Relationships between physico-chemical and microbiological parameters in the monimolimnion of a forest meromictic lake

The small meromictic Lake Zapadłe in North-Eastern Poland was the subject of our research in the vegetation period between April and November. Our study were to aim a better recognition of meromixis phenomenon and find connections between hydrochemical and microbiological parameters. Here, the monimolimnion layer was below 10 m depth with the chemocline between 13-14 m. Highly significant Spearman’s ranks correlations of P<0.05 were found between conductivity and biochemical oxygen demand (0.91), ammonium nitrogen (0.96), phosphate (0.91), iron (0.77) and manganese (0.82). Favourable conditions for bacterioplankton growth and function here included; the absence of water circulation, the presence of anaerobic conditions and hydrogen sulphide, a constant water temperature and highly significant correlations between total bacterial counts (TBC), bacterial biomass (BB) and biochemical oxygen demand (BOD), conductivity, total organic carbon (TOC) and dissolved organic carbon (DOC). The pool of bacteria-forming biomass increased significantly in the lower part of the monimolimnion. A highly significant correlation (P<0.05) existed between bacterial biomass (BB) and their anaerobic metabolic products: ammonium (r=0.75), hydrogen sulphide (r=0.45) and phosphate (r=0.68) anaerobic metabolic products. This correlation indicated the significant proportion of anaerobic sulfate-reducing bacteria. The impact of physico-chemical parameters on bacterioplankton biomass during the June-November growth season was clearly illustrated in the correspondence canonical analysis (CCA). This recorded its greatest mass at 15 to 17 metres above the lake bed. Although no clear seasonal variations were noted in bacterioplankton composition described by Denaturing Gradient Gel Electrophoresis (DGGE). The monimolimnion lake layer contained 46 Operational Taxonomic Units (OTUs). Subsequent comparison of the upper and lower minimolimnion layers showed 37 of these OTUs were common, while 5 were unique in the upper monimolimnion strata and 4 in the lower layers.