Vertical migration of autotrophic micro-organisms during a vernal bloom at the coastal Baltic Sea — coexistence through niche separation

Abstract

Vertical migration of two dinoflagellate species (Peridiniella catenata and Scrippsiella hangoei) and a phototrophic ciliate (Mesodinium rubrum) 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 of P. catenata and M. rubrum were observed in the deeper layers with elevated nutrient concentrations, while S. hangoei remained in the upper nutrient depleted mixed layer. Using a correspondence analysis the vertical distribution patterns of the species and chlorophyll a were examined 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 that the co-existence of the two dominant dinoflagellate species during the vernal bloom is due to niche separation through behavioural adaptations.