Combined effects of elevated carbon dioxide and temperature on phytoplankton-zooplankton link: A multi-influence of climate change on freshwater planktonic communities

Abstract

It is essential to understand the combined effects of elevated CO₂ and temperature on phytoplankton-zooplankton link when attempting to predict climate change responses of freshwater ecosystems. Phytoplankton species differ in stoichiometric and fatty acids composition, and this may result in phytoplankton-mediated effect on zooplankton at elevated CO₂ and temperature. Beyond the isolated analysis of CO₂ or temperature effect, few studies have assessed zooplankton growth under the phytoplankton-mediated effects of elevated CO₂ and temperature. In this study, three algal species (green alga, diatom, cyanobacteria) were fed on zooplankton Daphnia magna, under the conditions of CO₂ concentrations of ambient (390 ppm) and elevated (1000 ppm) levels and temperatures at 20, 25 and 30 °C. Elevated CO₂ increased the algal biomass, while it reduced the phosphorus (P) and ω3 polyunsaturated fatty acids (ω3 PUFAs) to carbon (C) ratios. Elevated temperature decreased the P/C ratios in all algal cultures and ω3 PUFAs/C ratios in the diatom and the cyanobacteria cultures. Phytoplankton-mediated effect of elevated CO₂ reduced the growth of zooplankton fed on the green and the mixed three algae culture. The stimulation of zooplankton fed on the diatom and the cyanobacteria by elevated temperature can be offset by decreasing food P and ω3 PUFAs contents. The combined effects of elevated CO₂ and temperature on the growth of daphnids were mainly mediated by ω3 PUFAs/C ratios in the phytoplankton. Rising temperature as a combined direct and indirectly phytoplankton-mediated effect on zooplankton may be able to ameliorate the negative effects of elevated CO₂. The results indicated that the combined effects of increased CO₂ and temperature increased the fatty acid content of the green alga but not the other algae. This study highlighted that climate change with simultaneously increasing temperature and CO₂ may entangle the carbon transfer in freshwater planktonic communities.

Keywords: Algae; Climate change; Ecotoxicology; Food chain; Stoichiometry; Zooplankton.