Kratzer and Tett (2009), for example found that in summer (Western Gotland Basin) the measured TSM ranged from only 0.48–1.34 gm −3 in the open sea ( n = 18) and 0.48–2.77 gm −3 in coastal areas ( n = 22). Off-shore waters showed concentrations of up to 9.3 gm −3 which is much higher than found in the Baltic Sea. (2008) estuarine waters typically have much higher concentrations of TSM (8.2–73.8 gm −3) compared to coastal waters (3.0–24.1 gm −3). Examples for this are the Thames Estuary where, according to Devlin et al. In some coastal areas, there is also a strong influence of TSM due to both high river run-off and tidal influence. Many marine waters, however, are also strongly influenced by TSM, especially waters that have a strong tidal influence where the resuspension of sediments may reach much further off-shore than in the Baltic Sea. Baltic Sea waters are generally classified as optical case 2 waters with a relatively strong optical influence from CDOM ( Kowalczuk et al., 2006 Kratzer and Tett, 2009 Kratzer and Moore, 2018). Optical case 2 waters are coastal waters that are influenced by terrestrial run-off, and thus are optically also influenced by total suspended matter (TSM, also termed suspended particulate matter, SPM) and by CDOM. Typical examples of case 1 waters are clear ocean waters, and the clearest water known is the Sargasso Sea ( Kirk, 1985). Optical case 1 waters are waters where the optical properties, are dominated by phytoplankton and co-varying colored dissolved organic matter (CDOM), besides the optical properties of water itself. Marine waters are usually divided into optical case 1 and optical case 2 waters ( Morel and Prieur, 1977). The TSM transect data from areas with low wind exposure and a stable thermocline showed a gradient distribution perpendicular to the coast for summer seasons 2009, 2010, 2011, and a 3-year summer composite, confirming a previous bio-optical study from the Western Gotland basin. The effect of wind-wave stirring on the distribution of TSM from source (shore) to sink (open sea) can be assessed using satellite data from European Space Agency's (ESA) MEdium Resolution Imaging Spectrometer (MERIS) mission (2002–2012) with 300 m resolution. We also evaluate a coastal TSM transect in Himmerfjärden bay, which is located at the Swedish East coast in the Western Gotland Basin. The Gdansk basin and the Gulf of Riga were distinguished both by relatively high TSM loads with high standard deviations, indicating strong fluvial input and/or resuspension of sediments. The total suspended matter (TSM) loads during summer vary substantially in the different basins, with the south-eastern Baltic overall being most influenced by cyanobacteria blooms. In this paper, we evaluate the distribution of TSM across the Baltic Sea using remote sensing data and statistically compare the TSM loads in the different Helsinki Commission (HELCOM)-defined basins. In coastal waters, the optical properties are strongly influenced by inorganic suspended matter, which may originate from coastal erosion and from run-off from land, streams, and rivers. Remote sensing images of TSM reveal large-and mesoscale features and currents, especially in the Southern Baltic, which are influenced both by atmospheric Rossby waves and the Coriolis force. The Baltic Sea is optically dominated by CDOM, apart from cyanobacteria blooms that often cover most of the Baltic proper during summer. In essence, it is the spectral absorption and scattering properties of each optical component that govern the underwater light field, and also the color of the sea that we can perceive, and that can also be measured remotely from space. There are three optical in-water components that, besides water itself, govern the under-water light field: phytoplankton, total suspended matter (TSM), and colored dissolved organic matter (CDOM). Department of Ecology Environment and Plant Sciences, Stockholm University, Stockholm, Sweden.
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