Benthic cyanobacteria of the genus Nostoc are a source of microcystins in Greenlandic lakes and ponds

Benthic primary producers are recognised for their important role in contributing to ecosystem productivity and nutrient cycling in lake and stream ecosystems, particularly in polar environments. In Arctic lakes, benthic producers often comprise mats or colonies of cyanobacteria capable of producing cyanotoxins. However, the extent to which benthic communities contribute cyanotoxins in polar regions remains poorly described. We evaluated the potential for benthic colonies of the cyanobacterium Nostoc pruniforme from lakes in Kangerlussuaq, Greenland, to contribute microcystins (MCs) to lake water using three approaches. First, we dissected field‐collected Nostoc colonies and measured MCs within multiple layers of fresh colony tissue. Second, we conducted a laboratory experiment to evaluate the temporal dynamics of MC release by incubated, intact colonies. Finally, we quantified whether MC concentrations in water and sediment samples in the field were higher in and above dense bands of benthic Nostoc as compared to bare sediment. Field‐collected Nostoc colonies contained MCs throughout the colony tissue, suggesting that damage to colonies from grazers or physical disturbance could facilitate the release of toxins into the water. Undamaged Nostoc colonies incubated in high‐nutrient conditions in the laboratory leaked MCs into the surrounding water at a steady mass‐specific rate over the course of 7 days. Microcystin concentrations in water and sediment from two Greenlandic lakes were highly variable, but slightly higher in lake water immediately above dense bands of Nostoc than in water immediately above bare sediments, suggesting that benthic Nostoc colonies contribute cyanotoxins to lake water and that MCs vary at very fine, 1–2 m spatial scales. Benthic cyanobacteria may be important in releasing MCs into aquatic ecosystems, especially in systems where benthic producers dominate, such as polar environments.