A detailed study of wind-induced coastal upwelling (CU) in the south-eastern Baltic Sea is presented based on an analysis of multi-mission satellite data. Analysis of moderate resolution imaging spectroradiometer (MODIS) sea surface temperature (SST) maps acquired between April and September of 2000-2015 allowed for the identification of 69 CU events. The Ekman-based upwelling index (UI) was applied to evaluate the effectiveness of the satellite measurements for upwelling detection. It was found that satellite data enable the identification of 87% of UI-based upwelling events during May-August, hence, serving as an effective tool for CU detection in the Baltic Sea under relatively cloud-free summer conditions. It was also shown that upwelling-induced SST drops, and its spatial properties are larger than previously registered. During extreme upwelling events, an SST drop might reach 14 • C, covering a total area of nearly 16,000 km 2 . The evolution of an upwelling front during such intensive events is accompanied by the generation of transverse filaments extending up to 70 km offshore. An analysis of the satellite optical data shows a clear decline in the chlorophyll-a concentration in the coastal zone and in the shallow Curonian Lagoon, where it drops down by an order of magnitude. It was also shown that a cold upwelling front alters the stratification in the atmospheric boundary layer, leading to a sudden drop of air temperature and near-surface winds.upwelling fronts also importantly modifies the vertical stratification and turbulent regime in the marine-atmosphere boundary layer (MABL), resulting in a change in the surface wind stress and direction in the coastal zone [3,9].Nutrient-rich waters brought up from deeper layers to the surface, particularly in the summer time, and the exposure of upwelled phytoplankton to surface radiation enhances the primary production and phytoplankton biomass during upwelling events [10,11], hence influencing the coastal pelagic communities and higher trophic levels [12]. Moreover, upwelling-related coastal ocean dynamics may eventually modify the spatial patterns of background algae blooms in the coastal zone by transporting them farther offshore.During the summer-time holiday season, CU might also have a negative impact on particular tourist areas as a result of a rapid drop in water and air temperatures near the shore [2]. Furthermore, bathing tourism depends on good water quality, while post-upwelling phytoplankton blooms might significantly reduce it, making coastal waters unattractive for recreation [13].The need to study coastal upwelling in the Baltic Sea is also essential in order to assess the regional variability of water and energy exchange, salinity dynamics, and the response of marine ecosystems to extreme events-some of the "Grand Challenges" of the Baltic Earth Science Plan [14], established in 2016. Thus, the upwelling phenomenon is indeed of certain interest to researchers, fishermen, and coastal managers [15]. The availability of wide spatial and temporal co...

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Based on the analysis of multispectral satellite data, this work demonstrates the influence of coastal upwelling on the variability of chlorophyll-a (Chl-a) concentration in the south-eastern Baltic (SEB) Sea and in the Curonian Lagoon. The analysis of sea surface temperature (SST) data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua/Terra satellites, together with Chl-a maps from Medium Resolution Imaging Spectrometer (MERIS) onboard Envisat, shows a significant decrease of up to 40–50% in Chl-a concentration in the upwelling zone. This results from the offshore Ekman transport of more productive surface waters, which are replaced by cold and less-productive waters from deeper layers. Due to an active interaction between the Baltic Sea and the Curonian Lagoon which are connected through the Klaipeda Strait, coastal upwelling in the SEB also influences the hydrobiological conditions of the adjacent lagoon. During upwelling inflows, SST drops by approximately 2–8 °C, while Chl-a concentration becomes 2–4 times lower than in pre-upwelling conditions. The joint analysis of remotely sensed Chl-a and SST data reveals that the upwelling-driven reduction in Chl-a concentration leads to the temporary improvement of water quality in terms of Chl-a in the coastal zone and in the hyper-eutrophic Curonian Lagoon. This study demonstrates the benefits of multi-spectral satellite data for upscaling coastal processes and monitoring the environmental status of the Baltic Sea and its largest estuarine lagoon.

Abstract. We analyse the cumulative impacts of climate change in a complex basin-lagoon-sea system continuum, which covers the Nemunas River basin, Curonian Lagoon, and the south-eastern part of the Baltic Sea. A unique state-of-the-art coupled modelling system, consisting of hydrological and hydrodynamic models, has been developed and used for this purpose. Results of four regional downscaled models from the Rossby Centre high-resolution regional atmospheric climate model have been bias-corrected using in situ measurements, and were used as forcing to assess the changes that the continuum will undergo until the end of this century. Results show that the Curonian Lagoon will be subjected to higher river discharges that in turn increase the outgoing fluxes into the Baltic Sea. Through these higher fluxes, both the water residence time and saltwater intrusion event frequency will decrease. Most of these changes will be more pronounced in the northern part of the lagoon, which is more likely to be influenced by the variations in the Nemunas River discharge. The southern part of the lagoon will experience lesser changes. Water temperatures in the entire lagoon and the south-eastern Baltic Sea will steadily increase, and salinity values will decrease. However, the foreseen changes in physical characteristics are not of the scale suggesting significant shifts in the ecosystem functioning, but are expected to manifest in some quantitative alterations in the nutrient retention capacity. However, some ecosystem services such as ice fishing are expected to vanish completely due to the loss of ice cover.

Multiple stressors, such as overfishing, pollution, climate change, biological invasions etc., are affecting fish communities, and thus can have versatile effects on marine ecosystems and socio-economic activities as well. Understanding the changes in the fish community structure is ecologically and economically important, yet a very complex issue, requiring comprehensive analysis of multiple factors. The role of regional oceanographic variability, namely, coastal upwelling, is often neglected when it comes to the analysis of fish assemblages. In this perspective, we were aiming, for the first time in the Baltic Sea, to assess the upwelling influence on fish communities and fish community-based ecological indices used under Marine Strategy Framework Directive. The study covered a long-term period (2000–2019) for upwelling identified by satellite data analysis and fish gillnet surveys, performed in three distinct locations in the coastal waters of the SE Baltic Sea. Overall, our study revealed that temporal dynamics of fish abundance and community composition were associated with the presence of coastal upwelling. The study outcomes suggest that the fish community was more diverse and a higher number of some fish species was observed before upwelling. During upwelling, there was more evident dominance of 1–2 main marine fish species. Through the changes in fish abundance and species composition upwelling was also responsible for the changes in fish community structure-based indices for marine environment status, i.e., in the majority of the cases a decrease in Trophic, Piscivorous Fish, and Diversity indices were observed. Our study demonstrates that upwelling can affect both, the quantitative and qualitative characteristics of coastal fish communities, therefore, it is important to consider this when predicting shifts in the distribution of fish stocks or assessing environmental status indicators, especially under changing climate. We believe that our approach adds novel information to the study of coastal ecosystems of the Baltic Sea and is important for better management of socio-economic activities in the coastal zone.