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Benthic primary production and light climate on Northeastern Gulf of Mexico Shelf sands.

(Funded through the NOAA-NGI project, E. Chassignet , W. Dewar Co-PIs)

This goal of this study, conducted in collaboration with Kevin Speer, Thorsten Dittmar, Bill Landing and Allan Clarke, is the quantification of benthic primary production on the Northeastern Gulf of Mexico shelf in order to assess its relative importance for the carbon fixation in the shallow shelf (Fig. 1).


Fig. 1. Diver at station K-Tower inspecting S-T logger  


Microphytobenthos consist of photosynthetic microorganisms on the sea floor including cyanobacteria, benthic diatoms, and flagellates, and can contribute a significant portion to the overall continental shelf primary production (Gattuso et al. 2006) . Highest production of microphytobenthos occurs on shelves where the overlying water is low in nutrients and therefore relatively clear as on the West Florida Shelf. Light limitation results in a declining gradient of microphytobenthos production with increasing depth, however, on the West Florida Shelf primary production may reach as deep as 70 m as shown by corals and algae growing at Pulley ridge. (Jahnke et al. 2000) found that on the South Atlantic Bight shelf, benthic primary production extends at least down to 40 m water depth and contributes approximately 60% to the total primary production, emphasizing the role of the microphytobenthos as base for the food chain in the shelf. However, also pelagic phytoplankton organisms are filtered into permeable shelf sediments (Huettel et al. 2007), thereby contributing organic matter to the benthic food web (Fig. 2)


  Fig. 2. Pelagic diatoms filtered into shelf sand. These diatoms can survive for months if they remain close to the sediment surface.


Organisms living over continental shelves utilize the primary production of microphytobenthos directly (e.g. shrimp) or indirectly (e.g. fish preying on shrimp and bottom dwellers). Anthropogenic nutrient input into the coastal zone enhances phytoplankton growth thereby increasing water turbidity. Ensuing shading may decrease microphytobenthos productivity and shift communities from assemblages that rely on benthic primary production towards those that rely on planktonic primary production (Okey et al. 2004) . This shift could have far reaching implications for benthic–pelagic coupling, cycling of carbon, nutrients and pollutants and fisheries yield.

We investigate benthic primary production at three stations along a transect crossing in southeasterly direction the Northereastern Gulf of Mexico Shelf from the intertidal zone to the K-Tower at 40m water depth (Fig. 3, 4). Sediment samples taken at monthly time intervals are used for the quantification of benthic primary production. Water samples taken at the same time intervals from bottom water and surface water is analyzed chlorophyll and subsamples are used to assess water column primary production.


  Fig. 3. Location of the transect study in the Northeastern Gulf of Mexico. The distance to station K-Tower is 18 miles.


  Fig. 4. Left: ADCP prepared for the deployment on the transect. Right: K-Tower, a communication tower maintained by the NAVY.  



Gattuso, J. P., B. Gentili, C. M. Duarte, J. A. Kleypas, J. J. Middelburg, and D. Antoine. 2006. Light availability in the coastal ocean: impact on the distribution of benthic photosynthetic organisms and their contribution to primary production. Biogeosciences 3: 489-513.

Huettel, M., P. Cook, F. Janssen, G. Lavik, and J. J. Middelburg. 2007. Transport and degradation of a dinoflagellate bloom in permeable sublittoral sediment. Marine Ecology-Progress Series 340: 139-153.

Jahnke, R. A., J. R. Nelson, R. L. Marinelli, and J. E. Eckman. 2000. Benthic flux of biogenic elements on the Southeastern US continental shelf: influence of pore water advective transport and benthic microalgae. Continental Shelf Research 20: 109-127.

Okey, T. A., G. A. Vargo, S. Mackinson, M. Vasconcellos, B. Mahmoudi, and C. A. Meyer. 2004. Simulating community effects of sea floor shading by plankton blooms over the West Florida Shelf. Ecological Modelling 172: 339-359.