Oceanography
Lecture
Estuary and Coastal Waters
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Structure of
estuaries:
Estuaries
are semi-enclosed,
tidally-influenced, coastal waters with variable salinity.
Why is salinity variable?
The physiography of estuaries is usually
the result of a submerging coastline
or
rising sea level.
The physiography of the US
east coast is mostly the result of rising sea levels flooding coastal
plains as the Pleistocene ice sheet melted. Typical
estuaries along the U.S. coast:
- Drowned
river valleys Why
do these types of estuaries look like reservoirs such as Allatoona?
- Lagoon estuary formed by deposition of
sediments to form barrier islands
As a result, estuaries are often
shallow. Turbidity is often
high. Why?
The density
differences due to salinity
difference among water masses have a dominant influence on the vertical
structure of estuaries (relative to temperature differences).
Salt wedges
are typical of estuaries and move up and down the estuary over each
tidal cycle.
Two consequences of this saltwater-freshwater interface:
- As freshwater moves seaward, a counterflow of saline water moves
upstream, retaining organic matter and nutrients, and providing
transport for larval crabs
and oysters.
- Flocculation (precipitation) of organic and inorganic particles
occurs as cations (e.g. Na+ and Mg2+) interact with river silt, forming
larger clumps that settle toward the bottom ("salting out").
The floc may further sorb inorganic and organic compounds (e.g.
phosphates and DOM).
The counterflow helps retain (trap) the settled floc within the estuary.
So, what type of organisms are going
to take advantage of this process?
Food
webs and energy flow in estuarine ecosystems
Not
only can organic matter in floc be utilized by bacterial decomposers (as DOM is in
the open ocean), but also floc is used by deposit
feeders on and in the bottom sediment, as well as by suspension/filter feeders, both
benthic and planktonic.
So,
how then does the food web/energy transfers of an estuary differ from
that of the open ocean?
Is
the exchange of energy one-way between
the upper water column compartment and the benthic one?
Can organisms consuming bacteria and
organic matter be larger?
What is a further advantage to being an
infaunal benthic organism in an environment in which salinity
fluctuates dramatically?
What
is a yet another advantage to being a infaunal benthic organism in an
environment in which water movement is substantial?
Ecology of estuaries
Species diversity of
estaurine organisms (globally) tends to be low relative to freshwater
and ocean systems.
Why
is this?
Which type of
organism would occur more frequently in estuarine inhabitants,
osmoregulator and osmoconformer?
Was diversity higher in the
freshwater
samples that we collected? How do marine systems differ from
freshwater systems in terms of connectiveness?
"Bottom-up" regulation of estuarine ecosystems is suggested by seasonal
succession of trophic levels in which phytoplankton blooms are followed
by increase in zooplankton abundance, followed by fish.
Many fish and crustaceans spawn or spend their juvenile stage within
estuaries (besides high primary productivity, nearby marshes provide
refuge from predators). Some animals take advantage of currents,
either spawning outside the estuary (e.g. freshwater and marine
systems) in such a way that larvae move into the estuary, or larvae of
estuarine organisms exhibit behaviors that retain the organism within
the estuary.
For example, older larvae of the American Osyter seek more saline,
deeper water of the incoming tide.
Why?
While estuaries tend to be fairly robust ecosystems, many systems are
threatened (e.g. excessive
nutrients in the Chesapeake Bay)
Coral reef have higher diversity but tend to be more susceptable to
threats compared with estuaries. Why might this be?
