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Marine Ecology Progress Series

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MEPS 735:63-76 (2024)  -  DOI:

Growth and mortality rates of picophytoplankton in the Baltic Sea Proper

Javier Alegria Zufia1, Christien P. Laber1, Catherine Legrand1,2, Elin Lindehoff1, Hanna Farnelid1,*

1Centre for Ecology and Evolution in Microbial model Systems—EEMiS, Department of Biology and Environmental Science, Linnæus University, 391 82 Kalmar, Sweden
2School of Business, Innovation and Sustainability, Halmstad University, 301 18 Halmstad, Sweden
*Corresponding author:

ABSTRACT: 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.

KEY WORDS: Picophytoplankton · Picoeukaryotes · Synechococcus · Grazing · Viral lysis · Carbon transfer · Baltic Sea

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Cite this article as: Alegria Zufia J, Laber CP, Legrand C, Lindehoff E, Farnelid H (2024) Growth and mortality rates of picophytoplankton in the Baltic Sea Proper. Mar Ecol Prog Ser 735:63-76.

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