'Blooms like it hot', argue two prominent biologists in this week’s issue of Science. Global warming creates favorable conditions for harmful cyanobacteria, because they respond more strongly to rising temperatures than most other algal species. This is likely to affect the water quality of many aquatic ecosystems worldwide, especially during summer heatwaves.
Blooms of harmful cyanobacteria (also known as bluegreen algae) are a growing nuisance in many lakes in Africa, Australia, China, the USA, and in many European waters. Professor Hans Paerl of the University of North Carolina (USA) and professor Jef Huisman of the University of Amsterdam (Netherlands) conclude on the basis of several recent studies that the worldwide proliferation of harmful cyanobacterial blooms is linked to climate change. Cyanobacteria flourish at high temperatures, especially in nutrient-rich waters with high concentrations of nitrogen and phosphorus. The surface water of lakes is heated during prolonged periods of warm weather. Warm water expands, and floats on the colder water underneath. This results in stratification of lake water, which suppresses vertical mixing. Cyanobacteria profit from these stratified conditions. They make small gas vesicles inside their cells, providing buoyancy. Buoyant cycanobacteria float upwards when vertical mixing is weak and accumulate in dense surface blooms. These surface blooms shade underlying nonbuoyant phytoplankton (such as green algae and diatoms). Thus, surface blooms of buoyant cyanobacteria effectively suppress other species by monopolizing all available light.
Changes in precipitation patterns and summer droughts
Cyanobacteria also profit from changes in precipitation patterns and summer droughts. Climate models predict more intense precipitation interspersed by longer periods of drought as a result of global warming. Intense precipitation leaches nutrients from soils, flushing them into rivers and lakes. As the discharge subsides during subsequent periods of warm, dry weather, cyanobacteria can capture the extra nutrient load, promoting their bloom development. Attempts to control the water table by closure of dams and sluices during summer droughts may further aggrevate the problem. This will increase the residence time of cyanobacteria in these stagnant waters, thus providing a longer time window for bloom development. Moreover, cyanobacteria appear to be more salt-tolerant than other freshwater algal species. Rising salinities due to increased evaporation or salt water intrusion from sea level rise may thus give cyanobacteria an additional competitive advantage.
Paerl and Huisman discuss the example of a tropical cyanobacterium responsible for a severe outbreak of hepatitis on Palm Eiland, Australia. This tropical species invaded southern Europe in the 1930s, and has subsequently expanded its range northwards to lakes in the Netherlands and Northern Germany. Likewise, the species also invaded Florida several decades ago, and is now widespread across the US southeast and midwest, where it proliferates in warm and nutrient-rich waters. ‘Water managers will have to anticipate a worldwide expansion of harmful cyanobacteria’, says Huisman. ‘This is another important reason to curb the emission of greenhouse gases’.
Cyanobacteria can produce a variety of different toxins, causing damage to the liver and nervous system of birds and mammals in particular. Ingestion of these toxins can be fatal to cattle, waterfowl, and pets, and is also a serious threat for human health. Bodies of waters are closed for recreation and agricultural use, when their concentrations of cyanobacteria exceed a critical threshold level.