PhD Graduation

In aquatic food webs, the role of body size is notoriously strong. It is also well known that temperature has an effect on body size. For instance, Bergmann’s rule states that body size increases from warm to cold climates. This thesis addresses the question how climate shapes the size structure of fish and zooplankton communities, and how this affects the strength of the trophic cascade from fish to plankton. I combine three different approaches: a space-for-time substitution study of data from the 83 shallow lakes distributed along a latitudinal gradient in South America, simple mathematical models to explore climate effects on the dynamics of trophic interactions, and an experimental analysis of trophic interactions using outdoor mesocosms.

First, in chapter 2, I analyze how the size structure of fish communities changes with latitude and show that lakes along this climatic gradient tend to be dominated by either small or large fish. Moreover, I show that this pattern suggests a catastrophe fold in the body size structure of fish communities, with higher probabilities of a small fish-dominated state at warmer climates. This suggests that factors that tend to reduce fish body size such as fishing for the largest individuals could change a large-fish dominated state into a small-fish dominated state more easily in warmer climates. In chapter 3 I present an analysis of a smaller set of lakes, in which we estimated the trophic position of fish using stable isotopes (¹⁵N). This revealed that while in cold lakes large fish forage higher in the food web than small fish, in tropical lakes large fish forage at equal or lower trophic levels than small fish. This pattern suggests that top-down control of small fish by large fish may be weaker towards warmer climates.

Chapter 4 shows that, just as in fish, the body size of zooplankton decreases from higher to lower latitudes. This pattern is stronger for Cladocera and Calanoid copepods than for smaller organisms such as rotifers. Also, the size gradient is strong for pelagic species but not for macrophyte-associated species, suggesting that fish predation (to which larger pelagic species are particularly sensitive) may be an important driver of the patterns. Indeed, differences in pelagic cladoceran body length were correlated strongly to the proportion of small fish in the lakes. In chapter 5 we used a model to analyze the potential implications for zooplankton of two characteristics of fish in warm regions: continuous reproduction and omnivory. Continuous reproduction leads to permanent presence of small fish, while omnivory boost their overall abundance. Our results indicate that while both aspects lead to reduced chances of top-down control of phytoplankton, continuous fish reproduction by itself may in principle lead to the reduction of large-sized Daphnia populations and reduce chances of top-down control of phytoplankton. This implies that in temperate shallow lakes, seasonality in fish reproduction might be essential to leave a window of opportunity for the zooplankton in early spring resulting in a clear-water phase in which zooplankton drives phytoplankton to very low densities. In chapter 6, we experimentally analyze the ability of the zooplankton typically found in subtropical lakes to control phytoplankton, under different levels of fish predation. Experimentally raised zooplankton densities did not lead to lower phytoplankton densities, indicating that the present zooplankton community had little top-down effect at all on the phytoplankton communities we tested. The experiments also revealed that very low fish densities were enough to eliminate large zooplankton from the communities.

In Chapter 7 I link the results presented in the different chapters. I argue that top-down control of phytoplankton tends to be weaker in warmer climates due mostly to a cascading effect of the temperature-driven increase in small fish densities leading to a lack of larger zooplankton in warm lakes. However, I also point out a range of questions, that would need to be addressed if we wish to understand the causality behind the patterns better.

Promotor: Prof.Dr. Marten Scheffer