20. Monitoring the Bio-optical State of the Baltic Sea Ecosystem with Remote Sensing and Autonomous In Situ Techniques

This chapter focuses on recent advances in water quality monitoring of the Baltic Sea using remote sensing techniques in combination with optical in situ measurements. Here the Baltic Sea ecosystem is observed through its bio-optical properties, which are defined by the concentration of optical in-water constituents governing the spectral attenuation of light. In the introduction, typical geographical patterns and seasonal variations of optical properties and the cause of the mass occurrence of cyanobacterial blooms in summer are discussed. The optical characteristic of Baltic Sea waters is clearly dominated by a relatively high load of dissolved organic matter and, during the productive season, by phytoplankton growth, stimulated by nutrients mostly originating from land. In the coastal zone, inorganic suspended matter also has a significant effect on the light attenuation, which increases with proximity to land. The ecological status of the coastal zone may be synthesized using a bio-optical model, summarizing important ecosystem state variables such as terrestrial runoff and phytoplankton production. The optical properties can also be observed with visible, spectral satellite remote sensing providing repetitive and optically consistent data for the whole Baltic Sea basin. Such observations have already significantly influenced our understanding of Baltic Sea dynamics and provide us with a new look into this brackish ecosystem. The focus in this chapter is on the ocean colour sensor ‘Medium Resolution Imaging Spectrometer’ (MERIS), which since 2002 is flying continuously onboard the European Environmental Satellite ENVISAT, developed by the European Space Agency (ESA). The advantage of MERIS data is its good spatial resolution of 300 m allowing the analysis of coastal features and bays of the Baltic Sea. Algorithms to retrieve bio-optical parameters from MERIS data are continuously improved and extended. The MERIS mission will be continued with the Ocean and Land Colour Instrument (OLCI), an optically similar sensor which will be flown on SENTINEL-3, scheduled to be operational until 2023, to assure long-term monitoring of water quality from space. The wide aerial coverage, the frequent repetition and continuity of the satellite observations, the consistency of the measured data, and a relative cost-effectiveness clearly respond to the demands of a modern operational monitoring system, and the requirements of effective Baltic Sea management. An overview of existing monitoring approaches is given, and operational online systems that combine remote sensing and autonomous in situ measurements are discussed.