Intracellular, environmental and biotic interactions influence recruitment of benthic Microcystis (Cyanophyceae) in a shallow eutrophic lake

Microcystis is known to overwinter on sediment surfaces and provide an inoculum to support water column blooms in lakes. There is uncertainty as to whether recruitment is an active process triggered by internal changes in buoyancy, or a passive process induced by resuspension. The effect on Microcystis recruitment of ammonium, light and temperature was assessed in laboratory experiments using sediments collected in a small eutrophic lake from two habitats: near-edge (containing Microcystis and Aphanizomenon gracile) and mid-lake (predominantly Microcystis). In all experiments single cells, rather than colonies, accounted for the majority (>55%) of recruited cells. Microcystis recruitment was significantly lower (P < 0.001) in near-shore samples suggesting that A. gracile elicits allelopathic effects on Microcystis. In mid-lake samples, Microcystis recruitment was significantly higher at moderate ammonium concentrations (0.1, 0.2 and 0.5 mg L-1; P < 0.001), at two temperatures (16 and 25 degrees C; P < 0.001) and high light intensities (50 and 100 mu mol m(-2) s(-1); P < 0.01). Microcystis cells were isolated from sediment in spring, early and late summer and assessed using transmission electron microscopy. The percentage of cell area filled with gas vesicles increased significantly (P < 0.001). These data demonstrate that allopathic interactions, ammonium, light and temperature can individually and synergistically regulate gas vesicle synthesis and Microcystis recruitment.