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The variation in phytoplankton composition and its nutritional values were estimated in conjunction with changes in sea waters physicochemical parameters at the Eastern Harbor, Alexandria from September 2018 to August 2019. The results explain the relation between the biochemical contents and phytoplankton abundance, especially diatoms and chlorophyll-a concentration. Proteins as the major formed 54.55% of the total biomass, followed by much lower contribution of both carbohydrates (26.56%) and lipids (18.89%). Each macromolecular content exhibited its maximum concentration during autumn, decreasing by the order of magnitude in 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 ), and strongly connected with food material (FM) and the relative caloric content (RCC). The physical characters of the sea water contributed significantly with the BPC content rather than the measured chemical ones. The short-term monitoring may act as an important background data for further studies in a marine system to understand the potential food quality under changing environmental conditions that directly influence fish production and sustainability of marine food webs.Phytoplankton group is a key microorganism that serves as a primary producer for aquatic biota at higher trophic levels in the food chain. Thus, t h e changes in the phytoplankton community controls the quantity of food sources for higher trophic level organisms, and could consequently affect the biomass and the production of fishery resources (Kang et al., 2017). At the same time, phytoplankton can generate many essential biomolecules such as carbohydrates, proteins, and lipids, which are potentially highly bioactive compounds (Jónasdóttir, 2019). The changes in the relative abundance and biochemical composition rely on factors such as the growth phase (de Viçose et al., 2012), environmental factors (Ahn et al., 2019), species composition