Östersjön är ett unikt ekosystem, i akut behov av bättre skydd och omsorg. Havet är utsatt för stora påfrestningar från föroreningar, övergödning, klimatförändringar och överfiske, och därför krävs fortsatt kraftfulla insatser. Här presenteras ett antal viktiga åtgärder som kan förbättra havsmiljön.

Vi människor är beroende av havet för vår existens, för syret vi andas och för ett klimat vi kan leva i. För många är också mat från havet en viktig del av födan. Transporter, fiske, turism och rekreation visar på havets stora ekonomiska och sociala värden, och mycket tyder på att välmående kustmiljöer också kan motverka klimatförändringarna. Havet behöver vårdas – både för vår egen skull och för de marina ekosystemens.

I Östersjön har mänskliga aktiviteter haft stor påverkan på miljön. Vissa bestånd av fisk, marina däggdjur och sjöfåglar har minskat kraftigt, andra är påverkade av miljögifter eller i dåligt skick, vilket bland annat syns i lägre tillväxt hos arter som sill och torsk. Syrefattiga bottnar, förändrad artsammansättning och skadade livsmiljöer är andra tydliga tecken på mänsklig påverkan. Klimatförändringarna förvärrar en del av miljöproblemen och gör det ännu mer angeläget att agera snabbt.

Det begränsade vattenutbytet i det inneslutna Östersjön gör att föroreningar blir kvar under lång tid och fördröjer effekten av olika åtgärder. Men vårt inneslutna hav med ett begränsat antal aktörer ger också möjlighet att agera unisont. I den här broschyren presenterar forskare och experter från Stockholms universitet ett antal viktiga åtgärder som kan förbättra miljön i Östersjön, från Bottenviken till Kattegatt.

Det behövs gemensamma ansträngningar och modiga beslut för att minska utsläppen av näringsämnen och farliga ämnen, och för att säkerställa en ekosystembaserad fiskeförvaltning som tar hänsyn till de ekologiska förutsättningarna. Bara så kan Östersjön bli basen för en hållbar blå ekonomi. Om hela havet förvaltas klokt, samtidigt som orörda områden skyddas och förstörda miljöer återställs, så finns goda möjligheter att på sikt återfå ett välmående och mer motståndskraftigt hav – till nytta för både människor och natur.

Excessive nutrient inputs have caused eutrophication of coastal ecosystems worldwide, triggering extensive algal blooms, oxygen-depletion, and collapse of local fisheries. In the Baltic Sea, inputs of nitrogen (N) and phosphorus (P) have been significantly reduced since the 1980s, but the environmental state shows little to no signs of recovery. However, a simulation with continued high loads from the mid-1980s demonstrates that while the state has not improved yet, it would be considerably worse today without the load reductions (e.g., 82% larger oxygen-free bottom areas and 104% and 58% higher wintertime concentrations of inorganic N and P, respectively, in the Baltic Proper). Additional simulations with current nutrient loads continuing into the future indicate that conditions will likely improve in the coming decades. This study underscores the significance of acting on early warning signs of eutrophication, and furthermore how sustained efforts to decrease nutrient loads can mitigate the severity of eutrophication.

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.

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.

Havsförsurningen väntas de kommande decennierna bli påtaglig även i Östersjön. För det redan hårt pressade ekosystemet innebär det ytterligare en belastning. Den samlade effekten av försurningen och annan miljöpåverkan kan stressa arter och minska den biologiska mångfalden. För att skydda den unika miljön och den framtida matproduktionen krävs både kraftigt minskade koldioxidutsläpp och åtgärder mot övergödning, överfiske och utsläpp av farliga ämnen.

In the coming decades, ocean acidification is expected to become significant also in the Baltic Sea. For an already stressed ecosystem, it represents an additional pressure, and the cumulative effect of this and other environmental impacts can stress species and reduce biodiversity. Protecting the unique environment and future food production requires both significant reductions in carbon dioxide emissions and measures against eutrophication, overfishing and emissions of hazardous substances.

Havsförsurningen väntas de kommande decennierna bli påtaglig även i Östersjön. För det redan hårt pressade ekosystemet innebär det ytterligare en belastning. Den samlade effekten av försurningen och annan miljöpåverkan kan stressa arter och minska den biologiska mångfalden. För att skydda den unika miljön och den framtida matproduktionen krävs både kraftigt minskade koldioxidutsläpp och åtgärder mot övergödning, överfiske och utsläpp av farliga ämnen.

A large part of the anthropogenic emissions of carbon dioxide have been absorbed by the oceans. However, many Swedish coastal areas are currently affected by eutrophication, making them a source of greenhouse gases, mainly in the form of methane. Reducing eutrophication is crucial for limiting methane emissions and thus mitigating climate change.

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.

En stor del av de mänskligt orsakade utsläppen av koldioxid har tagits upp av haven. Men många av de svenska kusterna är idag påverkade av övergödning, vilket gör dem till en källa till växthusgaser, främst i form av metan. Att minska övergödningen är avgörande för att begränsa metanutsläppen och därmed motverka klimatförändringarna.

In marine and brackish waters, the acidity of the water is mainly controlled by the inorganic carbon system. Anthropogenic CO2 emissions will – unless reduced – gradually move the Baltic Sea towards a state where acidification becomes harmful for some organisms. The effect is caused by the uptake of CO2 in the water, but can be further enhanced by other climate effects, such as increased water temperature and a possible freshening of the sea water. This is expected to lead to changes in species composition, both directly (competitive advantages/disadvantages) and indirectly (altered food availability), potentially influencing ecosystem functioning.

Coastal seas, such as the Baltic Sea, are highly influenced by their catchment areas, which means that pH dynamics is generally more complex than in the open ocean. The reason is that pH, in addition to the response to increasing CO2, is also influenced by changes in hydrology and changes in the supply of carbon and nutrients. High-productive waters typically experience larger seasonal pH variations than low-productive waters, with higher pH peaks in spring/summer and also a more pronounced pH decline in winter. The comparatively weak long-term acidification trend can be masked behind much larger short-term variations. Furthermore, since acidification is a slow process, organisms can to varying degrees adapt to the changes.

Model simulations performed as a part of the OMAI (Operational Marine Acidification Indicator) project indicate that the expected acidification in the Baltic Sea generally follows the same trajectory as the open oceans, with a pH decline of almost 0.4 by year 2100 and a further decline of 0.3 by year 2300 in the worst-case scenario. Due to large regional differences in the area, the annual mean pH in the Bothnian Bay might decline from present-day 7.8 to 7.4 by year 2100, whereas in the Gotland Sea and Southern Kattegat mean pH could decline from present-day 8.1 to 7.7. The degree of eutrophication has a comparatively small effect on the annual mean pH, but on the other hand a considerable impact on the seasonal amplitude and thus minimum and maximum values.

The complex situation in the Baltic Sea gives a strong incentive to improve the temporal and spatial coverage of acidification monitoring. This would broaden the understanding of current acidification trends and also improve the capacity to predict future changes. An indicator for acidification in the Baltic Sea is currently under development. Monitoring of parameters relevant for acidification, i.e., the inorganic carbon system parameters, would as an added value also provide an additional handle in terms of assessing changes in primary production and eutrophication trends.