Total suspended matter derived from MERIS data as indicator for coastal processes in the Baltic Sea

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 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 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, 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 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.