PARENT SESSION
Oral Session # 82: Aquatic Ecology: Freshwater.
Presiding: A Soja
Thursday, August 7. 1:30 PM to 5:00 PM, SITCC Meeting Room 200.

Water exchange, nutrients and "top-down" vs. "bottom-up" limitation of plankton: Mesocosm and modeling studies.

Petersen, John^*,1, Schromen-Wawrin, Lindsey¹, Kemp, W. Michael², ¹ Oberlin College, Oberlin, OH² University of Maryland, Cambridge, MD

ABSTRACT- The magnitude and quality of biological, material and energetic exchange are recognized as key factors governing the dynamics of coastal ecosystems. Additionally, ecologists invoke two distinct mechanisms to explain organism abundance: "bottom-up" control by resource availability and "top-down" control by organism removal (i.e. herbivory, predation and other removal mechanisms). The situation is further complicated by the fact that organisms at different trophic levels often have very different generation periods (e.g. in our system producers reproduce in 1 d, and primary consumers in 10 d). We used mesocosm experiments and developed a simulation model to explore how water exchange rate and nutrient concentration govern trophic interactions to control the abundance of phytoplankton and zooplankton. Specifically, experiments were conducted in 1 m³ mesocosms with 0%, 10% and 30% d^-1 water exchange using both high and low nutrient influent waters. We found that with high nutrient influent, phytoplankton abundance and primary productivity increased with increasing exchange while zooplankton abundance declined. In contrast, with low-nutrient influent waters, zooplankton, phytoplankton and primary productivity all increased with increasing exchange. Modeling studies corroborate the hypothesis that a transition in the effects of exchange occurs under these two scenarios: in the high nutrient experiment primary producers were controlled from the top-down by zooplankton grazing and zooplankton abundance was, in turn, controlled from by the rate at which these organisms washed out of the mesocosm. In contrast, in the low nutrient experiment phytoplankton were controlled from the bottom-up by availability of nutrients, and zooplankton were controlled from the bottom-up by phytoplankton availability. The complexity of possible interactions even in this relatively simple system points towards challenges and opportunities for interpreting dynamics in natural coastal ecosystems and the potential value of mesocosms and simulation models for interpreting such dynamics.

Key words: limitation, water exchange, mesocosm, simulation model