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In the North Sea, Tripos and Dinophysis are commonly occurring mixotrophic planktonic dinoflagellate genera. In order to understand their bloom dynamics, an occurring bloom dominated by T. furca and D. norvegica was followed for several days. High cell abundances of these species were located to estimate: in situ growth rates from cell cycle analyses, depth distributions, growth rates sustained by photosynthesis, and parasite infection prevalence in all T. furca, T. fusus, D. norvegica and D. acuminata. Cell abundances were over 10000 cells L−1 for T. furca and up to 18000 cells L−1 for D. norvegica. Cells accumulated between 15-25 m depth and presented low specific in situ growth rates of 0.04-0.15 d−1 for T. furca and 0.02-0.16 d−1 for D. norvegica. Photosynthesis could sustain growth rates of 0.01-0.18 d−1 for T. furca and 0.02 to 0.14 d−1 for D. norvegica, suggesting that these species were relying mainly on photosynthesis. Parasite infections where generally low, with occasional high prevalence in D. norvegica (by Parvilucifera sp.) and T. fusus (by Amoebophrya sp.), while both parasites showed comparable prevalence in D. acuminata, which could offset in situ growth rates by parasite-induced host mortality. The restructuring effect of parasites on dinoflagellate blooms is often overlooked and this study elucidates their effect to cell abundances and their growth at the final stages of a bloom.

Picophytoplankton (<2 µm diameter), a diverse group of picocyanobacteria and photosynthetic picoeukaryotes, are significant contributors to primary production. Predatory mortality controls picophytoplankton biomass and thereby energy transfer in the marine food web. The 2 major pathways of picophytoplankton mortality are grazing and viral lysis. Grazing passes carbon directly to higher trophic levels, while lysis products are passed into the viral loop. Picophytoplankton are abundant in the Baltic Sea but little is known about their predatory mortality. Using a modification of the dilution approach, we calculated growth and mortality rates of picophytoplankton and studied the effect of predation on community structure during late August and September. The experiments were conducted coinciding with the peak in picophytoplankton abundance (similar to 10(5) cells ml(-1)) at the Linnaeus Microbial Observatory in the Baltic Sea Proper. The results showed that grazing is an important controller of picocyanobacteria and photosynthetic picoeukaryote populations, while no significant viral lysis effect was detected. Grazing on picocyanobacteria was proportional to growth rates, while grazing on photosynthetic picoeukaryotes exceeded growth. Selective grazing of phylogenetically distinct picocyanobacterial clades had a significant effect on community structure, suggesting that grazing has an impact on the seasonal dynamics of co-occurring clades. Picocyanobacteria had a higher carbon transfer contribution to higher trophic levels than photosynthetic picoeukaryotes at the time of the experiments. The study shows that picophytoplankton are important contributors to carbon cycling in the Baltic Sea microbial food web and should be considered for future ecological models.

Picophytoplankton (<2 µm diameter), a diverse group of picocyanobacteria and photosynthetic picoeukaryotes, are significant contributors to primary production. Predatory mortality controls picophytoplankton biomass and thereby energy transfer in the marine food web. The 2 major pathways of picophytoplankton mortality are grazing and viral lysis. Grazing passes carbon directly to higher trophic levels, while lysis products are passed into the viral loop. Picophytoplankton are abundant in the Baltic Sea but little is known about their predatory mortality. Using a modification of the dilution approach, we calculated growth and mortality rates of picophytoplankton and studied the effect of predation on community structure during late August and September. The experiments were conducted coinciding with the peak in picophytoplankton abundance (∼105 cells ml-1) at the Linnaeus Microbial Observatory in the Baltic Sea Proper. The results showed that grazing is an important controller of picocyanobacteria and photosynthetic picoeukaryote populations, while no significant viral lysis effect was detected. Grazing on picocyanobacteria was proportional to growth rates, while grazing on photosynthetic picoeukaryotes exceeded growth. Selective grazing of phylogenetically distinct picocyanobacterial clades had a significant effect on community structure, suggesting that grazing has an impact on the seasonal dynamics of co-occurring clades. Picocyanobacteria had a higher carbon transfer contribution to higher trophic levels than photosynthetic picoeukaryotes at the time of the experiments. The study shows that picophytoplankton are important contributors to carbon cycling in the Baltic Sea microbial food web and should be considered for future ecological models.