Navigation
Return to
Table of
Content

Return to
Oceanography
Dr. D.'s
Home
Page
KSU Home
 Biological Oceanography Lecture
Benthic interactions


The area off U.S. coast ("South Atlantic Bight" or SAB) is characterized by a broad, relatively shallow continental shelf.  The bottom is typically composed of soft sediments (see the geomorphology lecture).





Marine sediments
Why do sediment layers tend to be thicker nearer to continental margins

Common taxa found in marine soft sediments
Bacteria
A diverse assemblage responsible for various geochemical transformations


Benthic microalgae, also known as microphytobenthos
Form thin layer at surfaces of sediment.  Diatoms are often a major component of this community.
How can primary productivity be measured in sediments


Numerous macro-invertebrates including:
Polychaete annelids (marine segmented worms) and other worms (e.g. sipunculids)
 Worms tend to be important in benthic / water column coupling.  They contribute significant to bioturbation and bioirrigation.  Some are suspension feeders, others deposit feeders


Bivalve molluscs (clams, oysters, ...) and Gastropod molluscs (snails)
   
 
Crustacean arthropods (crab, shrimp, amphipods, isopods)



Echinoderms (sea stars, brittle stars, urchins, sand dollars, sea cucumbers)



Chordates (including cephalochordates, teleosts)


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

Numerous taxa of demersal zooplankton:
"mobile, benthic organisms which periodically emerge from the benthos and move up into the water column" including ostracodes, copepods, and amphipods.

Do these benthic organisms affect the coupling between sediment and the overlying water column?


Geochemical processes
Because organic matter produced in the water column above tends to settle out, bacteria in bottom
sediment are important in remineralization (the transformation of organic molecules to inorganic forms) through aerobic respiration using dissolved oxygen as electron acceptor and anaerobic processes using a variety of inorganic species (i.e. nitrate, nitrite, manganese and iron oxides, sulfate).

The vertical distributions of light and oxygen in sediment

Most light is attenutated within the first few millimeters of the sediment surface (light pentration is greater where sediment grain size is greater).

Where is photosynthesis by benthic microalgae the greastest and what is the consequence of this on the distribution of oxygen with depth within the sediment?

What factors determine the rate exchange of pore water (the water held in the interstitial spaces between sediment particles) with the overlying water?

What factor determines what size particles settle in a given area?




Because light penetration and pore water exchange is limited in sediments, numerous important geochemical transformation can occur here.  The degree of coupling between these benthic processes and the overlying water column is of intense interest






Traditionally,
denitrification was thought of as reduction of nitrate to N2 with organic matter as the electron donor (heterotrophic denitrification) though recently other denitrification pathways to N2 have been discovered.


Phosphorus availability from the sediment to the water column is also tied in tightly with sediment oxygen concentration because phosphate in the presence of iron and oxygen will precipitate (i.e. insoluable) as iron phosphate (FePO4) making less available.



Sources of organic matter and nutrients on the shelf

Salt wedges are typical of some estuaries and move up and down the estuary over each tidal cycle.  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) and be utilized by both planktonic animals and benthic animals (both suspension feeders and deposit feeders).

Productivity is further enhanced by coupling with salt marshes (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).
  
Salt marshes are defined by the presence of emergent vascular plants that are distributed across an intertidal area.  In Georgia, smooth cordgrass (Spartina alterniflora), forms a near monoculture in the lower marsh and is responsible for much of the marsh's productivity.  Salt marsh systems are extremely productive (both primary and secondary).  Why are these systems so productive?





What determines to what degree this estuarine production influences the benthic community offshore on the continential shelf?



Navigation
Return to
Table of
Content
Return to
Oceanography
Dr. D.'s
Home
Page
KSU Home