Baltic Earth Assessment Report on the biogeochemistry 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, 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.

Climate Change in the Baltic Sea Region: A Summary

“…Since there are no statistically significant trends in annual river discharge (Section 3.2.2.1), these reductions are attributed to socio-economic development, including expansion of the wastewater treatment and reduction of atmospheric nitrogen deposition (Gauss et al, 2021) over the entire Baltic Sea drainage basin, and not to climate related effects (HELCOM, 2018a;Svendsen and Gustafsson, 2020). As an example, the coastal point sources of TN and TP decreased three-and ten-fold, respectively, comparing to the 1990s (Savchuk et al, 2012) and today contribute to the Baltic Sea less nutrients than they did in 1900 (Savchuk et al, 2008;Kuliński et al, 2021).…”

“…2. Baltic Earth Assessment Report on the biogeochemistry of the Baltic Sea (Kuliński et al, 2021): terrestrial biogeochemical processes and nutrient loads to the Baltic Sea, transformations of C, N, P in the coastal zone, organic matter production and remineralization, oxygen availability, burial and turnover of C, N, P in the sediments, the Baltic Sea CO2 system and seawater acidification, role of specific microorganisms in Baltic Sea biogeochemistry, interactions between biogeochemical processes and chemical contaminants.…”

“…environmental factors. These factors change both in time (phenological changes, long-term trends and lags due to land cover processes) and space (north-south gradients in climate and land use, east-west gradients in socio-economic features and climate) thus determining heterogeneity and variation of land nutrient inputs that drive long-term eutrophication of the Baltic Sea (Savchuk, 2018, and references therein;Kuliński et al, 2021).…”

“…Knowledge gaps and research needs have been intensively discussed within the grand challenge working groups of Baltic Earth and are summarized by the BEARs (Lehmann et al, 2021;Kuliński et al, 2021;Rutgersson et al, 2021;Weisse et al, 2021;Christensen et al, 2021;Gröger et al, 2021a;Meier et al, 2021a;Viitasalo, 2021;Reckermann et al, 2021).…”

“…The environmental and biological factors favoring certain biogeochemical pathways through complex interactions, the pools of dissolved organic matter, and sediment biogeochemical processes are poorly understood . Although initial studies on the coastal filter capacity have been made, coastal zone models for the entire Baltic Sea and an overall estimate of bioavailable nutrients and carbon loads from land to the open sea do not exist (Kuliński et al, 2021). Considering the large internal variability, investigations of changes in extremes are limited because high-resolution observational time series are too short and model ensembles too small (Rutgersson et al, 2021).…”