Biogeochemical functioning of the Baltic Sea

Abstract

Abstract. Location, specific topography, and hydrographic setting together with climate change and strong anthropogenic pressure are the main factors shaping the biogeochemical functioning and thus also the ecological status of the Baltic Sea. The recent decades have brought significant changes in the Baltic Sea. First, the rising nutrient loads from land in the second half of the 20th century led to eutrophication and spreading of hypoxic and anoxic areas, for which permanent stratification of the water column and limited ventilation of deep-water layers made favourable conditions. Since the 1980s the nutrient loads to the Baltic Sea have been continuously decreasing. This, however, has so far not resulted in significant improvements in oxygen availability in the deep regions, which has revealed a slow response time of the system to the reduction of the land-derived nutrient loads. Responsible for that is the low burial efficiency of phosphorus at anoxic conditions and its remobilization from sediments when conditions change from oxic to anoxic. This results in a stoichiometric excess of phosphorus available for organic-matter production, which promotes the growth of N2-fixing cyanobacteria and in turn supports eutrophication. This assessment reviews the available and published knowledge on the biogeochemical functioning of the Baltic Sea. In its content, the paper covers the aspects related to changes in carbon, nitrogen, and phosphorus (C, N, and P) external loads, their transformations in the coastal zone, changes in organic-matter production (eutrophication) and remineralization (oxygen availability), and the role of sediments in burial and turnover of C, N, and P. In addition to that, this paper focuses also on changes in the marine CO2 system, the structure and functioning of the microbial community, and the role of contaminants for biogeochemical processes. This comprehensive assessment allowed also for identifying knowledge gaps and future research needs in the field of marine biogeochemistry in the Baltic Sea.

Erosion of carbonate-bearing sedimentary rocks may close the alkalinity budget of the Baltic Sea and support atmospheric CO2 uptake in coastal seas

“…climate change, dredging, and bottom trawling. Moreover, eutrophication, rising temperatures and the increase in CO 2 accumulation in oxygendepleted bottom waters (Kulinski et al, 2022) may have supported further CaCO 3 dissolution. It should, however, be noted that more work on erosion rates and an improved reconstruction and understanding of temporal changes in the chemical composition of Baltic Sea water are required to consolidate these findings.…”

“…It released CO 2 prior to the onset of eutrophication (Omstedt et al, 2009), mainly driven by the mineralization of terrigenous organic matter, and has apparently acted as a net sink for atmospheric CO 2 over the last decades, even though the strong seasonal, interannual and spatial variability complicates the assessment of a basin-wide CO 2 budget (Thomas et al, 2010;Schneider et al, 2014;Gustafsson et al, 2015). Eutrophication in the widely stratified marginal sea led to enhanced CO 2 removal from the atmosphere and organic carbon deposition at the seabed while increasing atmospheric CO 2 levels amplified the air-sea disequilibrium (Kulinski et al, 2022).…”

On physical mechanisms enhancing air–sea CO2 exchange

“…This leads to a dynamic carbon system with significant spatio-temporal variability. A thorough assessment of the biogeochemical functioning of the Baltic Sea was recently published by Kuliński et al (2022). In terms of the air-sea CO 2 exchange, several approaches have been used to estimate the regional fluxes; however, no consensus has been reached on the role of the Baltic Sea as a net source or sink of atmospheric CO 2 (Thomas et al, 2010;Kuliński and Pempkowiak, 2011;Norman et al, 2013b;Parard et al, 2017).…”

Benthic alkalinity fluxes from coastal sediments of the Baltic and North seas: comparing approaches and identifying knowledge gaps

“…The Baltic and North seas form a continuum of water and material transport (Gustafsson, 1997;Kuliński et al, 2022;Maar et al, 2011), deeply linking the carbon cycles of these two ocean basins. While the nearshore regions of the southern North and western Baltic seas are located at a similar latitude, have a similar water depth, and contain similar sediment types, they experience very different physical forcing.…”

“…In contrast, limited tidal forcing combined with finer-grained sediments in the western Baltic Sea could promote stronger biogeochemical zonation and a greater importance of anaerobic over aerobic processes (Böttcher et al, 2000). While we know that these factors shape benthic biogeochemical processing, sedimentwater fluxes in these shallow coastal environments are still poorly constrained, limiting our understanding of their role in the regional carbon cycle (Kuliński et al, 2022).…”