Entire Baltic Sea Research Articles

Phosphorus fractions and their vertical distribution in seabed sediments of the eastern Baltic Sea

Phosphorus fractions and their vertical distribution in seabed sediments were studied in short cores from four coastal sites of western Estonia (Western Gulf of Finland, Väinameri Sea, Suur Strait, and Gulf of Riga) representing accumulation and erosion/transport settings. As a result of recent large-scale discharges of nutrients into the marine environment, abundant phosphorus has accumulated in the seabed sediments, from where it can, under the expansion of hypoxic or anoxic conditions, be remobilized and released back into the water column. A sequential extraction method with a neutral, a reducing, a basis, and an acidic solution was used to evaluate the distribution of phosphorous in pools of five different carrier fractions. Potentially mobile phosphorus is mostly associated with redox-sensitive iron- and manganese oxyhydroxides, and this fraction is significant in the Western Gulf of Finland and the Gulf of Riga. As oxygen conditions gradually deteriorate, the remobilization of a considerable amount of potentially mobile phosphorus from sediments can affect the entire Baltic Sea ecosystem.

Bioaccumulation of Cadmium and Copper in Different Areas of The Eastern Part of The Gulf of Finland

The Gulf of Finland (with the estuary of the Neva River) is located in the northeastern part of the Baltic Sea and plays a key role in the formation of biological resources and the quality of the natural environment of the entire Baltic Sea. Along with eutrophication, its ecosystem is subject to anthropogenic pollution with toxic elements, including heavy metals. In this work, we analyzed the content of cadmium and copper in bottom sediments of the Gulf of Finland and in the tissues of the dominant species of benthic macrofauna — the deposit-feeder polychaete Marenzelleria arctia and the predatory isopod Saduria entomon. Significant variability was found in the spatial distribution of these elements in both environments. The average concentrations of cadmium (0.67±0.1 mg/kg of dry matter) in bottom sediments were correspondingly two times higher, and copper (34.4±4.0 mg/ kg) 1.5 times lower than the threshold levels established for the Baltic Sea. A significantly higher content of these metals was found in the tissues of isopods than polychaetes; in some locations it is an order of magnitude higher in isopods than in polychaetes. The factor of trophic transfer of metals in the Gulf food web showed biomagnification, i. e. accumulation of metals during the transition from low-level consumers (M. arctia) to predatory consumers (S. entomon), cadmium by 3.7 times and copper by 8.7 times. Thus, both species of benthic animals have high accumulative activity in relation to cadmium and copper, which contributes to the active transport of both metals from bottom sediments to marine biota, and, ultimately, transfer to the higher levels of the food web (fish, birds and mammals).

Offshore wind power plant site selection in the Baltic Sea

The selection of suitable sites for offshore wind power plants is an important marine spatial planning problem in the Baltic Sea. The application of multiple and different decision support tools (DSTs) can facilitate site selection. This study employs a geographical information system (GIS) integrated with the levelized cost of energy (LCOE) model, and GIS with multi-criteria decision analysis (MCDA) using an analytical hierarch process (AHP), fuzzy logic, weighted linear combination (WLC), and the technique for order of preference by similarity to ideal solution (TOPSIS) as an extension of MCDA. The core novelty of the study is the use of several concurrent state-of-the art techniques to quantify the detailed spatial pattern of suitability of different sea areas for offshore wind power plant installations for the entire Baltic Sea at an unprecedented, for such a large sea area, resolution. Important parameters were identified and data from the Global Wind Atlas, HELCOM, and EMODnet portals were used. The most appropriate areas exhibit LCOE values between 42 and 58 €/MWh, MCDA values in the range of 0.78–0.92, and capacity factor values from 0.52 to 0.55. The areas identified as the least appropriate were characterized by LCOE values ranging from 107 to 210 €/MWh, MCDA values between 0.34 and 0.59, and capacity factor values within the range of 0.30–0.43. The outcomes using both methodologies were similar. The decision preferred are mostly locations close to shore in shallow water and with a high capacity factor. The largest suitable areas are in the south-west Baltic Sea whereas some suitable locations are identified further to the north and east.

Emission, Transport and Retention of Floating Marine Macro-Litter (Plastics): The Role of Baltic Harbor and Sailing Festivals

Every year, harbor and sailing festivals attract close to 20 million visitors in the Baltic Sea region, but their consequences on marine litter pollution are still unknown. We combine field studies with model simulations and literature reviews to quantify the annual emissions of floating macro-litter and to assess its retention in estuaries and role in Baltic Sea pollution. Results focusing on Hanse Sail in Rostock and Kiel Week are extrapolated to the entire Baltic Sea region. After the Hanse Sail 2018, the harbor pollution amounted to about 950 floating macro-litter particles/km²; 85–90% were plastics. We calculated an emission between 0.24 and 3 particles per 1000 visitors, depending on the year and the waste management system. About 0.02% of all waste generated during a festival ends up in the harbor water. The Hanse Sails contributes less than 1% to the total annual macro-litter emissions in the Warnow estuary. Model simulations indicate that over 99% of the emitted litter is trapped in the estuary. Therefore, Hanse Sails are not relevant to Baltic Sea pollution. The extrapolated Baltic-Sea-wide annual emissions are between 4466 and (more likely) 55,830 macro-litter particles. The over-30 harbor and sailing festivals contribute an estimated <0.05% to the total annual macro-litter emissions in the Baltic Sea region.

Spatiotemporal variability of wave climate in the Gulf of Riga

Basic properties of wind wave climate in the Gulf of Riga, the Baltic Sea, are evaluated based on modelled wave fields, instrumentally measured and historical visually observed wave properties. Third-generation spectral wave model SWAN is applied to the entire Baltic Sea for 1990–2021 with a spatial resolution of 3 nautical miles (nmi, about 5.5 km) forced by the wind field of ERA5, to the Gulf of Riga and its entrance area with a resolution of 1 nmi (about 1.85 km), and to nearshore areas of this gulf with a resolution of 0.32 nmi (about 600 m). The calculations are performed for an idealised ice-free climate. Wave properties are represented by 36 directional and 32 frequency bins. The simulations are complemented by five sessions of instrumental measurements in the 2000s and two sets of historical visual wave observations from the island of Ruhnu and the Sõrve Peninsula for 1954–2011. Predominantly representing fetch-limited windseas, the wave climate in the gulf is milder and more intermittent than in the open Baltic Sea. The average significant wave height is mostly in the range of 0.6–0.8 m and peaks at 0.82 m inside the gulf. Typical wave periods are shorter than in the Baltic proper. The spatial pattern of wave heights, with higher wave intensity in the northern and eastern parts of the basin, follows anisotropy in wind conditions. Interannual variations are highly synchronised in different parts of the gulf. Their magnitude is less than 10% of the long-term average wave height. No long-term trend has been found in significant wave height and no distinct decadal variation exists inside the gulf.

A Comparative Analysis of Seaports in Terms of the Development of Maritime Tourism in the Area of the Baltic Sea

This study examines maritime tourism in the Baltic Sea region. The first part presents basic information related to the maritime tourism market, along with the key aspects and effects that the maritime tourism market has on the regions where it takes place. The next part concerns the Baltic Sea. The location and characteristics of the Baltic Sea are briefly presented. Passenger shipping connections between Baltic ports are described. The last part is devoted to the analysis of statistical data in the Baltic Sea region, focusing on passenger exchange in selected ports and in the entire Baltic Sea over the years. Finally, the potential directions of the development of maritime tourism in the Baltic Sea are characterized.

Reassessing the management criteria of growing seal populations: The case of Baltic grey seal and coastal fishery

The unintended consequences of marine mammal recoveries have created complex issues for resource managers to solve. In the Baltic Sea, the grey seal (Halichoerus grypus) population has increased rapidly during recent decades, and the conflict between seal conservation and fishery has escalated. Although the magnitude of economic losses varies depending on the type of fisheries, there is strong evidence that the grey seal population negatively impacts coastal fisheries and indirectly threatens the cultural heritage connected to it. The current management paradigm is biased towards the preservation of seal populations and it is failing to adequately consider socio-economic impacts of seal population. There is a need to strike a balance between seal conservation and the viability of coastal fisheries, taking into consideration local circumstances. This paper contributes to resolving this problem by assessing the existing governance arrangement. We conclude that the inconsistencies between and within different regulatory frameworks in HELCOM recommendations and European Union law are a structural constraint to tackling the problem. Further to that, some of the existing management criteria applicable to Baltic grey seal population need to be revisited by giving more consideration to regional conditions within the Baltic Sea. For instance, if the data shows that the Baltic grey seal population in its core distribution area has reached a sustainable status and is no longer at risk, then the use of peripheral areas as an indication of inadequate state of the entire Baltic Sea grey seal population is questionable.

The Bornholm intermediate waters: Origination, pathway, and detection within the cold intermediate layer of the Baltic Sea

The Cold Intermediate Layer (CIL) is a distinctive feature of the Baltic Sea thermohaline structure: it is evident annually from March to December between the upper freshened layer and the deep layer with saline waters of oceanic origin. Water parameters are vertically inhomogeneous within the CIL, reflecting a complicated process of its formation that hasn’t been exhaustively described yet. On the base of field measurements and remote sensing data, a detailed study of the CIL formation in spring 2006 was performed. It showed that the core of the CIL (the coldest water) has a specific T,S-index that indicates the source of these waters in the south-western part of the sea. The data of CTD measurements, satellite-derived sea surface temperature (SST), and meteorological information were used to distinguish and trace the CIL core waters. The abrupt transformation of the SST field was observed in March 2006 when the coldest waters (0.9–2.1 °C) of moderate salinity (7.6–8.1) plunged from the upper mixed layer in the vicinity of Bornholm Island into intermediate layers. These waters were later identified as the sublayer of the CIL in the Baltic Proper in April and May 2006. The drivers of the penetration of winter-cold surface waters into the intermediate layers are suggested to be the negative surface buoyancy flux due to warming of cold surface waters with temperature below that of the density maximum and the intensification of easterly winds. Considering the importance of the CIL formation process for the entire Baltic Sea conveyor belt, these CIL core waters were termed the Bornholm Intermediate Waters (BIW). The T,S-index of the BIW shall vary from year to year, reflecting the severity of the past winter and the conditions of the particular spring. However, the BIW location right above the pycno/halocline, the lowest (specific for the current year) water temperature and its characteristic salinity of 7.6–8.1 are repeatedly confirmed by field observations in the Baltic Proper in spring. Joint application of satellite observations, field measurements, and meteorological data provides a coherent view on the process of the BIW waters origination. The results are particularly important it the context of growing hypoxia of deep Baltic waters, providing evidence of the mechanism of seasonal ventilation of intermediate water layers.

Variation of blubber thickness for three marine mammal species in the southern Baltic Sea.

Evaluating populational trends of health condition has become an important topic for marine mammal populations under the Marine Strategy Framework Directive (MSFD). In the Baltic Sea, under the recommendation of Helsinki Commission (HELCOM), efforts have been undertaken to use blubber thickness as an indicator of energy reserves in marine mammals. Current values lack geographical representation from the entire Baltic Sea area and a large dataset is only available for grey seals (Halichoerus grypus) from Sweden and Finland. Knowledge on variation of blubber thickness related to geography throughout the Baltic Sea is important for its usage as an indicator. Such evaluation can provide important information about the energy reserves, and hence, food availability. It is expected that methodological standardization under HELCOM should include relevant datasets with good geographical coverage that can also account for natural variability in the resident marine mammal populations. In this study, seasonal and temporal trends of blubber thickness were evaluated for three marine mammal species-harbor seal (Phoca vitulina), grey seal (Halichoerus grypus) and harbor porpoise (Phocoena phocoena)-resident in the southern Baltic Sea collected and investigated under stranding networks. Additionally, the effects of age, season and sex were analyzed. Seasonal variation of blubber thickness was evident for all species, with harbor seals presenting more pronounced effects in adults and grey seals and harbor porpoises presenting more pronounced effects in juveniles. For harbor seals and porpoises, fluctuations were present over the years included in the analysis. In the seal species, blubber thickness values were generally higher in males. In harbor seals and porpoises, blubber thickness values differed between the age classes: while adult harbor seals displayed thicker blubber layers than juveniles, the opposite was observed for harbor porpoises. Furthermore, while an important initial screening tool, blubber thickness assessment cannot be considered a valid methodology for overall health assessment in marine mammals and should be complemented with data on specific health parameters developed for each species.

Pollen in the Baltic Sea as viewed from space

Multi-sensor satellite images often show recurrent image slicks in the Baltic Sea. Based on historical reports, laboratory experiments, and spectral diagnostics, these image slicks are determined to be caused by pine (Pinus sylvestris) pollen aggregations as opposed to surface scums of cyanobacteria or other floating matters. This attribution is because the reflectance spectral shapes of these image slicks resemble those of pollen grains, which are all different from other floating matters. They all show rapid reflectance increase from ∼400 to ∼500 nm, beyond which the reflectance shape appears to plateau. During the 22-year period of 2000–2021, satellite images indicated large amounts of pollen grains in 14 years, with the earliest day being May 10 (in 2000) and the latest day being June 16 (in 2006). The longest duration in a single year is 22 days, from May 12 to June 2 of 2018. The waters containing pollen grains have expanded significantly in recent years, encompassing nearly the entire Baltic Sea. Because pollen grains contain a significant amount of carbon and have distinctive optical properties from other particles, these findings have significant implications on carbon sequestration, marine ecology, bio-optics, ocean color calibration/validation, and remote sensing of marine debris.

Ten years of remote sensing and analyses of the Baltic Sea primary production (2010–2019)

The aim of the work is to present the possibilities of using advanced systems combining data obtained by remote sensing methods with data from other sources and bio-optical models for tracking and analyzing long-term changes in marine ecosystems. For this purpose, the 2010–2019 primary production data set provided by the SatBałtyk System, a platform that monitors the Baltic Sea based on satellite data, was used. As a result, the paper provides a quantitative description of the variability of the organic matter primary production (PP) in the Baltic Sea. In the individual months, the mean daily PP for the entire Baltic Sea, ranges from ca. 5 mgC m-2day-1 in winter (December and January) to over 700 mgC m-2day-1 in July. The overall annual PP of this basin ranges from 37 to 45 *106 tC year-1, but it is clearly noticeable that the yearly distributions of PP vary significantly from year to year over the 2010–2019 period and different regions. On the basis of the presented results for selected regions, it can be also seen that the maximum values of PP characteristic for the Baltic Sea in the summer months in the Gdańsk Basin can be observed from June to July. However, in the Bornholm Basin and the East Gotland Basin, PP does not peak until July and occasionally in August. The obtained results indicate a slight increase in the productivity of the Baltic Sea (ca. 11%) over the 10 years’ period and it also varies regionally.

Chronic Oil Pollution from Vessels and Its Role in Background Pollution in the Southeastern Baltic Sea

The results of long-term satellite monitoring of oil pollution of the sea surface in the southeastern Baltic Sea (SEB) are discussed in this paper. From June 2004 to December 2020, in total, 2780 Synthetic Aperture Radar (SAR) images from different satellites were received and analyzed. There were 788 oil spills detected in the study area. The oil spills were concentrated along the main shipping routes in the SEB. The volume of the detected oil spills was estimated. The average size of the spill was about 2 km2 or 0.8 m3. Seasonal variability of oil pollution shows a decrease in the number of oil detections in the autumn–winter period, which is associated with the prevalence of unfavorable wind conditions that limit the use of SAR technology for oil spill detection and navigation for small ships. In situ measurements show that seasonal variation in the concentration of oil products in seawater is characterized by a maximum in April and a minimum in July. Since 2007, a decrease in oil detections has been observed for the entire Baltic Sea, including the study area. The interannual variability also shows a decrease in the concentration of oil products in the water column. In the southeastern Baltic Sea, the volume of oil products released yearly to the sea surface from ships does not exceed 0.1% of the average instantaneous presence of oil products in the water column.

Emission, Transport, and Deposition of visible Plastics in an Estuary and the Baltic Sea\u2014a Monitoring and Modeling Approach

Aimwas to assess whether a comprehensive approach linking existing knowledge with monitoring and modeling can provide an improved insight into coastal and marine plastics pollution. We focused on large micro- and mesoplastic (1–25 mm) and selected macroplastic items. Emission calculations, samplings in the Warnow river and estuary (water body and bottom sediments) and a flood accumulation zone monitoring served as basis for model simulations on transport and behavior in the entire Baltic Sea. Considered were the most important pathways, sewage overflow and stormwater. The coastline monitoring together with calculations allowed estimating plastics emissions for Rostock city and the Warnow catchment. Average concentrations at the Warnow river mouth were 0.016 particles/m³ and in the estuary 0.14 particles/m³ (300 µm net). The estuary and nearby Baltic Sea beaches were hot-spots for plastic accumulation with 6–31 particles/m². With increasing distance from the estuary, the concentrations dropped to 0.3 particles/m². This spatial pattern, the plastic pollution gradients and the observed annual accumulation values were consistent with the model results. Indicator items for sewer overflow and stormwater emissions exist, but were only found at low numbers in the environment. The considered visible plastics alone can hardly serve as indicator for microplastic pollution (<1 mm). The use of up-scaled emission data as input for Baltic Sea model simulations provided information on large scale emission, transport and deposition patterns of visible plastics. The results underline the importance of plastic retention in rivers and estuaries.

Increasing the cost-effectiveness of nutrient reduction targets using different spatial scales

In this paper, we investigate the potential gains in cost-effectiveness from changing the spatial scale at which nutrient reduction targets are set for the Baltic Sea, with particular focus on nutrient loadings from agriculture. The costs of achieving loading reductions are compared across five levels of spatial scale, namely the entire Baltic Sea; the marine basin level; the country level; the watershed level; and the grid square level. A novel highly-disaggregated model, which represents decreases in agricultural profits, changes in root zone N concentrations and transport to the Baltic Sea is used. The model includes 14 Baltic Sea marine basins, 14 countries, 117 watersheds and 19,023 10-by-10 km grid squares. The main result which emerges is that there is a large variation in the total cost of the program depending on the spatial scale of targeting: for example, for a 40% reduction in loads, the costs of a Baltic Sea-wide target is nearly three times lower than targets set at the smallest level of spatial scale (grid square). These results have important implications for both domestic and international policy design for achieving water quality improvements where non-point pollution is a key stressor of water quality.

Integrated ecosystem impacts of climate change and eutrophication on main Baltic fishery resources

The Baltic Sea is a heavily impacted ecosystem with multiple pressures acting simultaneously. In order to quantify ecosystem impacts of integrated climate change and eutrophication pressures under constant high fishing pressure, and to support decision-making and policies in generating environmental and economic sustainable systems, the Baltic Atlantis holistic and mechanistic ecosystem model was applied. The overall aim was to run scenarios of separate and integrated impacts of climate and riverine nutrient load changes, taking into account the interactions of the full food web in the entire Baltic Sea. This was done to identify which of those two pressures will likely dominate the future of the Baltic Sea ecosystem, and to test effects of different riverine nutrient forcing sources as well as the Baltic Atlantis functions in relation to hydrographic spawning thresholds. By integrating the hydrography, the biology covering all trophic levels of the food web, and multiple pressures, i.e. eutrophication, climate change and fishery, we were able to evaluate relative impacts of 3 climate scenarios and 3 nutrient load scenarios, using two sources of nutrient forcing and predict likely trends in ecosystem effects. With focus on major fish stocks, our model, with its assumptions, indicated that nutrient loads are the main driver of the changes in the ecosystem as long as the hydrographic thresholds for spawning are not reached. If the thresholds are reached for the Baltic cod, climate change impact will become most important. Furthermore, higher nutrient loads resulted in cod decrease, and increase in sprat and herring. This effect is amplified by stronger climate change. Overall, it is of crucial importance for the future of the Baltic Sea fisheries and stocks that potential impacts are considered both separate and integrated in a dynamic ecosystem-based management approach.

Urban Microplastics Emissions: Effectiveness of Retention Measures and Consequences for the Baltic Sea

Urban sewage water pathways seem most important for microplastics emissions to the Baltic Sea. We use microplastics emission data for the entire Baltic Sea region, calculate emissions for three sewage water related urban pathways and develop emission scenarios for the majority of microplastics particles. All plastics are divided into potentially floating (density 0.8–1.0 g/cmł) and sinking (1.1–1.5 g/cmł) polymers and we address the size class of 20–500 μm. 6.7 × 1013 microplastics particles enter the Baltic Sea annually from urban pathways. 62% result from stormwater runoff including sewer overflow, 25% from wastewater treatment plants (WWTPs) and 13% from untreated wastewater. The emission scenarios serve as input for 3D-model simulations, which allow estimating transport, behaviour and deposition in the Baltic Sea environment. Our model approach suggests average annual microplastics concentrations in the water body of the central Baltic Sea of 1–4 particles/m2 sea surface and 1 particle/m3 in the upper 2 m sea surface layer. The majority of the particles is accumulated in upper sea surface layers. The model suggests that only between 6% (Arcona Basin) and 21% (Gotland Basin) of the particles are below a depth of 25 m. In coastal waters, the concentrations can exceed 10 particles/m3 in the upper 2 m surface water layer (e.g., Gulf of Riga, Gulf of Gdańsk) and 1 particle/m2 on the sediment surface. Usually within weeks, emitted microplastics are washed ashore causing annual coastal accumulations of up to 109 particles/m coastline within a few kilometres distance to emission sources. On average, above 106 particles/m are annually accumulated and trapped at coasts around the Baltic Sea. The reduction of the annual sewer overflow from presently 1.5% of the annual wastewater loads to 0.3% would reduce the total emissions to the Baltic Sea by 50%. If all sewage water would be connected to WWTPs and undergo a tertiary treatment, a reduction of 14.5% of the total emissions could be achieved. The effect of retention in rivers seems limited in the Baltic Sea region, because near coast emissions contribute around 50% of the total microplastics emissions.

Seasonal variability, long-term distribution (2001\u20132014), and risk assessment of polar organic micropollutants in the Baltic Sea

From 2001 to 2014, 13 surveys were conducted in the Baltic Sea, to determine its pollution of 50 micropollutants. The investigations focused mostly on the German western Baltic Sea; in 2008, one survey covered the entire Baltic Sea. Various groups of herbicides (such as triazines, phenoxyacetic acid, phenylurea), perfluoroalkyl substances, pharmaceuticals, and industrial products were analyzed during these surveys. The highest concentrations (median 1 to 4 ng/L) were observed for atrazine, simazine, chloridazone, 2,4-dichlorophenoxyacetic acid, benzotriazole, primidone, and carbamazepine. Most micropollutants exhibited a relatively homogenous spatial distribution, though some herbicides show elevated concentrations in certain regions (e.g., Odra estuary), indicating a riverine input. The data set was analyzed, both for seasonal influences and long-time trends. Some herbicides exhibited higher concentrations during summertime. Both upward- and downward-directed time trends could be identified for some herbicides and perfluorinated compounds. For most of the detected compounds, a low-risk quotient was calculated. Only the occurrence of carbendazim could potentially pose a higher risk to the Baltic Sea.

The Baltic Health Index (BHI): Assessing the social–ecological status of the Baltic Sea

Abstract Improving the health of coastal and open sea marine ecosystems represents a substantial challenge for sustainable marine resource management, since it requires balancing human benefits and impacts on the ocean. This challenge is often exacerbated by incomplete knowledge and lack of tools that measure ocean and coastal ecosystem health in a way that allows consistent monitoring of progress towards predefined management targets. The lack of such tools often limits capabilities to enact and enforce effective governance. We introduce the Baltic Health Index (BHI) as a transparent, collaborative and repeatable assessment tool. The Index complements existing, more ecological‐oriented, approaches by including a human dimension on the status of the Baltic Sea, an ecosystem impacted by multiple anthropogenic pressures and governed by a multitude of comprehensive national and international policies. Using a large amount of social–ecological data available, we assessed the health of the Baltic Sea for nine goals that represent the status towards set targets, for example, clean waters, biodiversity, food provision, natural products extraction and tourism. Our results indicate that the overall health of the Baltic Sea is suboptimal (a score of 76 out of 100), and a substantial effort is required to reach the management objectives and associated targets. Subregionally, the lowest BHI scores were measured for carbon storage, contaminants and lasting special places (i.e. marine protected areas), albeit with large spatial variation. Overall, the likely future status of all goals in the BHI averaged for the entire Baltic Sea is better than the present status, indicating a positive trend towards a healthier Baltic Sea. However, in some Baltic Sea basins, the trend for specific goals was decreasing, highlighting locations and issues that should be the focus of management priorities. The BHI outcomes can be used to identify both pan‐Baltic and subregional scale management priorities and to illustrate the interconnectedness between goals linked by cumulative pressures. Hence, the information provided by the BHI tool and its further development will contribute towards the fulfilment of the UN Agenda 2030 and its Sustainability Development Goals. A free Plain Language Summary can be found within the Supporting Information of this article.

Remote Sensing Supported Sea Surface pCO2 Estimation and Variable Analysis in the Baltic Sea

Marginal seas are a dynamic and still to large extent uncertain component of the global carbon cycle. The large temporal and spatial variations of sea-surface partial pressure of carbon dioxide (pCO2) in these areas are driven by multiple complex mechanisms. In this study, we analyzed the variable importance for the sea surface pCO2 estimation in the Baltic Sea and derived monthly pCO2 maps for the marginal sea during the period of July 2002–October 2011. We used variables obtained from remote sensing images and numerical models. The random forest algorithm was employed to construct regression models for pCO2 estimation and produce the importance of different input variables. The study found that photosynthetically available radiation (PAR) was the most important variable for the pCO2 estimation across the entire Baltic Sea, followed by sea surface temperature (SST), absorption of colored dissolved organic matter (aCDOM), and mixed layer depth (MLD). Interestingly, Chlorophyll-a concentration (Chl-a) and the diffuse attenuation coefficient for downwelling irradiance at 490 nm (Kd_490nm) showed relatively low importance for the pCO2 estimation. This was mainly attributed to the high correlation of Chl-a and Kd_490nm to other pCO2-relevant variables (e.g., aCDOM), particularly in the summer months. In addition, the variables’ importance for pCO2 estimation varied between seasons and sub-basins. For example, the importance of aCDOM were large in the Gulf of Finland but marginal in other sub-basins. The model for pCO2 estimate in the entire Baltic Sea explained 63% of the variation and had a root of mean squared error (RMSE) of 47.8 µatm. The pCO2 maps derived with this model displayed realistic seasonal variations and spatial features of sea surface pCO2 in the Baltic Sea. The spatially and seasonally varying variables’ importance for the pCO2 estimation shed light on the heterogeneities in the biogeochemical and physical processes driving the carbon cycling in the Baltic Sea and can serve as an important basis for future pCO2 estimation in marginal seas using remote sensing techniques. The pCO2 maps derived in this study provided a robust benchmark for understanding the spatiotemporal patterns of CO2 air-sea exchange in the Baltic Sea.

Quantifying Contemporary Organic Carbon Stocks of the Baltic Sea Ecosystem

The identification of carbon pools and the quantification of carbon stocks is necessary to (1) track changes in ecosystem dynamics, (2) inform science-based ecosystem and blue-carbon management, and (3) evaluate ecosystem and food web models. However, estimates of organic carbon stocks in marine ecosystems are incomplete or inconsistent. Therefore, we provide a first consistent estimate of relevant organic carbon stocks of a distinct marine ecosystem- the Baltic Sea. We estimate its contemporary standing stocks of 18 non-living and living organic carbon pools using data from literature and open-access databases. In contrast to existing data, our estimates are valid for the entire Baltic Sea, include necessary pools and are verifiable, as we describe data sources, methods and the associated uncertainties in detail to allow reproduction and critical evaluation. The total organic carbon (TOC) in the Baltic Sea ecosystem amounts to 1,050 ± 90 gC/m2 (440 ± 40 Mt). The non-living stocks account for about 98.8% and the living stocks for 1.2% of the TOC. Our estimates indicate that benthos has the highest living organic carbon stock and that the stock of particulate organic carbon (POC) has been underestimated in some previous studies. In addition, we find a partially inverted biomass distribution with a higher stock of primary consumers than primary producers. Our estimates provide a baseline of the size and distribution of the organic carbon in the Baltic Sea for the current period. Analyses of inorganic carbon stocks and the interplay between inorganic and organic stocks must follow to further define the baseline of total carbon stocks in the Baltic Sea.