In this study, the Level-2 products of the Ocean and Land Colour Instrument (OLCI) data on Sentinel-3A are derived using the Case-2 Regional CoastColour (C2RCC) processor for the SentiNel Application Platform (SNAP) whilst adjusting the specific scatter of Total Suspended Matter (TSM) for the Baltic Sea in order to improve TSM retrieval. The remote sensing product “kd_z90max” (i.e., the depth of the water column from which 90% of the water-leaving irradiance are derived) from C2RCC-SNAP showed a good correlation with in situ Secchi depth (SD). Additionally, a regional in-water algorithm was applied to derive SD from the attenuation coefficient Kd(489) using a local algorithm. Furthermore, a regional in-water relationship between particle scatter and bench turbidity was applied to generate turbidity from the remote sensing product “iop_bpart” (i.e., the scattering coefficient of marine particles at 443 nm). The spectral shape of the remote sensing reflectance (Rrs) data extracted from match-up stations was evaluated against reflectance data measured in situ by a tethered Attenuation Coefficient Sensor (TACCS) radiometer. The L2 products were evaluated against in situ data from several dedicated validation campaigns (2016–2018) in the NW Baltic proper. All derived L2 in-water products were statistically compared to in situ data and the results were also compared to results for MERIS validation from the literature and the current S3 Level-2 Water (L2W) standard processor from EUMETSAT. The Chl-a product showed a substantial improvement (MNB 21%, RMSE 88%, APD 96%, n = 27) compared to concentrations derived from the Medium Resolution Imaging Spectrometer (MERIS), with a strong underestimation of higher values. TSM performed within an error comparable to MERIS data with a mean normalized bias (MNB) 25%, root-mean square error (RMSE) 73%, average absolute percentage difference (APD) 63% n = 23). Coloured Dissolved Organic Matter (CDOM) absorption retrieval has also improved substantially when using the product “iop_adg” (i.e., the sum of organic detritus and Gelbstoff absorption at 443 nm) as a proxy (MNB 8%, RMSE 56%, APD 54%, n = 18). The local SD (MNB 6%, RMSE 62%, APD 60%, n = 35) and turbidity (MNB 3%, RMSE 35%, APD 34%, n = 29) algorithms showed very good agreement with in situ data. We recommend the use of the SNAP C2RCC with regionally adjusted TSM-specific scatter for water product retrieval as well as the regional turbidity algorithm for Baltic Sea monitoring. Besides documenting the evaluation of the C2RCC processor, this paper may also act as a handbook on the validation of Ocean Colour data.

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Abstract. Total suspended matter (TSM) is an indicator of coastal processes and can be retrieved reliably from MERIS (Medium Resolution Imaging Spectrometer) data. In this project we used MERIS TSM data from a Swedish coastal monitoring system www.vattenkvalitet.se to evaluate the physical extend of coastal processes.The data set consisted of all viable MERIS scenes during summer (June-August) 2009-2011, covering the whole Baltic Sea area. Monthly composite images were produced for each year, and the monthly composites were subsequently evaluated 10 with regards to terrestrial influence, and the typical features caused by cyanobacteria blooms (typically during July and August).Next, a composite image from early June 2011 was generated in order to exclude a possible influence from cyanobacteria on the patterns of TSM distribution. This early June composite was then used as a basis to evaluate the extent of terrestrial influence using the NW Baltic Proper (Swedish coastal areas) and the SE Baltic Proper (Latvian, Lithuanian and Polish 15 coastal waters) as examples.In both areas the averaged TSM scenes from early June were used to extract transects of TSM data perpendicular to the coast, spanning from coastal to offshore waters. Based on previous bio-optical research in the NW Baltic Sea and on further statistical analysis of MERIS scenes local coastal water thresholds were defined for different areas in the Baltic Sea. Using these local thresholds, it was found that coastal processes in the NW Baltic Sea extend to approximately 15-27 km off-shore, 20 whereas in the SE Baltic Sea the coastal influence extended to about twice the distance i.e. to about 34-52 km off-shore.Next, the trendlines of TSM distribution along transects was evaluated mathematically. The trend line for the NW Baltic proper was best described by a polynomial equation, whereas the trend line from the transect in SE coastal waters was best described logarithmically in areas of high resuspension. These differing trends indicate different hydrological regimes in the two areas, which are mostly driven by a combination of land run-off distributed by diffusional processes, and coastal 25 dynamics driven by local wind exposure.The results demonstrate that ocean colour remote sensing can provide important information for Baltic Sea research and management, as well as for the monitoring of coastal processes. The method allows for an evaluation of the extent of coastal influence, and of seasonal fluctuations in river run-off and phytoplankton dynamics. Furthermore, the concentrations of total suspended matter in the different sub-basins of the entire Baltic Sea can be compared synoptically. 30Ocean Sci. Discuss.,

Monthly CHL-a and Secchi Depth (SD) data derived from the full mission data of the Medium Resolution Imaging Spectrometer (MERIS; 2002 were analysed along a horizontal transect from the inner Bråviken bay and out into the open sea. The CHL-a values were calibrated using an algorithm derived from Swedish lakes. Then, calibrated Chl-a and Secchi Depth (SD) estimates were extracted from MERIS data along the transect and compared to conventional monitoring data as well as to data from the Swedish Coastal zone Model (SCM), providing physico-biogeochemical parameters such as temperature, nutrients, Chlorophyll-a (CHL-a) and Secchi depth (SD). A high negative correlation was observed between satellite-derived CHL-a and SD (ρ = −0.91), similar to the in situ relationship established for several coastal gradients in the Baltic proper. We also demonstrate that the validated MERIS-based estimates and data from the SCM showed strong correlations for the variables CHL-a, SD and total nitrogen (TOTN), which improved significantly when analysed on a monthly basis across basins. The relationship between satellite-derived CHL-a and modelled TOTN was also evaluated on a monthly basis using least-square linear regression models. The predictive power of the models was strong for the period May-November (R 2 : 0.58-0.87), and the regression algorithm for summer was almost identical to the algorithm generated from in situ data in Himmerfjärden bay. The strong correlation between SD and modelled TOTN confirms that SD is a robust and reliable indicator to evaluate changes in eutrophication in the Baltic proper which can be assessed using remote sensing data. Amongst all three assessed methods, only MERIS CHL-a was able to correctly depict the pattern of phytoplankton phenology that is typical for the Baltic proper. The approach of combining satellite data and physio-biogeochemical models could serve as a powerful tool and value-adding complement to the scarcely available in situ data from national monitoring programs. In particular, satellite data will help to reduce uncertainties in long-term monitoring data due to its improved measurement frequency.Sea eutrophication is recognized as a major large-scale environmental pressure, partially attributed to an excess loading of anthropogenically-derived nutrients [2][3][4]. In marine systems, eutrophication occurs predominantly in coastal areas and semi-enclosed waterbodies [2,5,6]. Nutrient loads to coastal areas have significantly increased in recent decades due to population growth especially in coastal areas. In the Baltic Sea the problem is exalted due to the relatively large catchment area of the Baltic Sea (i.e., 4.2 times the Baltic Sea), and a relatively low water exchange rate with the North Sea. For the last three decades, it is has been acknowledged that excessive amounts of nutrients such as N, P and organic matter-often represented by organic particular carbon (POC)-result in anoxic bottom waters, the spreading of dead bottom zones and increased frequency and intensity of algal...

There are three optical in-water components that, besides water itself, govern the underwater light field: phytoplankton, total suspended matter (TSM), and colored dissolved organic matter (CDOM). 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. The Baltic Sea is optically dominated by CDOM, apart from cyanobacteria blooms that often cover most of the Baltic proper during summer. 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. In coastal waters, the optical properties are strongly influenced by inorganic suspended matter, which may originate from coastal erosion and from runoff from land, streams, and rivers. 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. The total suspended matter (TSM) loads during summer vary substantially in the different basins, with the southeastern Baltic overall being most influenced by cyanobacteria blooms. 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. We also evaluate a coastal TSM transect in Himmerfjärden bay, which is located at the Swedish East coast in 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. 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.