This study was conducted in the Pemba Channel off Tanga Region in northern Tanzania to investigate physical and chemical factors that drive changes in phytoplankton biomass. Three transects off Mwaboza, Vyeru and Sahare were selected. For each transect, ten stations were sampled. Phytoplankton biomass was determined as chlorophyll-a (Chl-a) concentration. Similarly, physico-chemical variables (temperature, salinity, dissolved oxygen, pH and nutri ents) were determined. It was observed that the Chl-a concentration was significantly higher during the northeast monsoon (median 1.44 mg m-3) as compared to the southeast monsoon (median 1.19 mg m-3; W = 2216, p = 0.029). The higher productivity during the northeast monsoon is attributed to the presence of high-nutrient water caused by coastal upwelling. It is concluded that indication of upwelling, observed through relatively low temperatures during the northeast monsoon season, could be responsible for bringing nutrient-rich waters to the surface, which in turn stimulated the increase in Chl-a concentration.Western Indian Ocean Marine Science Association (WIOMSA) through its MASMA Grant