Spatial and seasonal variability of picoplankton abundance and growth rates in the southern Bay of Biscay.

Autotrophic and heterotrophic picoplankton play fundamental roles in marine food webs and biogeochemical cycles. However, their growth responses have seldom been jointly assessed, including many temperate regions such as the Bay of Biscay. There, previous studies have shown their relevance in carbon fluxes. We describe here the spatio-temporal variability of the abundances and growth rates of the picoplanktonic groups routinely distinguished by flow cytometry (Synechococcus and Prochlorococcus cyanobacteria, two groups of differently sized picoeukaryotes and two groups of heterotrophic bacteria distinguished by their relative nucleic acid content) in the central Cantabrian Sea (S Bay of Biscay). To that end, from February to December 2021 we collected surface water on 5 occasions from 6 stations distributed along the S Bay of Biscay (6-3°W) and incubated it after removing protistan grazers in order to determine their dynamics along the seasonal cycle as well as the inshore-offshore and the west-east gradients. Seasonal variations in initial and maximum abundances generally matched previous knowledge of the region but growth rates were more variable, with Prochlorococcus and high nucleic acid (HNA) bacteria showing the maximum values (up to 2 d) while negative growth was observed in one third of Synechococcus incubations. Temporal differences generally overrode differences along the inshore-offshore gradient in trophic status while in situ and maximum abundances of most of the groups generally decreased towards the east following the increase in stratification and lower nutrient availability. Responses to stratification suggest Prochlorococcus and low nucleic acid (LNA) cells may prevail among autotrophic and heterotrophic bacteria, respectively, in a warmer ocean.