Near-Bottom Hypoxia Impacts Dynamics of Bacterioplankton Assemblage throughout Water Column of the Gulf of Finland (Baltic Sea)

Abstract

Over the past century the spread of hypoxia in the Baltic Sea has been drastic, reaching its 'arm' into the easternmost sub-basin, the Gulf of Finland. The hydrographic and climatological properties of the gulf offer a broad suite of discrete niches for microbial communities. The current study explores spatiotemporal dynamics of bacterioplankton community in the Gulf of Finland using massively parallel sequencing of 16S rRNA fragments obtained by amplifying community DNA from spring to autumn period. The presence of redoxcline and drastic seasonal changes make spatiotemporal dynamics of bacterioplankton community composition (BCC) and abundances in such estuary remarkably complex. To the best of our knowledge, this is the first study that analyses spatiotemporal dynamics of BCC in relation to phytoplankton bloom throughout the water column (and redoxcline), not only at the surface layer. We conclude that capability to survive (or benefit from) shifts between oxic and hypoxic conditions is vital adaptation for bacteria to thrive in such environments. Our results contribute to the understanding of emerging patterns in BCCs that occupy hydrographically similar estuaries dispersed all over the world, and we suggest the presence of a global redox- and salinity-driven metacommunity. These results have important implications for understanding long-term ecological and biogeochemical impacts of hypoxia expansion in the Baltic Sea (and similar ecosystems), as well as global biogeography of bacteria specialized inhabiting similar ecosystems.

Fig 1. Map of central Gulf of Finland supplemented with AP- and KERI-transects, stations are color-coded red and blue, respectively.

The timeline of the 2012 sampling cruises is provided with corresponding color-codes. Four samples were also collected on 8th November of 2011. The tan to bright green colours give an indication of the topography of the study area.

Fig 2. Water column profiles of stations AP5 and Keri3.

Temperature (°C, magenta), oxygen (mg/L, red) and chlorophyll a (mg/m³, green) are on upper x-axis. Salinity (g/kg, blue) is on bottom x-axis. The sampling date is marked as following: YYYY-MM-DD.

Fig 3. The relative abundance of dominant bacterial classes given in three depth ranges: 5–25 m (A), 25–65 m (B) and >65 m (C).

Fig 4. Occurrence patterns of relatively abundant OTUs (top 17).

OTUs are ordered by their co-localization.

Fig 5. NMDS plot of abundant and common OTUs fitted with environmental parameters (stress score: 0.125).

The added black lines divide OTUs into three major groups, which are marked with letters for discussion.

Fig 6. Cross-section of AP-transect on 2012-07-04 supplemented with relative abundances of top 17 OTUs.

Fig 7. Time series of Chlorophyll a at station AP5.

Pie charts indicate the relative abundance of OTUs that were associated with the spring phytoplankton bloom. Note that the second surface community (2012-04-23, 5 m depth) was recorded at station AP2 not AP5.