## 4 Results
### Discussion  
Figure 3 collects results of two days in which all three satellites have done an acquisition, showing from left to right the S3 Chl-a product, the S2 Chl-a product (whose validation is documented in EOMORES D5.3 [42]), the S1 WR index map and the spatial and temporal evolution of ECMWF reanalysis wind vector. Areas affected by cloud cover are masked out in Chl-a maps as well as in S1 images areas where \(w_{\text{ECMWF}}\) is lower than 2 m/s. Times are expressed in UTC (Coordinated Universal Time). Given the unavailability of synchronous SAR/optical images and the absence of in-field measurements in conjunction with the S1 acquisition, this research is focused to investigate whether the \"anomaly\" observed in SAR wind retrieval is compatible with a spatio-temporal evolution of scum observed in optical images. First line of Figure 3 refers to the 29 August 2018 and is an example of slow bloom evolution typical of calm or gentle wind conditions [16]. The area with Chl-a \(\gtrsim\) 100 mg/m\({}^{3}\), where visible, remains almost unvaried in the time between S3 and S2 acquisition. S1 image is acquired six hours later the S2 ones. The area colored in red in Figure 3c is a little wider and slightly shifted to East in respect to the high Chl-a concentration area observed in Figure 3a,b. It is reasonable to envision that the associated wind speed (\(\approx\)1 m/s) is too low to transport the surface accumulation of cyanobacteria elsewhere too far in the lagoon but large enough to be responsible for a slight convection. Considering the wind direction during the afternoon, see orange arrows in panel Figure 3d, this convection should be eastward. Moreover, as shown in [PERSON] et al. [41], Chl-a concentration tends to increase during the day, reaching a maximum at mid afternoon for calm weather condition. We can therefore conclude that the red area observed for the S1 afternoon image is overall compatible with the evolution of the Chl-a maps driven by vector spatio-temporal variation. These finding support the hypothesis that WR < 0.1 can indicates phytoplankton bloom. Second line in Figure 3 refers to 10 October 2018 and is an example of medium wind strength--wind speed (\(\approx\)3.5 m/s) is high enough to be able to transport the bloom on the water surface but not enough to increase the mixing layer depth and the turbulence level in the water column. Comparison between Chl-a products from S3/OLCI, panel (e) and S2/MSI, panel (f), highlights how the phenomenon can quickly evolve in less than two hours. WR map from S1, panel (g), shows an alert area consistent with a realistic spatio-temporal evolution of the high Chl-a concentration area observed from optical images and the whole day wind vector direction and intensity. These results seem to overall support the usability of the proposed approach, that in turn would confirm the leading role of solar radiance and wind speed and direction for daily spatial distribution and accumulation of positively buoyant cyanobacteria as discussed in References [20; 41]. In Figure 4, results from those days where acquisitions from S1 and S3 are available are shown. The first line refers to the 17 August 2018 and also presents a case where presumably bloom increases its area in the afternoon and moves accordingly with wind direction. Second line refer to the 18 October 2018 morning time. S3 indicates bloom area in the south-est region; WR map locate the alert area in a region that is almost congruent with that showed in panel (e) with no significant spatio-temporal evolution. Figure 5 collects results of those days where acquisition from S1 and S2 are available. Behaviours similar to those showed above are observed. 9 September 2018 (first line) WR map suggests again a case of afternoon spreading of bloom covered area consistent with wind direction. 18 October 2018 (second line) S1 acquisition is in the early morning and identifies an alert area bigger than the bloom area observed from S2 image acquired 5 h later.