No abstract

Microcystis aeruginosa is a single-celled cyanobacterium, forming large colonies on the surface of freshwater ecosystems during summer, and producing a toxin (microcystin) that in high concentration can be harmful to humans and animals. These toxic effects can be governed by abiotic environmental conditions including water temperature, light, nutrient abundance, and fluid motion. We investigated the effect of small-scale turbulence on the growth and metabolism of Microcystis aeruginosa using field measurements and laboratory bioreactor investigations. The laboratory setup included two underwater speakers, generating a quasi-homogeneous turbulent flow with turbulent kinetic energy dissipation rates up to 10 −6 m 2 /s 3 , comparable to field values in the lacustrine photic zone. The role of turbulence is quantified by comparing cell number, dissolved oxygen production/uptake, and inorganic carbon uptake in stagnant condition and two sets of experiments with turbulent conditions, quantified by the Taylor micro-scale Reynolds number at Reλ = 15 and Reλ = 33. The results suggest that turbulence mediates the metabolism of Microcystis aeruginosa measured by the net oxygen production, oxygen uptake, and inorganic carbon uptake. Furthermore, small-scale turbulence marginally influenced Microcystis growth rate estimated from cell population concentration (−5% and 11% for Reλ = 33 and Reλ = 15, respectively, as compared to stagnant conditions).

Harmful algal blooms (HAB) are ubiquitous ecological and public health hazards because they are composed of potentially toxic freshwater microorganisms called cyanobacteria. The abiotic drivers for toxic HAB are investigated using a research station deployed in a eutrophic lake in Minnesota in 2016. This research station provides full-depth water quality (hourly) and meteorological conditions monitoring (5 min) at the sampling site. Water quality monitoring provides chemical, physical, and biological measurements, that is, phycocyanin concentration, a photosynthetic pigment distinct to cyanobacteria. The high cyanobacteria biovolume (BV) in the epilimnion observed in mid-July persisted until late September when it was distributed uniformly throughout the water column. A scaling relationship was developed among BV heterogeneity, thermal stratification stability, and surface water temperature. This relationship was verified in a dimictic lake the following year. The proposed scaling relationship is relevant to sampling protocols of HAB as it informs if the sample depth is representative of the entire water column. During the strongly stratified period, BV accumulated above the thermocline and in the photic zone, with distinct peaks forming occasionally both near the water surface and at locations with photosynthetically active radiation approximately equal to 10 μE/m 2 s. Our observations suggest that the temporal/vertical variability of cyanobacteria BV is strongly influenced by lake dynamics, thermal structure, seasonal temperature variation, and light availability. These observations demonstrate that cyanobacteria tend to move and accumulate in specific warm water layers, confined by the thermocline and determined by well-defined light conditions.