Benthic-pelagic coupling in the population dynamics of the cyanobacterium Microcystis

Microcystis is one of the most notorious phytoplankton species, because it can form harmful toxic blooms that cause problems in freshwaters all over the world. Since Microcystis colonies are buoyant, high concentrations of toxic Microcystis accumulate at the water surface in scums during calm, warm conditions. The aim of this study was to investigate the physiology and population dynamics of benthic and pelagic Microcystis, and to use this knowledge to predict which lake management strategies could be effective in the battle against Microcystis blooms in Lake Volkerak-Zoommeer, The Netherlands. Changes in benthic and pelagic populations of Microcystis were monitored in the water and sediments of the lake. Sedimentation and recruitment rates were measured using traps, and the physiology of benthic colonies was investigated. In addition, the response of Microcystis to light, temperature, salinity and the aggregation of Microcystis to clay was determined in the laboratory. Large amounts of Microcystis sink from the water column in summer due to attachment of sediment particles. The attachment of clay particles to colonies depends on the concentration and type of clay and on the composition of the mucus that surrounds Microcystis. Colonies that have sunk overwinter in the sediments. A small amount of colonies overwinter in the water column. During autumn and winter, benthic colonies from shallow sediments are gradually transported to the deeper parts of the lake. The concentration of benthic Microcystis therefore increases with lake depth. Although all benthic colonies remain vital during winter, colonies in shallow euphotic sediments have a higher vitality then colonies in deep anoxic sediments. The overwintering benthic population inoculates the water column in spring. Since the density of benthic colonies does not change in spring, colonies do not actively recruit by using their buoyancy, but are probably passively resuspended by wind-induced mixing or bioturbation. Since colonies from shallow sediments have the highest vitality and are more frequently resuspended then deeper parts, most recruitment originates from the shallow sediments. Model simulations indicate that benthic recruitment contributes for 50-75% to the size of the summer bloom. To suppress harmful blooms of Microcystis in Lake Volkerak-Zoommeer, two management strategies have been suggested: flushing the lake with freshwater or reintroducing saline water. A mechanistic model of the population dynamics of Microcystis predicts that flushing with freshwater will suppress Microcystis blooms when the current flushing rate is sufficiently increased. Furthermore, the inlet of saline water will suppress Microcystis blooms for salinities exceeding 14 g/l. Both management strategies are technically feasible