Abstract. Submarine groundwater discharge (SGD) influences ocean chemistry,
circulation, and the spreading of nutrients and pollutants; it also shapes sea floor
morphology. In the Baltic Sea, SGD was linked to the development of terraces
and semicircular depressions mapped in an area of the southern Stockholm
archipelago, Sweden, in the 1990s. We mapped additional parts of the
Stockholm archipelago, areas in Blekinge, southern Sweden, and southern
Finland using high-resolution multibeam sonars and sub-bottom profilers to
investigate if the sea floor morphological features discovered in the 1990s
are widespread and to further address the hypothesis linking their formation
to SGD. Sediment coring and sea floor photography conducted with a remotely
operated vehicle (ROV) and divers add additional information to the
geophysical mapping results. We find that terraces, with general bathymetric
expressions of about 1 m and lateral extents of sometimes >100 m, are widespread in the surveyed areas of the Baltic Sea and are consistently formed in glacial clay. Semicircular depressions, however, are only found in a limited part of a surveyed area east of the island of Askö, southern Stockholm archipelago. While submarine terraces can be produced by several processes, we interpret our results to be in support of the basic hypothesis of terrace formation initially proposed in the 1990s; i.e. groundwater flows through siltier, more permeable layers in glacial clay to discharge at the sea floor, leading to the formation of a sharp terrace when the clay layers above seepage zones are undermined enough to collapse. By linking the terraces to a specific geologic setting, our study further refines the formation hypothesis and thereby forms the foundation for a future assessment of SGD in the Baltic Sea that may use marine geological mapping as a starting point. We propose that SGD through the submarine sea floor terraces is plausible and could be intermittent and linked to periods of higher groundwater levels, implying that to quantify the contribution of freshwater to the Baltic Sea through this potential
mechanism, more complex hydrogeological studies are required.
Effects of boating activities on aquatic vegetation in the Stockholm archipelago, Baltic Sea