We quantified excess N2 accumulation below the halocline of the Baltic Proper using in situ N2/Ar measurements combined with N2/Ar estimates derived from the three-dimensional ERGOM–MOM biogeochemical model. The observed temporal increase in excess N2 was subsequently converted to nitrogen loss. The analysis focused on the post-2014 Major Baltic Inflow period, when deep-water ventilation effects had largely subsided and stratification with progressive oxygen depletion was re-established. Both observations and model simulations showed increasing excess N2 below the halocline, particularly in the Eastern Gotland Basin (EGB). Observation-based estimates of nitrogen loss amounted to approximately 80.8 ± 22.7 kt N yr−1 in the EGB, ∼15.6 ± 7.8 kt N yr−1 in the Bornholm Basin (BB), and ∼9.3 ± 4.1 kt N yr−1 in the Gdańsk Basin (GB). Model-derived estimates were consistently lower, but basin-specific comparisons revealed no significant differences between observational and temporally matched model-derived excess N2 accumulation trends. The obtained values of net fixed nitrogen loss fall within the lower range of published Baltic Sea estimates, likely because the N2/Ar approach reflects integrated net accumulation of dissolved N2 rather than gross instantaneous denitrification rates. These results demonstrate that deep-water excess N2 accumulation provides an integrative basis for estimating first-order net fixed nitrogen loss in stratified marine systems such as the Baltic Sea.
Excess dinitrogen (ΔN2) is the net accumulation of N2 produced by nitrate-reducing processes in the water column and sediments of oxygen-deficient marine systems. Therefore, biological production and physical processes shape its spatial distribution. This study investigates ΔN2 distribution below the halocline in the Baltic Sea using a coupled biogeochemical model evaluated against observations from the Bornholm, Gdańsk, and Gotland Deeps during 2020–2021. Vertical distributions of temperature, salinity, oxygen (O2), nutrients, N2/Ar ratios, and ΔN2 were compared to assess model performance and identify controls on N2 production. A persistent spatial gradient in ΔN2 was observed, with highest concentrations in the Gotland Basin, intermediate levels in the Gdańsk Basin, and lowest values in the Bornholm Basin. These differences correspond to variations in stratification, ventilation, and organic matter (OM) accumulation. Seasonal variability was strongest in the Gdańsk Basin, where elevated winter ΔN2 suggests enhanced nitrogen-loss processes supported by OM export and mixing-driven nutrient redistribution. In contrast, the Gotland Basin exhibited relatively stable, persistently reducing conditions, while the Bornholm Basin showed strong interannual variability related to episodic North Sea inflows. The weak relationship between ΔN2 and modeled denitrification rates (ranging from �0.0010 to 0.0200 𝜇mol kg−1 day−1) indicates that ΔN2 should not be interpreted as a direct proxy for pelagic denitrification. Rather, it reflects the cumulative effects of both pelagic and benthic denitrification, together with physical transport processes. By comparison, denitrification rates were more strongly associated with particulate organic carbon (POC) availability. The reported ΔN2 patterns are representative of the investigated post-inflow period rather than a long-term Baltic Sea baseline.
The accelerating triple planetary crisis of climate change, biodiversity loss and pollution is heavily impacting the Baltic Sea. However, the region offers an encouraging example of successful science-policy cooperation in addressing climate change: Since 2018 the Helsinki Commission (HELCOM) and Baltic Earth have established a fruitful exchange underpinning informed, science-based decision-making within HELCOM, exemplified by the HELCOM-Baltic Earth Climate Change Fact Sheet, as well as the previous 2007 and 2013 HELCOM thematic assessments of climate change in the Baltic Sea Area, which were based on the BALTEX and Baltic Earth Assessments of Climate Change for the Baltic Sea Basin (BACC). As HELCOM implements its 2021 Baltic Sea Action Plan and starts looking beyond the Plan’s 2030 target date, cooperation between the two organizations will remain a key element in regional efforts to address the impacts of a shifting climate.
Atlantic salmon (Salmo salar) is a fish species valued for human consumption. It is a predator that feeds intensively and, as a consequence, may be exposed to the accumulation of harmful factors, such as parasites and heavy metals. Salmon were collected during a standard ichthyological analysis in spring 2023 in the southern Baltic Sea. All 40 specimens were heavily infected with the intestinal cestode parasite Eubothrium sp. This study aimed to evaluate the concentrations of the following elements: toxic metals (Cd, Hg, and Pb) and the metalloid As, macroelements (Ca, K, Na, Mg, and P), and microelements (Cr, Cu, Fe, Mn, Sr, and Zn), in the tissues of salmon (liver and muscle) and Eubothrium sp. The relationships between elements concentrations in the parasites and salmon tissues were expressed as bioconcentration factors (BCFs). In the parasite-host system (with respect to the muscle tissue of salmon), cestodes primarily accumulate microelements, which may negatively affect the condition of the fish. A high BCF ratio of parasite/host muscle tissue for Cd concentration may be beneficial for the host because parasite accumulation may influence metal distribution and potentially reduce metal availability for host tissues. In the parasite-host system (with respect to the liver), parasites accumulate highly toxic Hg but also absorb microelements (Sr, Zn, and Mn) and macroelements at much higher concentrations than those found in the host.
This study presents the development, implementation, and validation of the high-resolution three-dimensional bio-geochemical 3D CEMBS-PolSea model for the southern Baltic Sea. The model is based on the NPZD (Nutrient–Phytoplankton–Zooplankton–Detritus) framework and simulates key ecosystem components and processes, including phytoplankton, zooplankton, chlorophyll a, dissolved oxygen, nutrients (NO 3 , PO 4 , SiO 3 , NH 4 ), and organic matter dynamics. The model was developed within the CSI-POM 2 project and integrated into an operational environmental information system for Polish marine areas. Model performance was evaluated using in situ observations from the ICES Hydrochemistry databases and satellite-derived chlorophyll-a products from the SatBałtyk system. Validation results demonstrated the highest agreement for dissolved oxygen, with correlation coefficients reaching 0.87 and 0.72 for the ICES Low Resolution and High Resolution datasets, respectively. Moderate agreement was obtained for nutrients and chlorophyll a, with correlation coefficients ranging from 0.51 to 0.68. Additional performance metrics, including bias, normalised RMSE (nRMSE), the RMSE-observation standard deviation ratio (RSR), and the coefficient of variation (CV), indicate that model errors remain comparable to or lower than the variability of the observational datasets. These results demonstrate that the model reproduces a substantial fraction of the observed environmental variability despite the high natural variability of biogeochemical conditions in the Baltic Sea. The integration of satellite-derived chlorophyll a assimilation improves the representation of phytoplankton dynamics and enhances the operational applicability of the modelling system. The developed 3D CEMBS-PolSea model provides a valuable tool for ecosystem assessment, environmental monitoring, forecasting, and marine management in the southern Baltic Sea.
