Blue-green bacteria in the lake

When checking the water clarity on the 30th July I saw a green scum on the surface near the jetties. It looked like blue-green bacteria, so I took a sample for microscopic examination. The scum proved to be mainly a coiled blue-green bacterium (first image). There were smaller amounts of Aphanizomenon flos-aquae (second image) and two or three other species (remaining images below). There are a large number of blue-green bacteria that occur in the UK and it's a specialist matter to identify most species, and beyond my resources. The Environment Agency takes a precautionary approach to blue-green scums, recognising the need to regard all cyanobacterial species and strains as capable of producing toxins, and potentially posing a threat to animal and human health and safety (A DECADE OF MONITORING AND MANAGEMENT OF FRESHWATER ALGAE, IN PARTICULAR CYANOBACTERIA, IN ENGLAND AND WALES. Krokowski, J. & Jamieson, J. 2002). Particular concerns with our lake will be the risk to those undertaking watersports, anglers and, to a lesser extent, the general public on the lakeside path. Some dog walkers encourage their animals into the water, which could put dogs at particular risk.

One can only speculate on the reasons why we have got a bloom this year. Blue-green bacteria thrive in nutrient-rich water in warm summer conditions and it's possible that waterweed clearance has stirred up nutirents normally locked away in the sediments of the lake bed. The lake is already rich in phosphates, but algal growth is probably limited by nitrogen availability. The ability of blue-greens to fix atmospheric nitrogen could be another factor leading to the bloom.