Responsibility of microzooplankton and parasite pressure for the demise of toxic dinoflagellate blooms

摘要

ABSTRACT: Two mechanisms proposed to control dinoflagellate blooms—parasitism by eukaryotes (e.g. Amoebophrya sp.) and grazing by microzooplankton—have been explored through previous laboratory and field studies but lack quantitative assessment. We modelled the relative effect of these mechanisms. We used literature values to embed an Anderson-May host-microparasite model into a microbial food web model (nano- and microphytoplankton, nano- and microzooplankton, a toxic dinoflagellate, a dinoflagellate parasite). Three scenarios were examined to simulate the introduction of a toxic dinoflagellate and its parasite into an environment: (1) a food web, including autotrophic nano- and microplankton, heterotrophic flagellates, and ciliates; (2) as for Case 1 but with a toxic dinoflagellate; (3) as for Case 2 but with a dinoparasite. This mimics observations in a French estuary, where a toxic dinoflagellate began blooming in 1988; since 1998, blooms appear to have become regulated, and numerous parasitic infections by Amoebophrya sp. occurred from 2004 to 2006. After supporting parasite control of dinoflagellate blooms, we assessed the effects of observed ranges of key variables associated with the parasite and other components of the food web on parasite control of the dinoflagellate population. Population dynamics were examined over 50 d. In Case 1, all taxa had 10 to 20 d blooms. In Case 2, the toxic dinoflagellate population dynamics mimicked that of the microphytoplankton, and this dinoflagellate was reduced in numbers but not extirpated by microzooplankton grazing. In Case 3 population blooms occurred, and the parasite virtually eliminated the dinoflagellate over ~10 d. Sensitivity analysis indicated that our assessment was robust. We propose that the decline in toxic dinoflagellates in the French estuary may have been due to an introduced dinoparasite. In general, we suggest that parasites may have greater impact on toxic dinoflagellate blooms than microzooplankton grazers; the parasites have the potential to eliminate the toxic dinoflagellates. We recommend that such parasites be incorporated into more complex food web models.