To disentangle the effect of macronutrient availability on cell-size in native populations of coccolithophores we surveyed the seasonality of the prevalent coccolithophores in the northern Red Sea and the Eastern Mediterranean and performed a series of incubations assays. The size variations of coccospheres and coccoliths of the species Gephyrocapsa huxleyi and G. ericsonii native populations from samples of monitoring stations in the Gulf of Aqaba (Station A, 29.475 °N, 34.940 °E) and in the Eastern Mediterranean Sea (Themo2, 32.820 °N, 34.380 °E) were analysed using scanning electron microscopy and the relation to specific macronutrient availability was examined.