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NASA (National Aeronautics and Space Administration) has acquired to the present-day data on the temporal and spatial distribution of chlorophyll-a (Chl-a) biomass from the Moderate-Resolution Imaging Spectroradiometer (MODIS) Aqua sensor for ocean color. In our work, the daily Chl-a biomass was estimated from the MODIS product OC3M bio-optical standard algorithm. The Seasonal-Trend decomposition of time series based on Loess (STL) identified the temporal variability of the dynamical features in the MODIS products for Chl-a concentration time series in the Algerian Basin (AB) surface waters. The STL has the specificity to identify seasonal components changing over the time series composition, perfectly detects the presence of outliers, and is responsive to nonlinear trends. In the current work, this method was applied to a time series of 16 years in AB (Southwestern Mediterranean Sea) at 120 km offshore [0 to 120 km]. Decomposing the MODIS products into a trend, seasonal, and remainder components, the Chl-a indicated the dominance of the seasonal components with 108.7% from 2003 to 2018. Furthermore, interannual seasonal variation for Chl-a biomass showed the influence of the same sources of enrichment each year in the Algerian Basin.