Environmental temperature is an important determinant of species distributions. Temperature influences the performance of an organism both directly, by altering physiological processes, but also indirectly by influencing food availability or the intensity of species interactions such as predation and competition. One predicted consequence of anthropogenic climate change is that certain regions of the world are expected to increase in temperature by as much as 8 degrees C during the next century. As such, it is important to understand how both direct and indirect effects of changes in temperature may alter the future distribution of both native and non-native species. This research project has two main objectives. First, it will examine how temperature influences interactions between native and non-native species in freshwater ecosystems, focusing on the case study of a non-native freshwater invertebrate (Daphnia lumholtzi) and its interaction with native Daphnia species. Second, it will examine how temperature might impact lake food webs indirectly via the surrounding watershed. Terrestrial ecosystems provide important energy and material contributions to aquatic ecosystems, and autumn leaf drop in deciduous forests represents a direct and potentially important contribution from terrestrial to aquatic ecosystems. The leaves of deciduous trees provide nutrients and organic carbon that can increase productivity in both decomposer-based "brown" and plant-based "green" food webs. As leaf chemistry is sensitive to changes in temperature, the objective of this research is to determine how changes in temperature, through altering terrestrial vegetation, will impact lake food webs.
This research will contribute to an improved understanding of how changes in temperature can alter species composition and the ability of lakes to provide important ecosystem services, such as food production. It will also train future scientists by including both undergraduate and high school students as research assistants, including female research interns recruited through Dartmouth's Women in Science Project (WISP), and through hands-on research projects with K-12 students. In addition, both PIs currently work with the outreach and educational organization, the Lake Sunapee Protective Association, providing a route to disseminate findings to the broader public.
Our research had two main objectives: 1) to determine how increases in temperature may alter the establishment success of the non-native zooplankter Daphnia lumholtzi and 2) to understand how changes in temperature, both directly and by altering terrestrial vegetation, impact lake and pond food webs. We addressed our first objective using field experiments, laboratory experiments, and mathematical models. Our results supported several main conclusions. First, physiological measurements taken in a laboratory setting successfully predicted the ability of non-native species to thrive in more complex field settings, such as when predators and various food resources are present. Second, non-native Daphnia lumholtzi may benefit from warmer water temperatures both by gaining a competitive advantage over native Daphnia and by being eaten less by fish predators because of its morphological defenses. We addressed our second objective using a field mesocosm experiment manipulating the addition of deciduous leaves, an important subsidy for lake and pond ecosystems, from trees grown on ambient versus warmed soils. Experimental soil warming altered the chemical composition of deciduous leaves, the physical and chemical environment of the aquatic ecosystems to which leaves were added, and the aquatic food webs. We also demonstrated that the autumn leaf additions had lasting effects that persisted through the following spring. These results highlight the importance of conducting research across ecosystem boundaries to accurately predict the ecological consequences of climate change. Taken together, these studies assist in developing a mechanistic framework for predicting how lake and ponds may respond to changes in environmental temperature, and provide evidence that these ecosystems can exhibit large biological responses to even small changes in temperature.