Zooplankton grazing pressure is insufficient for primary producer control under elevated warming and nutrient levels

Within a given ecosystem, species persistence is driven by responses to the effects of biotic and abiotic stressors. Ongoing climatic shifts and increased pollution pressure have created the need to assess potential effects and interactions of physical and biotic factors on coastal ecosystem processes to project ecosystem resilience and persistence. In coastal marine environments, primary production dynamics are driven by the interaction between bottom-up abiotic effects and biotic effects induced by top-down trophic control. Given the many environmental and climatic changes observed throughout coastal regions, we assessed the effects of interactions among temperature, nutrients and grazing in a laboratory-based microcosm experiment. We did this by comparing chlorophyll-a (chl-a) concentrations at two temperatures in combination with four nutrient regimes. To test for subsequent cascading effects on higher trophic levels, we also measured grazing and growth rates of the calanoid copepod Pseudodiaptomus hessei. We observed different phytoplankton and zooplankton responses to temperature (17 °C, 24 °C) and nutrients (nitrogen only (N), phosphates only (P), nitrogen and phosphates combined (NP), no nutrient additions (C)). Contributions of predictors to model fit in the boosted regression trees model were phosphates (42.7%), copepods (23.8%), nitrates (17.5%) and temperature (15.9%), suggesting phosphates were an important driver for the high chl-a concentrations observed. There was an increase in total phytoplankton biomass across both temperatures, while nutrient addition affected the phytoplankton size structure prior to grazing irrespective of temperature. Phytoplankton biomass was highest in the NP treatment followed by the N treatment. However, the phytoplankton size structure differed between temperatures, with microphytoplankton being dominant at 24 °C, while nanophytoplankton dominated at 17 °C. The P and C treatments exhibited the lowest phytoplankton biomass. Copepod abundances and growth rates were higher at 17 °C than at 24 °C. This study highlights that bottom-up positive effects in one trophic level do not always positively cascade into another trophic level. It was, however, evident that temperature was a limiting factor for plankton abundance, productivity and size structure only when nutrients were limiting, with top-down pressure exhibiting minimal effects on the phytoplankton.