The biomass size structure is an intrinsic feature of phytoplankton communities that changes along the gradients of temperature, light intensity, nutrient concentrations, water column turbulence, as well as with changes in the size and the composition of the grazer community. The variations in it alter the trophic structuring in pelagic food webs leading to the quantitative changes in the flow of organic matter to the fisheries and to the deep ocean via the biological pump. The spatio-temporal dynamics of bulk and size-fractionated phytoplankton chl a biomass were studied in relation to the environmental factors at two coastal sites along the Dardanelles Strait, to test the suitability of its use as a nutrient enrichment indicator. The magnitude and the pattern of temporal change of the bulk phytoplankton biomass were very similar at two sampling sites, despite the significant variations in the spatial distributions of nutrients. Nevertheless, the spatial variation in the size fractionation of phytoplankton biomass, with significantly higher contributions of nano- and picoplankton at the nutrient-poor site reflected the nutrient gradient. Nanoplankton size fraction influenced the dynamics of the size and the size fractionation of the phytoplankton biomass, revealed by the statistically significant relations of the biomass and the relative abundance of it with each other and with the bulk phytoplankton biomass. Thus, the study altogether demonstrated; (i) the partitioning of bulk chlorophyll a biomass into size fractions may undergo changes without an accompanying change in its magnitude along the nutrient gradients, (ii) biomass size structure is a promising attribute of phytoplankton to be used as a nutrient enrichment indicator, (iii) identification and an in-depth analysis of factors influencing nanoplankton abundance are necessary for a through understanding of the spatio-temporal dynamics of the bulk and size-fractionated phytoplankton chl a biomass in the study area. (C) 2019 Elsevier B.V. All rights reserved.