Regulation of dinoflagellate populations: relative importance of grazing, resource limitation, and recruitment from sediments
We investigated the relative importance of resource, grazing, and life-history processes for the dynamics of the dinoflagellate Peridinium limbatum in two bog lakes with very different population densities. These lakes have a number of common chemical features but differ considerably in their morphometry. We tested the hypotheses that differences in dinoflagellate populations were regulated by differences in (i) growth processes as influenced by nutrient limitation, (ii) loss processes via zooplankton grazing, and (or) (iii) emergence from resting cysts as a function of lake morphometry. Nutrient concentrations and zooplankton density were manipulated in two 12-day enclosure experiments conducted simultaneously in each lake. Dinoflagellates in treatments receiving nutrients did not increase and also showed no response to zooplankton grazing. Overall, our results suggest that resource availability and zooplankton grazing are not dominant controlling mechanisms for dinoflagellate populations in either lake. Also, neither mechanism explains the differences in dinoflagellate population densities between the lakes. In contrast, a simple model based on measured emergence rates and lake morphometry successfully accounted for the dissimilar population densities of the two bogs. Our results suggest that interactions between life history and lake morphometry can play an important role in regulating phytoplankton populations.