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 They were formed by the ““drowningdrowning”” of low land of low land

around the mouths of rivers when sea level rose after around the mouths of rivers when sea level rose after

the last ice agethe last ice age

–– Ex: Chesapeake BayEx: Chesapeake Bay

Types of EstuariesTypes of Estuaries

•• Bar built estuaryBar built estuary--

–– Built by the Built by the

accumulation of accumulation of

sediments into sand sediments into sand

bars or barrier islandsbars or barrier islands

–– Ex: North Carolina Ex: North Carolina

(seen in lower part of (seen in lower part of

the photo to the right the photo to the right

near Cape Hatteras)near Cape Hatteras)

Types of EstuariesTypes of Estuaries

•• Tectonic estuaries Tectonic estuaries ––

–– Formed as a result of land sinking due Formed as a result of land sinking due

to movements of the Earthto movements of the Earth’’s crusts crust

–– Ex: San Francisco BayEx: San Francisco Bay

Types of EstuariesTypes of Estuaries

•• Fjords Fjords ––

–– Deep channels Deep channels

cut in the coastal cut in the coastal

zone as a result zone as a result

of retreating of retreating

glaciersglaciers

–– Ex: Alaska, Ex: Alaska,

NorwayNorway

–– Puget sound is a Puget sound is a

type of glacial type of glacial

fjordfjord

• Over millions of years canadianglaciers formed and retreated at least 4 times

• 20,000 years ago, glacier 1 mile deep

• After glaciers was a large lake

• Sea levels eventually rose and filled the estuary

• http://exhibits.pacsci.org/puget_sound/graphics/ps_glaciationsm.mov

Development of EstuariesDevelopment of Estuaries

•• Estuaries are the best developed in areas Estuaries are the best developed in areas

where the coastal plain is flat and the where the coastal plain is flat and the

continental shelf is wide. i.e. East Coast continental shelf is wide. i.e. East Coast

USUS

•• The opposite is true in areas with steep, The opposite is true in areas with steep,

narrow continental shelves and coastal narrow continental shelves and coastal

plains, i.e., Washington, west coast USplains, i.e., Washington, west coast US

Physical Characteristics of Physical Characteristics of

EstuariesEstuaries•• SalinitySalinity--

–– Can vary from 5 Can vary from 5 –– 30 30 pptppt

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

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

–– Depth also contributes Depth also contributes to salinityto salinity

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

Dealing with Salinity ChangesDealing with Salinity Changes

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

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

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

Osmoregulation in fishes

• Ion transport, and urine production

Dealing with Salinity ChangesDealing with Salinity Changes

•• 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 mangrove and mangrove

trees using salt glands) or concentrating trees using salt glands) or concentrating

solutes in specific tissues (such as in solutes in specific tissues (such as in

SalicorniaSalicornia, a.k.a. , a.k.a. ““pickleweedpickleweed”” or glasswort 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 hard Mud or sand can also shift unlike hard substrate which is a challenge for substrate which is a challenge for organisms as wellorganisms as well

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

Water TemperatureWater Temperature

•• Like in the intertidal community (and some Like in the intertidal community (and some

areas of estuaries are intertidal), areas of estuaries are intertidal),

temperature can vary greatly. temperature can vary greatly.

•• Temperatures can vary especially at times Temperatures can vary especially at times

when water levels are low.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 difficult This reduced clarity makes it more difficult

for photoautotrophs in the waterfor photoautotrophs in the water

•• Most primary production is the result of Most primary production is the result of

flowering plants in the salt marsh as flowering plants in the salt marsh as

opposed to algae or planktonopposed to algae or plankton

Representative Inhabitants in a Representative Inhabitants in a

Mudflat of an EstuaryMudflat of an Estuary

http://www.youtube.com

/watch?v=HK6IdWTodV4

http://www.youtube.com/watch?v=0PZwkx-YSbs

Mudflat ecology

• Only the upper few mm of the sediment is oxygenated, the rest is anoxic and rich in hydrogen sulfide

• Very little hard substrate which is preferred by many organisms

• Burrowing organisms can oxygenate their environment and also provide oxygen for other organisms that live in their burrows– Still can have problems when exposed at low tide – anoxia and

shorebirds

• Seagrasses can provide substrate and also oxygenate the sediment

• Species diversity tends to be higher in subtidalsediments that are sand, and lower in mud

Communities within an EstuaryCommunities within an Estuary

•• ““Oyster reefsOyster reefs”” are found are found

in many estuariesin many estuaries

•• These congregations of These congregations of

oysters are the platform oysters are the platform

on which dozens of on which dozens of

other species grow and other species grow and

thrivethrive

•• The oysters provide the The oysters provide the

only hard substrate in only hard substrate in

many estuariesmany estuaries

Communities within an EstuaryCommunities within an Estuary

•• SaltmarshSaltmarsh

•• The vegetated The vegetated area of an estuaryarea of an estuary

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

Zostera marina Eel grass

seagrasses

Organisms living in eelgrass meadow

Eelgrass distribution in pugetsound – 33% reduced

• lack of beds in southern Puget Sound

• attributed to a combination of high tidal amplitudes and timing of low tides during the summer

• During low tide events, especially during hot summer middays, desiccation/heat stress limits the upper distribution,

• at high tides, enough water covers the plants to limit net photosynthesis at depth.

• the limit of distribution corresponds to the 10.1-foot tidal amplitude isobar.

• The problem is exacerbated by the fact that the timing of extreme low tides in southern Puget Sound is in midday, when temperatures are the highest.

Additional factors affecting eelgrass

• Competitors– Introduced brown seaweed Sargassum muticum– the sand dollar (Dendraster excentricus) disturbs substrate

– crabs can uproot eelgrass

• Excessive nutrients– algal species such as sea lettuce (Ulva spp.) will– overgrow eelgrass

– overgrowth by epiphytes on the blades, blocking light, nutrients and gas exchange.

• Facilitation– Several herbivores (the snail Lacuna spp. and the marine isopod Idotea

spp.) can control epiphyte density and thereby benefit the underlying eelgrass

• Direct herbivory– usually not significant.

– Black Brandt, a small seagoose, can eat large quantities

• Storms can bury or kill eelgrass by sand overwash

Communities within an EstuaryCommunities within an Estuary

•• Mangroves or mangalsMangroves or mangals

•• Found in tropical and subtropical coastal areasFound in tropical and subtropical coastal areas

•• Replace saltmarshes in these areas normallyReplace saltmarshes in these areas normally

Worldwide Distribution of Worldwide Distribution of

Saltmarshes and Mangrove Saltmarshes and Mangrove

ForestsForestsAerial View of a MangroveAerial View of a Mangrove

Communities within an EstuaryCommunities within an Estuary

•• Some mangrove tree Some mangrove tree

species possess species possess

pneumatophores, or pneumatophores, or

vertical root vertical root

extensions, that assist extensions, that assist

the plant with the the plant with the

exchange of gases exchange of gases

(pictured at right)(pictured at right)