The black water chemocline of meromictic Lower Mystic Lake, Massachusetts, USA

Lower Mystic Lake, Massachusetts, USA, has an anoxic black water layer just below the top of the chemocline (15.5-16.0 mj. Bacterial concentrations averaged 10.4 x 10(6) cells/ml in the black water laver and 4.0 x 10(6) cells/ml below 17 m. Below the chemocline. microbial concentrations were linear Iv correlated to the vertical light absorption coefficient. r = 0.82. Phototrophic bacteria were not detected below the top of the chemocline, due to a low PAR that never exceeded 0.0001% surface illumination. Sulfate-reducing bacteria and methanogens were enriched from the monimolimnion in selective media. Below the chemocline, H,S concentrations were in excess of 11 mmoles/l and Fe, Mn, CH4 and CO2 concentrations ware elevated compared to the mixolimnion. Nuisance releases of HIS occurred from the lake in 1965. Although the monimolimnion remains a highly reduced environment rich in H2S, the potential of further nuisance releases is small due to the diminished volume of the monimolimnion and the relatively deep chemocline.