Would you swim in this?

Algal scum on polluted water

Toxic algal blooms are caused when pollution by agricultural fertilisers or sewage stimulates excessive algal growth. For example, two lakes on the River Tamar that supply drinking water to Cornwall have experienced huge algal blooms in recent years as a result of pollution from local farms. This has led to a massive increase in the cost of treating water, both from the lakes themselves and from the downstream river. At their worst, the algal blooms have also made the lakes unsafe for recreational use. Even when the source of pollutant is removed, the pollutant itself remains trapped in the silt, so such lakes can take decades to restore.