Date of Award

Summer 2019

Project Type


Program or Major

Biological Sciences

Degree Name

Master of Science

First Advisor

Jeff Schloss

Second Advisor

James F Haney

Third Advisor

Shane Bradt


Cyanobacteria blooms take different forms: surface scums, subsurface epilimnetic blooms, benthic mats, and in some lakes, metalimnetic layers. There is limited field research on the seasonal behavior of metalimnetic layers and the contribution of layer-produced cyanotoxins to the overall toxin profile of New Hampshire lakes. Three lakes in New Hampshire were monitored monthly pre-summer stratification through post-fall mixis in 2018, tracking the formation and persistence of deep-water cyanobacteria layers and the physical/chemical/light environments where they were found. Cyanobacteria abundance and dominance were quantified through cell counts using an Imaging Flow Cytobot (IFCB) and with phycocyanin fluorescence estimates. Persistent, nearly monospecific populations of Planktothrix isothrix were detected in the metalimnion of all three lakes multiple years in a row, despite differing lake size, trophic status, and ecoregion. The layers appeared to migrate up from the sediments, finding a depth with high nutrients, thermal stability, and low light levels that still allowed for photosynthesis. The levels of microcystin toxin were measured in both the surface and cyanobacteria layer and although we found overall low levels of microcystin in the lakes, surface microcystin levels were often higher than samples collected from the dense accumulations of cyanobacteria in the metalimnion. A partial least squares regression suggests microcystin levels are more related to toxin-producing species of cyanobacteria other than the Planktothrix isothrix found dominating the layer samples in all the lakes monitored. This raises the possibility that the dominant strain found in the metalimnetic layers may not produce microcystin or did not experience conditions in the 2018 growing season that favored the production of microcystin. While these dense Planktothrix isothrix layers did not appear to be producing microcystin at levels considered harmful to human health, we did not test for other toxins that Planktothrix sp. are known to produce and therefore cannot say that these layers do not represent a human health risk.