High Benthic Microalgal Biomass Found on Ship Shoal, North-central Gulf of Mexico

Recent studies have highlighted the biomass and ecological roles of benthic microalgae (BMA) on the continental shelf. However, few studies have specifically examined BMA in the north-central Gulf of Mexico. We examined the origin and biomass of sediment associated algae on Ship Shoal, a submerged sandbank with depth and sediment characteristics favorable to benthic primary production. Light levels at the sediment surface ranged from < 1% to 30% of surface PAR, and were judged sufficient for benthic photosynthesis throughout the year. Sediment algal biomass, as chlorophyll a, was highest in the spring and summer and was not consistently correlated with any of the physical parameters recorded including sediment light levels and water depth. Photosynthetic pigment analysis indicated sedimentary algae across Ship Shoal were predominately diatoms on all sampling dates. Microscopic analysis suggested the diatoms were primarily BMA with only a minor fraction of settled phytoplankton. Our results differ from studies of muddy sediments which have typically found that sediment algae on the Louisiana shelf were derived from phytoplankton. Comparison of pigments from the sediment and bottom water suggested weak exchange of benthic and pelagic algae between the two compartments. Algal biomass in the sediment exceeded that of the overlying water column over much of Ship Shoal during the spring and summer. The high benthic algal biomass (equivalent to typical estuarine values) suggests benthic primary product (BPP) may contribute to the Ship Shoal food web and that BPP may be an important ecosystem component on the other large shoals found on Louisiana's inner shelf.