Beyond hydraulic flushing: Deep water mixing takes the harm out of a haptophyte algal bloom

Inflows are linked to water quality, food web dynamics, and the incidence of harmful algal blooms (HABs). It may be that inflows can be manipulated to mitigate some blooms and accelerate recovery of living natural resources, such as fisheries. Lack of available water, however, can limit this approach to management. Utilizing source water from deeper depths to displace surface waters, however, might effectively mimic aspects of inflow events, such as disrupting ecological processes and community succession through hydraulic displacement. We tested this notion by conducting in-lake mesocosm experiments with natural plankton communities plagued by Prymnesium parvum where we manipulated hydraulic flushing. We found that P. parvum cell densities were reduced by up to 69% and 53%, and ambient toxicity ameliorated, during pre-bloom and bloom development periods. Furthermore, other phytoplankton taxa and zooplankton benefited from these pulsed flushing events. In other words, hydraulic flushing with deep waters not only suppressed P. parvum bloom initiation and development, but also proved beneficial to other aspects of the lower food web. These observations provide the first demonstration that P. parvum bloom initiation may be interrupted, and bloom development may be arrested, through hydraulic manipulations.