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 Oceanography Lecture
  Salt Marsh and Intertidal Mudflat Ecosystems

  
Salt Marshes
Structure of salt marsh ecosystems
Salt marshes are composed mainly of emergent vascular plants that are distributed across the intertidal.  How does this differ from seagrass ecosystems?


The boundaries of the salt marsh roughly concide with high spring tide and low spring tide.
How often then is the very upper edge of the marsh covered by high tide?  The mid-marsh?


http://static.howstuffworks.com/gif/barrier-island-profile1.jpg
Despite this relatively small change in elevation, marshes extend many kilometers inland (400,000 acres in Georgia alone) from the backside of barrier islands along much of the U.S. east coast and in other areas of the world.  Is it just coincidental that this land is so flat?
 
So how are marshes like coral reefs in terms of the interaction between geology and biology?


Why is this "land" of fine silt relatively stable?
  

When and how does it change?


Salt marsh vegetation is interupted by tightly meandering tidal creeks and salt pannes.
     
Why doesn't vegetation grow on salt pannes?



Food webs and energy flow in salt marsh ecosystems
Salt marsh systems are extremely productive (both primary and secondary).  Why are these systems so productive?

 Yet, as with seagrasses, there are limited number of animal species consuming living salt marsh grass tissues (blades are toughened with cellulose and silca, and may contain secondary metabolites).


How can secondary production (i.e. consumer production) be high if few species are consuming salt marsh plant tissue?


Other pathways of energy flow:
    
         
marsh periwinkles

So, what would a food web/energy diagram look like for a salt marsh ecosystem?


Ecology of salt marsh ecosystems

Dominant plants
In Georgia, smooth cordgrass (Spartina alterniflora), forms a near monoculture in the lower marsh and is responsible for much of the marsh's productivity.  It is considered an invasive species in other parts of the world.  Further north along the eastern coast of North America, another species of Spartina (S. patens; salt marsh hay) becomes more common in the upper marsh.
Spartina grows better when cultivated in freshwater, so why isn't abundant in freshwater marshes?

Black needle rush (Juncus roemerianus) can also form entensive meadows in areas within the marshes of Georgia.

Overall, diversity of plants is relatively low.  What are the major stresses affecting salt marsh plants?

Like desert plants, many marsh plants have narrow leaves and sunken stomata to reduce water loss.  Plants adapted to saltwater (halophytes) remove excess salt by various strategies. For example,  Spartina alternaflora  has glands through which salt is excreted leaving visible crystals of salt on its leaves.  Salicornia  sends salt to its tips and, in the autumn, these compartments dry up and break off at the joints.  

Salt marsh plants must also cope with low oxygen in sediements due to continual submergence The light brownish-red color around the roots reveals this oxidation process.  In the low marsh oxygenation by the rhizoshperes of nearby plants results in stimulation of con-specifics
Nitrogen tends to be the limiting nutrient in marshes, and changes in supply can alter competitive outcomes among marsh plant species


Important marsh animals
Detritivores include crabs, snails, and amphipods as well as deposit feeders such as polychaetes and shrimp.

Filter feeders are also abundant (why?) and include the ribbed mussel (it's Geukensia demissa, you Donax) and the american oyster (Crassostraea virginica).

Predators include the blue crab (Callinectes sapidus) which limit the lower distribution of ribbed mussels, and force migration of marsh periwinkles.


Importance of marshes
As much 40% of Spartina production from Georgia marshes is exported from the marsh, though the idea that the marshes the themselves serve as important "nursey" grounds for marine animals has been questioned.


Threats to salt marshes
Do excessive nutrients in water pose the same threat to salt marshes as they do seagrasses?

Over 50% of salt marshes in the U.S. have been destroyed, mostly due to filling of marshes to create more land area for homes, industry and agriculture and by ditching for mosquito control and diking.
 
An invasive species, the common reed (Phragmites spp.), has displaced native species in some regions.

Salt marsh dieback is a phenomenon documented recently in numerous areas, including Georgia.


Sea-level rise as a result of global warming is another potential problem inudating coastal marshesWhy?  Why hasn't this been a problem over the last several thousand years of sea level rise?

As with of marine ecosystem, global warming may not only affect the system, but the system itself may influence global warming.


Intertidal mudflat ecosystems
Like subtidal estaurine bottoms,  deposit feeders and suspension/filter feeders are common on and in the sediment of intertidal mudflats.  In fact, there is surprisingly little vertical zonation in soft-sediment communities (relative to rocky intertidal and marsh communities).  Why might organisms living within sediments not be as strongly influenced by tidal flucuations?

Diversity often appears low, but sampling with shovel and sieve will often reveal numerous taxa.

What is the source of primary production in intertidal mudflats?

Secondary producers:
Meiofauna are microscopic invertebrates (42-500 µm) that live between sediment grains (up to 2 billion individual/m2) and feed on detritus, bacteria, and protozoans.

Filterfeeders, such as bivalves, link water column and benthic process by filtering plankton and detritus, and by expelling regenerated nutrients back to the water column.

Deposit feeders, such as polychaetes, amphipods, sea cucumbers, and snails utilize a wide range of foods including decaying plant material, bacteria, protozoans, diatoms, fungi, and meiofauna.  Many deposit feeders process a large volume of sediment (up to 10 times the body weight daily) and the result is bioturbation of sediments enhancing sediment resuspension which may stimulate productivity.



Stress on soft-sediment organisms:
Not only do intertidal organisms have to deal with anoxic sediments, but also with dissecation  at low tide.

Predators high relative to subtidal soft-sediments and to marshes and seagrass beds.  Why?






Primary source: Bertness MD. 1998.. The Ecology of Atlantic Shorelines. Sinauer Associates Inc.  Sunderland, MA


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