Hydraulic flushing as a Prymnesium parvum bloom-terminating mechanism in a subtropical lake

Abstract

Prymnesium parvum blooms have increased in frequency and magnitude in the south-central USA in recent years, resulting in large fish kills and economic losses. Here, we document seasonal and system-wide plankton dynamics of Lake Granbury, TX, over a period spanning the formation and termination of a large, highly toxic, P. parvum bloom that occurred from January through March 2007. High-resolution spatial mapping showed that this bloom was system-wide and patchy during its peak densities. Consistent with laboratory studies, the highest in-lake toxicity to fish occurred during peak bloom density and under the most stressful in-lake conditions (based on salinity, temperature, light and inorganic nutrients). As with other P. parvum blooms, this bloom at its peak density was near monospecific, with P. parvum accounting for ∼92% of the phytoplankton biomass, and diatoms, cyanobacteria and green algae comprising most of the remaining biomass. A large inflow event in April obliterated this bloom, dramatically reducing population densities by 89% and completely removing toxicity to fish. Interestingly, the bloom had already started to decline somewhat before this hydraulic flushing event affected the lake. During this decline, in-lake conditions were not likely stressful to P. parvum, and predation did not appear to be a factor. The role of pathogens of P. parvum was not assessed during this study, however. Our findings show a strong link between hydrology and bloom termination, which raises concerns given that in-stream flows are predicted to decline as human population increases in this region. In addition, flow reduction may be exacerbated by climate change. Increased understanding of factors that influence P. parvum blooms is paramount given the possible need to offset the effect of diminished hydraulic flushing.