Marine Plankton during the Polar Night: Environmental Predictors of Spatial Variability.

Simple Summary: Plankton are a key component of Arctic marine ecosystems, connecting all trophic levels and being involved in organic matter recycling. Our main purpose was to reveal differences in plankton characteristics in two Arctic sites that were strongly affected by the inflow of warm Atlantic waters during the period of polar night. We detected similar bacterial abundances in both regions, while other plankton parameters were significantly different. Temperature, salinity, and sampling depth shaped the plankton communities. Phytoplankton biomass also had a marked influence on microbial abundance and zooplankton assemblages. Changes in the proportions of boreal taxa suggested the Atlantification of pelagic communities, a phenomenon reported in the Arctic due to global warming. We studied the spatial patterns of the planktonic ecosystems at two Arctic sites strongly affected by Atlantic Inflow (FS, the Fram Strait; and BS, the Barents Sea). A high degree of similarity in the bacterial abundance (mean: 3.1 × 105 cells mL−1 in FS vs. 3.5 × 105 cells mL−1 in BS) was found, while other plankton characteristics were different. Bacterial biomass reached a maximum in BS (3.2–7.9 mg C m−3), while viral abundances tended to be higher in FS (2.0–5.7 × 106 particles mL−1). Larger bacterial cells were found in BS, suggesting the presence of different bacterial populations at both locations. The virus-to-bacteria ratio was significantly higher in FS than in BS (13.5 vs. 4.7). Chlorophyll a concentration was extremely low (<0.25 mg m−3). The highest zooplankton abundance was in the surface layer (919 individuals m−3 in FS vs. 602 ind. m−3 in BS). Zooplankton biomass strongly varied (1–39 mg C m−3), with the maximum in BS. High proportions of boreal taxa in the total zooplankton abundance indicate the Atlantification of pelagic ecosystems in the Arctic. Plankton indicators are correlated with temperature, salinity, and sampling depth. Strong intercorrelations were found between major plankton groups, suggesting tight links in the studied plankton ecosystems. [ABSTRACT FROM AUTHOR]