Nutrient pulse scenarios drive contrasting patterns in the functional stability of freshwater phytoplankton

Climate change is increasing the frequency, intensity, and stochasticity of extreme weather events such as heavy rainfall, storm‐induced mixing, or prolonged drought periods. This results in more variable regimes of dissolved nutrients and carbon in lakes and induces temporal fluctuations in the resource availability for plankton communities, which can further lead to changes in growth and the cellular ratio of essential elements, such as carbon, nitrogen, and phosphorus. However, the current understanding of the effects of variations in regularity and frequency of precipitation events on both producer and consumer levels is limited by the lack of experimental studies examining processes at multiple trophic levels. In our mesocosm study, we added the same total amount of nitrate, phosphate, and colored dissolved organic matter (cDOM) to each mesocosm at pulses differing in frequency (daily, intermittent, or one extreme addition) and regularity (regular, irregular) over a simulated run‐off period followed by a recovery period. Our results showed that phytoplankton biomass fully recovered to control conditions from one extreme nutrient and cDOM pulse, whereas pulses of higher frequency gradually increased the biomass. In terms of stoichiometry, the extreme pulse led to the lowest stability in particulate C : P and N : P ratios. At the zooplankton level, copepod biomass decreased across all nutrient and cDOM additions, but no effects between the treatments were found. Overall, our study demonstrates that phytoplankton stability depends on the regularity and frequency of nutrient additions and differs substantially between biomass and stoichiometry, but the effects may be buffered on zooplankton level.