The Baltic Sea as a time machine for the future coastal ocean

Abstract

Coastal global oceans are expected to undergo drastic changes driven by climate change and increasing anthropogenic pressures in coming decades. Predicting specific future conditions and assessing the best management strategies to maintain ecosystem integrity and sustainable resource use are difficult, because of multiple interacting pressures, uncertain projections, and a lack of test cases for management. We argue that the Baltic Sea can serve as a time machine to study consequences and mitigation of future coastal perturbations, due to its unique combination of an early history of multistressor disturbance and ecosystem deterioration and early implementation of cross-border environmental management to address these problems. The Baltic Sea also stands out in providing a strong scientific foundation and accessibility to long-term data series that provide a unique opportunity to assess the efficacy of management actions to address the breakdown of ecosystem functions. Trend reversals such as the return of top predators, recovering fish stocks, and reduced input of nutrient and harmful substances could be achieved only by implementing an international, cooperative governance structure transcending its complex multistate policy setting, with integrated management of watershed and sea. The Baltic Sea also demonstrates how rapidly progressing global pressures, particularly warming of Baltic waters and the surrounding catchment area, can offset the efficacy of current management approaches. This situation calls for management that is (i) conservative to provide a buffer against regionally unmanageable global perturbations, (ii) adaptive to react to new management challenges, and, ultimately, (iii) multisectorial and integrative to address conflicts associated with economic trade-offs.

Fig. 1. The Baltic Sea time machine.

(A) The Baltic Sea, its neighboring countries, and the catchment area. (B) Sea surface temperature (SST) change per decade since 1980. The Baltic Sea is at the center of the map. (C) Left: High-resolution surface seawater CO₂ variability in 2014 at a coastal Baltic Sea site (Kiel Fjord Time Series, 54.2°N, 10.9°E; red symbols) in comparison to an oceanic site close to Hawai’i (Woods Hole—Hawaii Ocean Time-series Site, 22.7°N, 157.9°W; blue symbols) and coastal sites in Florida (Cheeca Rocks, 24.9°N, 80.6°W), California [California Current Ecosystem Mooring 2 (CCE2), 34.3°N, 120.8°W; green symbols], Alaska (Kodiak, 57.7°N, 152.3°W; black symbols), and Washington (Twanoh, 47.4°N, 123°W; orange symbols). Right: Mean Pco₂/xco₂ values and SD for 2014 data from selected time series stations (see above). All data are seawater (SW) Pco₂ (in microatmospheres) except for station Twanoh [xco₂, in parts per million (ppm), dry]. Kie, Kiel; Twa, Twanoh, Kod, Kodiak; Cal, CCE2; Che, Cheeca Rocks; Haw, Hawai’i. (D) Expansion of hypoxic zones in the Baltic Sea during 115 years of monitoring. Black shading shows the situation for the period 1900–1910, whereas red shading indicates the period 2001–2010. Coastal hypoxia is depicted by red dots. For data sources, see data S3.

Fig. 2. Examples of long-term time series available for the Baltic Sea.

(A) Temperature (0 to 10 m). (B) Pco₂ in the bottom waters (>150 m) for station BY15 in the central Gotland Basin. (C) Secchi depths after Baltic Sea Environmental Proceedings no. 133. (D) Benthic area with anoxic conditions (<2 mg O₂ liter⁻¹). (E) Abundance of cyanobacteria in the Gulf of Finland. (F) Abundance of zooplankton (Acartia spp.) in Pärnu Bay, Estonia. (G) Eastern Baltic cod total spawning stock biomass. (H) Herring total spawning stock biomass data. (I) DDT concentration in liver of sea eagles. (J) Counts of NIS. Green-, red-, and blue-colored areas indicate the time period when policies for fisheries management, the reduction of nutrients, and the ban of DDT were implemented, respectively. For data sources, see data S3.

Fig. 3. Governance structure in the Baltic Sea region.

(A) Baltic fisheries management is an exclusive EU competence under the Common Fisheries Policy (2013). Fishing is based on the maximum sustainable yield principle resulting in total allowable catches (TACs) and national quotas. TACs are developed in a process involving the following steps: Advice from stakeholder groups is collected by Advisory Councils (ACs), and scientific advice is provided by ICES and communicated to the EU Commission by the EU Scientific, Technical, and Economic Committee for Fisheries (STECF). The EU Commission suggests TACs to the EU Council of Ministers that makes the final decisions. On the basis of the TACs, national quotas are distributed, implemented, and monitored by member states. Bilateral agreements integrate Russia into the EU environmental management. (B) In the management of hazardous substances, HELCOM carries a significant role for monitoring, assessing, and agenda setting, whereas the EU provides legal basis and enforcement. HELCOM works through its recommendations, the BSAP, and ministerial declarations. The EU has addressed the issue via, for example, the Registration, Evaluation, Authorization and restriction of Chemicals (REACH) regulation, the Marine Strategy Framework Directive (MSFD), and the Water Framework Directive (WFD). The EU Commission initiates and proposes new legislation to be approved by both the Council of Ministers and the European Parliament. The EU and HELCOM closely interact. For example, the BSAP was initiated in 2007 following the EU MSFD. ICES provides scientific data to HELCOM and was involved in the development of the MSFD. (C) Governance of eutrophication. HELCOM targets the sources of eutrophication via several recommendations (for example, Rec 28E/4 on measures to hinder land-based pollution) and the BSAP with reduction targets for emissions of nitrogen and phosphorus. EU has adopted several directives to deal specifically with eutrophication including the Urban Waste Water Treatment Directive (UWWTD), the Nitrate Directive (ND), and the National Emission Ceilings Directive (NECD). The EU Common Agricultural Policy (CAP) strongly influences nutrient management. Within the CAP, member states implement specific agricultural measures targeted at nutrient reduction from agriculture that (partly) reflect measures recommended by HELCOM. For detailed references and sources, see data S3.

Fig. 4. Nutrient input into the Baltic Sea.

Five-year moving average values of N and P loads (in 1000 metric tons per year) to the Baltic Sea together with the BSAP targets. Along the x axis, the timing of countries joining the EU and the introduction of key EU environmental legislation are shown. WWTP, wastewater treatment plans; HELCOM, signing of the Helsinki Convention; UWWD, urban wastewater directive. Key developments of the EU CAP are indicated by arrows at top of the diagram. Supply mgmt, supply management; DE, Germany; DK, Denmark; SE, Sweden; FI, Finland; EST, Estonia; LIT, Lithuania; LT, Latvia; PL, Poland. For detailed references and sources, see data S3.