Understanding how climate change alters interactions among species is one of the big challenges facing ecologists today. The consequences of changes to existing interactions among species are especially important in lake ecosystems where there is the potential for shifts between alternate states. Lakes often can be found in either a clear-water state with little algal growth, or a turbid state in which there are frequent dense algal blooms. Two factors which largely determine whether a lake will be clear or turbid are nutrients and algal consumers. Nutrients, especially nitrogen and phosphorus, increase the growth rate of algae leading to algal blooms and a turbid state. Algal consumers, particularly of the genus Daphna, are very effective grazers of algae, limiting their populations and promoting the clear-water state. The increase in global temperatures is decreasing the length and severity of winters, which increases the abundance of over-wintering Daphnia. This study focuses on understanding how lake nutrient status affects the climate driven differences in over-wintering of Daphnia. Experimental ponds will be used to perform the critical experiment to determine where the tipping point between a turbid and a clear-water state is across the range of lake nutrient concentrations and overwintering Daphnia abundance seen in natural lakes.
Determining the conditions that bring about shifts between ecosystem states is an important part of the management of ecosystems under a changing climate. This research will answer fundamental questions about alternate states in lakes and provide a useful tool for lake managers to use for promoting the clear-water state in lakes. Results of this study will be published in a management oriented journal; as well an ecological journal to ensure they reach the audience that can make use of them. This study will determine the densities of Daphnia required to be achieved for a given nutrient level, providing lake managers with targets for their management activities to maintain beautiful clear lakes. Additionally, this project will provide an undergraduate student with training and mentoring in the conduct of scientific research.
With the work supported by this NSF award we were able to make significant progress toward understanding how climate change will affect the plankton living in temperate lakes. We conducted two experiments that were designed to test how the small planktonic organisms which make up the base of lake food webs would respond to altered conditions brought on by climate change. Our goal was to understand when the new conditions brought on by climate change would favor the uncontrolled growth of algae which leads to harmful algal blooms. The results of these studies will help lake managers to anticipate changes to ecosystems brought on by a changing climate and allow them to take steps to prevent undesirable outcomes. In our first experiment, we measured how warming affected the growth of algae and Daphnia, an important consumer of algae in lakes. We found that the effect of warming changed between different temperatures. At moderate temperatures warming results in increased algal growth rates, but feeding rates increased even faster. As a result, warming of typical spring and fall temperatures will improve the performance of Daphnia, helping them to keep algae populations in check. At very cold and very warm temperatures; however, warming favored algal growth over Daphnia feeding. As a result, more moderate winters and extremely hot summers are likely to promote algal growth, resulting in more algal blooms under these conditions. In our second experiment we tested how changes to the number of algae and Daphnia which are able to survive through the winter will impact the growth of algae in the spring. To make this experiment more realistic, we conducted it across a range of water nutrient conditions experienced by lakes. On one extreme are pristine, low nutrient lakes which have had little human impacts, while the other extreme corresponds to lakes which are strongly impacted by human pollution from things like fertilizers or sewage inputs, resulting in high nutrient contents. We found that the number of algae and Daphnia which are able to survive the winter becomes much more important in high nutrient lakes than in pristine waters. In the high nutrient waters a small increase in the number of Daphnia that are able to survive the winter can result in a large reduction in the amount of algae that is able to grow in the spring.
