AME 33:163-172 (2003)  -  doi:10.3354/ame033163

ABSTRACT: The temporal and vertical distribution of the infection of the dinoflagellate Dinophysis norvegica by the endoparasite Amoebophrya sp. was investigated at a fixed sampling location in the Baltic Sea during 2000 and 2001. Infected hosts were detected by epifluorescence microscopy after DAPI staining. The maximum depth-averaged parasite prevalence was 2.3% in July 2000 and 1.8% in August 2001. The percentage of infected hosts was usually higher close to the thermocline than at the surface. The highest parasite prevalence at a specific depth (4.8%) was found at 20 m in August 2001. No correlation was observed between parasite prevalence and host abundance or dissolved nutrient (N and P) concentrations, neither for depth-averaged nor discrete depth measurements. However, temperature seemed to be an important factor influencing infection of D. norvegica by Amoebophrya sp., with infected host cells observed only above 12°C. Amoebophrya sp. was only sporadically observed inside other dinoflagellate species, indicating specificity towards D. norvegica. The seasonal pattern of infection suggests the existence of a dormancy stage of the parasite dinospores. The low prevalence observed during this study indicates that parasitism by Amoebophrya sp. is not a relevant loss factor for D. norvegica in the Baltic Sea.

KEY WORDS: Phytoplankton · Interspecific interactions · Parasitism · Dinoflagellates · Amoebophrya · Dinophysis norvegica · Baltic Sea