GE-I-6: Contamination of bathing waters by cyanobacteria burdens

Subject to weather conditions during the bathing season, health risks can develop in bathing waters owing to high concentrations of cyanobacteria. Based on satellite images, the analyses of data from up to 16 lakes distributed over different natural spaces demonstrate that there are distinctly no trends discernible in the time series which is rather short and gappy so far. However, in very warm summers the proportion of lakes with major burdens can be higher than average.

Cyanobacteria – impairment of recreational bathing waters

If temperatures rise in future summers, the desire of humans to take a cooling bath in lakes, rivers or the sea will increase. At the same time however, climate change can affect the quality of recreational bathing waters. A much discussed health risk in connection with climate change is the contamination of recreational waters with cyanobacteria, commonly known as blue-green algae.

Increased concentrations of plant nutrients – especially total phosphorus – in standing water or in slow-flowing watercourses, are the main cause of mass developments of cyanobacteria, the so-called algal bloom. In waters only moderately contaminated with nutrients, cyanobacteria have to compete for available nutrients with both higher water plants and with other phytoplankton, thus rarely achieving dominance. Furthermore, some cyanobacteria are favoured by a stable thermal layering of waters which develops especially at high temperatures and stable weather conditions. Stable layering also leads to the deposition of some cyanobacteria at the surface thus potentially resulting in further localised accumulations of cyanobacteria. Owing to these relationships, climate change has been under discussion as one of the causes of increased health risks from cyanobacteria.

The absorption of major quantities of cyanotoxins – the toxic substances contained in cyanobacteria (nerve gases and liver toxins) – can lead to serious impairments of the liver, kidneys and nerves during bathing in the wild, and it can also lead to bathing accidents (‘near-drowning’). The risk is particularly high for young children and children of primary-school age who are apt to swallow relatively large amounts of water inadvertently when playing in a shallow-water area. However, in the case of unspecific symptoms such as irritations of the skin and mucous membranes, allergic reactions or stomach, bowel and respiratory illnesses which have been observed to occur increasingly owing to bathing in waters rich in blue-green algae, it is more likely that these symptoms have been caused by other pathogenic bacteria or substances. Cyanobacteria are found within the framework of bathing water inspections and the European Water Framework Directive (WRRL), although they are not always recorded quantitatively or in sufficient frequencies. Moreover, the flowering of cyanobacteria within a lake can be marked by very different characteristics, because the cyanobacteria can drift to different locations within the water body or move up and down in the water column in the course of a day. It therefore depends on a number of factors, whether the occurrence of cyanobacteria by means of measurements taken in situ, (typically) on a monthly basis, is indeed captured. This is why methods of remote sensing are better suited to the detection and recording of cyanobacteria blossoms. There are satellites which are capable of measuring the reflexion of the pigment phycocyanin responsible for photosynthesis in cyanobacteria. This process also permits the detection of cyanobacteria drifting below the water surface and – provided there are no clouds in the sky – to collect data very frequently (in a cycle of one to three days, as required for the size of the water). The relevant analysis of satellite data can be validated by individual surveys carried out in situ.

For the years from 2003 to 2011 and from 2016 onwards, suitable satellites were available. In the years between, there were no satellites active that were equipped with the requisite satellite sensors. The indicator is underpinned by data covering four lakes in the area of the Alps and associated foothills, three lakes in the central uplands region and eight lakes in the North German Plain. Lake Constance was covered in two ways (upper part and lower part). Work is ongoing to expand the selection of lakes. The overflights during the relevant months between July and September were assessed in respect of the occurrence of cyanobacteria. As a requirement for such occurrences, it was stipulated that positive cyanomarkers (phycocyanin) are found in at least 30 % of correctly captured pixels (grid cells). If in a maximum of 20 % of all valid overflights occurrences of cyanobacteria are recorded, this is taken as a medium burden; if the value exceeds 20 % of all valid overflights, that is taken as a high burden for the year in question.

There is no distinct trend discernible for the time series to date. However, owing to the four-year data gap, the trend analysis is of limited information value. Fluctuations in the cyanobacteria burden between the years can be attributed, for instance, to weather patterns and thus to associated stratification regimes in the lakes in question. If stratification sets in earlier in the year – owing to higher spring temperatures – this can in turn result in cyanobacteria developing higher biomass maxima earlier in summer (cf. Indicator WW-I-9). Compared to other species of phytoplankton, cyanobacteria species grow slowly and therefore benefit particularly from persistent high water temperatures. However, warm years are not stringently associated with stable stratification. For example, in the hot summer of 2003, the summer stratification was distinctly less stable than in 2006 – a year in which the months of May, June, July as well as the entire autumn were much warmer than average, thus leading to stable stratifications which forced the development of cyanobacteria. The year of 2018, too, stands out distinctly from the time series. In early April, the weather turned within a few days from winter to summer thus resulting in rapid warming of water temperatures. May and June remained warm too, and in July and August followed the longest and most intense heat period to date. These conditions were extremely favourable for the development of cyanobacteria and entailed that in half of the lakes surveyed, medium and high burdens were recorded. It must be noted, however, that apart from the temperature increase, the nutrient content of the lakes also played a crucial role. As long as the nutrient concentration in a water body amounts to less than 30 µg (microgram) total phosphate per litre, the growth of cyanobacteria is distinctly limited. In water bodies, phosphorus is considered a growth-inhibiting factor. Any efforts to reduce the nutrient inputs into lakes – thus achieving an associated mitigation of the cyanobacteria burden – might be frustrated by a prolonged and more stable stratification, as long as the phosphorus concentrations do not fall below the threshold mentioned above.

GE-I-6: Contamination of bathing waters by cyanobacteria burdens | Umweltbundesamt