The variation in phytoplankton composition and its nutritional values were estimated in conjunction with changes in sea waters physicochemical parameters at the Eastern Harbour, Alexandria from September 2018 to August 2019. The results explain the relationship between the biochemical contents, phytoplankton abundance, especially of diatoms, and chlorophyll-a concentration. Proteins as the major formed 54.55 % of the total biomass, followed by a much lower contribution of both carbohydrates (26.56%), and lipids (18.89 %). Each macromolecular content exhibited its maximum concentration during autumn, decreasing by order of magnitude in the spring, summer, and winter seasons. The lipids/carbohydrates ratio at < 1, indicates a high nutritional content of the phytoplankton dissolved organic matter (DOM). The highest phytoplankton biopolymeric particulate organic carbon (BPC) was observed in late July (6491.95 mg C l⁻¹), connected strongly with food material (FM), and the relative caloric content (RCC). The physical characteristics of the seawater contributed significantly to the BPC content rather than the measured chemical ones. The short-term monitoring may act as important background data for further studies in a marine system to understand the potential food quality under changing environmental conditions that directly influence the fish production and sustainability of marine food webs.