Collaborative Research: Trophic Structure and the Stoichiometry of Nitrogen and Phosphorus in the Pelagic Food Web

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Ecologists are increasingly realizing that patterns of storage and processing of energy and chemical elements in ecosystems are strongly influenced by the composition of the ecological communities present in the ecosystem. In lakes, the abundance of "top predators" (i.e. fish-eating fish species like bass) can have strong effects on all levels of the system, including the abundance and productivity of plant producers and the cycling of chemical elements. This occurs because predatory fish consume plankton-feeding fish, like minnows, which in turn influence herbivorous consumers (zooplankton). Finally, zooplankton both regulate the abundance of plant producers (phytoplankton) and decomposers (bacteria) and recycle nutrients. This project focusses on the effects of food web structure on the relative availability of nutrient elements critical for microbial growth (nitrogen and phosphorus). It tests the predictions of a recently-developed, physiologically-based theory designed to understand how different zooplankton species regenerate nitrogen and phosphorus present in their food. By altering the dynamics of these herbivorous zooplankton, fish predation can therefore influence the cycling of nitrogen and phosphorus. Based at the Experimental Lakes Area in Canada, the main research approaches will in involve experimental manipulations of nutrients and zooplankton in large enclosures in lakes as well as whole-lake manipulations of food-web structure (via introduction of pike to two lakes which lack such a fish-eating species). THese studies have direct application to understanding the processes controlling elemental cycling in ecosystems; such processes play a critical role in various areas of direct societal, including global climate change, the dynamics of contaminants in the environment, and degradation of water supplies.