Summary
Determining the limiting factors of aquatic primary production is central in the management of freshwaters. The finding that the phosphorus generally limits the algae growth in freshwaters is recently challenged by evidence that the primary production of some lakes is nitrogen limited. We conducted 10 in situ phytoplankton limitation bioassay experiments during one growing season (May ?October). The experiments were done with natural phytoplankton communities, with and without pre-screening of herbivores, on different depths in the vertical light gradient, and with and without addition of phosphorus and nitrogen. Chlorophyll fluorescence and pigment absorbance spectra, measured in a micro-plate reader, were used as response parameters. All experiments had a highly significant effect of light, while the effect of screening out herbivores was generally small. Significant limitation effects of both N and P were demonstrated, but the majority of experiments from the period with colonial cyanobacteria dominance had very small responses to nutrient addition.