Nuisance blooms of cyanobacteria are a clear expression of the eutrophic state of lakes, rivers, and coastal waters in which they occur. There are three prerequisites for formation of surface-water blooms: i) a high biomass, ii) buoyancy of the cells, iii) a stable water column. I studied these conditions and the formation of blooms in relatively shallow lakes in the Netherlands. Gas vesicles provided buoyancy to the cyanobacteria. Supported by high nutrient concentrations in the lakes the cyanobacteria established a high biomass, benefiting from their buoyancy under conditions of partial water-column stability. During these periods mixing was restricted to a shallow near-surface layer. The colonial cyanobacteria floated up into this layer, thus increasing their daily light dose, and outcompeting non-buoyant competitors in the phytoplankton. Periods with intensive mixing arrested growth of colonial cyanobacteria, presumably because the cyanobacteria are not well adapted to the fast changing light conditions that result from mixing (see second paragraph). The likelihood of persistent bloom formation, however, was higher after a short period of deep mixing, following a period of stable conditions. Deep mixing imposed a low average irradiance on the phytoplankton; conditions that stimulated gas vesicle synthesis. The now over-buoyant colonies possessed 50% more gas vesicles than needed for neutral buoyancy. They could no longer lower their buoyancy sufficiently to sink, and remained at the lake surface (the main mechanism for the regulation of buoyancy in these lakes was through the accumulation of dense photosynthate, mainly glycogen). However, colonies that are acclimated to low average irradiance prior to bloom formation can easily get photodamaged during exposure to high irradiance at the lake surface. This was found in the top layer of blooms by measuring rates of photosynthesis using oxygen microelectrodes, that allowed analysis with sufficient high spatial resolution.

Acclimation to high irradiance, as well as to fluctuating light was studied in more detail in cultures of cyanobacteria and green algae receiving computer controlled variable light regimes. These regimes simulated the natural light environments resulting from different degrees of wind induced mixing. Acclimation of the photosynthetic apparatus to fluctuating light was more reluctant in the cyanobacterium. Partially because of a conservative carotenoid composition, that showed no acclimation to the diel variation in irradiance. The sustained presence of the photoprotective xanthophyll zeaxanthin led to a detrimental loss of excitation energy at low irradiance (as judged from patterns of fluorescence quenching). In contrast, eukaryotic green algae have a xanthophyll cycle, in which zeaxanthin is only present during over-excitation of photosystem II. The study of acclimation to high irradiance, in a light regime simulating surface bloom formation, showed that the cyanobacterium was also less flexible in this respect. These results underlined the need for cyanobacteria to have a fine tuned buoyancy regulation. This reduces fluctuations in irradiance by allowing the colonies to float into the illuminated near surface mixed layer, but at the same time avoids over-exposure.