Increased nutrient input, from fertilizer and other sources, to aquatic systems (eutrophication) leads to degradation of water quality as a result of increases in algae (phytoplankton), and in particular, phytoplankton species that produce toxins. These toxins are a serious threat to human and livestock uses of surface waters in both freshwater and marine environments. One strategy for improving water quality is to manage the food web by increasing the abundance of small animals (zooplankton) that eat phytoplankton. Laboratory studies have suggested, however, that food-web manipulation may fail because of the strong negative effects of phytoplankton toxins on zooplankton growth and reproduction. Previous research has demonstrated that individuals of a common species of zooplankton (Daphnia pulicaria) vary greatly in their ability to grow on a diet of toxic phytoplankton. This project examines the consequences of adaptation to toxins for water quality in lakes in Michigan. In addition to the obvious potential impact of this research on the management of surface waters, the project focuses on two little-studied general phenomena in community ecology: how predator adaptation and trait variation within species affect species interactions. Consequently, the field experiments to be conducted will advance both basic and applied ecology. The project will also provide hands-on training in experimental ecology to graduate and undergraduate students at Michigan State University and Auburn University and provide related outreach activities to middle and high school students.
We found large effects of consumer adaptation on aquatic community structure and function. Using multiple genotypes of the herbivorous zooplankter, Daphnia pulicaria, that vary in their tolerance of toxic phytoplankton (namely, cyanobacteria), we demonstrated that D. pulicaria genotypes collected from productive lakes (that harbor toxic cyanobacteria) strongly reduced toxic cyanobacteria and primary production, while D. pulicaria genotypes collected from unproductive lakes (that lack toxic cyanobacteria) had little or no effect. The magnitude of the herbivore-genotype effect on community- and ecosystem-level properties was essentially the same as the effect of herbivore presence/absence. The latter has major implications for understanding and predicting how species interact. We extended our project to examine and quantify trait variation in the toxic phytoplankter, Microcystis aeruginosa. M. aeruginosa is a major producer of noxious and toxic water blooms in lakes and large rivers worldwide. By isolating singe colonies of M. aeruginosa and rearing them clonally under uniform laboratory conditions, we have established a library of genotypes of this one species from across lakes with widely varying environmental conditions. This library allows us to quantify the magnitude of genetically-based intraspecific trait variation in this important species. We have demonstrated that chemical resistance to grazing by zebra mussels varies among M. aeruginosa genotypes from completely invulnerable (not eaten at all by mussels) to completely susceptible (eaten at maximal rates), and that this variation exists among genotypes isolated from a single population. Thus, the magnitude of variation in this critical trait is as large within this species as it is across all species of phytoplankton. We have also shown that genotypes isolated from high-nutrient lakes have substantially (as much as 5 times higher) maximal growth rates as genotypes isolated from low-nutrient lakes. Our results for both D. pulicaria and M. aeruginosa challenge the fundamental assumption in population and community ecology that species are packages of relatively fixed traits and argues that variation in species and food-web interactions may depend crucially on adaptive trait variation within as well as among species. In addition to sharing project findings in the peer-reviewed literature and at conferences, our research team, which consisted of graduate, undergraduate, and high school students, organized a variety of outreach events at nature centers, a zoo, daycares, and prisons to broaden the impact of our research. These events have connected diverse audiences with aquatic ecology.
