## 4 Discussion
### Water Hyacinth Assessment  
Our NDVI maps at 10 m spatial resolution showed that water hyacinth expansion varied both in space and in time. In January 2020, the water hyacinth extended along the entire eastern shore with the highest biomass observed in the northern part. The cover progressively decreased, showing its lowest values in July 2020. Thereafter, the water hyacinth started to proliferate rapidly along the shore, although the highest cover was observed in the northeastern part of the lake. The highest cover and NDVI were observed in October 2020 at the end of the wet season (Figure 8), subsequently decreasing from November to December 2020. These results confirm the previous findings by [PERSON] et al. (2020), who found that the highest water hyacinth coverage was observed at the end of the wet season (October-November 2017) [26]. However, [PERSON] et al. (2020) found that the highest cover shifted somewhat over the years from October in 2016 and 2017 to December in 2018 and 2019 [37]. Such peaks at the end of the wet season have been observed elsewhere, such as in Lake Victoria [34; 39; 66; 67] (Table S1). Mechanical removal interventions carried out by the local communities and coordinated by the Federal Government of Ethiopia were reported by the Lake Tana Protection Agency for the study year (Figure 9). In December 2020, it was reported that about 80% of the water hyacinth cover and biomass were removed from Lake Tana (Lake Tana Protection Agency, 2020). Although we recorded a further decrease in cover and biomass, in contrast to a previous study [37], our results did not confirm the reported amount of change in December 2020, suggesting that the percentage of the removal in the report was overestimated. If, however, the water hyacinth removed was indeed that high, the fact that, only a month later, the biomass level had returned to previous levels only indicates the rapid growth capacity of the water hyacinth and demonstrates that the management practices are not effective. Removal practices have been employed since 2012 and yet the expansion rate of water hyacinth has not been reduced [37]. In Lake Victoria and other areas, mechanical removal of water hyacinth succeeded in reducing cover with increased labor and logistics costs, however [11; 68]. [PERSON] et al. (2020) and [PERSON] et al. (2020) suggested that the optimal season for physical removal is during the dry season when its cover is minimal [26; 37]. However, it should be noted that water hyacinth also acts as a nutrient filter. In fact, the water hyacinth is being used as a eutrophication control tool [9; 69]. Therefore, management practices should also consider those aspects carefully. The application of satellite images at higher temporal resolution (5 day-revisit with the Sentinel-2 mission) could allow more precise short-term estimation of the extent and effectiveness of such management routines. The peak in water hyacinth, cover, and biomass occurred just one month after the peaks of Chl-a and TSM. In addition, the spatial pattern of TSM, Chl-a, and water hyacinth was similar, with the highest values in the same northeastern part of the lake during the same periods. Previous studies in the lake showed a similar trend in TSM [26] with the highest values coinciding with the rainy period and the influx of material from the watershed. In terms of the primary producers, Lake Tana is a nutrient-limited fresh-water lake during the dry season and receives a substantial amount of nutrients from its catchment in the wet season [70; 71]. This input of nutrients and sediment load from the upper catchment through the main rivers, two of which discharge into the eastern part of the lake, could trigger the proliferation of both water column phytoplankton and water hyacinth. In addition, both the water hyacinth and phytoplankton could potentially benefit from the nutrients stored in the soils of the cultivated land adjacent to the lake shores that are inundated during the wet season, as previously suggested by [PERSON] et al. (2020).