RAPID: Effects of Enhanced Circulation on Vertical Mixing and Algal Blooms In Freshwater Reservoirs

North Carolina State University, Raleigh NC

Investigators

Abstract

1545624 Obenour Jordan Lake, like many similar reservoirs around the world, is impaired due to excessive algal growth and harmful algal blooms are becoming more prevalent. The lake provides drinking water to 300,000 people, and it serves as a recreational attraction for many more. The use of enhanced circulation has been proposed as a means of suppressing the formation of harmful algal blooms in freshwater and coastal waterbodies. This study represents a rare opportunity to directly investigate how large-scale enhanced circulation affects mixing and algal dynamics (i.e. growth) in a large waterbody. At present, characterization of artificial mixing and its effect on phytoplankton community structure has been limited to whole-lake mixing (i.e. destratification), which may be impractical for larger waterbodies. The proposed research represents the first comprehensive characterization of how enhanced surface-layer circulation affects vertical diffusion rates and HABs formation potential. The project takes advantage of a uniquely large deployment of surface-layer circulators, to answer scientific and engineering questions at multiple scales. Overall, the project will help provide critical insight into the potential role of geoengineering as a tool for the abatement of HABs. For this research project, the principle investigators will perform a series of field activities to determine how these circulators are affecting physical and biological conditions within the lake. Activities will be conducted in different arms of the lake, representing treatment (circulators present) and control (circulators absent) conditions. Physical measurements will include vertical profiles of temperature micro-structure to infer vertical diffusivity, near-surface current velocities to determine extent of circulation, and wind velocities. Biological measurements will include vertical profiles of chlorophyll (total algae) and phycocyanin (indicating blue-green, cyanobacteria algae). Monitoring will be coordinated with the North Carolina Department of Water Resources, who will be measuring a series of conventional water quality parameters that will also be used in this study. Using the data from these field activities, we will determine how the synergistic impacts of wind speed and artificial circulation affect vertical diffusion within the surface layer, which is expected to be a primary control on algal dynamics. We will further test hypotheses related to how mixing (natural and artificial) affects the abundance and vertical distribution of total chlorophyll and cyanobacteria.