Vertical heterogeneities of cyanobacteria and microcystin concentrations in lakes using a seasonal In situ monitoring station

Abstract

A high frequency, high resolution, seasonal research station was deployed to quantify a wide range of local meteorological conditions, water temperature, and water chemistry, including phycocyanin, in two different eutrophic stratified Minnesota lakes. The monitoring effort was coupled with discrete weekly sampling measuring nutrients, cyanobacteria composition, and microcystin concentrations. Our objective was to describe the vertical and seasonal distributions of cyanobacteria biovolume (BV) and microcystin concentrations (MC) using physical lake variables. Two types of BV distributions were observed above the thermocline upward in the water column. The first distribution depicted BV uniformly distributed over the diurnal surface layer (hSL), and the second BV distribution displayed local BV maxima. A quantitative relationship was developed to determine the anticipation of observing a uniform distribution as a function of the surface layer Reynolds number (ReSL), the dimensionless ratio of inertial to viscous forces. The uniform distribution was observed systematically for ReSL > 50,000. MC was observed to accumulate above the thermocline and have a vertical distribution similar to BV, thus depending on ReSL. This is important for directing sampling efforts, because it narrows the range of BV and MC heterogeneity above the thermocline, and suggests a vertical sampling protocol to detect potential maxima and compute representative depth-average concentrations. We explored the temporal variability of the MC to BV ratio, spatially averaged in the epilimnion (MCep/BVep). The maximum MCep/BVep occurred before the maximum BVep and specifically, during the onset of significant biomass growth in both lakes. This observation is notable because the maximum MCep occurs before the visual signs of enhanced cyanobacterial accrual are less recognizable to the public and to monitoring efforts. Our findings could have important implications for predicting MC distribution and guiding monitoring strategies for quantifying MC concentrations in small stratified lakes.