## 2 Materials and Methods  
### Study Area  
The Estonian coastal sea is divided into 16 water bodies: Gulf of Riga (Northwest, Northeast, center), Parnu Bay, Hara and Kolga bays, Hiiu Shallow, Matsalu Bay, Haapsalu Bay, Moonsund Sea, Eru-Kasmu Bay, Muuga-Tallinna-Kakumae Bay, Kassari-Ounaku Bay, Narva-Kunda Bay, Soela Strait, Pakri bays, and Kihelkonna Bay (Table 1 and Figure 1) [38; 39]. The varying environmental states of the different areas have led to contrasting cyanobacterial bloom conditions. Narva-Kunda Bay are open areas in the southeastern Gulf of Finland. Narva-Kunda Bay is heavily influenced by currents and waves. Rivers are the main sources of pollution from heavy industry in the adjacent areas. Due to the lack of human activity and freshwater inflow, Eru-Kasmu is much less polluted. The Hara-Kolga, Muuga-Tallinna-Kakumae, and Pakri bays are all semi-enclosed shallow bays with deeper parts of the sea open to waves and currents. The influence of fresher and nutrient-rich water from the open part of the Gulf of Finland can also occur. The main sources of pollution in these areas are the city of Tallinn and maritime traffic. Hiiu Shallow, Kihelkonna Bay, and Soela Strait can be described as being strongly influenced by the Baltic Sea due to their openness. These areas are minimally impacted by the adjacent land. Haapsalu Bay is mostly shallow and semi-enclosed. The area is under the influence of effluents from the coastal city of Haapsalu. Due to poor water exchange, nutrients also accumulate in the inner part of the bay. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Frequency of In Situ Sampling**} & \\
**Coastal Area** & **Chl-a (In Situ)** & **Average Number** & **Minimum Years** & **Average Salinity,** & **Predominant** \\  & **mg m\({}^{-3}\)** & **of Sampling** & **between** & **psu** & **Depth Range, m** \\  & & **per Year** & **Measurements** & & \\ \hline Narva-Kunda Bay & 5.7 (0.7–29.5) & 5.4 & 0 & 4–5 & 5–25 \\ Eru-Käsmu Bay & 5.1 (1.8–32.1) & 4.0 & 1 & 4.5–5 & 20–50 \\ Hara and Kolga bays & 3.8 (0.2–20.3) & 4.4 & 0 & 5–5.5 & 25–80 \\ Muuga-Tallima- & 4.0 (0.2–18.0) & 11.6 & 0 & 5.5–6 & 20–50 \\ Kakumae Bay & 4.5 (0.7–8.4) & 7 & 6 & 5.5–6.5 & 10–50 \\ Pakri bays & 3.5 (1.8–7.7) & 6 & 6 & 6–7 & 10–25 \\ Haapsalu Bay & 3.5 (0.9–9.1) & 4.5 & 2 & 3–6 & 1–3 \\ Matsalu Bay & 2.8 (1.5–6.3) & 4 & 6 & 3–6 & 1–3 \\ Soela Strait & 2.7 (1.1–6.6) & 5 & 6 & 6.5–7 & 20–50 \\ Kihelkoma Bay & 1.9 (0.8–4.9) & 5 & 6 & 6.5–7 & 10–25 \\ Parnu Bay & 7.7 (2.9–47.7) & 5.3 & 0 & 4–5 & 3–5 \\ Kassari-Ounaku Bay & 1.8 (0.8–6.8) & 5.5 & 2 & 6–7 & 3–7 \\ Moonsund Sea & 2.4 (0.3–5.8) & 3.3 & 2 & 5.5–6.5 & 5–10 \\ Gulf of Riga (NW) & NA * & NA * & NA * & 5.5–6.5 & 10–25 \\ Gulf of Riga (NE) & 8.7 (2.2–71.9) & 5.1 & 0 & 5–5.5 & 10–20 \\ Gulf of Riga (central) & 3.1 (1.4–5.7) & 2.8 & 1 & 5–5.5 & 25–40 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Names and environmental parameters of the study areas. Median summer chl-a is given, minimum and maximum in brackets. Average salinity is given as the range of values between different sampling stations in a coastal area (April–October). Figure 1: Coastal areas of Estonia and in situ sampling locations are represented as white points. (a) Eru-Käsmu Bay, (b) Haapsalu Bay, (c) Hara and Kolga bays, (d) Hiiu Shallow, (e) Kassari-Ounaku Bay, (f) Kihelkonna Bay, (g) Gulf of Riga (central), (h) Gulf of Riga (NE), (i) Gulf of Riga (NW), (j) Matsalu Bay, (k) Muuga-Tallima-Kakumae Bay, (l) Narva-Kunda Bay, (m) Pakri bays, (n) Pármu Bay, (o) Soela Strait, (p) Moonsund Sea. Base image: Google Hybrid. Matsalu Bay is a typical estuary where fresh water in the eastern part transitions to brackish water (5-6 PSU) at the mouth of the bay. Kassari-Ounaku Bay is open from the west, while the eastern side is separated from the neighboring water bodies by an archipelago. In the Moonsund Sea, active north-south currents affect the physicochemical properties of the water body.