Analyses of high-resolution multibeam and sub-bottom profiling data, acquired during marine geological field cruises between 2017 and 2019 in the eastern Gulf of Finland (Baltic Sea), enabled the detailed mapping of Quaternary deposits, and revealed diverse submerged glacial and post-glacial landforms (e.g. streamlined moraine ridges, large retreat moraine ridges, De Geer moraines and kettle holes). The morphology of these glacial features provides evidence of the ice-sheet retreat direction and rate throughout the deglaciation of the region, which occurred between 13.8 and 13.3 ka BP (Pandivere­­–Neva Stage) and 12.25 ka BP (Salpausselkä I Stage). Analysis of sub-bottom profiling, supported by piston long-core sampling, indicates periods of bottom erosion/non-deposition during the Holocene caused by relative water-level regressions. Significant negative relief features are also observed in the area for the first time. These linear and curved V-shaped furrows are several kilometres long and 5 m deep, and are tentatively ascribed to bottom current and gas-seepage processes.

The Curonian Lagoon is the largest coastal lagoon of the Baltic Sea. The Curonian Lagoon is a hypereutrophic water body beset with two major problems: eutrophication and algal blooms. Biological and chemical data for the study of water eutrophication and algal blooms were collected from 4 sampling points in the coastal and off-shore areas at distances of 1 km and 4–5 km from the Curonian Spit during the period from April 2007 to November 2016. The ratio of mineral nitrogen/phosphorus forms created conditions for regular Cyanobacteria hyperblooms during the summer and early autumn. Such blooms are followed by an increase in the concentration of ammonia nitrogen, pH and BOD5, their values exceeding the threshold limits for fishery water reservoirs. A distinct peak of chlorophyll a concentration was observed in the period of freshwater Cyanobacteria hyperbloom from July to September or October. During the “hyperbloom” of Cyanobacteria, their accumulation and decomposition, which was caused by a constant wind direction, also led to the local oxygen deficit and fish mortality in the coastal zone. Chlorophyll a concentration was always at the level of intensive bloom (10–100 μg/l) and over the period of 6 years (2008, 2010, 2011, 2012, 2014, 2016) it reached the hyperbloom state (above 100 μg/l). Water temperature appeared to be one of the key factors determining seasonal and long–term variability in phytoplankton abundance and, therefore, the level of eutrophication in the Curonian Lagoon.

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