Global warming: Algae thrive in European Lakes

Global warming causes many problems in the seas, oceans … and European lakes. Its effects will be even vicious. It would frustrate many conservation efforts to restore these aquatic freshwater and limit the mass growth of potentially toxic algae.

The cyanobacterium Planktothrix rubescens is a filamentous alga. It is up to 2 mm long and 0.005 mm wide. Despite its sizeit dominates many phytoplankton communities, for example within the Lake of Bourget. © Limnologische Station, UZH

Lakes in Central Europe are polluted by arrivals of waste water since it is rich in nutrients for decades. These contributions, phosphorus compounds or nitrates, have often been responsible for eutrophication phenomena with important algal blooms and decreased oxygen concentration in deep waters. Realizing these problems, our societies undertook, in recent years, many efforts to minimize the pollution concerned. The amount of phosphorus in the lake of Zurich (area of 66 sq km) has been divided by 4.8 in 40 years, changing the ratio between the concentrations of phosphates and nitrates which themselves have hardly diminished.

This change in the ratio of phosphorus / nitrate would, according to Thomas Posch of the University of Zurich, both greatly favored the development of a photosynthetic cyanobacteria that produce toxins harmful to animals: such as Burgundy blood algae (Planktothrix rubescens). The population of the planktonic algae in Lake Zurich is followed for many years by the local water association who must deal with the precious liquid, during its abstraction, to make it drinkable.

But another trend remained to explain more than 40 years and despite efforts to reduce nutrient inputs, algal blooms, which take place in autumn, tend to become increasingly dense. According to the authors of the study, global warming would be involved. The explanation of the phenomenon has been described in the journal Nature Climate Change.

An incomplete mixing of lakes favorable to algae

The beginning of every spring (March-April), the different water masses of Lake Zurich, at a given time and under normal circumstances, all have the same temperature (homothermy phenomenon) and therefore the same density. The wind then promotes their mixing. Surface waters are carried towards the bottom (up to 130 m), carrying with them the oxygen in quantity and especially the cyanobacteria that have survived the winter. Not supporting the pressure of 14 bar, these organisms die in large numbers, which explains why their population is minimal from May to July. When mixing is complete, the lake is said holomictique.

But global warming has caused, during the past four decades a rise in average temperature of surface waters from 0.6 to 1.2 ° C. As a result of warm winters, the status is no longer systematically homothermy reached, a thermal stratification is set up as the water masses have different temperatures and therefore different densities. The mixing of the lake can not therefore be complete. Cyanobacteria are not brought forward into the background and then survive in greater numbers. When environmental conditions permit, they will form the basis of a new population explosion.

Global warming would thus be opposed to efforts to restore the health of lakes in central Europe. There would be one solution to stop the massive proliferation of algae: be cold and windy winters, like that of 2011-2012.

A bloom of Burgundy blood algae (Planktothrix rubescens).
Photo by : UZH

Tagged algae, global warming, nitrate, phosphorus, toxic