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 Oceanography Lecture
  Estuary and Coastal Waters


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:
      


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).
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Salt wedges are typical of estuaries and move up and down the estuary over each tidal cycle. 
Two consequences of this saltwater-freshwater interface:
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?


Productivity is further enhanced by tight coupling with the nearby salt marsh and seagrass beds (i.e. export of decaying vegetation), and with intertidal mudflats (sediments are usually black due to reduced sulfides resulting in absorption of heat that is released into estuary waters at high tide).
  

Plankton are also an important component of estuarine systems.  Compared to open ocean ecosystems, would light or nutrient limitation be more likely in estuaries?





Given that the benthic component of estuaries is so important, how might estuarine zooplankton differ from oceanic forms in terms of life cycle?








The end result
of this nutrient/organic matter trap is that estuaries tend to be highly productive ecosystems!



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?



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