© 2006 Thomson-Brooks Cole Chapter 14 Estuaries

© 2006 Thomson-Brooks Cole Key Concepts Estuaries form in embayments where freshwater from rivers and streams mixes with seawater. The salinity of water in estuaries varies both vertically and horizontally. Mixing of nutrients from saltwater and fresh water, combined with plentiful sunlight and relatively shallow water, makes estuaries very productive ecosystems.

© 2006 Thomson-Brooks Cole Key Concepts Animals and plants that live in estuaries must be able to adapt to changing salinity. The physical characteristics of estuaries tend to favor benthic organisms. Many commercially valuable fishes and shellfishes spend a portion of their life cycle in estuaries.

© 2006 Thomson-Brooks Cole Key Concepts Estuarine communities include oyster reefs, mud flats, seagrass meadows, salt marshes, and mangrove forests (mangals).

© 2006 Thomson-Brooks Cole Physical Characteristics of Estuaries Formation of an estuary –embayments—coastal areas where portions of the ocean are partially cut off from the rest of the sea –rivers and streams carry freshwater runoff from land into some embayments –estuary forms where fresh and salt water are mixed –all estuaries are partially isolated from the sea by land, and diluted by fresh water

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Types of Estuaries Coastal plain estuary—forms between glacial periods when melting glaciers raise the sea level and flood coastal plains –found along the Gulf of Mexico and eastern Atlantic coasts Drowned river valley estuary—forms when melting glaciers raise the sea level and flood low-lying rivers –e.g. Chesapeake Bay, Long Island Sound

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Types of Estuaries Tectonic estuary—forms when an earthquake causes the land to sink, allowing seawater to cover it –e.g. San Francisco Bay

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Types of Estuaries Fjord—estuary formed when a deep valley cut into the coast by retreating glaciers fills with water –found in Alaska and Scandinavia

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Types of Estuaries Tidal flats—deltas formed in the upper part of a river mouth by accumulated sediments, which divide and shorten an estuary Bar-built estuary—estuary in which deposited sediments form a barrier between the fresh water from rivers and salt water from the ocean –e.g. Cape Hatteras region of North Carolina, Texas/Florida Gulf Coasts, etc.

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Salinity and Mixing Patterns Salinity varies horizontally –salinity increases from the mouth of the river toward the sea Salinity varies vertically –uniform salinity results when currents are strong enough to thoroughly mix salt and fresh water from top to bottom –layered salinity may occur, with the layers moving at different rates

© 2006 Thomson-Brooks Cole Salinity and Mixing Patterns Mixing patterns –tidal overmixing—seawater at the surface moves upstream more quickly; denser seawater at the surface sinks as lighter freshwater beneath it rises, creating a mixing action

© 2006 Thomson-Brooks Cole Salinity and Mixing Patterns Water circulation patterns –positive estuary influx of fresh water from the river more than replaces the amount of water lost to evaporation surface water is less dense and flows out to sea denser salt water from the ocean flows into the estuary along the bottom most estuaries are positive estuaries

© 2006 Thomson-Brooks Cole Salinity and Mixing Patterns Water circulation patterns (continued) –negative estuary occur in hot, arid regions lose more water through evaporation than the river is able to replace surface water flows toward the river; its salinity is increased by evaporation water along the bottom moves out to sea usually low in productivity e.g. Laguna Madre estuary in Texas

© 2006 Thomson-Brooks Cole Salinity and Mixing Patterns Salt-wedge estuary –occur in the mouths of rivers that are flowing into seawater –freshwater flows rapidly out to sea at the surface –denser saltwater flows upstream along the river bottom –rapid flow of the river prevents saltwater from entering and produces an angled boundary between the freshwater moving downstream and the seawater moving upstream called a salt wedge

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Salinity and Mixing Patterns Well-mixed estuary –river flow is low and tidal currents play a major role in water circulation –seaward flow of water and uniform salinity at all depths –lines of constant salinity move back and forth with the tides

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Salinity and Mixing Patterns Partially-mixed estuary –strong surface flow of freshwater and a strong influx of seawater –tidal currents force seawater upward to mix with surface water –rapid exchange of surface water between the estuary and ocean

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Salinity and Mixing Patterns Other mixing patterns –e.g. Galveston Bay (Texas), fjords

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Temperature Shallowness of estuaries allows temperatures to fluctuate dramatically Warmth comes from solar energy and warm tidal currents In some estuaries, winter turnover results when cooler surface water sinks and warmer deep water rises –circulates nutrients vertically between water and bottom sediments

© 2006 Thomson-Brooks Cole Estuarine Productivity Nutrients in fresh and saltwater complement one another –freshwater contains nitrogen, phosphorus and silica –surface seawater has less nitrogen and silica but more phosphorus Silt and clay dumped by rivers hold, then release excess nutrients Filter feeders consume more plankton than they can absorb, producing pseudofeces which provide food for bottom feeders

© 2006 Thomson-Brooks Cole Life in an Estuary Many are species are generalists, and can feed on a variety of foods depending on what is available Species that tolerate temperature and salinity changes can exploit estuaries and grow large populations So, estuaries contain abundant individuals from relatively few species

© 2006 Thomson-Brooks Cole Life in an Estuary Maintaining osmotic balance –osmoconformers—animals with tissues and cells that tolerate dilution e.g. tunicates, jellyfishes, sea anemones

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Life in an Estuary Maintaining osmotic balance –osmoregulators—animals that maintain an optimal salt concentration in their tissues, regardless of the salt content of the environment concentrate or excrete salts, or shield themselves from their environment

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Life in an Estuary Remaining stationary in a changing environment –natural selection favors benthic organisms because of the difficulty in staying still to feed in constantly-moving water –non-benthic animals (e.g. crustaceans, fishes) maintain position by actively swimming or by moving back and forth with the movement of the tides

© 2006 Thomson-Brooks Cole Life in an Estuary Estuaries as nurseries –high level of nutrients + few predators makes a great habitat for juveniles –juveniles live in the estuary until they grow large enough to be successful in the open sea –e.g. striped bass, shad, bluefish, blue crabs, white shrimp

© 2006 Thomson-Brooks Cole Estuarine Communities Many hardy organisms are euryhaline —species that can tolerate a broad range of salinity Oyster reefs –reefs form from numerous oysters growing on the shells of dead oysters –provide a habitat for many organisms, which may depend on oysters for food, protection, and a surface for attachment –oyster drill snails prey on oysters

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Estuarine Communities Mud flats –contain rich deposits of organic material + small inorganic sediment grains –bacteria and other microbes thrive in the mud, producing sulfur-containing gases –mud provides mechanical support for organisms –cohesiveness permits construction of a permanent burrow

© 2006 Thomson-Brooks Cole Estuarine Communities Mud flats (continued) –mud flat food webs main energy base = organic matter consisting of decaying remains and material deposited during high tides bacterial decomposition channels organic matter to other organisms, and recycles nitrogen and phosphate back to the sea floor deposit feeders prey on bacteria larger organisms eat secondary consumers of bacteria, and so forth

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Estuarine Communities Mud flats (continued) –animals of the mud flats most are burrowers living just below surface closely-packed silt prevents good water circulation, so many animals have a “snorkel” soft-shelled clams use a siphon to filter feed and obtain oxygenated water, then metabolize anaerobically during low tide lugworms are common mud flat residents innkeeper worms house many other organisms in their burrows, as do ghost shrimp

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Estuarine Communities Seagrass meadows –seagrass productivity depends on the ability of seagrasses to extract nutrients from the sediments depends on activity of symbiotic, nitrogen- fixing bacteria also depends on productivity of algae that grow on and among seagrasses nutrients from drawn from sediments are released into the water by seagrasses, for use by algae

© 2006 Thomson-Brooks Cole Estuarine Communities Seagrass meadows (continued) –seagrass food webs seagrasses are tough, and seldom consumed directly by herbivores seagrasses are a food source to many animals as detritus, when their dead leaves are eaten by bacteria, crabs, sea stars, worms, etc. organisms from other communities feed in seagrass meadows during high tide, exporting nutrients to other communities

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Estuarine Communities Seagrass meadows (continued) –seagrass meadows as habitat epiphytes and epifauna attach to seagrasses filter feeders live in the sand among blades rhizoids and root complexes provide more permanent attachment sites, and protect inhabitants from predators larvae and juveniles of many species live here, protected from predators by changing salinity, plentiful hiding places, and shallow water

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Estuarine Communities Salt marsh communities –distribution of salt marsh plants low marsh—region covered by tidal water much of the day and typically flushed twice each day by the tides high marsh—region covered briefly by saltwater each day and only flushed by the spring tides cordgrass dominates the low marsh short, fine grasses dominate the high marsh

© 2006 Thomson-Brooks Cole Estuarine Communities Salt marsh communities (continued) –salt marsh productivity tides bring in replenishing supplies of nutrients most primary production supports detrital food chains bacteria eat decaying plant material deposit feeders eat bacteria some salt marshes export large amounts of detritus to nearby communities; in others, resident organisms consume most of the detritus

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Estuarine Communities Salt marsh communities (continued) –animals of the salt marsh permanent residents include periwinkles, tidal marsh snails, ribbed mussels, purple marsh crabs, fiddler crabs, amphipods, grass shrimp burrowing animals play an important role in bringing nutrient-rich mud from deeper down to the surface, while oxygenating deeper sediments tidal visitors that come to the salt marsh to feed include predatory birds, herbivorous animals from land, fishes and blue crabs

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Estuarine Communities Salt marsh communities (continued) –succession in salt marshes salt marshes can be the first stage in a succession process that produces more land roots of marsh plants trap sediments until the area becomes built up with sand/silt that combine with organic material to make mud mud islands appear and merge, and high tide covers less and less of them tall cordgrass is replaced by short cordgrass, which is replaced by rushes and then land plants

© 2006 Thomson-Brooks Cole Estuarine Communities - Mangals

© 2006 Thomson-Brooks Cole Estuarine Communities Mangrove communities –distribution of mangrove plants red mangroves are usually pioneering species, and grow close to the water where the amount of tidal flooding is greatest black mangroves occupy areas that experience only shallow flooding during high tide white mangroves and buttonwoods (not true mangroves) live closest to land, but can tolerate flooding during high tide and saline soil

© 2006 Thomson-Brooks Cole Estuarine Communities Mangrove communities (continued) –mangrove root systems shallow, widely spread root systems anchor the plants and provide oxygen for parts buried in the mud red mangroves have prop roots, and black mangroves have many pneumatophores prop roots and pneumatophores slow water movement, causing suspended materials to sink to the bottom eventually, this sediment build-up can transform the estuary into a terrestrial habitat

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Estuarine Communities Mangrove communities (continued) –mangal productivity primary producers (mangroves, algae and diatoms) support a productive detrital food web; burrowing/climbing crabs eat the leaves

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Estuarine Communities Mangrove communities (continued) –mangroves as habitat many animals live on prop roots and pneumatophores, such as bivalves and snails roots provide habitat for many organisms found in salt marshes and mud flats sheltered waters provide a nursery as well

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