Abiotic Mechanisms for Cyanobacteria Physiology and Distribution in Lakes: A Multi-Scale Approach

Abstract

Harmful Algal Blooms (HABs) are a ubiquitous ecological and public health hazard because they are comprised of potentially toxic freshwater microorganisms, called cyanobacteria. Cyanobacteria are capable of accumulating in large concentrations in fresh-water ecosystems during summer and producing a toxin (microcystin) that in high concentration can be harmful to humans and animals. The occurrences of toxic HABs are highly spatially and temporarily variable in freshwater ecosystems and are difficult to predict. These HABs can be governed by abiotic environmental conditions including water temperature structure, light, nutrient abundance, and mixing. This dissertation increases the understanding of abiotic environmental conditions, i.e. different mixing scales, on the physiology and distribution of cyanobacteria in nutrient invariant eutrophic systems using field and laboratory studies. In the laboratory, we investigated the effect of small-scale turbulence on the growth and metabolism of Microcystis aeruginosa. The laboratory bioreactor setup included two underwater speakers, generating a quasi-homogeneous turbulent flow, comparable to field values in the lacustrine photic zone (Reλ =0, Reλ =33 and Reλ =15). The results suggest that turbulence mediates the metabolism of Microcystis aeruginosa, quantified by the net oxygen production, oxygen uptake, and inorganic carbon uptake, which is not manifested in changes in growth rate. In the field, we investigate the abiotic drivers for cyanobacteria and microcystin vertical distribution using a research station 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 distributions of cyanobacteria biovolume (BV) and microcystin concentrations (MC) using easily measurable physical lake parameters. The analysis of vertical heterogeneity of cyanobacteria in the entire water column revealed high positive correlations among BV stratification, surface water temperature, stratification stability, quantified by the Schmidt stability. During strong stratification, the MC and BV accumulated above the thermocline and were highly correlated. Although, the cyanobacteria BV is significant only above the thermocline during stratification where cyanobacteria are exposed to high phosphate, temperature and light, there is still further vertical variability to explain within this region. Two types of BV distributions were observed above the thermocline. The first distribution depicted BV uniformly distributed over the diurnal surface mixed layer (SL). The second BV distribution displayed local BV maxima near and under the surface in the SL. A quantitative relationship was developed to determine the probability of observing a uniform distribution as a function of the surface Reynolds number (ReSL), the dimensionless ratio of inertial to viscous forces, over the SL. The uniform distribution was observed for ReSL>50,000. The outcome of this analysis is the first step towards the quantification and prediction vertical stratification of cyanobacteria biovolume and microcystins as a function of local meteorological and physical conditions in a stratified lake.