Regulation of dinoflagellate populations: Relative importance of grazing, resource limitation, and life history processes
I investigate the relative importance of resource, food web and life history processes for the population dynamics of dinoflagellates in a relatively deep and a relatively shallow lake. These lakes have a number of common chemical features but differed not only in lake morphometry but also cell density of the dinoflagellate P. limbatum. I tested the hypotheses that differences in dinoflagellate populations were regulated by differences in 1) growth processes driven by nutrient limitation 2) loss processes driven by zooplankton grazing and/or 3) emergence from resting cysts which is a function of lake morphometry. Nutrient concentrations (N and P) and zooplankton density were manipulated in two, 12-day enclosure experiments conducted simultaneously in each lake. Zooplankton did not graze dinoflagellates, suggesting that large cell size rendered them resistant to grazing. Dinoflagellate populations in treatments receiving nutrients did not exhibit increased growth and in one experiment exhibited significantly lower densities than non-nutrient treatments. Results from these enclosure experiments suggest that resource availability and zooplankton grazing were not dominant controlling mechanisms of dinoflagellate populations in these systems, nor did they explain differences in population density between these two systems. Alternatively, measurements of emergence suggest that life history processes may more strongly determine population density in these systems. A simple model based on measured emergence rates and lake morphometry predicted the dissimilar population densities of the two bogs. These results suggest that life history processes are an important regulating factor for dinoflagellate population dynamics during the summer stratified season and may be important for other species with similar life history stages.