Effects of mercury on enclosed plankton communities in the Rosfjord during POSER

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Abstract
During POSER 2 experiments were formed to study the effects of a single addition of HgCl2 on the development of plankton communities enclosed in large plastic bags, containing 1.5-30 m3 water and ragning from 3-40 m in depth. The experiments lasted 1 and 3 wk, respectively. In one experiment, the influence of an additional dose of nutrients was studied. During both experiments Hg concentrations in the water decreased after the addition. Part of the Hg was found in the sediment colllected on the bottom of the bags during the experiment, but most of the Hg probably volatilized and was lost to the atmosphere. Addition of 1 or 5 .mu.g Hg/l in the 1st experiment resulted in lower numbers of bacteria than in the controls. The first experiment was too short to show effects on phytoplankton and zooplankton. Addition of 5 .mu.g Hg/l in the 2nd experiment inhibited bacteria and phytoplankton and increased the mortality of copepods, which formed the principal part of the zooplankton. Results were comparable to those obtained during other experiments with natural marine plankton communities also enclosed in large plastic bags in fairly different sea areas. Addition of nutrients stimulated phytoplankton growth, but had no observable effects on bacteria or zooplankton.