Outwelling is a hypothesized process by which coastal
salt
marshes and mangroves, “hot spots” of production, produce an excess amount
of carbon each year and “outwell” these organic nutrients and detritus into the
surrounding coastal embayment or ocean, thus
increasing the productivity of local fisheries or
other coastal plants. Outwelling also nourishes plankton
communities and causes a spike in activity.
The majority of outwelling is dissolved organic carbon (DOC) and some
particulate organic carbon (POC)[2]
Outwelling expels salt
(90 g salt/m2), silicate
(1.0 mmol/m2), orthophosphate (0.03 mmol/m2), and nitrate
(0.04 mmol/m2) during each tidal cycle.
Outwelling is affected by a number of different factors. For
one, the amount of outwelling is dependent upon the primary production of an estuary, thus,
highly productive salt marshes result in increased outwelling. It is also
dependent on tidal amplitude and geomorphology
of the estuary. Outwelling is not a steady process, and is affected by large rainfalls or inundation
events (the larger the inundation, the greater the outwelling).
Outwelling does not occur in every estuary. It is more
evident and occurs more in estuaries bordering extensive coastal marshes. For example,
a study done in a New England salt marsh found no evidence of outwelling, and
in fact found that the salt marshes import carbon; however, another study done
in Louisiana
near the extensive salt marshes where tidal amplitude is larger found that
outwelling contributed a significant amount of organic carbon to the nearby
waters.
Outwelling occurs as pulses that correlate to inundation and
precipitation events, productivity and
tidal fluctuations. In some cases, it is macrofauna
and algae that are
pulsed out of the salt marsh into the water column rather than nutrients, but
this has a similar effect of attracting small fish and nourishing the marine
environment.
Ramifications
Because of this hypothesis, many states have passed laws
protecting estuaries based on the rationale that protecting estuaries will
protect the food source of local fish populations.
Controversy
The Outwelling hypothesis has been hotly debated for
decades. There are many studies examining this hypothesis, but there has not
yet been a firm conclusion. It appears that it may occur in some estuaries at
some times, but there is a body of evidence contesting the claimed amount of
organic carbon export, and even whether marshes export carbon at all.
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