Assessing Sediment Carbon Stocks and Microbial Turnover in Shallow Coastal Areas of the Baltic Sea

Coastal sediments are known for their role in long-term carbon sequestration. Bathymetric depressions in coastal settings accumulate fine grained organic-rich material, removing carbon from biogeochemical cycles in the water column. However, the importance of shallow near-shore settings as carbon sinks is yet to be explored. Sediments in such settings may contain large amounts of carbon from various marine and terrestrial sources, that may be either buried in-situ or transported to deeper areas. Transport and deposition of organic matter in the littoral zones of the Baltic Sea are highly complex. In this study, hand core sediments and porewaters were analysed to quantify the spatial distribution of dissolved and particulate carbon over a strong gradient of salinity and wave exposure in shallow locations (3–4 m water depth). We compared four groups of locations in the coastal zone: sheltered estuary, sheltered archipelago, semi-sheltered archipelago, exposed shoreline, in each case determined sediment carbon stock and microbial turnover as a fraction of the stock. Results were also compared against data for deeper known sites of sediment accumulation in the Baltic Sea. In the shallow near-shore settings, highest carbon stocks (~4000g C/m2 in the uppermost 25 cm) were observed in sediments in the low wave energy systems (sheltered estuary and sheltered archipelago). Grain size analysis confirms that these areas are characterized by relatively fine material (d50 is <63μm). In contrast, exposed shoreline areas were generally sandy and had carbon stocks one to two orders of magnitude lower than in the sheltered locations. Using porewater profiles to estimate diffusive fluxes of dissolved inorganic carbon, methane, and dissolved organic carbon, we observed variable rates of carbon turnover between the sampling locations, expressed as a fraction of the stock remineralised in a one-year period. Our findings highlight the importance of sheltered near-shore sediments in the carbon budget and dynamics of this coastal system and underscore the need to capture small-scale spatial heterogeneities when quantifying the fate of organic carbon in coastal settings. 

Keywords: Carbon sequestration, Coastal sediments, Sediment dynamics, Baltic Sea