PRODUCTION AND DECOMPOSITION PROCESSES IN A SALINE MEROMICTIC LAKE

Bacterial and phytoplankton cell number and productivity were measured in the mixolimnion and chemocline of saline meromictic Mahoney Lake during the spring (Apr.-May) and fall (Oct.) between 1982 and 1987. High levels of bacterial productivity (methyl 3H-thymidine incorporation), cell numbers, and heterotrophic assimilation of 14C-glucose and 14C-acetate in the mixolimnion shifted from near surface (1.5 m), at a secondary chemocline, to deeper water (4-7 m) as this zone of microstratification gradually weakened during a several year drying trend in the watershed. In the mixolimnion, bacterial carbon (13-261 μgC 1-1) was often similar to phytoplankton carbon (44-300 μgC 1-1) and represented between 14-57% of the total microbial (phytoplankton + bacteria) carbon depending on the depth interval. Phototrophic purple sulphur bacteria were stratified at the permanent primary chemocline (7.5-8.3 m) in a dense layer (POC 250 mg 1-1, bacteriochlorophyll a 1500-70001μ 1-1), where H2S changed from 0.1 to 2.5 mM over a 0.2 m depth interval. This phototrophic bacterial layer contributed between 17-66% of the total primary production (115-476 mgC m-2 d-1) in the vertical water column. Microorganisms in the phototrophic bacterial layer showed a higher uptake rate for acetate (0.5-3.7 μC 1-1 h-1) than for glucose (0.3-1.4 μgC 1-1 h-1) and this heterotrophic activity as well as bacterial productivity were 1 to 2 orders of magnitude higher in the dense plate than in the mixolimnetic waters above. Primary phytoplanktonic production in the mixolimnion was limited by phosphorus while light penetration appeared to regulate phototrophic productivity of the purple sulphur bacteria. © 1990 Kluwer Academic Publishers.