VERTICAL MIGRATION OF AUTOTROPHIC MICROORGANISMS DURING A VERNAL BLOOM AT THE COASTAL BALTIC SEA - COEXISTENCE THROUGH NICHE SEPARATION

Vertical migration of two dinoflagellate species (Peridiniella catenat a and Scrippsiella hangoei) and a phototrophic ciliate (Mesodinium rub rum) were studied during the peak and decline of a vernal bloom at the SW coast of Finland. During the diel cycle, part of the populations o f P. catenata and M, rubrum were observed in the deeper layers with el evated nutrient concentrations, while S. hangoei remained in the upper nutrient depleted mixed layer. Using a correspondence analysis the ve rtical distribution patterns of the species and chlorophyll a were exa mined over a temporal scale of hours and weeks. The vertical migration was reflected in much higher variability in the depth distribution of P. catenata and M, rubrum over a diel scale, compared to S. hangoei. The analysis revealed also significant differences in species specific depth distribution patterns over both time scales. It is discussed th at the co-existence of the two dominant dinoflagellate species during the vernal bloom is due to niche separation through behavioural adapta tions.