Heatwave-mediated decreases in phytoplankton quality negatively affect zooplankton productivity

Here we address this knowledge gap by growing natural assemblages of freshwater phytoplankton at ‘ambient’, ‘constant warming’, or ‘heatwave’ conditions. We next fed these phytoplankton communities to natural assemblages of zooplankton to test the prediction that zooplankton that consume heatwave-exposed phytoplankton will exhibit reductions in biomass. Our experiment demonstrated that zooplankton that consumed ‘heatwave’ phytoplankton attained lower community biomass than those fed ‘constant warming’ or ‘ambient’ phytoplankton.

Additionally, despite receiving similar total heat input, phytoplankton exposed to ‘heatwave’ conditions contained lower C, N, P, and fatty acid concentrations compare to phytoplankton grown in ‘constant warming’ conditions. Correlations between zooplankton biomass and all measured phytoplankton traits revealed that decreases in zooplankton biomass were best explained by low quantities of C, N, and monounsaturated fatty acids in ‘heatwave’ phytoplankton.

Our study demonstrates that the effects of heatwaves on phytoplankton quality are clearly distinct from those caused by constant warming temperatures, and that heatwave-mediated decreases in resource quality have immediate effects on consumer productivity.

Methods

The study was a laboratory experiment using wild-caught phytoplankton and zooplankton communities. All data were collected and analyzed by the authors except that phytoplankton were identified by Biologica Environmental Services in Victoria British Columbia, Canada. Phytoplankton taxonomy data were analyzed by the authors.