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.

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.

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.

Coastal eutrophication is a major issue worldwide, also affecting the Baltic Sea and its coastal waters. Coastal eutrophication management requires further understanding of the complex interlinkages between drivers, coastal processes and ecosystem impacts of eutrophication. In this study, we investigate how research on Baltic coastal eutrophication has handled this complexity and what key gaps and inconsistencies still remain. We do this through a scoping review to classify and quantify the research effort reported in scientific papers with a focus on Baltic coastal eutrophication. We have identified 832 such papers and classified them in terms of study focus on drivers, impacts, and characterization of eutrophication, as well as based on study methods, and links between land, open sea, and coastal system components (anthropogenic drivers, natural drivers, coastal functions, and management aspects). 

Classification results show that more studies focus on drivers (52%) or impacts (39%) than on characterizing (20%) the coastal eutrophication. Moreover, few studies investigate drivers and impacts together, indicating that ecosystem feedbacks to eutrophication are understudied. Regarding methods, more studies focus on data collection (62%) than on linking and synthetic methods (44%; e.g. modelling). The latter tend to be more cited, however, which indicates a need for studies that can link system components beyond direct measurements. Coastal links with land and open sea are well-mentioned but much less investigated. The main coastal components are largely well-studied, separately or considering some component connections, but specific coastal functions (coastal filter), drivers (sea-ice, legacy sources) and concrete management solutions are under-investigated. More research is needed on cross-scale and multi-solution management, and on investigating synergies, trade-offs, and incentivisation of various solutions, including in the coastal ecosystem itself, like mussel farming and fishery management.