Effects of microzooplankton growth and trophic interactions on herbivory in coastal and offshore environments

We performed serial dilution experiments to estimate rates of gross phytoplankton growth (k) and grazing mortality (m) in both eutrophic (Corpus Christi Bay, Texas, USA) and oligotrophic (offshore Gulf of Mexico) waters. Two parallel experiments were performed in both environments, with seawater pre-screened through 153 or 25 km mesh to observe the responses of microzooplankton (MZP) to dilution treatments. MZP biomass changed over the duration of the experimental incubations; in several treatments, MZP net growth rates were >1 d super(-1). Patterns of growth varied between dilutions and initial screening size. In the eutrophic system, the ratio of phytoplankton grazing mortality rate to gross phytoplankton growth rate (m/k) was 1.10 +/- 0.54 (mean +/- SD) versus 0.41 +/- 0.65 when screened through 153 and 25 km mesh, respectively. This difference was attributed to cascading trophic interactions among MZP size groups leading to suppression of the primary herbivores in the 25 km fraction and, in turn, a lower value of m. A food web model consisting of multiple trophic levels was constructed to examine the role of MZP growth and trophic interactions on measurements of k and m. The model, using 3 interacting groups of MZP, was able to reproduce experimental results. Model simulations demonstrated that MZP growth during incubation leads to an overestimation of m. Non-linearity in the phytoplankton growth response curves was due to MZP growth and trophic interactions in these model simulations, as variable feeding responses were not incorporated into the models. Trophic interactions among MZP can provide context to measurements of k and m and insight into microbial food web efficiency.