Chapter 12 Estuaries: Where Rivers Meet the Sea Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 (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

Development of Estuaries Estuaries are the best developed in areas where the coastal plain is flat and the continental shelf is wide. 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 The opposite is true in areas with steep, narrow continental shelves and coastal plains

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 euryhaline (can tolerate a wide variety of salinities) 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 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

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 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

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)

Water Temperature Like in the intertidal community (and some areas of estuaries are intertidal), temperature can vary greatly. 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. Temperatures can vary especially at times when water levels are low.

Water Clarity Water clarity is very poor in estuaries Water clarity is very poor in estuaries This is due to suspended sediments and particles from rivers in the water 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 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 Most primary production is the result of flowering plants in the salt marsh as opposed to algae or plankton

Communities within an Estuary Open water in bays and tidal creeks Open water in bays and tidal creeks Predominant organisms include plankton, floating algae, fish and many larval forms (crabs, shrimp, molluscs, fish, etc) Predominant organisms include plankton, floating algae, fish and many larval forms (crabs, shrimp, molluscs, fish, etc) 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 (Juncus or needlerush can be found near the water in low salinity estuaries < 10 ppt) 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

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

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

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