Bacterial Enrichments in Surface Films of Freshwater Lakes

Numerous freshwater lakes were examined for enrichment of bacteria in their surface films as compared to their subsurface waters. Surface films were sampled with a glass plate (60-100 μm) surface samples), a Teflon plate (∼7 μm samples), or a hydrophilic Nuclepore filter (∼40 μm samples). Subsurface samples (∼0.1 m) were obtained with a sterile flask. Total bacterial counts were determined by epifluorescent microscopy. Enumerations of benzoic acid-degrading microorganisms were performed using a14C-most-probable-number technique, with 14C-substrate added to lakewater media at μg/L concentrations. Most lakes examined did not show significant enrichments of either total bacteria or benzoic acid-degrading bacteria in their surface films (∼ 10–100 μm samples). Certain highly transparent lakes did show small enrichments of total bacteria in their surface films. A survey of lakes of various transparencies revealed a direct relationship (r = 0.94) between increasing transparency and increasing surface film enrichment of bacteria.