Parasitism of photosynthetic dinoflagellates in a shallow subestuary of Chesapeake Bay, USA

Rhode River (USA) populations of the red-tide dinoflagellates Gymnodinium sanguineum Hirasaka, 1922, Gyrodinium uncatenum Hulburt, 1957, and Scrippsiella trochoidea (Stein) Loeblich III, 1976, were commonly infected by their parasitic relative Amoebophrya ceratii Cachon, 1964, during the summer of 1992. Mean infection levels were relatively low, with data for vertically integrated samples averaging 1.0, 1.9, and 6.5 % for G. sanguineum, G. uncatenum, and S. trochoidea, respectively. However, epidemic outbreaks of A, ceratii (20 to 80 % hosts parasitized) occurred in G. uncatenum and S. trochoidea on several occasions, with peak levels of parasitism associated with decreases in host abundance. Estimates for parasite induced mortality indicate that A. ceratii is capable of removing a significant fraction of dinoflagellate biomass, with epidemics in the upper estuary cropping up to 54 % of the dominant bloom-forming species, G. uncatenum, daily. However, epidemics were usually geographically restricted and of short duration, with daily losses for the 3 host species due to parasitism averaging 1 to 3 % over the summer. Thus, A. ceratii appears capable of exerting a controlling influence on bloom-forming dinoflagellates of the Rhode River only when conditions are suitable for production of epidemic infections. Interestingly, epidemics failed to occur in multiple dinoflagellate taxa simultaneously, even when alternate host species were present at high densities. This observation, along with laboratory experiments demonstrating that parasites isolated from G. sanguineum were unable to infect G. uncatenum, S. trochoidea, and Ceratium furca, suggests that the dinoflagellate taxon A. ceratii may represent a cluster of relatively host-specific species.