Effect of climate driven shifts in initial population size on the phytoplankton spring bloom

Background/Question/Methods

Climate change is driving shifts in overwintering success and emergence timing of organisms, but these changes do not affect species equally.  The spring bloom of plankton in temperate lakes is especially vulnerable to changes in the relative abundance of algae and zooplankton because of close ties of these trophic levels during this period of rapid growth.  We use a modeling and experimental approach to understand what conditions makes changes in the spring inoculum most likely to drive changes in population dynamics.  We use the Rosenweig-MacArthur model to understand how vital rates of organisms affect the importance of initial conditions.  We then test the predictions of the model with an experiment manipulating temperature and initial abundance of Daphnia pulicaria.  Since even considerable warming would not prevent winter water temperatures from approaching freezing in most temperate lakes, we manipulate the rate the water warms.  Our warming treatments consist of a slow warming, consistent with growth beginning early in the season following a mild winter, and a fast warming, mimicking rapid warming following ice out later in the year.  In each warming treatment we stocked D. pulicaria at three densities and quantified the algal and D. pulicaria densities through a complete population cycle. 

Results/Conclusions

Our model results show that initial conditions are most important when the consumer growth rate is high relative to that of the resource.  Since warm temperatures increase the growth rate of consumers relative to resources, shifts in the inoculum should be most important when spring-time temperatures are warm.  We found that peak density of algae and Daphnia were significantly related to the initial density of D. pulicaria, the warming regime and the interaction of the two.  Increasing the initial D. pulicaria density from 0.25 to 3 L^-1 resulted in a 14 fold reduction in peak algal density in the fast warming treatment and a 23 fold reduction in the slow warming regime.  When consumers were initially present at high density they were able to limit algal growth to low levels, resulting in clear water.  When initial D. pulicaria densities were low algal densities reached high levels before D. pulicaria populations were able to limit algal growth especially when spring temperature increase was slow.  These results indicate that mild winters (slow warming spring) will result in increased algal densities unless there is an increase in the overwintering density of D. pulicaria.