Chapter 12 Estuaries. Types of Estuaries Drowned river valleys Drowned river valleys –Most common type of estuary –They were formed by the “drowning”

Chapter 12Chapter 12

EstuariesEstuaries

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 sand sediments into sand bars or barrier bars or barrier islandsislands

– Ex: North Carolina Ex: North Carolina barrier islands (seen barrier islands (seen in lower part of the in lower part of the photo to the right photo to the right near Cape Hatteras)near Cape 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 ________ (can tolerate a normally ________ (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

Types of species living in estuaries in relation to salinity

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 solutes (such as SpartinaSpartina and and mangrove trees using salt glands) or mangrove trees using salt glands) or concentrating solutes in specific concentrating solutes in specific tissues (such as in tissues (such as in SalicorniaSalicornia, a.k.a. , 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 and This is due to suspended sediments and particles from rivers in the waterparticles from rivers in the water

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

• Most primary production is the result of Most primary production is the result of flowering plants in the salt marsh or flowering plants in the salt marsh or mangrove forests as opposed to mangrove forests as opposed to phytoplanktonphytoplankton

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 phyto- and phyto- and zooplankton, floating zooplankton, floating macroalgae, fish and macroalgae, fish and many larval forms many larval forms (crabs, shrimp, (crabs, shrimp, molluscs, and fish)molluscs, and fish)

• At the right, this At the right, this diagram shows the diagram shows the extreme importance extreme importance of estuaries as a of estuaries as a “nursery” area for “nursery” area for larval specieslarval species

• 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

• SpartinaSpartina species species predominate predominate near the water in near the water in most estuaries. most estuaries. Halophytes Halophytes occur in higher occur in higher elevation parts elevation parts of the marsh.of the marsh.

Aransas National Wildlife Aransas National Wildlife RefugeRefuge

Salt marsh on Texas Coast with endangered whooping cranes that depend on the marsh ecosystem. Blue crab and halophytes are theirMain food sources.

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

Epifauna includes deposit feeding snailslike mudsnails and Predators like shorebirdsand moon snails.

Infauna includes clams,crustaceans like ghostshrimp, and annelids.

Meiofauna is lowdue to anoxy.

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

• Normally replace saltmarshes in these Normally replace saltmarshes in these areasareas

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

ForestsForests

Generalized Food Webs in Generalized Food Webs in Estuarine EcosystemsEstuarine Ecosystems

Food webs for salt mashes and mangals

Aerial View of a MangalAerial View of a Mangal

Communities within an Communities within an EstuaryEstuary

• In Texas, mangroveIn Texas, mangrove

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


• 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 species dozens of other species grow and thrivegrow and thrive

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

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Chapter 12 Estuaries. Types of Estuaries Drowned river valleys Drowned river valleys –Most common type of estuary –They were formed by the “drowning”