Many freshwater environments experience dramatic seasonal changes with some systems remaining ice-covered for most of the winter. Freshwater systems are also highly sensitive to environmental change. However, little is known about changes in microbial abundance and community composition during lake ice formation and times of persistent ice cover. The goal of this study is to characterize temporal dynamics of microbial communities during ice formation and persistent ice cover. Samples were collected in triplicate, five days per week from surface water in the Keweenaw Waterway between November and April. Environmental conditions along with microbial abundance and microbial community composition was determined. Distinct community composition was found between ice-free and ice-covered time periods with significantly different community composition between months. The microbial community underwent dramatic shifts in microbial abundance and diversity during the transitions into and out of ice cover. The richness of the microbial community increased during times of ice cover. Relatives of microbes involved in nitrogen cycling bloomed during times of ice cover as sequences related to known nitrifying taxa were significantly enriched during ice cover. These results help to elucidate how microbial abundance and diversity change over drastic seasonal transitions and how ice cover may affect microbial abundance and diversity.

The fungicides, phenyl mercuric dodecyl succinate (PMS) and phenyl mercuric dimethyldithiocarbamate (PMDDC), have been prepared labelled with 203Hg. Comparative exposure tests have been carried out on emulsion paints each containing one of the labelled compounds phenyl mercuric acetate (PMA), PMS, or PMDDC. After 236 days of indoor exposure, approximate losses from films are 60 (PMA), 50 (PMS), and 20% (PMDDC). Relative losses from films of low‐Hg content (0·02%) are a few per cent higher than those from comparable films of high‐Hg content (0·2%). After 56 days of outdoor exposure, approximate losses are 95 (PMA), 90 (PMS), and 40% (PMDDC). PMS and PMDCC films of high‐Hg content show relative losses slightly higher than those of low‐Hg content; a behaviour opposite to that of PMA films. This may be due to the distribution of PMS or PMDDC through a paint film being less uniform than that of PMA. A correlation between mercury loss and rainfall is observed for all three fungicides. The loss rates from all films decrease with time during indoor exposure. Out‐of‐doors, this decrease becomes well defined only when a substantial portion of the fungicide has been removed by rain. The results indicate that in emulsion paint films, under the exposure conditions used in this work, PMS is retained slightly better than PMA, but that PMDDC is retained much better than either. The high retention of PMDDC is probably due to its low saturation vapour pressure and to its low solubility in water. The solubility of PMDDC in water at 20° has been determined radiometrically as 5·7 ppm.