## 4 Discussion  
When comparing the spectral signature exhibited in Figure 4 with the spectral signature of phytoplankton, colored dissolved organic matter (CDOM), and total suspended matter (TSM) from [PERSON] et al. [15], it is possible to infer that the Aveiro lagoon plume is dominated by TSM, as its runoff carries particulate matter from the land wash towards the sea. The marine outfall plume, however, carries a high concentration of CDOM, as the water discharged derives from urban and industrial sewage and, subsequently, from the open ocean water as a typical Case 1 water. The substantial difference in the total number of products obtained for 2016 relative to the following years (Figure 5) was due to only one satellite being in orbit. The Sentinel-2A was launched in June 2015, while Sentinel-2B was launched in March 2017 [13]. Nevertheless, the number of cloud-free scenes for 2017, 2018, and 2019, when both satellites were in orbit, was almost identical. It indicates that there are, on average, 66 cloud-free images per year of the ROI. For the results obtained from the annual visibility analysis, it is important to consider the dilution induced due to the precipitation and the hydrodynamic condition of the coastal area. Moreover, the period of the day the wastewater is discharged into the sea--information that was not accessible--can also influence the Sao Jacinto sea outfall plume's visibility. The dark water mass color seen in Figure 10 can be explained by the anaerobic activities when the old sewage decomposes, acquiring a dark grey and black color, known as stale or septic color. Also, the color may be related to the formation of sulfide produced under anaerobic conditions reacting with the metals present in wastewater [40; 41]. Ocean true color and derived reflectance water quality parameters still present limitations. For instance, the events that make turbid plume detection and coastal monitoring through S2 imagery difficult are shown in Figure 11. The sunlight phenomenon (Figure 11A) happens with the formation of a silver-mirror-like cap when the sunlight reflects at the sea surface at the same angle the satellites view it. This limitation often occurs for Sentinel-2 imagery, as the viewing geometry of the satellite makes it vulnerable to sunlight contamination [42]. Additionally, morning fog (Figure 11B) is frequently detected on the west coast of Portugal, especially in the summer, as described by [PERSON] [43]. The fog develops from dawn to morning, lasting until the early afternoon, when the air becomes warmer than the sea and the breeze blows more intensely. Thus, as this study focused on extracting information from the true color products, the visible light spectrum of the spectrum used  
Figure 11: **(A)** Sunglint and (B) morning fog events detected through Sentinel-2 satellites. for the MSI does not obtain information from the Earth's surface when an area does not receive direct sunlight (when it is cloudy or at night). Thus, for turbid plume seasonal visibility, some factors are considered to have influenced the results:  
* Water column stratification incremented with temperature, which limits the plume's ascension, as observed by [PERSON] et al. (2018) and [PERSON] (2018). According to the latest results, coastal water stratification increases from January, when it is dominated by salinity, to June, when it starts to be ruled by the water temperature as it increases throughout the summer. Coastal water experiences a less stratified gradient when the seasons change, as the dominant parameters' range is altered;
* Less storm-water runoff carrying CDOM and sediments due to less precipitation in summer months;
* Sunglint and morning fog events which occur more often towards the end of spring and early summer. As the primary force influencing the nearshore circulation patterns on the Aveiro coast, the wind prevails from the northwestern quadrant of the study area, as observed by [PERSON] et al. (2018) and [PERSON] et al. (2018).