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Effects of warming temperatures on phytoplankton community composition and nutritional quality Kim, Jihyun


Climate change is resulting in ongoing temperature warming and increased frequencies of heatwaves. In aquatic ecosystems, temperature not only affects the distribution and abundance of phytoplankton, but warming may also be reducing the quantities of key nutrients produced by these important primary producers. However, thus far most research in this field has examined the effects of warming temperatures on nutrient production in individual species of phytoplankton. To characterize how warming affects phytoplankton-based nutrients at the community scale, we subjected naturally occurring phytoplankton assemblages to three temperature treatments (ambient, warming, heat wave) in a seven-week laboratory experiment. We used community-wide fatty acid composition and stoichiometric indicators (C:N, C:P and N:P ratios) as our measures of phytoplankton nutritional quality. By the end of the experiment, there was no effect of temperature on phytoplankton community composition. Phytoplankton communities from the heatwave treatment had decreased concentrations of C, N and P, but neither the heatwave nor warming affected community-wide C:N, C:P and N:P ratios. Both warming and the heatwave reduced phytoplankton polyunsaturated fatty acid (PUFA) content, but as the heatwave subsided, PUFA quantities in this treatment approached those found in the ambient temperature treatment. We then fed warmed phytoplankton communities to naturally-collected zooplankton assemblages and found that the PUFA composition of the zooplankton communities closely reflected that of their food source. Our results suggest that 1) temperature warming has negative effects on phytoplankton community nutritional quality, 2) these responses are not caused by broad-scale shifts in phytoplankton taxonomy, and 3) phytoplankton PUFA levels appear to closely track water temperature. Furthermore, we provide evidence that zooplankton communities experience indirect effects of temperature warming through nutritional shifts in their phytoplankton resource. Overall, this study improves our understanding of the types of phytoplankton nutrients that are affected by warming, how quickly these nutrients can respond to temperature change, and the down-stream effects of phytoplankton-based nutrients on zooplankton consumers.

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