Michelle Tseng

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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The field of climate change impacts on ecology primarily focuses on the measures of abundance and distribution of individuals to assess organism response, but the measure of organism quality is not often applied to assessing organism response within food webs. The abiotic factor of temperature has known effects on organism quality, but it is unclear how the biotic factor of predator presence impacts organism quality. The goal of my thesis was to investigate (a) the combined effects of warming and predator presence on prey nutrient quality, and (b) how temperature affects organism quality and the nutritional needs of consumers. To address the combined effects of warming and predator presence on one measure of prey quality - organism body size - I conducted a meta-analysis on 14 papers that tested both warming and predator presence on prey body size in aquatic systems. Across all studies, I found no net effect of warming on body size, a large increase in prey body size with predator presence, and an additive effect of the two factors combined. I then conducted a laboratory experiment using the primary producer Scenedesmus obliquus, the primary consumer Daphnia pulex, and the secondary consumer Chaoborus americanus to investigate temperature mediated changes in algal quality and consumer nutritional needs (measured using the fatty acid profile of algae that affected D. pulex population size and C. americanus growth rates). Overall, we observed changes in S. obliquus quality with temperature and mild cascading effects of these changes on D. pulex and C. americanus. Further investigation is needed into the effects of warming and predator presence on other nutrients (such as carbohydrates and proteins that may respond differently to temperature), and if the relationship between body size and quality holds true for all organisms. Overall, my thesis provides insight into how predator presence can have a stronger effect on organism body size than warming and suggests greater care must be taken when interpreting the results of studies that assess the effects of temperature on organism body size in the absence of biotic factors.

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