Diversity and community composition of pico- and nanoplanktonic protists in the Vistula River estuary (Gulf of Gdańsk, Baltic Sea)

Abstract

Pico-and nanoplanktonic protists (eukaryotic microorganisms with cell size of < 3 µm and 3-20 µm, respectively) are the key component of plankton communities. However, their diversity and distribution patterns along environmental factors are still poorly recognized, largely due to their enormous phylogenetic diversity that has been realized only via the application of molecular methods over the past two decades. Here, we compared diversity and composition of active communities of pico-and nanoplantonic protists from three zones of the Vistula River estuary (Gulf of Gdańsk): freshwater, mixing (salinity 3.5) and brackish (salinity 7), in four seasons, by pyrosequencing the V3-V4 fragment of 18S rRNA taxonomy marker gene libraries. Alpha diversity was the highest at the brackish site, but the OTU richness was characteristic for specific protist groups. The active protistan communities in the freshwater and mixing zones (salinity 0-3.5) were similar (sharing >72% of phylotypes) and included centric diatoms (Stephanodiscus minutulus), synurophytes from clades C, E and F, and cryptophytes. However, at salinity of 7 at the brackish site the communities were significantly different from those in freshwater/mixing zone, and showed higher contributions of Dinophyceae, Mamiellophyceae, Telonemia, and picobiliphytes. The high similarity between the freshwater and mixing site, as well as high dissimilarity of the brackish site was observed in all months, despite seasonal shifts in pico-and nanoplantonic protistan communities. Seventy five percent of the observed variability in the communities was explained by combinations of temperature, salinity, nutrients and geographical distance, indicating interplay between species sorting and mass effects in shaping the active protistan communities in the Vistula River estuary. Groups that were more active in freshwaters and mixing zone seemed to be more affected by mass effects of mixing water masses, while those from brackish site by environmental species sorting. Finally, we report, for the first time, presence of Radiolaria (Acantharea) from the Baltic Sea.

Weekly dynamics of abundance and size structure of specific nanophytoplankton lineages in coastal waters (Baltic Sea)

“…The changes in nanoplankton diversity were very dynamic in the Gulf of Gdansk, with values of species richness and Shannon entropy varying over twofold from 1 week to another (Fig. B,C), but within a range reported previously (Piwosz et al ). Such high dynamics points to the importance of temporal sampling in recovering the true picture of nanoplankton diversity on coastal waters.…”

“…Sequencing confirmed persistence of groups and phylotypes recovered previously from the Gulf of Gdańsk and the Baltic Sea (Hu et al ; Piwosz et al ). However, groups found abundant using CARD‐FISH were under‐represented in the sequencing libraries.…”

“…For instance, in the brackish Baltic Sea, marine groups, such as coccolitophores, were replaced by freshwater groups, such as synurophytes, along the salinity gradient from about 30 in Kattegat to < 1 in the Bothnian Bay (Hu et al 2016). Similar changes were observed also at a smaller scale in the estuary of the Vistula River (Gulf of Gda nsk, Baltic Sea; Piwosz et al 2018). Moreover, high *Correspondence: piwosz@alga.cz…”

“…Moreover, most studied groups are mixotrophic, which confounds their dependence on inorganic nutrients (Andersson et al ; Jones et al ; Tillmann ; Piwosz and Pernthaler ; Mckie‐Krisberg and Sanders ; Grujčić et al ). Finally, coastal waters are very dynamic systems, where plankton communities constantly mix (Bazin et al ; Piwosz et al ), affecting the observed dynamics in community composition and size structure of NPP. Experimental approaches are essential to verify whether the correlations observed here are meaningful, and to progress our understanding of NPP dynamics.…”

“…The focal groups, studied by catalyzed reported deposition‐fluorescence in situ hybridization (CARD‐FISH), were chlorophytes, cryptophytes (including lineage CRY‐1), pedinellids (including species Apedinella radians and Pseudopedinella elastica ), haptophytes (including Pavlovophyceae and genera Prymnesium , Haptolina, and Chrysochromulina ), chrysophytes, and pelagophytes. They were selected based on pyrosequencing of weekly collected samples and on the previous study in the region (Piwosz et al ). I hypothesized that (1) different algal groups would contribute to NPP maxima during the season, (2) their cell volume and cell SV ratio would vary over the season, and (3) abundance, biovolume, cell volume, and cell SV ratio of different NPP groups would correlate differently with the environmental variables, which together would suggest temporal niche separation between the studied groups.…”