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i Summary Phytoplankton species have different resource requirements and different sensitivities to important growth factors. Interactions between nutrients and physical factors, such as temperature and light should therefore influence the species composition. Understanding these interactions will provide insights into the consequences of climate and trophic change, which shift the relationship between nutrients, temperature and light. Because these interactions are poorly understood, this study investigated the interactive effects of temperature and photoperiod on phytoplankton growth controlled by fluctuating light, phosphorus (P) and silicon (Si). Growth and competition experiments were performed in the laboratory on Stephanodiscus minutulus, Nitzschia acicularis (both diatoms) and Limnothrix redekei (cyanobacterium). A model of factor interactions was developed and long-term field data from Lake Müggelsee (Berlin) were statistically analysed. The fluctuating light regime used in experiments on all three species was designed to simulate the natural variation of light intensity that algae experience when they are transported through the mixed layer due to mixing. Temperature and photoperiod had the same influence on growth under constant light as they did under fluctuating light induced by mixing in lakes. The photoperiod and short term light fluctuations had additive effects on growth, indicating that they are inherently related as different aspects of temporal variation of the light supply. Their combined effects were accounted for with a simple, yet accurate empirical equation, which avoids many of the inaccuracies of deriving growth rates from photosynthesis. The photoperiod did not influence the degree of limitation by P or Si (multiplicative interaction), nor did it influence relative P-uptake rates. In contrast, the temperature interactions were more complex and species-specific. Regardless of whether growth was controlled by P, Si or fluctuating light, S. minutulus (an early spring centric species) became more competitive under lower temperatures and short photoperiods, whereas N. acicularis (a late spring pennate species) became more competitive under higher temperatures and longer photoperiods. The field data analysis confirmed the predictions of the culture experiments, showing that Si, P, temperature and photoperiod were the most important predictors of centric diatom biomass. Pennate diatoms depended on temperature and light, but starting population size and zooplankton abundance also played an important role. ii Summary Contrary to established views, the minimum P-quota (internal P-content, Q 0) may be independent of temperature, reflecting instead a temperature dependent half-saturation coefficient (k Q), which fixed-form quota curves without a k Q like the Droop model cannot account for. In contrast to …

Interactive effects of nutrients and physical factors on phytoplankton growth