Biogeochemical controls on the oxygen, nitrogen and sulfur distributions in the water column of Golfo Dulce:: an anoxic basin on the Pacific coast of Costa Rica revisited

Chemical distributions, respiration rates, and bacterial distributions were measured in 1994 in the water column and sediments of a small, tropical, anoxic basin (Golfo Dulce, Pacific coast of Costa Rica) to examine the biogeochemical controls on anoxia, sulfide, dissolved inorganic nitrogen, and organic carbon consumption. As reported previously, the deepest 100 in of the water column were anoxic, and sulfide concentrations in the bottom waters were less than 7 mu M and then only transiently. Both free-swimming sulfide-oxidizing bacteria and Beggiatoa sp. (containing large vacuoles) were observed in the anoxic bottom waters or at the sediment-water interface. Aerobic respiration dominated the decomposition of organic matter in the surface waters and pycnocline, whereas sulfate reduction was principally restricted to the sediments. Bacteria were distributed in discrete zones and exhibited the highest densities where oxygen decreased below 1 mu M around 100 in depth, and near the sediment-water interface. The sub-oxic, sub-pycnocline water column was characterized by a dissolved inorganic nitrogen (DIN) deficit of 2.9 mole m(-2). With a water residence time of 35 - 57 d, estimated from a salt balance, this deficit corresponded to a DIN loss of 51 - 85 mmol m(-2) d(-1), comparable to the sub-pycnocline oxygen consumption. Sulfide in the water column was maintained at low concentrations by frequent inputs of oxygenated water from the Pacific Ocean. Sulfide production in the sediments due to bacterial sulfate reduction was scavenged by frequent deposition of iron-rich turbidites. Based on Pb-210 distributions, the most recent emplacement of a turbidite in the basin sediments was determined to have occurred between 1989 and 1992.