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Excerpt from introduction::

The planktonic realm from bacteria to zooplankton provides the baseline for pelagic aquatic food

webs. However, multiple trophic levels are seldomly included in time series studies, hampering a

holistic understanding of the influence of seasonal dynamics and species interactions on food web

structure and biogeochemical cycles. Here, we investigated plankton community composition,

focusing on bacterio-, phyto- and large mesozooplankton, and how biotic and abiotic factors correlate

at the Linnaeus Microbial Observatory (LMO) station in the Baltic Sea from 2011 to 2018. Plankton

communities structures showed pronounced dynamic shifts with recurring patterns. Summarizing

the parts of the planktonic microbial food web studied here to total carbon, a picture emerges

with phytoplankton consistently contributing > 39% while bacterio- and large mesozooplankton

contributed ~ 30% and ~ 7%, respectively, during summer. Cyanophyceae, Actinobacteria,

Bacteroidetes, and Proteobacteria were important groups among the prokaryotes. Importantly,

Dinophyceae, and not Bacillariophyceae, dominated the autotrophic spring bloom whereas

Litostomatea (ciliates) and Appendicularia contributed significantly to the consumer entities together

with the more traditionally observed mesozooplankton, Copepoda and Cladocera. Our findings of

seasonality in both plankton composition and carbon stocks emphasize the importance of time

series analyses of food web structure for characterizing the regulation of biogeochemical cycles and

appropriately constraining ecosystem models.