Effects of low concentrations of heavy metals on plankton community dynamics in a small, shallow, fertile lake

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Abstract
Significant differences in phytoplankton abundance developed among identical, untreated 2.3-m 3 enclosures, maintained in a shallow, fertile lake in northern Italy for 14 days in June 1981. Differences in soluble reactive phosphorus concentration were significant after 7 days and developed more rapidly if the water columns were isolated from the sediment. Three replicate enclosures were used per treatment in experiments carried out between November 1981 and June 1983 to investigate heavy metal toxicity. This enabled between-treatment variation caused by the heavy metal additions to be distinguished from within-treatment variation caused by containing the water column within enclosures. For plankton abundance and physico-chemical variables, the within-treatment coefficient of variation exceeded ±50% if experiments were >3 weeks. Containment within enclosures affected the ecosystem differently in different seasons but, in general, planktonic populations resembled those in the lake. Cadmium, copper and mercury additions of between 10 and 100 μg l −1 had little immediate effect upon phytoplankton biomass in summer. In winter, marked reductions occurred in the first 2 days of the experiment. Zooplankton was more sensitive than phytoplankton to the heavy metals in summer, cladoceran and rotifer populations declining precipitously, shortly after heavy metal addition in each experiment. Copepods were the most heavy-metal-tolerant zooplankters. As the direct toxicity of the heavy metals subsided, there was disruption of zooplankton predation and nutrient regeneration. In summer, reduced grazing by heavy-metal-sensitive zooplankton caused algae to proliferate, whereas in winter, inorganic nutrients, which accumulated in the treatments due to heavy-metal-induced decreases in plankton biomass, later supported larger populations of algae than would otherwise have occurred. Low nutrient concentrations in summer probably reflected rapid uptake in productive conditions. Mercury persisted in the water for a shorter period than did either cadmium or copper, and accumulated more rapidly in periphyton. Copper remained longest in the water column and was largely associated with suspended particulate material. Cadmium also persisted in the water column but remained largely in solution. Heavy metal accumulation in sediment was not evident, but may have been masked by high background concentrations.

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