Much of the Baltic Sea is currently classified as 'affected by eutrophication'. The causes for this are twofold. First, current levels of nutrient inputs (nitrogen and phosphorus) from human activities exceed the natural processing capacity with an accumulation of nutrients in the Baltic Sea over the last 50-100 years. Secondly, the Baltic Sea is naturally susceptible to nutrient enrichment due to a combination of long retention times and stratification restricting ventilation of deep waters. Here, based on a unique data set collated from research activities and long-term monitoring programs, we report on the temporal and spatial trends of eutrophication status for the open Baltic Sea over a 112-year period using the HELCOM Eutrophication Assessment Tool (HEAT 3.0). Further, we analyse variation in the confidence of the eutrophication status assessment based on a systematic quantitative approach using coefficients of variation in the observations. The classifications in our assessment indicate that the first signs of eutrophication emerged in the mid-1950s and the central parts of the Baltic Sea changed from being unaffected by eutrophication to being affected. We document improvements in eutrophication status that are direct consequences of long-term efforts to reduce the inputs of nutrients. The reductions in both nitrogen and phosphorus loads have led to large-scale alleviation of eutrophication and to a healthier Baltic Sea. Reduced confidence in our assessment is seen more recently due to reductions in the scope of monitoring programs. Our study sets a baseline for implementation of the ecosystem-based management strategies and policies currently in place including the EU Marine Strategy Framework Directives and the HELCOM Baltic Sea Action Plan.

It is estimated that up to half of global methane (CH4) emissions are derived from microbial processes in aquatic ecosystems. However, it is not fully understood which factors explain the spatial and temporal variability of these emissions. For example, light has previously been shown to both inhibit and stimulate aerobic methane-oxidizing bacteria (i.e., methanotrophs) in the water column. These contrasting results indicate that the mechanisms that light has on CH4 oxidation are not yet clearly known, even less so for benthic aerobic methanotrophs. Here, we tested whether light reaching the seafloor can inhibit methanotrophic activity on the sediment surface. We sampled and distributed over 40 intact sediment cores from two coastal sites (illuminated 10 m, and a dark site at 33 m water depth) into 0, 50, and 100 PAR light treatments. After 10 days, we found no difference between treatments for each site in pore-water CH4 concentrations, relative abundance of aerobic methanotrophs, or the number of RNA transcripts related to methane oxidation. Our results suggest that light attenuation in coastal waters does not significantly affect aerobic methanotrophs in coastal sediments.

Deoxygenation is a global problem in coastal and open regions of the ocean, and has led to expanding areas of oxygen minimum zones and coastal hypoxia. The recent expansion of hypoxia in coastal ecosystems has been primarily attributed to global warming and enhanced nutrient input from land and atmosphere. The largest anthropogenically induced hypoxic area in the world is the Baltic Sea, where the relative importance of physical forcing versus eutrophication is still debated. We have analyzed water column oxygen and salinity profiles to reconstruct oxygen and stratification conditions over the last 115 y and compare the influence of both climate and anthropogenic forcing on hypoxia. We report a 10-fold increase of hypoxia in the Baltic Sea and show that this is primarily linked to increased inputs of nutrients from land, although increased respiration from higher temperatures during the last two decades has contributed to worsening oxygen conditions. Although shifts in climate and physical circulation are important factors modulating the extent of hypoxia, further nutrient reductions in the Baltic Sea will be necessary to reduce the ecosystems impacts of deoxygenation.

Coastal pH and total alkalinity are regulated by a diverse range of local processes superimposed on global trends of warming and ocean acidification, yet few studies have investigated the relative importance of different processes for coastal acidification. We describe long-term (1972-2016) and seasonal trends in the carbonate system of three Danish coastal systems demonstrating that hydrological modification, changes in nutrient inputs from land, and presence/absence of calcifiers can drastically alter carbonate chemistry. Total alkalinity was mainly governed by conservative mixing of freshwater (0.73-5.17mmolkg(-1)) with outer boundary concentrations (similar to 2-2.4mmolkg(-1)), modulated seasonally and spatially (similar to 0.1-0.2mmolkg(-1)) by calcifiers. Nitrate assimilation by primary production, denitrification, and sulfate reduction increased total alkalinity by almost 0.6mmolkg(-1) in the most eutrophic system during a period without calcifiers. Trends in pH ranged from -0.0088year(-1) to 0.021year(-1), the more extreme of these mainly driven by salinity changes in a sluice-controlled lagoon. Temperature increased 0.05 degrees Cyr(-1) across all three systems, which directly accounted for a pH decrease of 0.0008year(-1). Accounting for mixing, salinity, and temperature effects on dissociation and solubility constants, the resulting pH decline (0.0040year(-1)) was about twice the ocean trend, emphasizing the effect of nutrient management on primary production and coastal acidification. Coastal pCO(2) increased similar to 4 times more rapidly than ocean rates, enhancing CO2 emissions to the atmosphere. Indeed, coastal systems undergo more drastic changes than the ocean and coastal acidification trends are substantially enhanced from nutrient reductions to address coastal eutrophication.

Recent studies on marine life show that the anthropogenic increase in atmospheric CO2 concentration can have negative impacts on growth and survival of groups of marine life such as corals and other calcifying organisms.

Increased CO2 concentration in the atmosphere, and hence in the oceans, lead to decreasing pH or increasing acidification, a process known as ocean acidification (OA). During the last century, the CO2 concentration in the atmosphere has risen from around 280 ppm to 400 ppm; this has led to a pH decrease in the oceans of 0.1. OA currently takes place at a rate corresponding to approximately -0.02 pH unit per decade and an increase in CO2 at around 2 ppm per year. The projections for atmospheric CO2 concentration is an increase to around 1000 ppm at the end of the century, which will lower pH in the oceans by 0.3-0.4. Although this may appear relatively small, a decrease in pH of 0.1 corresponds to an increase in acidity (“free” protons) of 25%, and 0.3-0.4 corresponds to an increase of 200-250%.

Coastal systems experience changes in pH over time exceeding those of the ocean by several orders of magnitude, but the field is poorly studied, and the spatial variation is large. The Baltic Sea is no exception to this. pH changes in the Baltic Sea are tightly coupled to nutrient input, alkalinity (AT) of freshwater sources in addition to increased CO2 levels and warming. Acidification trends vary substantially among coastal systems and time of year, but have been reported up to 10 times faster than OA. 

The TRIACID project has mapped acidification trends in the Baltic Sea during the past 40 years, in different regions, and identified areas with a general lack of data. The project has described spatial variation and trends  in pH status, and the main drivers of changing pH have been identified. Given the spatial variation, the data gaps, and all the different drivers a detailed projection of the development is complicated but since we expect increasing CO2 concentration in the atmosphere, rising temperatures and decreasing nutrient input, the acidification trend will continue or accelerate in most of the region.

Hypoxia has occurred intermittently over the Holocene in the Baltic Sea, but the recent expansion from less than 10 000 km(2) before 1950 to > 60 000 km(2) since 2000 is mainly caused by enhanced nutrient inputs from land and atmosphere. With worsening hypoxia, the role of sediments changes from nitrogen removal to nitrogen release as ammonium. At present, denitrification in the water column and sediments is equally important. Phosphorus is currently buried in sediments mainly in organic form, with an additional contribution of reduced Fe-phosphate minerals in the deep anoxic basins. Upon the transition to oxic conditions, a significant proportion of the organic phosphorus will be remineralized, with the phosphorus then being bound to iron oxides. This iron-oxide bound phosphorus is readily released to the water column upon the onset of hypoxia again. Important ecosystems services carried out by the benthic fauna, including biogeochemical feedback-loops and biomass production, are also lost with hypoxia. The results provide quantitative knowledge of nutrient release and recycling processes under various environmental conditions in support of decision support tools underlying the Baltic Sea Action Plan.

Submarine groundwater discharge (SGD) is an important process responsible for transporting terrestrial dissolved chemical substances into the coastal ocean, thereby impacting the marine ecosystem. Despites its significance, there are few studies addressing SGD in the northern Baltic Sea. Here we investigate the potential occurrence of SGD in an area characterized by seafloor terraces formed in varved glacial clay located around Fifång Island, Southern Stockholm Archipelago. We analyzed 222Rn activity and porewater geochemistry in both marine and terrestrial sediment cores retrieved from Fifång Island and its surrounding offshore areas. Results from 222Rn mass-balance calculations, water isotopes, salinity, chloride concentration, and dating (including 14C and helium-tritium dating) indicate that modern groundwater flows through varved glacial clay layers and fractured rocks on Fifång Island and discharges into Fifång Bay. Additionally, the offshore cores reveal a saline groundwater source that, dating of the dissolved inorganic carbon, appears systematically younger than the hosting clay varves dated using the Swedish clay varve chronology. Acoustic blanking in our acquired sub-bottom profiles may be related to this fluid migration. The occurrence of this saline groundwater seems to be independent from the distance to the submarine terraces. Collectively, our study confirms the occurrence of submarine groundwater in the varved glacial clay close to Fifång Island and further offshore. Our findings help establish the significance of submarine groundwater discharge in influencing the past and present coastal environment in the Baltic Sea region.

Intensive agriculture has been linked to increased nitrogen loads and adverse effects on downstream aquatic ecosystems. Sustained large net nitrogen surpluses have been shown in several contexts to form legacies in soil or waters, which delay the effects of reduction measures. In this study, detailed land use and agricultural statistics were used to reconstruct the annual nitrogen surpluses in three agriculture-dominated watersheds of Denmark (600-2700 km2) with well-drained loamy soils. These surpluses and long-term hydrological records were used as inputs to the process model ELEMeNT to quantify the nitrogen stores and fluxes for 1920-2020. A multi-objective calibration using timeseries of river nitrate loads, as well as other non-conventional data sources, allowed to explore the potential of these different data to constrain the nitrogen cycling model. We found the flux-weighted nitrate concentrations in the root zone percolate below croplands, a dataset not commonly used in calibrating watershed models, to be critical in reducing parameter uncertainty. Groundwater nitrate legacies built up in all three studied watersheds during 1950-1990 corresponding to & SIM;2% of the surplus (or & SIM;1 kg N ha yr-1) before they went down at a similar rate during 1990-2015. Over the same periods active soil nitrogen legacies first accumulated by approximately 10% of the surplus (& SIM;5 kg N ha yr-1), before undergoing a commensurate reduction. Both legacies appear to have been the drivers of hysteresis in the diffuse load at the catchments' outlet and hindrances to reaching water quality goals. Results indicate that the low cropland surpluses enforced during 2008-2015 had a larger impact on the diffuse river loads than the European Union's untargeted grass set-aside policy of 1993-2008. Collectively, the measures of 1990-2015 are estimated to have reset the diffuse load regimes of the watersheds back to the situation prevailing in the 1960s.

Experimental studies of intact sediment cores from the Gulf of Riga, Baltic Sea, were conducted to estimate the response of sediment nutrient fluxes to various near-bottom water oxygen conditions. The experiment was performed in the laboratory using a batch-mode assay type system on the sediment cores held at 4 degrees C and oxygen concentrations maintained at 1, 2, 3, 4 and 5 mg l(-1). The results from the experiment were subsequently used to optimise the fit of the sediment denitrification sub-model of the Gulf of Riga basin. Sedimentwater fluxes of phosphate were low and directed out of the sediments under all treatments, demonstrating a general decreasing tendency with increasing near-bottom water oxygen concentration. The sediment-water fluxes of ammonium and nitrate + nitrite demonstrated opposing trends: ammonium fluxes decreased whereas nitrate + nitrite fluxes increased with rising near-bottom water oxygen concentration. The modelled fluxes agreed well with the measured ones, with correlation coefficients of 0.75, 0.63 and 0.88 for ammonium, nitrate + nitrite and phosphate fluxes respectively. The denitrification rate in sediments was simulated at oxygen concentrations from -2 to 10 mg l(-1). At oxygen concentrations < 2 mg l(-1) the modelled denitrification was sustained by nitrate transport from water overlying the sediments. With increasing oxygen concentrations the simulated denitrification switched from the process fuelled by nitrates originating from the overlying water (D-w) to one sustained by nitrates originating from the coupled sedimentary nitrification - denitrification (D-n). D-n reached its maximum at an oxygen concentration of 5 mg l(-1).

Even though the effects of benthic fauna on aquatic biogeochemistry have been long recognized, few studies have addressed the combined effects of animal bioturbation and metabolism on ecosystem–level carbon and nutrient dynamics. Here we merge a model of benthic fauna (BMM) into a physical–biogeochemical ecosystem model (BALTSEM) to study the long-term and large-scale effects of benthic fauna on nutrient and carbon cycling in the Baltic Sea. We include both the direct effects of faunal growth and metabolism and the indirect effects of its bioturbating activities on biogeochemical fluxes of and transformations between organic and inorganic forms of carbon (C), nitrogen (N), phosphorus (P) and oxygen (O). Analyses of simulation results from the Baltic Proper and Gulf of Riga indicate that benthic fauna makes up a small portion of seafloor active organic stocks (on average 1 %–4 % in 2000–2020) but contributes considerably to benthic–pelagic fluxes of inorganic C (23 %–31 %), N (42 %–51 %) and P (25 %–34 %) through its metabolism. Results also suggest that the relative contribution of fauna to the mineralization of sediment organic matter increases with increasing nutrient loads. Further, through enhanced sediment oxygenation, bioturbation decreases benthic denitrification and increases P retention, the latter having far-reaching consequences throughout the ecosystem. Reduced benthic–pelagic P fluxes lead to a reduction in N fixation and primary production, lower organic matter sedimentation fluxes, and thereby generally lower benthic stocks and fluxes of C, N and P. This chain of effects through the ecosystem overrides the local effects of faunal respiration, excretion and bioturbation. Due to large uncertainties related to the parameterization of benthic processes, we consider this modelling study a first step towards disentangling the complex ecosystem-scale effects of benthic fauna on biogeochemical cycling.

Dynamic model simulations of the future climate and projections of future lifestyles within the Baltic Sea Drainage Basin (BSDB) were considered in this study to estimate potential trends in future nutrient loads to the Baltic Sea. Total nitrogen and total phosphorus loads were estimated using a simple proxy based only on human population (to account for nutrient sources) and stream discharges (to account for nutrient transport). This population-discharge proxy provided a good estimate for nutrient loads across the seven sub-basins of the BSDB considered. All climate scenarios considered here produced increased nutrient loads to the Baltic Sea over the next 100 years. There was variation between the climate scenarios such that sub-basin and regional differences were seen in future nutrient runoff depending on the climate model and scenario considered. Regardless, the results of this study indicate that changes in lifestyle brought about through shifts in consumption and population potentially overshadow the climate effects on future nutrient runoff for the entire BSDB. Regionally, however, lifestyle changes appear relatively more important in the southern regions of the BSDB while climatic changes appear more important in the northern regions with regards to future increases in nutrient loads. From a whole-ecosystem management perspective of the BSDB, this implies that implementation of improved and targeted management practices can still bring about improved conditions in the Baltic Sea in the face of a warmer and wetter future climate

Microbial sulfate reduction (MSR), which transforms sulfate into sulfide through the consumption of organic matter, plays an integral part in sulfur and carbon cycling. Yet, its magnitude beyond snap-shot conditions in specific surface water systems such as individual wetlands or lakes is with few exceptions unknown. We use stream water samples and a sulfur isotopic fractionation and mixing scheme to derive MSR at relatively large scales, both within and between five study areas located between southern Sweden and the Kola Peninsula, Russia. Results showed that the freshwater MSR ranged from 0 to 79% within the catchments and from 2 to 28% between the catchments, displaying a catchment-average value of 13%. The MSR-values showed differences between seasons, in particular in wetland/lake catchments where they were high during the spring flood, which is consistent with the mobilization of water that under low-flow winter periods can have developed anoxic conditions needed by the sulfate-reducing microorganisms (SRM). The combined abundance or deficiency of several landscape elements were found to indicate relatively well whether or not catchment-scale MSR would be high, such as the percentage of forest cover that can support MSR by the provision of organic matter to the SRM, and the areal percentage of lakes/wetlands providing longer hydrological residence times and the anoxic conditions needed by the SRM. This study, for the first time, shows compelling evidence in multiple catchments of MSR at levels around 10%, which is unaccounted for in global weathering budgets, implying for instance that the terrestrial pyrite oxidation may be underestimated.

Coastal seas receive large amounts of terrestrially derived organic carbon (OC). The fate of this carbon, and its impact on the marine environment, is however poorly understood. Here we combine underway CO₂ partial pressure (pCO₂) measurements with coupled 3D hydrodynamical-biogeochemical modelling to investigate whether remineralization of terrestrial dissolved organic carbon (tDOC) can explain CO₂ supersaturated surface waters in the Gulf of Bothnia, a subarctic estuary. We find that a substantial remineralization of tDOC, and that a strong tDOC induced light attenuation dampening the primary production, is required to reproduce the observed CO₂ supersaturated waters in the nearshore areas. A removal rate of tDOC of the order of one year, estimated in a previous modelling study in the same area, gives a good agreement between modelled and observed pCO₂. The remineralization rate is on the same order as bacterial degradation rates calculated from published incubation experiments, suggesting that this remineralization could be caused by bacterial degradation. Furthermore, the observed high pCO₂ values during the ice covered season argues against photochemical degradation as the main removal mechanism. All of the remineralized tDOC is outgassed to the atmosphere in the model, turning the northernmost part of the Gulf of Bothnia to a source of atmospheric CO₂.

High inputs of nutrients and organic matter make coastal seas places of intense air‐sea CO₂ exchange. Due to their complexity, the role of coastal seas in the global air‐sea CO₂ exchange is, however, still uncertain. Here, we investigate the role of phytoplankton stoichiometric flexibility and extracellular DOC production for the seasonal nutrient and CO₂ partial pressure (pCO₂) dynamics in the Gulf of Bothnia, Northern Baltic Sea. A 3‐D ocean biogeochemical‐physical model with variable phytoplankton stoichiometry is for the first time implemented in the area and validated against observations. By simulating non‐Redfieldian internal phytoplankton stoichiometry, and a relatively large production of extracellular dissolved organic carbon (DOC), the model adequately reproduces observed seasonal cycles in macronutrients and pCO₂. The uptake of atmospheric CO₂ is underestimated by 50% if instead using the Redfield ratio to determine the carbon assimilation, as in other Baltic Sea models currently in use. The model further suggests, based on the observed drawdown of pCO₂, that observational estimates of organic carbon production in the Gulf of Bothnia, derived with the method, may be heavily underestimated. We conclude that stoichiometric variability and uncoupling of carbon and nutrient assimilation have to be considered in order to better understand the carbon cycle in coastal seas.

The fate of terrestrial organic matter brought to the coastal seas by rivers, and its role in the global carbon cycle, are still not very well known. Here the degradation rate of terrestrial dissolved organic carbon (DOCter) is studied in the Baltic Sea, a subarctic semi-enclosed sea, by releasing it as a tracer in a 3-D circulation model and applying linear decay constants. A good agreement with available observational data is obtained by parameterizing the degradation in two rather different ways; one by applying a decay time on the order of 10 years to the whole pool of DOCter, and one by dividing the DOCter into one refractory pool and one pool subject to a decay time on the order of 1 year. The choice of parameterization has a significant effect on where in the Baltic Sea the removal takes place, which can be of importance when modeling the full carbon cycle and the CO2 exchange with the atmosphere. In both cases the biogeochemical decay operates on time scales less than the water residence time. Therefore only a minor fraction of the DOCter reaches the North Sea, whereas approximately 80% is removed by internal sinks within the Baltic Sea. This further implies that DOCter mineralization is an important link in land-sea-atmosphere cycling of carbon in coastal- and shelf seas that are heavily influenced by riverine DOC.

The East Siberian Sea and contiguous western Arctic Ocean basin are characterized by a subsurface nutrient maximum in the halocline, generally attributed to both Pacific inflow and intensive remineralization in shelf bottom waters that are advected into the central basin. We report nitrogen and oxygen isotopic measurement of nitrate from the East Siberian Sea and western Eurasian Basin, in order to gain insight into how nitrate is processed by the microbial community and redistributed in the Arctic Ocean. A large decoupling between nitrate delta N-15 and delta O-18 is reported, increasing and decreasing upward from the Atlantic temperature maximum layer toward the surface, respectively. A correlation between water and nitrate delta O-18 indicates that most of the nitrate (> 60%) at the halocline has been regenerated within the Arctic Ocean. The increase in nitrate delta N-15 correlates with the fixed N deficit, indicating a causal link between the loss of fixed N and the delta N-15 enrichment. This suggests that a significant share of benthic denitrification is driven by nitrate supplied by remineralization and partial nitrification, allowing residual delta N-15-enriched ammonium to diffuse out of the sediments. By increasing nutrient concentrations and fixed N deficit in shelf bottom waters, this imprint is attenuated offshore following advection into the halocline by nitrate regeneration and mixing. Estimation of the sedimentary isotope effect related to benthic denitrification yields values in the range of 2.4-3.8 parts per thousand, with its magnitude driven by both the degree of coupling between remineralization and nitrification, and fixed N concentrations in shelf bottom waters.

In the project ECOSUPPORT, three coupled physical-biogeochemical models (ER-GOM, RCO-SCOBI and BALTSEM) are used to produce an ensemble of combine climate change - nutrient load scenarios. The atmospheric driving forces for the models are produced by two global climate models, ECHAM5 and HadCM3; and these results are dynamically downscaled using the RCAO model at the SMHI, see Meier et al. (2011) and references therein. Since, results from dynamic hydrological models were not available, river runoff were constructed from the net P-E balance of the RCAO model results. Nutrient load scenarios wasto be explicitly given, but effects from climate change induced river runoff variations were estimated. In total, four different climate change scenarios produced and four different assumptions on direct anthropogenic changes in nutrient loads, thus in total 16 combinations. The nutrient load scenarios comprised of; one reference with approximately unchanged loads, one pessimistic business as usual and two more optimistic, current legislation and Baltic Sea Action Plan. All the latter based on previous work. This report describes the construction of these nutrient load scenarios.

The main intention of this report is to give a general description of how the marine CO₂ system is incorporated into BALTSEM, a coupled physical-biogeochemical Baltic Sea model . The CO₂ system involves four parameters; dissolved inorganic carbon (DICor CT), total alkalinity (TA or AT), CO₂ partial pressure (pCO₂), and pH. If any two of these parameters are known (together with other state variables such as temperature and salinity), the other two can be calculated. DIC and alkalinity are chosen as model state variables as they are conservative with regard to changes in temperature and salinity (as opposed to pCO₂ and pH that are strongly dependent especially on temperature). A comprehensive description of the marine CO₂ system in general, and the Baltic Sea CO₂ system in particular, is given by Schneider (2011). Furthermore, all definitions and calculations employed are described in detail by Dickson et al. (2007). Alkalinity and DIC are mainly transported to the Baltic Sea from rivers, but also from the North Sea. Alkalinity can be produced or consumed by different biogeochemical processes, and is in addition transported to the North Sea through the open boundary in Northern Kattegat. DIC is assimilated by phytoplankton and again released when organic carbon is mineralized. Burial of freshly produced organic material is a permanent sink for carbon, but the major carbon sink is outflow to the North Sea. Differences in CO₂ partial pressure between atmosphere and surface water together with wind speed determine the CO₂ uptake or outgassing in different sub-basins. The magnitude and direction of air-sea CO₂ exchange varies between sub-basins and also seasonally within basins. The Gulf of Bothnia has earlier been described as a source of CO₂ to the atmosphere, whereas conflicting results have been reported for the Baltic Proper. Formation and precipitation of calcium carbonate (CaCO₃)is a sink term for alkalinity and DIC, whereas dissolution of CaCO₃ on the other hand is a source term for both variables. In the Baltic Proper as well as the eastern and northern gulfs, formation of CaCO₃ seems to be negligible. The reason is believed to be unsaturation of calcite and aragonite mineral forms of CaCO₃ in winter and early spring. In the Kattegat and Belt Sea, calcite seems to be oversaturated throughout the year, and the coccolithophore Emiliania Huxleyicanat times form massive blooms in the Skagerrak-Kattegat area. Nevertheless, precipitation and dissolution of CaCO₃ are at this pointnot included as processes in the model, but may be a topic for future studies.A short description of how calcite and aragonite saturation states (ΩCA and ΩAR) are calculated is presented in Appendix B. The main processes that affect the concentrations of DIC and alkalinity are discussed in detail in this report and followed by a description of model forcing and boundary conditions, and an overview of the available observations of CO₂ system parameters. Model performance on both long-term average scale, and on short-term scale is analysed in terms of comparisons between model results and observations. Finally, budget calculations reveal the overall main source and sink terms of both DIC and alkalinity in the Baltic Sea.

Increasing atmospheric CO₂ drives ocean acidification globally. In coastal seas, acidification trends can however be either counteracted or enhanced by other processes. Ecosystem effects of acidification are so far small in the Baltic Sea, but changes should be anticipated unless CO₂ emissions are curbed. Possible future acidification trends in the Baltic Sea, conditional on CO₂ emissions, climate change, and changes in productivity, can be assessed by means of model simulations. There are uncertainties regarding potential consequences for marine organisms, partly because of difficulties to assign critical thresholds, but also because of knowledge gaps regarding species’ capacity to adapt. Increased temporal and spatial monitoring of inorganic carbon system parameters would allow a better understanding of current acidification trends and also improve the capacity to predict possible future changes. An additional benefit is that such measurements also provide quantitative estimates of productivity. The technology required for precise measurements of the inorganic carbon system is readily available today. Regularly updated status evaluations of acidification, and the inorganic carbon system in general, would support management when assessing climate change effects, eutrophication or characteristics of the pelagic habitats. This would, however, have to be based on a spatially and temporally sufficient monitoring program.

A coupled physical-biogeochemical model (BALTSEM) is used to estimate carbon fluxes in the Baltic Sea over the 1980-2006 period. Budget calculations for organic carbon indicate that of the total allochthonous organic carbon (TOCT) supplied to the system, on average 56% is mineralized, 36% is exported out of the system, and the remainder is buried. River discharge is the main source of dissolved inorganic carbon (DIC) to the Baltic Sea. However, model results indicate that in the Gulf of Bothnia (northern Baltic Sea), the contribution to the DIC stock by TOCT mineralization is of the same order as direct river input of DIC In the Kattegat and Danish Straits (southwestern Baltic Sea) on the other hand, net uptake of atmospheric CO2 comprises the major DIC source. Despite large variations within the system, with net outgassing from some sub-basins and net absorption in others, the Baltic Sea as a whole was estimated to be a net sink for atmospheric CO2. Mineralization of allochthonous dissolved organic carbon (DOCT) influences air-sea CO2 exchange. A sensitivity study indicates that depending on the labile fraction of DOCT, the contribution from CO2 absorption to total external DIC sources can amount to 10-25%.

Future acidification of coastal seas will depend not only on the development of atmospheric CO2 partial pressure (pCO(2)), but also on changes in the catchment areas, exchange with the adjacent ocean, and internal cycling of carbon and nutrients. Here we use a coupled physical-biogeochemical Baltic Sea model to quantify the sensitivity of pH to changes both in external forcing and internal processes. The experiments include changes in runoff, supply of dissolved inorganic carbon (DIC) and total alkalinity (A(T)), nutrient loads, exchange between the Baltic and North Seas, and atmospheric pCO(2). We furthermore address the potential different future developments of runoff and river loads in boreal and continental catchments, respectively. Changes in atmospheric pCO(2) exert the strongest control on future pH according to our calculations. This CO2-induced acidification could be further enhanced in the case of desalination of the Baltic Sea, although increased concentrations of A(T) in the river runoff due to increased weathering to some extent could counteract acidification. Reduced nutrient loads and productivity would reduce the average annual surface water pH but at the same time slightly increase wintertime surface water pH (the annual pH minimum). The response time of surface water pH to sudden changes in atmospheric pCO(2) is approximately one month, whereas response times to changes in e.g. runoff and A(T)/DIC loads are more related to residence times of water and salt (> 30 years). It seems unlikely that the projected future increase in atmospheric pCO(2) and associated pH reduction could be fully counteracted by any of the other processes addressed in our experiments.

Methane (CH4) cycling in the Baltic Sea is studied through model simulations that incorporate the stable isotopes of CH4 (12C-CH4 and 13C-CH4) in a physical-biogeochemical model. A major uncertainty is that spatial and temporal variations in the sediment source are not well known. Furthermore, the coarse spatial resolution prevents the model from resolving shallow-water near-shore areas for which measurements indicate occurrences of considerably higher CH4 concentrations and emissions compared with the open Baltic Sea. A preliminary CH4 budget for the central Baltic Sea (the Baltic Proper) identifies benthic release as the dominant CH4 source, which is largely balanced by oxidation in the water column and to a smaller degree by outgassing. The contributions from river loads and lateral exchange with adjacent areas are of marginal importance. Simulated total CH4 emissions from the Baltic Proper correspond to an average ∼1/41.5 mmol CH4 m-2 yr-1, which can be compared to a fitted sediment source of ∼1/418 mmol CH4 m-2 yr-1. A large-scale approach is used in this study, but the parameterizations and parameters presented here could also be implemented in models of near-shore areas where CH4 concentrations and fluxes are typically substantially larger and more variable. Currently, it is not known how important local shallow-water CH4 hotspots are compared with the open water outgassing in the Baltic Sea.

Enhanced release of alkalinity from the seafloor, principally driven by anaerobic degradation of organic matter under low-oxygen conditions and associated secondary redox reactions, can increase the carbon dioxide (CO₂) buffering capacity of seawater and therefore oceanic CO₂ uptake. The Baltic Sea has undergone severe changes in oxygenation state and total alkalinity (TA) over the past decades. The link between these concurrent changes has not yet been investigated in detail. A recent system-wide TA budget constructed for the past 50 years using BALTSEM, a coupled physical-biogeochemical model for the whole Baltic Sea area, revealed an unknown TA source. Here we use BALTSEM in combination with observational data and one-dimensional reactive transport modelling of sedimentary processes in the Fårö Deep, a deep Baltic Sea basin, to test whether sulfate reduction coupled to iron (Fe) sulfide burial can explain the missing TA source in the Baltic Proper. We calculated that this burial can account for 26% of the missing source in this basin, with the remaining TA possibly originating from unknown river inputs or submarine groundwater discharge. We also show that temporal variability in the input of Fe to the sediments since the 1970s drives changes in sulfur burial in the Fårö Deep, suggesting that Fe availability is the ultimate limiting factor for TA generation under anoxic conditions. The implementation of projected climate change and two nutrient load scenarios for the 21st century in BALTSEM shows that reducing nutrient loads will improve deep water oxygen conditions, but at the expense of lower surface water TA concentrations, CO₂ buffering capacities and faster acidification. When these changes additionally lead to a decrease in Fe inputs to the sediment of the deep basins, anaerobic TA generation will be reduced even further, thus exacerbating acidification. This work highlights that Fe dynamics play a key role in the release of TA from sediments where Fe sulfide formation is limited by Fe availability, as exemplified for the Baltic Sea. Moreover, it demonstrates that burial of Fe sulfides should be included in TA budgets of low oxygen basins.

Measurements from the SWERUS-C3 and ISSS-08 Arctic expeditions were used to calibrate and validate a new physical-biogeochemical model developed to quantify key carbon cycling processes on the East Siberian Arctic Shelf (ESAS). The model was used in a series of experimental simulations with the specific aim to investigate the pathways of terrestrial dissolved and particulate organic carbon (DOC_ter and POC_ter) supplied to the shelf. Rivers supply on average 8.5 Tg C yr⁻¹ dissolved inorganic carbon (DIC), and further 8.5 and 1.1 Tg C yr⁻¹ DOC_ter and POC_ter respectively. Based on observed and simulated DOC concentrations and stable isotope values (δ¹³C_DOC) in shelf waters, we estimate that only some 20 % of the riverine DOC_ter is labile. According to our model results, an additional supply of approximately 14 Tg C yr⁻¹ eroded labile POC_ter is however required to describe the observed stable isotope values of DIC (δ¹³C_DIC). Degradation of riverine DOC_ter and POC_ter results in a 1.8 Tg C yr⁻¹ reduction in the uptake of atmospheric CO₂, while degradation of eroded POC_ter results in an additional 10 Tg C yr⁻¹ reduction. Our calculations indicate nevertheless that the ESAS is an overall small net sink for atmospheric CO₂ (1.7 Tg C yr⁻¹). The external carbon sources are largely compensated by a net export from the shelf to the Arctic Ocean (31 Tg C yr⁻¹), and to a smaller degree by a permanent burial in the sediments (2.7 Tg C yr⁻¹).

In this study, C-12 and C-13 contents of all carbon containing state variables (dissolved inorganic and organic carbon, detrital carbon, and the carbon content of autotrophs and heterotrophs) have for the first time been explicitly included in a coupled physical-biogeochemical Baltic Sea model. Different processes in the carbon cycling have distinct fractionation values, resulting in specific isotopic fingerprints. Thus, in addition to simulating concentrations of different tracers, our new model formulation improves the possibility to constrain the rates of processes such as CO2 assimilation, mineralization, and air-sea exchange. We demonstrate that phytoplankton production and respiration, and the related air-sea CO2 fluxes, are to a large degree controlling the isotopic composition of organic and inorganic carbon in the system. The isotopic composition is further, but to a lesser extent, influenced by river loads and deep water inflows as well as transformation of terrestrial organic carbon within the system. Changes in the isotopic composition over the 20th century have been dominated by two processes the preferential release of C-12 to the atmosphere in association with fossil fuel burning, and the eutrophication of the Baltic Sea related to increased nutrient loads under the second half of the century.

In this study, the BALTSEM model is used to estimate how air-water CO₂ fluxes in the Baltic Sea respond to parameterizations of organic alkalinity (Aorg), gas transfer, and phytoplankton growth, and further to changes in river loads. The forcing data include the most complete compilation of Baltic river loads for dissolved inorganic and organic carbon (DIC and DOC) and total alkalinity (TA). In addition, we apply the most recent estimates of internal TA generation in the system. Our results clearly demonstrate how air-water CO₂ fluxes of a coastal sea depend on river loads of carbon, TA, and nutrients as well the freshwater import itself. Long-term changes in DIC loads are shown to be compensated by corresponding changes in air-water CO₂ exchange. By adding Aorg, a discrepancy in the carbonate system calculations was removed, and the simulated net CO₂ absorption of the system decreased by 11%. A new parameterization for cyanobacteria growth significantly improved the seasonal development of pCO₂ in the central Baltic Sea, although the net effect on CO₂ fluxes was below 5%. By applying either a linear, quadratic, or cubic wind speed dependence for gas transfer, the long-term net CO₂ exchange was adjusted by less than 5%. There is no clear indication that any one of these parameterizations provides a more accurate estimate of CO₂ fluxes than the other two. Our findings are applicable in other coastal areas that are heavily influenced by river loads of TA, DIC, and DOC.

In this study we first present updated riverine total alkalinity (TA) loads to the various Baltic Sea sub-basins, based on monthly measurements in 82 of the major rivers that represent 85% of the total runoff. Simulations in the coupled physical-biogeochemical BALTSEM (BAltic sea Long-Term large Scale Eutrophication Model) model show that these river loads together with North Sea water inflows are not sufficient to reproduce observed TA concentrations in the system, demonstrating the large influence from internal sources. Budget calculations indicate that the required internal TA generation must be similar to river loads in magnitude. The nonriverine source in the system amounts to about 2.4mmolm(-2) d(-1) on average. We argue here that the majority of this source is related to denitrification together with unresolved sediment processes such as burial of reduced sulfur and/or silicate weathering. This hypothesis is supported by studies on sediment processes on a global scale and also by data from sediment cores in the Baltic Sea. In a model simulation with all internal TA sources and sinks switched on, the net absorption of atmospheric CO2 increased by 0.78mol C m(-2) yr(-1) compared to a simulation where TA was treated as a passive tracer. Our results clearly illustrate how pelagic TA sources together with anaerobic mineralization in coastal sediments generate a significant carbon sink along the aquatic continuum, mitigating CO2 evasions from coastal and estuarine systems.

Seabirds redistribute nutrients between different ecosystem compartments and over vast geographical areas. This nutrient transfer may impact both local ecosystems on seabird breeding islands and regional biogeochemical cycling, but these processes are seldom considered in local conservation plans or biogeochemical models. The island of Stora Karlsö in the Baltic Sea hosts the largest concentration of piscivorous seabirds in the region, and also hosts a large colony of insectivorous House martins Delichon urbicum adjacent to the breeding seabirds. We show that a previously reported unusually high insectivore abundance was explained by large amounts of chironomids—highly enriched in δ¹⁵N—that feed on seabird residues as larvae along rocky shores to eventually emerge as flying adults. Benthic ammonium and phosphate fluxes were up to 163% and 153% higher close to the colony (1,300 m distance) than further away (2,700 m) and the estimated nutrient release from the seabirds at were in the same order of magnitude as the loads from the largest waste-water treatment plants in the region. The trophic cascade impacting insectivorous passerines and the substantial redistribution of nutrients suggest that seabird nutrient transfer should be increasingly considered in local conservation plans and regional nutrient cycling models.

The Baltic Sea is characterized by the largest area of hypoxic (oxygen (O₂) < 2 mg L⁻¹) bottom waters in the world’s ocean induced by human activities. Natural ventilation of these O₂-depleted waters largely depends on episodic Major Baltic Inflows from the adjacent North Sea. In 2014 and 2015, two such inflows led to a strong rise in O₂ and decline in phosphate (HPO₄²⁻) in waters below 125 m depth in the Eastern Gotland Basin. This provided the opportunity to assess the impact of such re-oxygenation events on the cycles of manganese (Mn), iron (Fe) and phosphorus (P) in the sediment for the first time. We demonstrate that the re-oxygenation induced the activity of sulphur (S)-oxidising bacteria, known as Beggiatoaceae in the surface sediment where a thin oxic and suboxic layer developed. At the two deepest sites, strong enrichments of total Mn and to a lesser extent Fe oxides and P were observed in this surface layer. A combination of sequential sediment extractions and synchrotron-based X-ray spectroscopy revealed evidence for the abundant presence of P-bearing rhodochrosite and Mn(II) phosphates. In contrast to what is typically assumed, the formation of Fe oxides in the surface sediment was limited. We attribute this lack of Fe oxide formation to the high flux of reductants, such as sulphide, from deeper sediments which allows Fe(II) in the form of FeS to be preserved and restricts the penetration of O₂ into the sediment. We estimate that enhanced P sequestration in surface sediments accounts for only ∼5% of water column HPO₄²⁻ removal in the Eastern Gotland Basin linked to the recent inflows. The remaining HPO₄²⁻ was transported to adjacent areas in the Baltic Sea. Our results highlight that the benthic O₂ demand arising from the accumulation of organic-rich sediments over several decades, the legacy of hypoxia, has major implications for the biogeochemical response of euxinic basins to re-oxygenation. In particular, P sequestration in the sediment in association with Fe oxides is limited. This implies that artificial ventilation projects that aim at removing water column HPO₄²⁻ and thereby improving water quality in the Baltic Sea will likely not have the desired effect.

Coastal areas are an important source of methane (CH4). However, the exact origins of CH4 in the surface waters of coastal regions, which in turn drive sea-air emissions, remain uncertain. To gain a comprehensive understanding of the current and future climate change feedbacks, it is crucial to identify these CH4 sources and processes that regulate its formation and oxidation. This study investigated coastal CH4 dynamics by comparing water column data from six stations located in the brackish Tvärminne Archipelago, Baltic Sea. The sediment biogeochemistry and microbiology were further investigated at two stations (i.e., nearshore and offshore). These stations differed in terms of stratification, bottom water redox conditions, and organic matter loading. At the nearshore station, CH4 diffusion from the sediment into the water column was negligible, because nearly all CH4 was oxidized within the upper sediment column before reaching the sediment surface. On the other hand, at the offshore station, there was significant benthic diffusion of CH4, albeit the majority underwent oxidation before reaching the sediment-water interface, due to shoaling of the sulfate methane transition zone (SMTZ). The potential contribution of CH4 production in the water column was evaluated and was found to be negligible. After examining the isotopic signatures of δ13C-CH4 across the sediment and water column, it became apparent that the surface water δ13C-CH4 values observed in areas with thermal stratification could not be explained by diffusion, advective fluxes, nor production in the water column. In fact, these values bore a remarkable resemblance to those detected below the SMTZ. This supports the hypothesis that the source of CH4 in surface waters is more likely to originate from ebullition than diffusion in stratified brackish coastal systems.

The main objective of this work was to develop regional settings of the NANI budgeting tool that will address the significant variation in agricultural practices and resulting nutrient accountings among European countries. NANI (Net Anthropogenic Nitrogen Inputs), first introduced by Howarth et al. (1996), estimate the human‐induced nitrogen inputs to a watershed and have been shown to be a good predictor of riverine nitrogen export at a large scale, multi‐year average basis. NANI have been calculated as the sum of four major components: atmospheric N deposition, fertilizer N application, agricultural N fixation, and net food and feed imports, which in turn are composed of crop and animal N production (negative fluxes removing N from watersheds) and animal and human N consumption (positive fluxes adding N to watersheds). Assuming approximate steady-state behavior, riverine N export is a fixed proportion of net nitrogen inputs.

Similar calculations can be made for phosphorus (P) inputs, though because atmospheric deposition of P is usually considered negligible and there is no analog in P for atmospheric fixation, the calculation of Net Anthropogenic Phosphorus Inputs (NAPI) reduces to accounting for P fertilizer and P in net food/feed terms. While this document is primarily concerned with calculating NANI, we also describe the data sources and assumptions used to make the parallel calculations of NAPI.

Version 2.0 of the Toolbox described in this document is an improvement of version 1.0 developed for US watersheds (http://www.eeb.cornell.edu/biogeo/nanc/nani/nani.htm; Hong et al. 2011). Version 1.0 allows the user to calculate NANI in any area within the contiguous United States (e.g., watershed, county, etc.) from nationally available databases downloadable from the Internet. The toolbox consists of a set of tools that:

(1) calculate the proportions of various regions (political or gridded) in which data are collected that fall into areas of interest such as watersheds (“NANI‐GIS tools”),

(2) extract and organize relevant data downloaded from web‐based datasets to be used by the accounting tools (“NANI‐extraction tools”), and

(3) calculate NANI, their components, and other relevant items such as animal excretion (“NANI-.‐accounting tools”).

While attempting to apply version 1.0 of the toolbox to Baltic Sea catchments, we found that the calculation of NANI in Baltic Sea catchments is more challenging than in US watersheds, mainly for two reasons:

• Watersheds span international boundaries. Significant variation in agricultural practices and resulting nutrient accountings among European countries exist. For example, a substantial gradient in agricultural practices is expected among the former EU countries, new EU member states with transitional economies, and Belarus and Russia.

• Gaps and uncertainties in the available data are much greater than those in the US. In general, the problem of missing information is more severe for the transitional countries, Belarus, and Russia, requiring numerous assumptions and guesswork to be made to deal with the insufficient data issue.

Version 2.0 of the Toolbox describe in this document  has several modules and improvements added to version 1.0 (which assumes spatially uniform agricultural practices, i.e., fixed values for all the NANI parameters, supported by the availability of well‐established and standardized datasets) to address the above difficulties. These improvements include:

• Allowing spatial variation of NANI parameters (in this example, country‐specific NANI parameters) (Sections 4, 5.1, and 5.2)

• Distribution of regional data (e.g., country-level crop production) into smaller spatial units (e.g., grid cells containing crop area information) (Section 5.3)

• Making post‐calculation adjustments and refinements by accepting auxiliary datasets and manual calculations from the user (Section 3) In the following sections we describe the calculation of NANI and their components in the Baltic Sea catchments, with details of data availability, input preparation, and step-by‐step procedure of the use of various tools, and provide some preliminary results. In addition,  Appendix 1 described parameter values used to create NAPI estimates following an accounting methodology in parallel to that for NANI.

Aquatic ecosystem services are important for human wellbeing, but they are much less studied than terrestrial ecosystem services. The objectives of this study are to broaden, itemize and exemplify the human‐nature interactions in modeling the future provision of aquatic ecosystem services. We include shared socioeconomic and representative concentration pathways, used extensively in climate research, as drivers of change for the future development of the Baltic Sea. Then we use biogeochemical and ecosystem models to demonstrate the future development of exemplary supporting, provisioning and cultural ecosystem services for two distinct combinations of regionally downscaled global climate and socioeconomic futures. According to the model simulations, the two global futures (“Sustainable well‐being” vs. “Fossil‐fuelled development”) studied lead to clearly deviating trajectories in the provision of marine ecosystem services. Under the “Sustainable well‐being”‐scenario primary production decreases by 20%, catches of demersal fish increases and the recreation opportunities increase significantly by the end of the ongoing century. Under the “fossil‐fuelled development”‐scenario primary production doubles, fisheries focus on less valued pelagic fish and the recreation possibilities will decrease. Long‐term projections of aquatic ecosystem services prepared for alternative global socioeconomic futures can be used by policy makers and managers to adaptively and iteratively adjust mitigation and adaptation effort with plausible future changes in the drivers of water pollution.

Drastic changes have occurred in the Eastern Gulf of Finland ecosystem after recent invasion and establishment of polychaete Marenzelleria spp. Possible mechanisms of these changes are explored with the help of three-dimensional ecosystem model SPBEM. Relative significance of bioirrigation activity is studied by comparison of two climate change scenario simulations, which include or disregard Marenzelleria effects. The novel results obtained with this approach demonstrate that on a system level biogeochemical consequences of both implemented climate changes scenario and polychaete activity are equivalent to a weakening of vicious circle of the Baltic Sea eutrophication. The eutrophication-mitigating effects of the Marenzelleria invasion into the Eastern Gulf of Finland, revealed by the long-term field measurements, are explained by simulation-based considerations.

Despite a wide-ranging research, there is almost no information regarding the major biogeochemical fluxes that could characterize the past and present state of the European Lake Onego ecosystem and be used for reliable prognostic estimates of its future. To enable such capacity, we adapted and implemented a three-dimensional coupled hydrodynamical biogeochemical model of the nutrient cycles in Lake Onego. The model was used to reconstruct three decades of Lake Onego ecosystem dynamics with daily resolution on a 2 × 2 km grid. A comparison with available information from Lake Onego and other large boreal lakes proves that this hindcast is plausible enough to be used as a form of reanalysis. This model will be used as a form of studies of Lake Onego ecosystem, including long-term projections of ecosystem evolution under different scenarios of climate change and socio-economic development.

Modern models of the Baltic Sea eutrophication describe only a bioavailable fraction of the nutrient input from land, thus introducing uncertainty into forcing. In order to alleviate this uncertainty, the coupled 3D hydrodynamical-biogeochemical St. Petersburg Eutrophication Model (SPBEM) has been expanded with variables representing dissolved organic nutrients. The model modification involves an explicit description of the labile and refractory fractions of dissolved organic nitrogen and phosphorus, in addition to their particulate forms, represented by the detritus variables. The modified SPBEM-2 allows for a full account of the total amounts of nutrients reported in field measurements and presented in environmental documents. Particularly, a model description of detritus, as the only bulk organic matter variable, has been replaced by more realistic parameterizations with adequate rates of settling and mineralization. The extensive validation and verification of the model performance in the Gulf of Finland from 2009 to 2014, based on over 4000 oceanographic stations, shows that SPBEM-2 plausibly reproduces all the major large-scale features and phenomena of the ecosystem dynamics in the Gulf of Finland, especially in its surface productive layer. These demonstrated capabilities of SPBEM-2 make the model a useful tool, both in studies of biogeochemical interactions and in historical and scenario simulations.

In high-latitude continental shelf environments, late Pleistocene glacial overdeepening and early Holocene eustatic sea-level rise combined to create restricted marine basins with a high vulnerability to oxygen depletion. Here we show that ongoing glacio-isostatic rebound during the Holocene may have played an important role in determining the distribution of past hypoxia in these environments by controlling the physical exchange of water masses and the distribution of large-scale phosphorus (P) sinks. We focus on the Baltic Sea, where sediment records from a large, presently oxic sub-basin show evidence for intense hypoxia and cyanobacteria blooms during the Holocene Thermal Maximum. Using paleobathymetric modeling, we show that this period was characterized by enhanced deep-water exchange, allowing widespread phosphorus regeneration. Intra-basin sills then shoaled over a period of several thousand years, enhancing P burial in one of the sub-basins. Together with climate forcing, this may have caused the termination of hypoxia throughout the Baltic Sea. Similar rearrangements of physical and chemical processes likely occurred in response to glacio-isostatic rebound in other high-latitude shelf basins during the Holocene.

Hypoxia has occurred intermittently in the Baltic Sea since the establishment of brackish-water conditions at ∼8,000 years B.P., principally as recurrent hypoxic events during the Holocene Thermal Maximum (HTM) and the Medieval Climate Anomaly (MCA). Sedimentary phosphorus release has been implicated as a key driver of these events, but previous paleoenvironmental reconstructions have lacked the sampling resolution to investigate feedbacks in past iron-phosphorus cycling on short timescales. Here we employ Laser Ablation (LA)-ICP-MS scanning of sediment cores to generate ultra-high resolution geochemical records of past hypoxic events. We show that in-phase multidecadal oscillations in hypoxia intensity and iron-phosphorus cycling occurred throughout these events. Using a box model, we demonstrate that such oscillations were likely driven by instabilities in the dynamics of iron-phosphorus cycling under preindustrial phosphorus loads, and modulated by external climate forcing. Oscillatory behavior could complicate the recovery from hypoxia during future trajectories of external loading reductions.

Population oscillations in multi-species or even single species systems are well-known but have rarely been detected at the lower trophic levels in marine systems. Nitrogen fixing cyanobacteria are a major component of the Baltic Sea ecosystem and sometimes form huge surface accumulations covering most of the sea surface. By analysing a satellite-derived 39-year (1979–2017) data archive of surface cyanobacteria concentrations we have found evidence of strikingly regular interannual oscillations in cyanobacteria concentrations in the northern Baltic Sea. These oscillations have a period of ~3 years with a high-concentration year generally followed by one or two low-concentration years. Changes in abiotic factors known to influence the growth and survival of cyanobacteria could not provide an explanation for the oscillations. We therefore assume that these oscillations are intrinsic to the marine system, caused by an unknown, probably mainly biological mechanism that may be triggered by a combination of environmental factors. Interactions between different life cycle stages of cyanobacteria as well as between predator-prey or host-parasite are possible candidates for causing the oscillations.

Changes in the phenology of physical and ecological variables associated with climate change are likely to have significant effect on many aspects of the Baltic ecosystem. We apply a set of phenological indicators to multiple environmental variables measured by satellite sensors for 17-36 years to detect possible changes in the seasonality in the Baltic Sea environment. We detect significant temporal changes, such as earlier start of the summer season and prolongation of the productive season, in several variables ranging from basic physical drivers to ecological status indicators. While increasing trends in the absolute values of variables like sea-surface temperature (SST), diffuse attenuation of light (Ked490) and satellite-detected chlorophyll concentration (CHL) are detectable, the corresponding changes in their seasonal cycles are more dramatic. For example, the cumulative sum of 30 000 W m(-2) of surface incoming short-wave irradiance (SIS) was reached 23 days earlier in 2014 compared to the beginning of the time series in 1983. The period of the year with SST of at least 17 degrees C has almost doubled (from 29 days in 1982 to 56 days in 2014), and the period with Ked490 over 0.4 m(1) has increased from about 60 days in 1998 to 240 days in 2013 -i.e., quadrupled. The period with satellite-estimated CHL of at least 3 mg m(-3) has doubled from approximately 110 days in 1998 to 220 days in 2013. While the timing of both the phytoplankton spring and summer blooms have advanced, the annual CHL maximum that in the 1980s corresponded to the spring diatom bloom in May has now shifted to the summer cyanobacteria bloom in July.