Examining the effects of warming on freshwater phytoplankton communities and their nutritional quality for zooplankton consumers

Background/Question/Methods

Global warming significantly impacts aquatic organisms in a multitude of ways. The responses of phytoplankton and zooplankton to warming are particularly important as these organisms form the foundation of the aquatic food web and changes at these trophic levels have the potential to affect the productivity of higher trophic levels. Effects of temperature warming on the food resources (phytoplankton) of zooplankton could manifest through temperature-mediated shifts in the taxonomic groups that make up the phytoplankton community, and/or through shifts in nutrient production within each phytoplankton group. The relative importance of each of these mechanisms (species turnover vs. within-species plasticity) is unknown, and we aim to address this question by investigating how temperature affects nutrient availability and transfer through simple aquatic food webs. We conducted a laboratory experiment to examine the effects of ambient (AT), warming (W) and heatwave (HW) temperature treatments in a wild-collected phytoplankton assemblage. Temperature treatments were applied to phytoplankton communities for 48 days, with sampling for community composition, stoichiometry and fatty acid (FA) content occurring at three sampling events. Through controlled feeding of these phytoplankton communities to a wild-caught zooplankton assemblage, the effect of shifts in phytoplankton on zooplankton were also examined.

Results/Conclusions

For phytoplankton community composition, we found that species richness decreased in all treatments over time and primary differences among treatments were colony sizes, with the proportion of single-celled phytoplankton highest in AT, followed by HW then W treatments. Phytoplankton showed no differences in C,N,P ratios among treatments, however HW phytoplankton had significantly lower C,N and P concentrations than the other treatments at the final sampling event (SE3). There were also notable differences in FA among treatments in SE3, with W treatment phytoplankton and zooplankton having the highest saturated-FA:polyunsaturated-FA ratios (indicating lower nutritional quality). For zooplankton, results suggest that nutritional quality differences among treatments were likely due to food-induced physiological changes rather than community-level species changes. Our results also support previous findings that changes in FA composition of phytoplankton is reflected in that of their zooplankton consumers. Overall, W had greater effects on phytoplankton nutritional quality than HW and zooplankton experienced indirect temperature-mediated changes as evidenced from their FA composition. Altogether, this research improves our understanding of the direct and indirect effects of warming and heatwaves on phytoplankton and zooplankton communities, with wild-collected plankton assemblages allowing these data to be more applicable to natural systems (compared to single-species experiments).