Cascading effects of algal warming in a freshwater community

Michelle Tseng, Carla M. Di Filippo, Madeline Fung, Jihyun O. Kim, Ian P. Forster, Yilin Zhou

Daphnia
Daphnia

Certain fatty acids confer considerable health benefits on animals including humans. The majority of fatty acids in aquatic and terrestrial ecosystems originate in algae. There is some worry that with continued global warming, algae will produce lower quantities of beneficial fatty acids, and that this reduction will result in decreased health and growth in the organisms, like zooplankton and fish, that depend on these algal-based fatty acids. In many species of algae, these fatty acids decrease in quantity with warming, but depending on how the cell repackages its fatty acids at warmer temperatures, there may not always be a negative relationship between temperature and algal fatty acid content (specifically, polyunsaturated fatty acids, PUFA). In this study we grew algae at one of three temperatures (12˚C, 20˚C, 28˚C) and then fed these algae to Daphnia zooplankton (water fleas) that were themselves grown at one of three temperatures. We next fed a subset of these Daphnia/algae combinations to Chaoborus (midge) insect larvae. We found that PUFA content per cell size increased with warming, but that because algal size decreases with warming, there was no net effect of temperature on algal PUFA content. Interestingly, Daphnia that were fed 12˚C algae maintained higher population sizes than Daphnia that were fed algae reared at the other two temperatures, and this might be due the fact that 12˚C-reared algae contained slightly more of the beneficial omega-3 PUFA. This result suggests that the 12˚C-reared algae were of higher quality than the algae grown at higher temperatures. Furthermore, this effect of algal quality attenuated as Daphnia environmental temperature increased, suggesting that at warmer temperatures, Daphnia nutritional needs are lower or at least different than at colder temperatures. Lastly, there was little effect of algal rearing temperature on Chaoborus growth rates, suggesting that any nutritional benefits found in 12˚C algae have largely disappeared by the time they are processed through Daphnia. Results from this study thus suggest that beneficial fatty acids in algae may indeed decrease with warming, but that because the nutritional requirements of consumers change with temperature, the reduction in algal fatty acids may not necessarily result in lower productivity at higher trophic levels.

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