Satellite-Based Assessment of Chlorophyll-a Variability, Seasonal Trends, and Eutrophication Risk for Water Sustainability in a Strategic Desert Reservoir

Lake Nasser represents Egypt’s principal strategic freshwater reservoir; however, its long-term phytoplankton dynamics at the basin scale remain insufficiently characterized. This study presents a detailed spatiotemporal investigation of chlorophyll-a (Chl-a) variability across the reservoir during the 2002–2020 period using exclusively MODIS-Aqua Level-3 satellite observations. As no concurrent in situ measurements were available for validation, the reported values are treated as satellite-derived Chl-a estimates rather than direct observations of phytoplankton biomass. A non-parametric statistical approach, incorporating the Mann–Kendall test, Theil–Sen slope estimator, coefficient of variation (CV), and relative anomaly analysis, was used to characterize pixel-level climatology and temporal trends. The findings demonstrate that the reservoir exhibits pronounced spatial heterogeneity and a well-defined bimodal seasonal pattern associated with the annual Blue Nile flood pulse during August–October and winter convective mixing processes between November and February. Although annual-scale analysis did not indicate a statistically significant monotonic trend, seasonal investigations revealed strongly contrasting temporal responses throughout the year. At the pixel level, significant increases dominated ten of the twelve calendar months, from September through June, with median slopes among significant pixels of +0.36 to +1.47 mg m−3 yr−1. July and August reversed this pattern, combining significant declines along the northern and central main channel with the steepest increases in the record at the extreme southern inflow. The directional contrast was unaffected by control of the false discovery rate within each calendar month (q = 0.05), which retained 66% of the significant pixels. Summer conditions were additionally marked by substantial temporal variability (CV > 60%) and pronounced positive anomalies, reaching +274.51% during August. Notably, these anomalies were spatially concentrated within the central and southern sectors of the reservoir, rather than within the conventionally productive northern region. This spatial redistribution, together with the broad enrichment tendency indicated by the pixel-level trends, points to an intensification of the summer maximum within the southern inflow zone and to a possible emergence of localized eutrophication risk, a hypothesis that requires confirmation through concurrent nutrient, transparency, and dissolved-oxygen observations. Overall, the study provides valuable geospatial insights for the environmental management of Lake Nasser, emphasizing the importance of implementing targeted early-warning systems for harmful algal bloom monitoring. The results also demonstrate the critical role of satellite-based Earth observation technologies in supporting adaptive water resource management under evolving climatic and hydrological conditions.