Colonization of diatom aggregates by the dinoflagellate Noctiluca scintillans

Abundance and vertical distribution of the dinoflagellate Noctiluca scintillans were studied during a diatom bloom in the Benguela current. Video observations showed the occurrence of abundant Chaetoceros spp. aggregates colonized by N. scintillans. The diatom aggregates were formed by regular coagulation of diatom cells and not by mucus feeding behavior of N. scintillans. N. scintillans can be positively buoyant, and estimates of encounter rates between N. scintillans and diatom aggregates during ascent demonstrates that this mechanism is sufficient to account for the observed colonization. The attached N. scintillans were feeding on the diatoms in the aggregates, as revealed by food-vacuole content. In the particular environment studied, N, scintillans appeared not to depend on aggregate feeding since shipboard experiments showed that clearance rates (10-20 mu l h(-1)) on unaggregated cells were sufficient to account for the high growth rates measured (up to 0.7 d(-1)).