Estuaries under pressure surveying the extreme shallow water environments

River estuaries are diverse coastal ecosystems that have significant ecological, social, cultural and economic value. Estuaries worldwide are stressed by increasingly intensive human activities, also in the Baltic Sea, a European inland sea. Human pressures include e.g., dredging, port constructions, river water acidification and pollutants. In the latest assessment of threatened habitat types in Finland, coastal estuaries were assessed as an Endangered (EN) habitat complex due to historical abiotic and biotic quality changes. As estuaries are often very shallow environments with turbid water column, it is not easy to acquire detailed seabed information from those areas. In the ongoing SeaMoreEco project we use remote sensing methods such as shipborne acoustic surveys, floating drones, flying drones and satellites, as well as seabed sampling and underwater video observations to map and monitor shallow water areas of the Gulf of Bothnia (GoB), northern Baltic Sea. We provide information e.g., on seabed geology and underwater vegetation. Here, we focus on seabed sediment data produced in the SeaMoreEco and in some other projects. Anthropogenic radionuclides and heavy metal pollution are typical pressures widely affecting river estuaries and other marine ecosystems. For example, the fallout from the April 1986 Chernobyl nuclear power plant accident has rendered the Baltic Sea as the most polluted marine body in the world with respect to 137Caesium (137Cs). In the present study we determined the levels of 137Cs activity and heavy metal content in the bottom sediments, and their spatial and vertical distribution in the subsurface sediments of the GoB. Activity contents of 137Cs and heavy metal contents in seabed surface sediments of the GoB have generally declined over the last decades. In some estuaries however, 137Cs values in subsurface sediments remain at elevated levels relative to values measured from other areas of the Baltic Sea. In some areas, also the contents of heavy metals (e.g., cadmium, lead, zinc) in the subsurface sediments are quite high. This is typical for areas close to e.g., the metal industry and the areas affected by the loading from acid sulfate soils. Data on harmful substances (e.g., radionuclides) in seabed sediments is important for coastal management and marine spatial planning while assessing risks associated with dredging and other operations. Dredging in areas where bottom sediments contain a lot of harmful substances can cause the re-mobilization and transport of these contaminants. Increasing anthropogenic pressures in coastal and marine areas will likely increase risk associated with polluted bottom sediments. Climate change might also shift many of the parameters (precipitation, river discharge) that affect sediment distribution and pollution in the coastal and marine areas, also in the GoB. This study is part of the Interreg Aurora funded SeaMoreEco project, the EMODnet Geology project funded by The European Climate, Environment, and Infrastructure Executive Agency (CINEA) through contract EASME/EMFF/2020/3.1.11/Lot 2/SI2.853812 - EMODnet Geology, the EMODnet Ingestion 3 project funded by the CINEA through contract CINEA/EMFAF/2021/3.4.10/02/SI2.868290, and the MAAMERI project funded by the Ministry of Environment, Finland. The study utilized research infrastructure facilities provided by FINMARI (Finnish Marine Research Infrastructure network).