Ecological constraints on planktonic nitrogen fixation in saline estuaries. II. Grazing controls on cyanobacterial population dynamics

Blooms of nitrogen (N)-fixing cyanobacteria are common in freshwater lakes of moderate to high productivity. In contrast, blooms of N-fixing cyanobacteria are largely absent from the water columns of N-limited estuaries. In a companion study, we reported that the abundance and N-fixation rates of planktonic filamentous heterocystous cyanobacteria are strongly suppressed by the presence of zooplankton consumers in saline estuarine mesocosms. Here, we show that bloom formation in these estuarine cyanobacterial N-fixers (Anabaena sp.) is strongly dependent on their ability to grow into sufficiently large filamentous colonies that are capable of developing heterocysts and thereby fixing N. This basic physiological constraint on N fixation results in a sensitivity of N-fixer bloom initiation to suppression by herbivorous zooplankton. In the presence of zooplankton consumers, small populations of heterocystous cyanobacteria exhibited reduced colonial filament size, decreased heterocyst frequencies, and suppressed growth rates. In short-term grazing assays, estuarine zooplankton (Acartia tonsa) directly consumed heterocystous cyanobacteria. This consumption occurred at high rates and resulted, at times, in marked reductions in colonial filament size. These results provide a mechanistic understanding of how trophic interactions can mediate the functional composition of phytoplankton communities and thereby constrain the biogeochemical response of estuaries to N limitation.