Description: Aim Following the biogeographical approach implemented by Longhurst for the epipelagic layer, we propose here to identify a biogeochemical 3-D partition for the mesopelagic layer. The resulting partition characterizes the main deep environmental biotopes and their vertical boundaries on a global scale, which can be used as a geographical and ecological framework for conservation biology, ecosystem-based management and for the design of oceanographic investigations. Location The global ocean. Methods Based on the most comprehensive environmental climatology available to date, which is both spatially and vertically resolved (seven environmental parameters), we applied a combination of clustering algorithms (c-means, k-means, partition around medoids and agglomerative with Ward's linkage) associated with a nonparametric environmental model to identify the vertical and spatial delineation of the mesopelagic layer. Results First, we show via numerical interpretation that the vertical division of the pelagic zone varies and, hence, is not constant throughout the global ocean. Indeed, a latitudinal gradient is found between the epipelagic–mesopelagic and mesopelagic–bathypelagic vertical limits. Second, the mesopelagic layer is shown here to be composed of 13 distinguishable Biogeochemical Provinces. Each province shows a distinct range of environmental conditions and characteristic 3-D distributions. Main conclusions The historical definition of the mesopelagic zone is here revisited to define a 3-D geographical framework and characterize all the deep environmental biotopes of the deep global ocean. According to the numerical interpretation of mesopelagic boundaries, we reveal that the vertical division of the zone is not constant over the global ocean (200–1,000 m) but varies between ocean basin and with latitude. We also provide evidence of biogeochemical division of the mesopelagic zone that is spatially structured in a similar way than the epipelagic in the shallow waters but varies in the deep owing to a change of the environmental driving factors.

Description: 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.

Description: Reducing anthropogenic nutrient inputs is a major policy goal for restoring good environmental status of coastal marine ecosystems. However, it is unclear to what extent reducing nutrients would also lower fish production and fisheries yields. Empirical examples of changes in nutrient loads and concurrent fish production can provide useful insights to this question. In this paper, we investigate to what extent a multi-fold increase in nutrient loads from the 1950s to 1980s enhanced forage fish production in the Baltic Sea. We use monitoring data on fish stock dynamics covering the period of the nutrient increase, combined with nutrient concentrations from a 3-dimensional coupled physical-biogeochemical ocean model. The results suggest that nutrient enrichment enhanced the biomass level of forage fish by up to 50 % in some years and areas due to increased body weight of fish. However, the trends in fish biomasses were generally decoupled from changes in nutrient concentrations

Description: The current paper summarizes catch and effort data for Swedish fisheries in the Baltic Sea prior to the standardized recording of fish catches, evaluates spatial and temporal variations and gives interpretations of selected findings. Long-term datasets (1868–1913) of species having different environmental preferences (and of different origin) – marine, migratory and freshwater – are presented for several Swedish counties extending from the Bothnian Bay to the Bornholm Basin. Herring (Clupea harengus membras) was the most important commercial fish-based on landings and also on the amount of historical records available. In addition, herring landings have undoubtedly fluctuated the most. Other commercially important fish were cod (Gadus morhua), salmon and trout (Salmo spp.), flounder (Platichthys flesus), eel (Anguilla anguilla) and whitefish (Coregonus lavaretus). Different species exhibited different patterns of catch dynamics. In general, the 1880s and the 1890s can be characterized as ‘good fishing years’ for the Swedish Baltic fisheries: catches of herring and cod were high while fishing effort indices were relatively stable. Sprat was not represented in the catch data and may have been recorded as herring. For several species, regions and time periods, catch–effort relationship exhibited clear coupling. However, in several cases, insufficient or lack of effort-related information hampered data interpretations. In summary, the Swedish fisheries during the late 19th century generally exploited the same species as presently (including a dominance by clupeids), but the relative contributions of individual species have changed.

Description: Highlights: • We unveil regional progressive climate modifications fostering the entire plankton food web. • Hydroclimate shifts have progressively impacted the seasonality of plankton. • The profound plankton reorganization warns on the replacement of local species by cosmopolitan ones. Abstract: Growing evidence has shown a profound modification of plankton communities of the North East Atlantic and adjacent seas over the past decades. This drastic change has been attributed to a modification of the environmental conditions that regulate the dynamics and the spatial distribution of ectothermic species in the ocean. Recently, several studies have highlighted modifications of the regional climate station L4 (50° 15.00′N, 4° 13.02′W) in the Western English Channel. We here focus on the modification of the plankton community by studying the long-term, annual and seasonal changes of five zooplankton groups and eight copepod genera. We detail the main composition and the phenology of the plankton communities during four climatic periods identified at the L4 station: 1988–1994, 1995–2000, 2001–2007 and 2008–2012. Our results show that long-term environmental changes underlined by Molinero et al. (2013) drive a profound restructuration of the plankton community modifying the phenology and the dominance of key planktonic groups including fish larvae. Consequently, the slow but deep modifications detected in the plankton community highlight a climate driven ecosystem shift in the Western English Channel.

Description: The Baltic Sea is a dynamic environment responding to various drivers operating at different temporal and spatial scales. In response to climate change, the Baltic Sea is warming and the frequency of extreme climatic events is increasing (Lima & Wethey 2012, BACC 2008, Poloczanska et al. 2007). Coastal development, human population growth and globalization intensify stressors associated with human activities, such as nutrient loading, fisheries and proliferation of invasive and bloom-forming species. Such abrupt changes have unforeseen consequences for the biodiversity and the function of food webs and may result in loss of ecological key species, alteration and fragmentation of habitats. To mitigate undesired effects on the Baltic ecosystem, an efficient marine management will depend on the understanding of historical and current drivers, i.e. physical and chemical environmental conditions and human activities that precipitate pressures on the natural environment. This task examined a set of key interactions of selected natural and anthropogenic drivers in space and time, identified in Task 3.1 as well as WP1 and WP2 (e.g. physico-chemical features vs climate forcing; eutrophication vs oxygen deficiency vs bio-invasions; fisheries vs climate change impacts) by using overlay-mapping and sensitivity analyses. The benthic ecosystem models developed under Task 2.1 were used to investigate interactions between sea temperature and eutrophication for various depth strata in coastal (P9) and offshore areas (P1) of the Baltic Sea. This also included investigation on how the frequency and magnitude of deep-water inflow events determines volume and variance of salinity and temperature under the halocline, deep-water oxygen levels and sediment fluxes of nutrients, using observations and model results from 1850 to present (P1, P2, P6, P9, P12). The resulting synthesis on the nature and magnitude of different driver interactions will feed into all other tasks of this WP3 and WP2/WP4. Moreover, the results presented in this report improve the process-based and mechanistic understanding of environmental change in the Baltic Sea ecosystem, thereby fostering the implementation of the Marine Strategy Framework Directive.

Description: Eutrophication and climate change will affect habitats of species and more generally, the structure and functioning of ecosystems. We used a three‐dimensional, coupled hydrodynamic‐biogeochemical model to investigate potential future changes in size and location of potential habitats of marine species during the 21st century in a large, eutrophicated brackish sea (the Baltic Sea, northern Europe). We conducted scenario projections under the combined impact of nutrient load and climate change. Possible future changes of the eutrophication state of this sea were also assessed through two policy‐relevant indicators. The results imply a physiologically more stressful environment for marine species by the end of the 21st century: volumes of higher salinity water become more hypoxic/anoxic and the volumes of low salinity, oxic water increase. For example, these results impact and reduce cod reproductive habitats. The decrease is mainly climate change induced in the Baltic basins less directly influenced by inflows of saline, oxic water to the Baltic Sea (E Gotland and Gdansk Basins). In basins more directly influenced by such inflows (Arkona and Bornholm Basins), the combined effect from climate change and nutrient loads is of importance. The results for the eutrophication state indicators clearly indicate a more eutrophic sea than at present without a rigorous nutrient reduction policy, that is, the necessity to implement the Baltic Sea Action Plan. The multidisciplinary, multiscenario assessment strategy presented here provides a useful concept for the evaluation of impacts from cumulative stresses of changing climate and socioeconomic pressures on future eutrophication indicators and habitats of marine species.