SALINITY CONTROL OF BENTHIC MICROBIAL MAT COMMUNITY PRODUCTION IN A BAHAMIAN HYPERSALINE LAGOON

The purpose of this study was to determine the production and N2 fixation responses of a hypersaline mat community following a reduction in salinity and nutrient enrichment. Cyanobacteria-dominated microbial mat samples were collected from hypersaline Storr's Lake and normal seawater salinity Pigeon Creek and preincubated at ambient (90 parts per thousands) and reduced (45 parts-per-thousand) salinities following no nutrient as well as inorganic nutrient (NO3-, PO4-, trace metals) and dissolved organic carbon (DOC, as mannitol) enrichment. CO2 and N2 fixation rates were determined 2 and 4 days later. In addition, DOC (trace concentrations of H-3-labeled glucose and amino acids) uptake was measured in mats under normal and hypersaline conditions. A reduction in salinity from 90 to 45 parts per thousand significantly enhanced CO2 and N2 fixation rates, but inorganic nutrient and DOC additions did not significantly enhanced rates compared with the controls. Dissolved organic carbon/dissolved organic nitrogen (DON) uptake was not influenced over the entire range of salinities (45-90 parts per thousand) used in this study. When salinity-induced osmotic stress was relieved, mats underwent enhanced primary production and nitrogen fixation. Abiotic stress, induced by hypersaline conditions in Bahamian lagoons, results in lower productivity of the microbial mat communities and this stress may outweigh the typical limiting factors regulating phototrophic community primary production.