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For a complete picture of the lake ecosystem, it is necessary to understand the mechanisms regulating biomass and nutrient status (nutrient limitation, optimal supply, or surplus) of benthic algae, which are important primary producers and a food resource for grazers. This thesis gives an overview of the natural variation of benthic algae at different scales of space and time and on different substrates, and unravels some of the underlying factors. Algal nutrient status was assessed using the C:N:P (carbon:nitrogen:phosphorus) ratio of the entire natural benthic community. A review, observations, and experiments confirmed that a C:N:P ratio of about 158:18:1 (molar basis) represented an optimal nutrient supply, and that substantially higher C:N, N:P, or C:P ratios reflected algal growth limitation caused by an N or P nutrient deficiency.

Horizontal variation of benthic algal biomass and nutrient status was patchy, of similar amount for all investigated distances, substrates, and lakes, and constituted a dominant proportion of the total variation. For example, patches of nutrient limited algae were found within only 10 m distance from patches with a nutrient surplus. Thus, horizontal variation should not be neglected when sampling benthic algae in lakes. Field observations suggested an impact of wind, nutrients, and grazers on the horizontal variation. Light and nutrients might have caused the observed vertical and temporal variation. Field experiments confirmed a simultaneous control of benthic algal biomass by nutrients and grazing, mediated by light and temperature. Grazing effects were larger than nutrient effects, but the comparison of natural communities in lakes of different trophy suggested that benthic algal biomass was controlled by nutrients in the long run.

An important nutrient supply was animal excretions, causing a low C:N:P ratio of epizoon on zebra mussels, and algal communities associated with macrograzers. A field experiment revealed that 15N circulated one week longer in epizoon associated with a sessile caddisfly than in surrounding epilithon. In conclusion, the regulation of benthic algal biomass and nutrient status in lakes is complex, and benthic animals should be looked at not only as grazers, but also as a nutrient source.