Cyanobacteria blooms are a worldwide phenomenon characterized by cyanobacterial taxa outcompeting eukaryotic phytoplankton through superior nitrogen fixation or antagonistic shading. Lake Mývatn, Iceland, is a naturally eutrophic lake where cyanobacteria blooms of Dolichospermum sp. occur with a frequency and severity that is not well characterized. Understanding patterns in cyanobacteria bloom formation is important because of their potential to inhibit benthic production, which accounts for more than 80% of the primary production of the lake. Our overall objective was to document spatial patterns in Dolichospermum blooms and examine the bloom’s influence on the phytoplankton and zooplankton communities. Specifically, we explored the possible abiotic drivers of cyanobacteria blooms. Throughout summer 2016 we sampled ten sites across Mývatn for standard limnological factors, including temperature, light, dissolved oxygen, pH, and pelagic and benthic chlorophyll. For each site and sampling date, we also determined Dolichospermum densities and characterized colony-level morphology, as an indicator of Dolichospermum performance.
We found that Dolichospermum colony length, density, and heterocyst density varied significantly both spatially and temporally in our field measurements. The abiotic factors most strongly associated with spatial variation colony morphology and density differed through time. Early in the season, light availability explained the most spatial variation in colony length, while light availability and water temperature explained differences in density. Midway through the season, temperature became the only significant predictor of colony length, while at the end of the field season none of the abiotic factors explained a significant amount of variation in either colony length or density. Thus, early stages of Dolichospermum blooms may depend on light availability, while bloom progression and severity depend on water temperature. Our results suggest that a sequence of multiple abiotic factors drive cyanobacteria blooms in Lake Mývatn The end of the cyanobacterial bloom was also concurrent with an increase in benthic chlorophyll, hinting at possible resting cell deposition and a possible contributor to bloom inoculation.