Several beaches in Madison closed for extended periods of time due to high bacteria levels this summer. Cyanobacteria, commonly known as blue-green algae, are photosynthetic bacteria that can bloom in extremely high numbers. Some strains produce compounds that are toxic to people and pets. Madison lakes have been especially prone to cyanobacteria blooms in recent years. Trina was recently interviewed by the Capital Times as part of their “Murky Waters” series (“Beach Bummer: Closures highlight algae, bacterial threats in Madison-area lakes”) about why cyanobacteria blooms have been so frequent. One of the biggest factors is the amount of phosphorus entering the lakes. Much of this is from agricultural fertilizer, but urban erosion and debris are also contribute phosphorus. The addition of this nutrient allows cyanobacteria to grow to high densities.
However, not all cyanobacterial strains are the same – the group Cyanobacteria is an entire phylum. This paper, published by the McMahon Lab in 2013, shows how the toxic group Microcystis consistently blooms in Lake Mendota following a bloom of nitrogen fixing microbes, including its non-toxic cousin Aphanizomenon. This suggests that nitrogen is also a limiting nutrient for the toxic cyanobacteria that result in beach closures. In order to reduce the frequency of these toxic blooms, a reduction in the amounts of both phosphorus and nitrogen may be necessary.
Has your local beach been closed due to blue-green algae? Let us know in the comments!