Spring heatwaves intensify negative effects by cyanobacteria blooms on polyunsaturated fatty acids of a brackish zooplankter

Abstract Marine heatwaves are projected to occur more frequently in near future and can favor the occurrence of cyanobacteria blooms. Cyanobacteria typically provide low dietary quality for consumers due to their lack of long‐chain polyunsaturated fatty acids (LC‐PUFA). This can alter the fatty acid composition of their primary consumers and trigger cascading effects throughout the food chain. In this study, we investigated the combined effects of a simulated spring heatwave and associated cyanobacteria bloom on intrinsic growth rates and fatty acid compositions of the zooplankter Brachionus plicatilis . The rotifers were fed three diets differing in quality and temporal occurrence, with (1) permanent high (“H”), (2) permanent low quality (“L”), or with (3) a diet switch from high to low quality, mimicking a cyanobacteria bloom (“CB”). Rotifers of each food treatment were additionally exposed to either ambient temperature (12°C, “‐HW”) or spring heatwave conditions (18°C, “+HW”). Population growth rates increased under heatwave conditions for all diets. Food quality affected fatty acids, both quantitatively and qualitatively. H rotifers showed highest contents of fatty acids and LC‐PUFA. The fatty acids of CB rotifers resembled those of L rotifers, indicating rotifers could not maintain their fatty acid compositions during a 5‐d cyanobacteria bloom. Most importantly, the simulated heatwave intensified diet‐mediated fatty acid differences, benefiting H rotifers but not L or CB rotifers. Therefore, the cyanobacteria bloom accompanied by warming resulted in fewer PUFA in rotifers. We thus conclude that the projected increasing occurrence of heatwaves can aggravate cyanobacteria's negative impacts on future food webs.