Abstract:The launch of Ocean and Land Colour Instrument (OLCI) on board Sentinel-3A in 2016 is the beginning of a new era in long time, continuous, high frequency water quality monitoring of coastal waters. Therefore, there is a strong need to validate the OLCI products to be sure that the technical capabilities provided will be used in the best possible way in water quality monitoring and research. The Baltic Sea is an optically complex waterbody where many ocean colour products, performing well in other waterbodies, fail. We tested the performance of standard Case-2 Regional/Coast Colour (C2RCC) processing chain in retrieving water reflectance, inherent optical properties (IOPs), and water quality parameters such as chlorophyll a, total suspended matter (TSM) and coloured dissolved organic matter (CDOM) in the Baltic Sea. The reflectance spectra produced by the C2RCC are realistic in both shape and magnitude. However, the IOPs, and consequently the water quality parameters estimated by the C2RCC, did not have correlation with in situ data. On the other hand, some tested empirical remote sensing algorithms performed well in retrieving chlorophyll a, TSM, CDOM and Secchi depth from the reflectance produced by the C2RCC. This suggests that the atmospheric correction part of the processor performs relatively well while IOP retrieval part of the neural network needs extensive training with actual IOP data before it can produce reasonable estimates for the Baltic Sea.

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<p>Climate change is expected to continue in the 21st century, but the magnitude of change depends on future actions. In the Baltic Sea, specifically in the Pärnu Bay region, this is predicted to mean warmer temperatures, less ice cover, more precipitations and a slight increase in average wind speed, furthermore extreme climatic events such as heavy rains, strong winds and storms will be more intense and frequent. The coastal waters play a central role in humans and nature's everyday lives as providing food, living and recreational opportunities. Since Pärnu Bay is one of the most eutrophied area in the Baltic Sea and provides living hood more the 800 fishermen, then regular monitoring is strongly recommended, but with traditional methods often unfeasible. The availability of free Sentinel satellites data with good spectral, spatial and temporal resolution has generated wide interest in how to use remote sensing capabilities to monitor coastal waters water quality, which affects the underwater light field and can lead even to changes in fish composition. However, these waters are optically complex and influenced independently by coloured dissolved organic matter, phytoplankton and an amount of suspended sediments. Therefore, the remote sensing of optically complex waters is more challenging, and standard remote sensing products often fail. In this study, we use satellite Sentinel-3 data to investigate weather phenomena as strong wind and precipitations effect to Pärnu Bay water quality parameters. We study the spatial and temporal scope of change of water quality parameters after the event. For that, we use optical water type classification based chlorophyll-a, suspended sediments and coloured dissolved organic matter algorithms on Sentinel-3 images and estimate underwater light field changes. Furthermore, we also use in situ data to analyses the frequency and the strength of weather events. Finally, we look at the composition of fish based on literature and we investigate the possible effects of the change of the underwater light field on fish composition.</p>

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