Free-living, photosynthetic marine dinoflagellates are frequently infected by microparasites of the genus Amoebophrya. Attacks by Amoebophrya can contribute to the termination of dinoflagellate blooms and have been suggested to influence the geographical distribution of certain host species. Because infection terminates with the killing of the host (i.e. Amoebophrya behaves like a parasitoid), the interaction can be considered, from a modeling point of view, similar to the process of predation, with the difference that it takes a longer time for the parasitoid to kill the host as compared to typical predator-prey interactions. In the present work, we explored the population dynamics in Amoebophrya and their dinoflagellate hosts using the Rosenzweig-MacArthur modification of the traditional Lotka-Volterra predation model. The model was parameterized for 3 systems, Akashiwo sanguinea, Gymnodinium instriatum, and Karlodinium micrum, and their respective Amoebophrya parasitoids, using published experimental data. Parameter validation was possible for parasitoid search rate and mortality. The potential for host control by Amoebophrya and the probability for host extinction were studied with respect to carrying capacity, a parameter that is influenced by e. g. eutrophication. The model may be useful to predict conditions under which Amoebophrya can control populations of its dinoflagellate hosts.

We studied allelopathy in the dinoflagellate genus Alexandrium by testing the effect of A. tamarense on a natural plankton community from Hopavagen Bay, Trondheimsfjord, Norway, and the effect of toxic and non-toxic strains of A. tamarense and a toxic strain of A. minutum on algal monocultures. Also, a possible relation between the allelopathic effect and the production of paralytic shellfish poison (PSP) toxin was investigated. A. tamarense affected the whole phytoplankton community by decreasing the growth rate and changing the community structure (relative abundance of each species, dominant species). A negative effect of A. tamarense was also observed on ciliates, but not on bacteria numbers, In the bioassay with algal monocultures, the diatom Thalassiosira weissflogii and the cryptophyte Rhodomonas sp. were exposed to the filtrate of Alexandrium spp. All tested Alexandrium strains negatively affected T weissflogii and Rhodomonas sp. cultures, independent of whether PSP toxins were produced. The compounds released by Alexandrium caused lysis of natural and cultured algal cells, suggesting that the allelopathic effect may be connected with previously described ichthyotoxic and haemolytic properties of Alexandrium. Furthermore, the observation that several components of the plankton community were affected by compounds released by A. tamarense emphasizes the importance of allelopathy for the ecology of this species.