Prymnesium parvum bloom termination: role of hydraulic dilution

The toxic haptophyte Prymnesium parvum is an alga that causes fish-killing blooms worldwide. In Texas (USA), P. parvum blooms occur in inland brackish waters and have recently increased in frequency and magnitude. Here, we document the development and termination of a localized bloom in Lake Whitney during spring. Initially, water conditions were non-toxic, and then became toxic to cladocerans and fish. As the bloom peaked, P. parvum populations reached ∼38 million cells L−1, and prymnesiophytes dominated the phytoplankton assemblage, comprising ∼92% of the biomass. In April, a sizeable inflow event terminated this bloom, completely removed toxicity and decreased P. parvum densities by 52%, under a 30% hydraulic dilution. The lake level rose, but no substantial outflow occurred. This demonstrated that flushing losses, previously shown to be important for P. parvum bloom termination elsewhere, did not contribute to the termination of this bloom. Following the inflow event, P. parvum might have lost its competitive edge due to the cessation of toxin production as a result of nutrient loading. Despite positive specific growth rates for P. parvum, population density did not increase. During the non-toxic, post-inflow conditions, zooplankton biomass increased ∼225-fold, suggesting that grazing losses contributed to the offset of P. parvum reproductive growth. Our findings emphasize the importance of inflows to P. parvum bloom dynamics, while demonstrating the impact of hydraulic dilution on bloom termination.