Coastal zones of the Baltic Sea in the Anthropocene: Current state and the impact of climate change

Coastal zones and estuaries are highly dynamic marine systems subject to anthropogenic pressure and to climate-related changes. The coastal zone of the Baltic Sea, shaped by extreme seasonality, strong bentho-pelagic coupling, and intense human use, represents a unique model system to study these interactions under accelerating climate change. While extensive research has been conducted on warming, eutrophication, and large scale hypoxia in the region, critical gaps remain in understanding how physical forcing, sediment type, and benthic–pelagic coupling control nutrient turnover, primary production, seed and egg bank dynamics, and the emission of climate-relevant gases under rapidly changing dynamic conditions. The Baltic Sea, with 26% of its area shallower than 15 m, harbor mostly sandy sediments along the southern coast, but the coastal nutrient filter remains poorly investigated because element fluxes are mostly controlled by advection. Sediments are home for phytoplankton resting stages, but the contribution of seed germination to the development of phytoplankton blooms is unknown. The resting eggs of zooplankton are also deposited in the sediment, but the timing of zooplankton and fish development in spring and the impact of progressive warming are poorly understood. Decreases in salinity strongly influence macrobenthos in the Baltic Sea, whereas increased eutrophication favors the growth of opportunistic species. Finally, coastal darkening is discussed, as it affects marine life in ways that are difficult to assess. Future studies of coastal zones will benefit from technical innovations like mooring systems that transmits data immediately to the shore and uses drones to aid in sampling along coastlines. This review synthesizes current knowledge on the state and functioning of the Baltic coastal zone, highlighting novel insights into the role of permeable sandy sediments as nutrient cycling hotspots. We identify key uncertainties arising from the high spatio-temporal variability of these systems, which limit the predictive capacity of existing models. Only by combining novel approaches we can improve projections of climate-change impacts, and provide a robust scientific basis for the management and protection of vulnerable coastal ecosystems. Accordingly, the measures that will enable stakeholders and politicians to improve the protection of coastal areas remain to be developed. The coastal zone differs greatly from the open Baltic Sea and is characterized by extreme seasonality, a strong link between benthic and pelagic fauna, and intensive human use.Benthic–pelagic coupling, bottom currents and hypoxia control nutrient turnover, primary production, seed and egg bank dynamics, and the emission of climate-relevant gases.The consequences of climate change are increasingly affecting coastlines.This makes it necessary to develop new research approaches for effective coastal protection measures. • The coastal zone differs greatly from the open Baltic Sea and is characterized by extreme seasonality, a strong link between benthic and pelagic fauna, and intensive human use. • Benthic–pelagic coupling, bottom currents and hypoxia control nutrient turnover, primary production, seed and egg bank dynamics, and the emission of climate-relevant gases. • The consequences of climate change are increasingly affecting coastlines. • This makes it necessary to develop new research approaches for effective coastal protection measures.