Alternate Title

Sand Bottom Microalgal Production and Benthic Nutrient Fluxes on the Northeastern Gulf of Mexico Nearshore Shelf

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Benthic microalgal production on the continental shelves may be an important contributor to the overall productivity of offshore ecosystems. We used light and dark benthic chambers to measure in situ production, respiration, and benthic nutrient flux on the nearshore quartzite sands of the northeast Gulf of Mexico shelf. Net exchange of O2, NH4+, NO3- + NO2-, PO4-3, and SiO2 was measured in samples taken from chambers at depths of 15 to 16 m offshore of Pensacola, FL. Phytoplankton production and respiration in near-bottom water was determined in paired light/dark BOD bottles to correct chamber measurements for water column processes. Sediment chlorophyll a (Chl a) averaged 4.8 μg Chl a/g. Phytoplankton averaged 5.5 μg Chl a/L. Pheophytin:chlorophyll ratios for the sediment were near 1 indicating an actively growing algal community. Phytoplankton net production ranged from 0.6 to 2.8 mg C/m3/hr. Benthic net production in three separate determinations was 17.7 ± 6.1, 9.5 ± 2.9, and 8.8 ± 1.6 mg C/m2/hr. Benthic respiration was 24.8 ± 0.7, 30.8 ± 1.4, and 11.3 ± 0.3 mg C/m2/hr, respectively. Benthic gross production was thus 42.5 ± 5.2, 40.3 ± 1.2, and 20.3 ± 1.7 mg C/m2/hr, respectively. Benthic nutrient fluxes were highly variable and generally low. Sediment uptake was observed for NH4+ and PO4-3 throughout the study. NO3- + NO2- and SiO2 uptake was observed in 2004 with sediment release seen in 2005.

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