Impact of a Major Inflow Event on the Composition and Distribution of Bacterioplankton Communities in the Baltic Sea

Abstract

Major Baltic inflow (MBI) events carry highly saline water from the North Sea to the central Baltic Sea and thereby affect both its environmental conditions and its biota. While bacterioplankton communities in the Baltic Sea are strongly structured by salinity, how MBIs impact the composition and distribution of bacteria is unknown. The exceptional MBI in 2014, which brought saline and oxygenated water into the basins of the central Baltic Sea, enabled the linkage of microbiological investigations to hydrographic and modeling studies of this MBI. Using sequence data of 16S ribosomal RNA (rRNA) and 16S rRNA genes (rDNA), we analyzed bacterioplankton community composition in the inflowing water and in the uplifted former bottomwater at stations reached by the MBI. Bacterial diversity data were compared with respective data obtained from previous, non-inflow conditions. Changes in bacterial community composition following the 2014 MBI were mainly apparent at the genus level. A number of specific taxa were enriched in the inflowing water, with large changes in the rRNA/rDNA ratios indicating the different activity levels between of the water masses. The relative similarity of the bacterial communities in the inflowing and uplifted waters as well as the results from an inflow-simulating numerical model showed that the inflowing water did not originate directly from the North Sea but mostly from adjacent areas in the Baltic Sea. This suggested that the inflow event led to a series of shifts in Baltic Sea water masses among the Baltic Sea basins and a gradual mixing of the water bodies. Dramatic changes in the bacterial community composition occurred when the bottomwater inflow reached the anoxic, sulfidic deep basins, resulting in an uplifting of the formerly anoxic bacterial community, dominated by Epsilonproteobacteria. Our study of the impact of MBIs on bacterioplankton communities therefore highlights two relevant underlying mechanisms that impact the distribution and possibly also the activities of planktonic bacteria in the Baltic Sea: (1) the successive dilution of inflowing North Sea water with ambient waters and (2) the uplifting of former bottom-water communities to higher water strata.

Relevance of Nutrient-Limited Phytoplankton Production and Its Bacterial Remineralization for Carbon and Oxygen Fluxes in the Baltic Sea

“…Four months before our study took place, the composition of bacterial communities in uplifted and renewed bottom water was analyzed. The successive dilution of inflowing North Sea water with Baltic Sea water during its passage to the Eastern Gotland Basin resulted in increasingly similar communities (Bergen et al, 2018). With regard to heterotrophic bacterial activity, our results suggest that these newly combined communities perform equally well under oxic, hypoxic and sulfide-free anoxic conditions.…”

Characterizing ventilation events in an anoxic coastal basin: Observed dynamics and the role of climatic drivers

“…Profound changes in phosphorus, nitrogen, and methane dynamics have been reported following ventilation of a deep anoxic lake (Lehmann et al 2015) and large changes to the bacterial community of an anoxic deep basin of the Baltic were detected following oxygenation (Bergen et al 2018). The fate of basin water H 2 S and methane during and following ventilation also deserves further investigation and are relevant to larger‐scale analogues (Capet et al 2016).…”

“…Renewal of the deep basin water L 3 (termed ventilation in cases of suboxic waters) is of particular interest as it can cause profound biogeochemical changes over a very short timeframe (Arneborg et al 2004; Lehmann et al 2015; Bergen et al 2018). Of particular concern are the potential impacts of releasing toxic hydrogen sulfide (H 2 S) gas, which has accumulated in anoxic deep water, to aquatic life in oxygenated surface waters (Luther et al 2004) or to the atmosphere (Capet et al 2016).…”

Baltic Sea Operational Oceanography—A Stimulant for Regional Earth System Research

“…Today we know that, on average, half of the total amount of salt imported into the Baltic is supplied by barotropic inflows of highly saline water (Mohrholz, 2018). In particular, the wellobserved, exceptionally strong Major Baltic Inflow (MBI) of 2014 (Mohrholz et al, 2015) enabled unprecedented, detailed studies of the dynamics of saltwater inflow events and of their implications for the ecosystem (e.g., Gräwe et al, 2015;Schmale et al, 2016;Holtermann et al, 2017;Bergen et al, 2018). In a long-term average MBIs contribute 20 to 25% to the total salt import into the Baltic (Mohrholz, 2018), beside this they are the solely mechanism for deep water ventilation of the central Baltic (Meier et al, 2006).…”