The habitat distribution of overwintered eggs, which were found to be the only source of spring recruitment of Argulus coregoni Thorell, was studied at a commercial fish farm in Central Finland. The frequency of occurrence of egg clutches in the deep parts of the canals and ponds was 50 to 80% and the percentage cover of the surface of stones with egg clutches was 1.7 to 6.4%, while in the shallow parts these values were 8 to 27% and 0.1 to 0.3%, respectively. A greater proportion of empty egg-shells was observed in shallow water in the mid-May, suggesting an earlier hatching there stimulated by the increased temperature and higher illumination. Under laboratory conditions, only elevated UV illumination, but not diurnally fluctuating temperature, significantly accelerated hatching. Normally overwintered eggs produced a pronounced peak of hatched larvae at the end of May and hatching continued at a much slower rate throughout the summer. Eggs that overwintered twice, first normally and then for a second time buried under sediments, were exposed to the same laboratory conditions simultaneously with normally overwintered eggs, but their hatching was delayed until August. The hatching rate was low, but markedly increased in December.

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Benthic and pelagic microalgae inhabiting shallow lakes compete mainly for the limiting resources phosphorus and light. The availability of these resources depends on the spatial structure of the algal habitats. Pelagic microalgae use phosphate that is dissolved in the water column, and in turn their biomass affects the light intensity that reaches benthic microalgae. Benthic algae are able to use sediment-associated nutrients and nutrients that are recycled within the algal mats. To investigate competition between benthic and pelagic microalgae in shallow lakes, we performed two microcosm experiments with a three-factorial design. Phosphorus concentration in both the artificial substrates and the water column as well as light intensity were manipulated. The manipulated factors significantly affected Chl a and particulate phosphate concentrations of the experimental communities. In both experiments, benthic algae dominated during the entire time. Irrespective of light intensity, phosphate concentrations in the sediments and in the water column, pelagic algae were unable to become competitively superior by producing a biomass that was dense enough to shade the benthic algal community. Nutrients dissolved in the water column were effectively used by benthic microalgae, which reduced their availability for pelagic algae. The assumption that increasing the nutrient concentration in the water column would result in the loss of benthic communities was not confirmed.

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