Source: Dissertation Abstracts International, Volume: 77-06(E), Section: B.

Adviser: David M. Post.

Anthropogenic climate change is pushing environmental conditions into new realms, changing the conditions that organisms are exposed to. Direct effects of warming are well documented, including the movement of species distributions toward higher elevations and altitudes, as well as advances in the seasonal timing of life history events. Changes to species interactions are also occurring, but the dynamics can be highly complex. The consequences of alterations to important interactions due to warming are especially important in systems such as temperate lakes where there is the potential for regime shifts between alternate states.

In temperate lakes the increase in global mean temperatures is decreasing the length and severity of winters, altering the ability of consumers, such as Daphnia, to overwinter. If phytoplankton growth in spring is sufficiently fast relative to Daphnia growth rates, however, changes in the grazing pressure will be unimportant as abiotic conditions will dominate phytoplankton growth. Conditions such as the rate of temperature warming in spring and the nutrient content of lake water are important for regulating the relation between the growth rate of Daphnia and phytoplankton.

I first quantified the feeding rates of two Daphnids and the growth rate the algal resource upon which they feed across the range of temperatures experienced in temperate lakes. By linking the temperature and density dependent functions for Daphnia consumption and algal growth, I was able quantify the density of Daphnia necessary to halt algal growth. I found that the impact of warming generally followed expectations, making both Daphnia species more effective grazers, with the increase in feeding rates outpacing the increases in algal growth rate. Between 25 and 30°C both species of Daphnia experienced a precipitous drop in feeding rates, while algal growth rates remained high, increasing the likelihood of algal blooms in warming summer temperatures.

My remaining work focuses on understanding how climate driven changes in the relative abundance of consumers and resources impacts their interaction. I first use the Rosenzweig-MacArthur model to show that changes in initial abundances create a more variable system when consumer growth rates are high relative to resource growth rates. Rising temperatures disproportionately increase the growth rate of consumers, compared to resources, and thus should increase the variability of the system. I tested the predictions of the model in a laboratory experiment by manipulating the initial abundance of two Daphnids and changing the average temperature they experience. In agreement with the model, I found that the Daphnid which is more effective in controlling algal growth produces a more variable system, but that increasing the average temperature only increased variability for the less effective Daphnid.

Communities experiencing additional stressors, such as nutrient enrichment, may be especially sensitive to alterations in the relative abundance of consumers and resources, especially when these changes have the ability to shift the ecosystem into an alternate regime. Temperate lake systems which I focus on can exist in two alternate states: the preferred clear water state in which abundant grazers limit phytoplankton growth and a degraded, turbid state with high algal densities. I conducted a manipulative experiment to determine the sensitivity of algal populations to shifts in the early spring abundance of Daphnia pulicaria in a range of nutrient conditions. I found a strong interaction between the initial Daphnia abundance and the water nutrient content in determining the timing and magnitude of the spring algal bloom. In high nutrient conditions, decreases in the initial abundance of Daphnia drove large increases in algal abundance. Enriched systems are especially vulnerable to climate induced shifts in phenology. In nutrient enriched temperate lakes, advances in spring algal growth which are not matched by zooplankton grazers could result in shifts to a turbid state.

For zooplanktivorous fish, spring is a time of limited planktivory as they concentrate energy on spawning. Nonetheless, there are systems where fish planktivory in spring is sufficient to eliminate large-bodied cladocerans from the water column. In between these extremes, there are a range of intermediate effects which zooplanktivorous fish can have on the spring plankton bloom. I used laboratory data and a theoretical model to determine how the functional form and magnitude of fish predation impact Daphnia and algae, as their populations grow from varying overwintering densities. Increased fish predation generally limits the importance of differences in the overwintering density of Daphnia. The presence of a refuge from predation for Daphnia when at low density, however, makes the initial abundances important even with high fish densities.