly as larvae.

Communities within an Estuary Saltmarsh Saltmarsh The vegetated area of an estuary The vegetated area of an estuary Spartina species predominate near the water in most estuaries. Halophytes occur in higher elevation parts of the marsh. Spartina species predominate near the water in most estuaries. Halophytes occur in higher elevation parts of the marsh.

Aransas National Wildlife Refuge Salt marsh on Texas Coast with endangered whooping cranes that depend on the marsh ecosystem. Blue crab and halophytes are their main food sources.

Representative Inhabitants in a Mudflat of an Estuary Epifauna includes deposit feeding snails like mudsnails and Predators like shorebirds and moon snails. Infauna includes clams, crustaceans like ghost shrimp, and annelids. Meiofauna is low due to anoxy.

Communities within an Estuary Mangroves or mangals Mangroves or mangals Found in tropical and subtropical coastal areas Found in tropical and subtropical coastal areas Normally replace saltmarshes in these areas Normally replace saltmarshes in these areas

Worldwide Distribution of Saltmarshes and Mangrove Forests

Generalized Food Webs in Estuarine Ecosystems Food webs for salt mashes and mangals

Aerial View of a Mangal

Communities within an Estuary In Texas, mangrove In Texas, mangrove forest estuaries are found at Boca Chica, where the Rio Grande River enters (sometimes) the Gulf of Mexico. It is rich in tropical marine life but like many estuaries and bays is becoming more saline.

Communities within an Estuary Seagrass beds can be found in some estuaries if the water clarity permits 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 Water clarity must be better than the average estuary to support this growth

Communities within an Estuary “Oyster reefs” are found in many estuaries “Oyster reefs” are found in many estuaries These congregations of oysters are the platform on which dozens of other species grow and thrive 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 The oysters provide the only hard substrate in many estuaries