Diurnal floc generation from neuston biofilms in two contrasting freshwater lakes.

Selective adaptation of biofilm-forming bacteria to the nutrient-rich but environmentally challenging conditions of the surface microlayer (SML) or neuston layer was evident in littoral regions of two physically and geochemically contrasting freshwater lakes. SML bacterial communities (bacterioneuston) in these systems were depleted in Actinobacteria, enriched in either Betaproteobacteria or Gammaproteobacteria, and either unicellular Cyanobacteria were absent or microbial mat forming Cyanobacteria enriched relative to communities in the underlying shallow water column (0.5 m depth). Consistent with the occurrence of biofilm-hosted, geochemically distinct microhabitats, As-, Fe-, and S-metabolizing bacteria including anaerobic taxa were detected only in the SML in both systems. Over diurnal time scales, higher wind speeds resulted in the generation of floc from SML biofilms, identifying a transport mechanism entraining SML accumulated microorganisms, nutrients, and contaminants into the underlying water column. The energy regime experienced by the SML was more important to floc generation as larger flocs were more abundant in the larger, oligotrophic lake (higher relative energy regime) compared to the sheltered, smaller lake, despite relatively higher concentrations of bacteria, organic carbon, Fe, and PO4(3-) in the latter system.