Occurrence, Prevalence and Dynamics of Parasitoids during Recurrent Blooms of the Toxic Dinoflagellate .

Dinoflagellate blooms are natural phenomena that often occur in coastal areas, which in addition to their large number of nutrient-rich sites are characterized by highly restricted hydrodynamics within bays, marinas, enclosed beaches, and harbors. In these areas, massive proliferations of dinoflagellates have harmful effects on humans and the ecosystem. However, the high cell density reached during blooms make them vulnerable to parasitic infections. Under laboratory conditions parasitoids are able to exterminate an entire host population. In nature, parasitoids infect the toxic dinoflagellate during bloom conditions but their prevalence and impact remain unexplored. In this study, we evaluated the occurrence, prevalence, and dynamics of parasitoids during recurrent blooms of in a confined site in the NW Mediterranean Sea as well as the contribution of parasitism to bloom termination. parasitoids were recurrently detected from 2009 to 2013, during seasonal outbreaks of . Parasitic infections in surface waters occurred after the abundance of reached 10-10 cells L, suggesting a density threshold beyond which transmission is enhanced and the number of infected cells increases. Moreover, host and parasitoid abundances were not in phase. Instead, there was a lag between maximum and densities, indicative of a delayed density-dependent response of the parasitoid to host abundances, similar to the temporal dynamics of predator-prey interactions. The highest parasitoid prevalence was reached after a peak in host abundance and coincided with the decay phase of the bloom, when a maximum of 38% of the population was infected. According to our estimates, infections accounted for 5-18% of the total observed mortality, which suggested that the contribution of parasitism to bloom termination is similar to that of other biological factors, such as encystment and grazing.