Multi-host–parasitoid interactions in marine phytoplankton blooms: temporal coexistence, succession and benthic–pelagic coupling

Parasitoid communities associated with summer dinoflagellate blooms in the north-western Mediterranean showed temporal partitioning, taxon-specific dynamics and contrasting associations with the water column and sediments. Several zoosporic parasitoid lineages co-occurred, including perkinsids, chytrids and Syndiniales such as Amoebophrya and Euduboscquella -like taxa. Members of the Amoebophrya complex (Dino-Group-II clades 2 and 4) had the highest relative sequence abundances in the water column, and their increases coincided with peaks or declines in dominant host populations, a pattern consistent with density-responsive infection dynamics. Perkinsids also exhibited patterns consistent with density-responsive dynamics, with peaks near declines in host abundance; they were detected in both the water column and sediments. Chytrids occurred sporadically, with low infection prevalence in the water column and only occasional detection in sediments. These contrasting patterns were consistent with temporal differentiation among host–parasitoid interactions associated with differences in host tracking, infection timing and habitat use. Overall, infection prevalence remained low and parasitoid peaks were not consistently associated with bloom termination, although persistent, low-intensity infections may have influenced competitive interactions among hosts and species coexistence. Differences in temporal occurrence and compartment associations may facilitate coexistence among parasitoid lineages. Sediments may act as reservoirs, particularly for perkinsids, suggesting that benthic–pelagic coupling contributes to the structure of marine parasitoid communities.