Coastal eutrophication is a major issue worldwide, also affecting the Baltic Sea and its coastal waters. Effective management responses to coastal eutrophication require good understanding of the interacting coastal pressures from land, the open sea, and the atmosphere, and associated coastal ecosystem impacts. In this study, we investigate how research on Baltic coastal eutrophication has handled these interactions so far and what key research gaps still remain. We do this through a scoping review, identifying 832 scientific papers with a focus on Baltic coastal eutrophication. These are categorized in terms of study focus, methods, and consideration of coastal system components and land-coast-sea interactions. The coastal component categories include coastal functions (including also socio-economic driver aspects), pressures that are natural (or mediated by a natural process or system) or directly anthropogenic, and management responses.

The classification results show that considerably more studies focus on coastal eutrophication pressures (52%) or impacts (39%) than on characterizing the coastal eutrophication itself (20%). Moreover, few studies investigate pressures and impacts together, indicating that feedbacks are understudied. Regarding methods, more studies focus on data collection (62%) than on linking and synthetic methods (44%; e.g., modelling), and very few studies use remote sensing (6%) or participatory (3%) methods. Coastal links with land and open sea are mentioned but much less investigated. Among the coastal functions, studies considering ecological aspects are dominant, but much fewer studies investigate human aspects and the coastal filter function. Among the coastal pressures, studies considering nutrient loads are dominant, but much fewer studies investigate the sources of these loads, especially long-lived legacy sources and possible solutions for their mitigation. Overall, few studies investigate synergies, trade-offs and incentives for various solutions to address cross-scale multi-solution management.

Assessments of ecosystem service and function losses of wetlandscapes (i.e., wetlands and their hydrological catchments) suffer from knowledge gaps regarding impacts of ongoing hydro-climatic change. This study investigates hydro-climatic changes during 1976–2015 in 25 wetlandscapes distributed across the world’s tropical, arid, temperate and cold climate zones. Results show that the wetlandscapes were subject to precipitation (P) and temperature (T) changes consistent with mean changes over the world’s land area. However, arid and cold wetlandscapes experienced higher T increases than their respective climate zone. Also, average P decreased in arid and cold wetlandscapes, contrarily to P of arid and cold climate zones, suggesting that these wetlandscapes are located in regions of elevated climate pressures. For most wetlandscapes with available runoff (R) data, the decreases were larger in R than in P, which was attributed to aggravation of climate change impacts by enhanced evapotranspiration losses, e.g. caused by land-use changes.

Eutrophication endangers coastal ecosystems all over the world and is most often associated with an increase in anthropogenic nutrient loads to coastal waters, which fuel the growth of algae and create a variety of environmental problems. This is also the case for the Baltic Sea where coastal waters may be affected by various land, coast-sea, and hydroclimatic drivers and feedbacks, over different scales, including the eutrophic open sea. This thesis aims at improving our understanding of how these drivers affect coastal eutrophication and its management opportunities across the various coupled scales of the Baltic land-coast-sea system. To achieve this aim, the interactions between land-catchment, coastal, and open sea processes, and their influences on coastal eutrophication have been investigated through water quality modelling with applications to specific Baltic coastal waters. Hydroclimatic influences on the propagation of change-impacts through the land-coast-sea continuum to coastal eutrophication have also been investigated via the water quality modelling and additional analysis of actual water quality trends over the last 30 years along the Swedish coast. Moreover, coastal eutrophication research on the Baltic Sea system has been investigated through scientific literature analysis with focus on how the reported research has accounted for and linked components in the land-coast-sea system, and the aim to identify possible research gaps.

Results show that impacts of water quality improvements in the open sea propagate to a large share of the coastal waters, especially for phosphorus and phytoplankton, while impacts of reducing nutrient loads from land are more localised and more pronounced for nitrogen than for phosphorus. Therefore, reducing coastal nitrogen, phosphorus and phytoplankton concentrations requires both regional measures for open sea improvements and local land-catchment measures for reduction of nutrient loads to the specific coast. Moreover, data analysis shows that trends in coastal Summer chlorophyll a (Chl-a) are well correlated with those in open sea Summer Chl-a and in riverine nitrogen loads. Regarding hydroclimatic drivers, warmer and wetter conditions are found to complicate remediation of coastal eutrophication in comparison to drier and colder conditions. In addition, trends in coastal Summer Chl-a are well correlated with those in sea-ice conditions. These results highlight the various land-based, coastal, open sea, and hydroclimatic drivers and conditions that mix, interact in and influence the coastal waters. The various driver, management, and ecosystem components involved are overall included in Baltic coastal eutrophication research. However, specific coastal management measures, and feedbacks between drivers and impacts of coastal eutrophication are under-investigated, and the social and ecological components of the whole land-coast-sea system are not well-connected in the research.

Furthermore, long-lived legacy sources on land, as well as at sea, have not been much accounted for in coastal eutrophication research so far. This calls for further research on recovery time scales and specific remediation measures that can be effective against such sources, like mussel farming and wetlands. Finally, coastal eutrophication management needs to account for the influences on local coastal conditions from a melting pot of multi-scale drivers and biogeochemical as well as ecological impacts and feedbacks.

Coastal eutrophication is a major environmental issue worldwide. In the Baltic Sea, eutrophication affects both the coastal waters and the open sea. Various policy frameworks aim to hinder its progress but eutrophication-relevant water quality variables, such as chlorophyll-a concentrations, still exhibit opposite temporal trends in various Baltic Sea marine and coastal waters. In this study, we investigate the temporal-trend linkages of measured water quality variables and their various anthropogenic, climatic and hydrospheric drivers over the period 1990–2020 with focus on the Swedish coastal waters and related marine basins in the Baltic Sea.

We find that it is necessary to distinguish more and less isolated coastal waters, based on their water exchanges with the open sea, to capture different coastal eutrophication dynamics. In less isolated coastal waters, eutrophication is primarily related to nitrogen concentrations, while it is more related to phosphorus concentrations in more isolated coastal waters. In the open sea, trends in eutrophication conditions correlate best with trends in climatic and hydrospheric drivers, like wind speed and water salinity, respectively. In the coastal waters, driver signals are more mixed, with considerable influences from anthropogenic land-based nutrient loads and sea-ice cover duration. Summer chlorophyll-a concentration in the open sea stands out as a main change driver of summer chlorophyll-a concentration in less isolated coastal waters. Overall, coastal waters are a melting pot of driver influences over various scales, from local land-based drivers to large-scale total catchment and open sea conditions. The latter in turn depend on long-term integration of pathway-dependent influences from the various coastal parts of the Baltic Sea and their land-based nutrient load drivers, combined with overarching climate conditions and internal feedback loops. As such, our results challenge any unidirectional local source-to-sea paradigm and emphasize a need for concerted local land-catchment and whole-sea measures for robust coastal eutrophication management.

Geography and associated hydrological, hydroclimate and land-use conditions and their changes determine the states and dynamics of wetlands and their ecosystem services. The influences of these controls are not limited to just the local scale of each individual wetland but extend over larger landscape areas that integrate multiple wetlands and their total hydrological catchment - the wetlandscape. However, the data and knowledge of conditions and changes over entire wetlandscapes are still scarce, limiting the capacity to accurately understand and manage critical wetland ecosystems and their services under global change. We present a new Wetlandscape Change Information Database (WetCID), consisting of geographic, hydrological, hydroclimate and land-use information and data for 27 wetlandscapes around the world. This combines survey-based local information with geographic shapefiles and gridded datasets of large-scale hydroclimate and land-use conditions and their changes over whole wetlandscapes. Temporally, WetCID contains 30-year time series of data for mean monthly precipitation and temperature and annual land-use conditions. The survey-based site information includes local knowledge on the wetlands, hydrology, hydroclimate and land uses within each wetlandscape and on the availability and accessibility of associated local data. This novel database (available through PANGAEA https://doi.org/10.1594/PANGAEA.907398; Ghajarnia et al., 2019) can support site assessments; cross-regional comparisons; and scenario analyses of the roles and impacts of land use, hydroclimatic and wetland conditions, and changes in whole-wetlandscape functions and ecosystem services.

Hydroclimatic change may affect the range of some infectious diseases, including tularemia. Previous studies have investigated associations between tularemia incidence and climate variables, with some also establishing quantitative statistical disease models based on historical data, but studies considering future climate projections are scarce. This study has used and combined hydro-climatic projection outputs from multiple global climate models (GCMs) in phase six of the Coupled Model Intercomparison Project (CMIP6), and site-specific, parameterized statistical tularemia models, which all imply some type of power-law scaling with preceding-year tularemia cases, to assess possible future trends in disease outbreaks for six counties across Sweden, known to include tularemia high-risk areas. Three radiative forcing (emissions) scenarios are considered for climate change projection until year 2100, incuding low (2.6 Wm−2), medium (4.5 Wm−2), and high (8.5 Wm−2) forcing. The results show highly divergent changes in future disease outbreaks among Swedish counties, depending primarily on site-specific type of the best-fit disease power-law scaling characteristics of (mostly positive, in one case negative) sub- or super-linearity. Results also show that scenarios of steeper future climate warming do not necessarily lead to steeper increase of future disease outbreaks. Along a latitudinal gradient, the likely most realistic medium climate forcing scenario indicates future disease decreases (intermittent or overall) for the relatively southern Swedish counties Örebro and Gävleborg (Ockelbo), respectively, and disease increases of considerable or high degree for the intermediate (Dalarna, Gävleborg (Ljusdal)) and more northern (Jämtland, Norrbotten; along with the more southern Värmland exception) counties, respectively.

Coastal eutrophication is a common problem worldwide, with main drivers including land-based freshwater and nutrient discharges, as well as hydroclimatic and open sea conditions. This study investigates the combined effects of different hydroclimatic and eutrophication management scenarios on coastal water quality and ecological status. As a case study we consider and simulate these scenarios for the Himmerfjärden Bay, situated in the semi-enclosed Baltic Sea. Effects on different eutrophication-relevant variables are assessed for several potential land, coast and/or sea-based management scenarios under different hydroclimatic conditions spanning the range of recent past observations.

Our results show that the land and sea-based management scenarios have different effects on each of the studied eutrophication-relevant coastal variable. In general, management strategies need to target both nitrogen and phosphorus reduction for robust coastal effects. We find hydroclimate as a key non-human eutrophication driver, which can substantially counteract management effects. For hydroclimatic conditions close to the recently experienced average, various management measures can improve water quality and ecosystem status in the studied local Baltic coast. Under projected climate change, however, such improvement will require combined land- and sea-based measures.

A storm surge is a complex phenomenon in which waves, tide and current interact. Even though wind is the predominant force driving the surge, waves and tidal phase are also important factors that influence the mass and momentum transport during the surge. Devastating storm surges often occur in the Bohai Sea, a semi-enclosed shallow sea in North China, due to extreme storms. However, the effects of waves on storm surges in the Bohai Sea have not been quantified and the mechanisms responsible for the higher surges that affect part of the Bohai Sea have not been thoroughly studied. In this study, we set up a storm surge model, considering coupled effects of tides and waves on the surges. Validation against measured data shows that the coupled model is capable of simulating storm surges in the Bohai Sea. The simulation results indicate that the longshore currents, which are induced by the large gradient of radiation stress due to wave deformation, are one of the main contributors to the higher surges occurring in some coastal regions. The gently varying bathymetry is another factor contributing to these surges. With such bathymetry, the wave force direction is nearly uniform, and pushes a large amount of water in that direction. Under these conditions, the water accumulates in some parts of the coast, leading to higher surges in nearby coastal regions such as the south coast of the Bohai Bay and the west and south coasts of the Laizhou Bay. Results analysis also shows that the tidal phase at which the surge occurs influences the wave–current interactions, and these interactions are more evident in shallow waters. Neglecting these interactions can lead to inaccurate predictions of the storm surges due to overestimation or underestimation of wave-induced set-up.

In semi-enclosed seas, eutrophication may affect both the coastal waters and the whole sea. We develop and test a modelling approach that can account for nutrient loads from land as well as for influences and feedbacks on water quality across the scales of a whole semi-enclosed sea and its coastal zones. We test its applicability in the example cases of the Baltic Sea and one of its local archipelagos, the Archipelago Sea. For the Baltic Sea scale, model validation shows good representation of surface water quality dynamics and a generally moderate model performance for deeper waters. For the Archipelago Sea, management scenario simulations show that successful sea measures may have the most important effects on coastal water quality. This highlights the need to consistently account for whole-sea water-quality dynamics and management effects, in addition to effects of land drivers, in modelling for characterisation and management of local water quality.

For the large-scale coastal wetland system of the Baltic Sea, this study develops a methodology for investigating if and to what degree the variability and changes in certain hydro-climatic drivers control key coastal-marine physical conditions. The studied physical conditions include: (a) water temperature, (b) water salinity, and (c) flow structures (magnitudes and directions of flows between marine basins and the associated coastal zones and wetlands). We use numerical simulations of three hydro-climatically distinct cases to investigate the variations in hydro-climatic drivers and the resulting physical conditions (a-c) among the cases. The studied hydro-climatic forcing variables are: net surface heat flux, wind conditions, saltwater influx from the North Sea, and freshwater runoff from land. For these variables, the available observation-based data show that the total runoff from land is significantly and positively correlated with precipitation on the sea itself, and negatively correlated with saltwater influx from the North Sea to the Baltic Sea. Overall, the physical condition (a-c) variability in the Baltic Sea and its coastal zones is found to be pairwise well-explained by simulation case differences as follows: (a) Net heat flux is a main control of sea water temperature. (b) Runoff from land, along with the correlated salt water influx from the North Sea, controls average sea salinity; with the variability of local river discharges shifting some coastal zones to deviate from the average sea condition. (c) Wind variability and change control the Baltic Sea flow structure, primarily in terms of flow magnitude and less so in terms of flow direction. For specific coastal wetland zones, considerable salinity differences from average Baltic Sea conditions (due to variability in local river discharges) are found for the coasts of Finland and Estonia, while the coastal wetland zones of south-eastern Sweden, and of Estonia and Latvia, emerge as particularly sensitive to wind shifts.