Application of Satellite-Derived Summer Bloom Indicators for Estonian Coastal Waters of the Baltic Sea  
Ian-Andreas Rahn  
1 Tartu Observatory, University of Tartu, Observatooriumi 1, 61602 Toravere, Estonia;  
are prevalent and dominate during the summer HAB events [14], whereas bloom-forming cyanobacteria, especially _Nodularia spunigena_, and _Dolichospermum_ spp., all may produce toxins [15]. Currently, in situ measurements taken sparsely and from singular measuring points are used to give an indication of the environmental status (ES) of the entire coastal area. This is problematic as the coastal areas differ from one another regarding currents, wind speed, and prevailing direction, as well as the species present (e.g., presence of vacuoles in some species), and as a result, HABs are not homogenous across the coast. Numerous problems arise from in situ sampling of cyanobacterial biomass. Using research vessels to gather water samples leads to unintended disturbances in the horizontal and vertical distribution of cyanobacteria [16]. Satellite images could provide a solution to monitoring large-scale cyanobacterial blooms [17; 18]. Satellites allow for much larger areas to be measured at much more frequent timescales (daily without cloud cover) [19]. This would permit the identification of possible parameters responsible for changes in time and space. However, the downside to using satellite data is that it can only give information about the surface of the waterbody, whereas cyanobacteria are mixed in the water column and extensive subsurface blooms can occur [20]. This is one of the reasons for match-up inconsistencies, as in situ measurements follow different methodologies and, in some instances, consider biomass in deeper layers of the water column, which satellite images cannot pick up. Furthermore, depending on the satellite, the large spatial resolution of Sentinel-3, for example, and even Sentinel-2 and Landsat, do not allow for monitoring the spatial distribution at very fine scales. This can be an issue, as Kutser (2004) has shown, as blooms can be patchy and differ at even 30 m scales in the Baltic Sea. One solution is ground-based remote sensing, as evidenced by Cook et al. [21], but this is better suited for smaller water bodies such as lakes and would not be fiscally viable for coastal areas. Currently, another issue is the inability to detect specific toxins or toxin-producing species via satellite images which hinders the usefulness of satellite data for the more accurate monitoring of blooms [22; 23]. Despite all of it, satellite remote sensing is a useful tool for monitoring large-scale bloom events through time and space. Monitoring the ES of coastal areas is required under the EU Water Framework Directive (WFD) [24]. Furthermore, monitoring and managing aquatic resources is necessary to ensure the safety of marine environments and is one of the key parameters under the United Nations' Sustainable Development Goal 6 [25]. Under the Marine Strategy Framework Directive (MSFD) (2008) of the European Commission, it is a priority for countries in the EU to monitor the ES of their waterbodies and achieve a good environmental state. One of the key parameters which must be monitored is eutrophication as it can lead to numerous cascading environmental impacts, such as ecosystem deterioration, cyanobacterial blooms, and anaerobic conditions of the benthic layers [26]. The Baltic Marine Environment Protection Commission, also known as the Helsinki Commission or HELCOM, is the organization responsible for implementing the principles of the MSFD in the Baltic Region and has coordinated the development of indicators for monitoring the ES of the region. Each HELCOM indicator evaluates a specific aspect of the Baltic Sea. The HELCOM indicators go through three stages: the candidate indicator, the pre-core indicator, and the core indicator. An indicator is officially approved when it fulfils rigorous criteria, for example, it has approved threshold values and data availability.