Cyanobacterial blooms

Cyanobacteria can form dense and sometimes toxic blooms in freshwater and marine environments, which threaten ecosystem functioning and degrade water quality for recreation, drinking water, fisheries and human health. Here, we review evidence indicating that cyanobacterial blooms are increasing in frequency, magnitude and duration globally. We highlight species traits and environmental conditions that enable cyanobacteria to thrive and explain why eutrophication and climate change catalyse the global expansion of cyanobacterial blooms. Finally, we discuss management strategies, including nutrient load reductions, changes in hydrodynamics and chemical and biological controls, that can help to prevent or mitigate the proliferation of cyanobacterial blooms

How rising CO2 and global warming may stimulate harmful cyanobacterial blooms

Climate change is likely to stimulate the development of harmful cyanobacterial blooms in eutrophic waters, with negative consequences for water quality of many lakes, reservoirs and brackish ecosystems across the globe. In addition to effects of temperature and eutrophication, recent research has shed new light on the possible implications of rising atmospheric CO2 concentrations. Depletion of dissolved CO2 by dense cyanobacterial blooms creates a concentration gradient across the air–water interface. A steeper gradient at elevated atmospheric CO2 concentrations will lead to a greater influx of CO2, which can be intercepted by surface-dwelling blooms, thus intensifying cyanobacterial blooms in eutrophic waters. Bloom-forming cyanobacteria display an unexpected diversity in CO2 responses, because different strains combine their uptake systems for CO2 and bicarbonate in different ways. The genetic composition of cyanobacterial blooms may therefore shift. In particular, strains with high-flux carbon uptake systems may benefit from the anticipated rise in inorganic carbon availability. Increasing temperatures also stimulate cyanobacterial growth. Many bloom-forming cyanobacteria and also green algae have temperature optima above 25 °C, often exceeding the temperature optima of diatoms and dinoflagellates. Analysis of published data suggests that the temperature dependence of the growth rate of cyanobacteria exceeds that of green algae. Indirect effects of elevated temperature, like an earlier onset and longer duration of thermal stratification, may also shift the competitive balance in favor of buoyant cyanobacteria while eukaryotic algae are impaired by higher sedimentation losses. Furthermore, cyanobacteria differ from eukaryotic algae in that they can fix dinitrogen, and new insights show that the nitrogen-fixation activity of heterocystous cyanobacteria can be strongly stimulated at elevated temperatures. Models and lake studies indicate that the response of cyanobacterial growth to rising CO2 concentrations and elevated temperatures can be suppressed by nutrient limitation. The greatest response of cyanobacterial blooms to climate change is therefore expected to occur in eutrophic and hypertrophic lakes

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