Blooms of green

High nutrient concentrations and summery weather produce algal blooms

Anyone in the vicinity of Berlin’s Lake Müggelsee these days will immediately notice that the lake is covered with an unusually thick carpet of algae. An unpleasant odour is also present, putting off bathers in spite of the continued good weather. What, then, causes this prolific growth of algae in rivers and lakes?

Due to cyanobacteria of the genus Microcystis swimming in Lake Müggelsee cannot be recommended at the moment. | Photo: IGB

Essentially, two factors encourage such algal blooms: a high nutrient concentration in the water and sustained high temperatures. As such, the latest algal bloom is primarily a result of the warm summer months. High temperatures stimulate the metabolism of bacteria and microbes, accelerating decomposition processes. The nutrients originate mainly from algae, aquatic plants and detritus which accumulated at the sediments during the previous winter and spring. This leads to the release of exceptionally large amounts of nutrients, including phosphorous and nitrogen – two of the main nutrients that plants, and hence also algae, need to grow.

In addition, warming results in the stable stratification of sufficiently deep lakes. Nutrients released from the sediment of a lake then are no longer able to reach the upper layer of water, and accumulate instead at deeper levels. At the same time, intensive decomposition processes at the bottom of the lake lead to higher oxygen consumption. Since stratification prevents oxygen from being transported from the top layer of water down to the bottom layer, oxygen deficiency occurs. This lack of oxygen encourages what is known as the self-fertilisation of lakes, which involves iron-bound phosphates from the sediment finding their way back into the water, further increasing the phosphorous concentration.

These mechanisms mainly affect nutrient-rich lakes where there is a lot of matter to decompose and where a lot of nutrients are stored in the sediment. Lake Müggelsee fits this description. The first cooler days of September, paired with a fresh breeze, caused its stratification to break down. The water column was mixed, enabling the algae to exploit the accumulated nutrients. The return of high temperatures over the past few days then led to a veritable explosion of algal growth. Cyanobacteria, commonly referred to as blue-green algae, are among the beneficiaries of this development. These organisms are able to avoid losses by grazing or sedimentation and do not need much light. Some cyanobacteria are also able to fix atmospheric nitrogen, leading to their mass distribution. This proliferation mainly causes problems for bathers, animals and water treatment plants, because some species of cyanobacteria can produce toxins.

The role played by nitrogen in the development of algal blooms is demonstrated in the IGB video “Preventing algal blooms in lakes” >