Cascading effects of algal warming in a freshwater community

<b>Abstract</b><br/><p>1. Much of our research on the effects of ongoing climate warming on ecological communities is focused on how temperature affects resource quantity. However, resource quality is also affected by warming, and changes in resource quality can have meaningful effects on the productivity of higher trophic levels.</p> <p>2. Aquatic communities in particular are likely to affected by temperature-mediated shifts in resource quality because the nutritional value of algae is highly sensitive to temperature. For example, the production of healthful omega-3 polyunsaturated fatty acids (n-3 PUFA) by algae often decreases with warming.</p> <p>3. Documented decreases in algal PUFAs with warming have led to the hypothesis that global warming should lead to an overall decrease in productivity in aquatic communities. However, this hypothesis: a) potentially oversimplifies the relationship between algal PUFAs and temperature, and b) assumes that the nutritional requirements of consumers are not affected by temperature warming. Here we test these assumptions using a freshwater community (Scenedesmus algae, Daphnia zooplankton, Chaoborus insects).</p> <p>4. We found that total algal PUFAs per unit cell size increased, but cell size decreased with warming, resulting no net effect of warming on PUFA per algal cell. We also found a negative relationship between algal neutral lipids and temperature. Daphnia fed 12˚C-reared algae maintained higher population sizes than those fed 20˚C or 28˚C-reared algae, but the effect of algal food type diminished with increasing Daphnia rearing temperature. The indirect effects of cold-reared algae on the growth rate of Chaoborus predators were minor.</p> <p>5. These data highlight the importance of investigating the effects of temperature on both resource quality and on the nutritional needs of consumers. Our results suggest that at warmer temperatures, consumer nutritional requirements may be reduced. We caution against broad claims that the negative relationship between some algal PUFAs and temperature should result in overall declines in aquatic productivity with ongoing climate warming.</p>