Sack Lunch Seminar (SLS)

Abstract:
The systematic change in a trait with size is a concise means of representing the diversity and organization of planktonic organisms. Using this simplifying principle, three size-structured nutrient-phytoplankton-zooplankton models are used to examine the influence of physiological rates, trophic dynamics, and nutrient regime on the emergent size structuring of planktonic communities. To parameterize our models, we use a synthesis of literature values.

Using these parameterizations, the models produce realistic planktonic size distributions, which increase in size range and biomass with increasing total nutrient concentrations. Under all combinations of grazing complexity and total nutrient concentration, both the phytoplankton and zooplankton have decreasing normalized biomass spectral slopes. This general spectral shape for each planktonic group is mainly determined by the rates of the other plankton type. That is, the zooplankton spectral slope is largely governed by the phytoplankton, and the phytoplankton spectrum is determined by the zooplankton parameters. The inclusion of grazing on multiple size classes of phytoplankton does not lead to appreciable changes in the phytoplankton spectra and only decreases the biomass of the smallest size classes of zooplankton. Omnivorous grazing on multiple size classes of plankton decreases the zooplankton biomass and, consequently, increases the phytoplankton biomass. Using a sensitivity analysis, changes in the size-dependence of the microzooplankton grazing rate was found to have the largest influence on both the phytoplankton and microzooplankton biomass.