COLONIZATION OF DIATOM AGGREGATES BY THE DINOFLAGELLATE NOCTILUCA SCINTILLANS

Abundance and vertical distribution of the dinoflagellate Noctiluca sc intillans 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 for med by regular coagulation of diatom cells and not by mucus feeding be havior of N. scintillans. N. scintillans can be positively buoyant, an d estimates of encounter rates between N. scintillans and diatom aggre gates during ascent demonstrates that this mechanism is sufficient to account for the observed colonization. The attached N. scintillans wer e feeding on the diatoms in the aggregates, as revealed by food-vacuol e content. In the particular environment studied, N, scintillans appea red not to depend on aggregate feeding since shipboard experiments sho wed 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)).