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Estuaries
An estuary is a semi-enclosed body of water that has a free connection with the sea
.
Estuaries have more food for organisms, but the organisms usually have to deal with large temperature and salinity changes, high silt content and pollution.
Hint How does this organism get its food?
Estuaries are regions of transition and sharp gradients.
Estuaries support fauna recruited mostly from the sea
For those organisms that can survive the problems of the estuary, there is a great deal of food.
These organisms tend to be in large numbers.
The Chesapeake Bay is a drowned river valley. This is the most common type of estuary. It was formed during the last ice age some 12,000 – 18,000 years ago.
106 ft. 56 ft 32 ft. 21 ft. 14ft. 9ft.
21 feet is the correct answer. The deepest spot in the bay is 174 feet near Annapolis Md.
Can you name the 6 states that make up the watershed of the Chesapeake Bay?
Virginia Maryland West Virginia Delaware New York Pennsylvania
From South to North
James York Rappahannoc
k Potomac Susquehanna
The Susquehanna River provides about 50% of the fresh water coming into the Bay.
The river empties an average of 19 million gallons of water per minute.
About 50,000 commercial vessels enter the Bay each year.
All these people and activities put a strain on the Bay ecology.
The Bay is home to over 3600 living organisms!
Marsh dwellers are located in and around marshes. They include small fish, birds, and marsh grasses.
Submerged Aquatic Vegetation Communities are important for many reasons. They include ducks, crabs, and eelgrass.
The plankton community includes the drifters of the Bay.
It includes phytoplankton, bacteria, and zooplankton.
Benthic refers to the bottom of the Bay. Benthic organisms include oysters, clams, barnacles, and mud crabs.
Nekton refers to the swimmers of the Bay.
Croaker, Spot, and menhaden use shallow water in the Bay as a nursery
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
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
Tectonic estuary—forms when an earthquake causes the land to sink, allowing seawater to cover it e.g. San Francisco Bay
Fjord—estuary formed when a deep valley cut into the coast by retreating glaciers fills with water found in Alaska and Scandinavia
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 oceane.g. Cape Hatteras region of North
Carolina, Texas/Florida Gulf Coasts, etc.
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
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
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
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
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
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
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
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
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
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
Maintaining osmotic balance osmoconformers—animals with tissues and cells
that tolerate dilution e.g. tunicates, jellyfishes, sea anemones
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
Mangrove communities (continued) mangal productivity
primary producers (mangroves, algae and diatoms) support a productive detrital food web; burrowing/climbing crabs eat the leaves
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