Chapter 12 Estuaries: Where Rivers Meet the Sea Copyright © The McGraw-Hill Companies, Inc....

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Chapter Chapter

Estuaries: Estuaries: Where Where Rivers Rivers

Meet the Meet the SeaSea

Types of EstuariesTypes of Estuaries

• Drowned river valleysDrowned river valleys – Most common type of estuaryMost common type of estuary– They were formed by the “drowning” of low They were formed by the “drowning” of low

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

– Ex: Chesapeake BayEx: Chesapeake Bay

• Bar built estuary- Bar built estuary- – Built by the Built by the

accumulation of accumulation of sediments into sediments into sand bars or barrier sand bars or barrier islandsislands

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

• Tectonic estuaries – Tectonic estuaries – – Formed as a result of land sinking Formed as a result of land sinking

due to movements of the Earth’s due to movements of the Earth’s crustcrust

– Ex: San Francisco BayEx: San Francisco Bay

• Fjords – Fjords – – Deep channels Deep channels

cut in the cut in the coastal zone coastal zone as a result of as a result of retreating retreating glaciersglaciers

– Ex: Alaska, Ex: Alaska, NorwayNorway

Development of EstuariesDevelopment of Estuaries

• Estuaries are the best developed in Estuaries are the best developed in areas where the coastal plain is flat areas where the coastal plain is flat and the continental shelf is wide.and the continental shelf is wide.

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

Physical Characteristics of Physical Characteristics of EstuariesEstuaries• Salinity-Salinity-

– Can vary from 5 – Can vary from 5 – 30 ppt30 ppt

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

– Can also vary as a Can also vary as a result of stormsresult of storms

– Depth also Depth also contributes to contributes to salinitysalinity

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

Dealing with Salinity Dealing with Salinity ChangesChanges• Organisms in the estuaries are Organisms in the estuaries are

normally euryhaline (can tolerate a normally euryhaline (can tolerate a wide variety of salinities)wide variety of salinities)

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

• Others are osmoconformers with Others are osmoconformers with internal concentrations that vary internal concentrations that vary with their surroundings with their surroundings

Dealing with Salinity Dealing with Salinity ChangesChanges

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

SubstrateSubstrate

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

hard substrate which is a challenge hard substrate which is a challenge for organisms as wellfor organisms as well

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

Water TemperatureWater Temperature

• Like in the intertidal community (and Like in the intertidal community (and some areas of estuaries are some areas of estuaries are intertidal), temperature can vary intertidal), temperature can vary greatly. greatly.

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

Water ClarityWater Clarity

• Water clarity is very poor in estuariesWater clarity is very poor in estuaries

• This is due to suspended sediments This is due to suspended sediments and particles from rivers in the waterand particles from rivers in the water

• This reduced clarity makes it more This reduced clarity makes it more difficult for photoautotrophs in the difficult for photoautotrophs in the waterwater

• Most primary production is the result Most primary production is the result of flowering plants in the salt marsh of flowering plants in the salt marsh as opposed to algae or planktonas opposed to algae or plankton

Communities within an Communities within an EstuaryEstuary• Open water in bays Open water in bays

and tidal creeksand tidal creeks • Predominant Predominant

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

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

Communities within an Communities within an EstuaryEstuary• SaltmarshSaltmarsh• The vegetated The vegetated

area of an area of an estuaryestuary

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

Representative Inhabitants Representative Inhabitants in a Mudflat of an Estuaryin a Mudflat of an Estuary

Communities within an Communities within an EstuaryEstuary• Mangroves or mangalsMangroves or mangals

• Found in tropical and subtropical coastal Found in tropical and subtropical coastal areasareas

• Replace saltmarshes in these areas Replace saltmarshes in these areas normallynormally

Worldwide Distribution of Worldwide Distribution of Saltmarshes and Mangrove Saltmarshes and Mangrove

ForestsForests

Generalized Food Webs in Generalized Food Webs in Estuarine EcosystemsEstuarine Ecosystems

Aerial View of a MangroveAerial View of a Mangrove

Communities within an Communities within an EstuaryEstuary

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

• Seagrass beds can be found in some Seagrass beds can be found in some estuaries) if the water clarity permitsestuaries) if the water clarity permits

• Water clarity must be better than the Water clarity must be better than the average estuary to support this growthaverage estuary to support this growth

• ““Oyster reefs” are Oyster reefs” are found in many found in many estuariesestuaries

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

• The oysters provide The oysters provide the only hard substrate the only hard substrate in many estuariesin many estuaries

Chapter 12 Estuaries: Where Rivers Meet the Sea Copyright © The McGraw-Hill Companies, Inc....

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