The aim of this study was to investigate nutrient availability for mussel growth in the coastal waters of NW Thessaloniki Gulf (Greece). Nutrient and chlorophyll-a measurements were taken on a monthly basis during 2014–15, along an experimental line of mussels growing in suspension and in the open sea, outside the area of intensive mussel farming. The results are based on the calculation of N:P ratio, f = NO3−-N/(NO3−-N + NH4+-N) ratio and chlorophyll-a measurements. N:P ratio was high throughout the year, showing phosphate limitation, while the f ratio revealed low values of ‘new’ nitrogen entering the system (input of nitrate). According to chlorophyll-a measurements, the coastal waters are occasionally oligotrophic. Size fractionation revealed that phytoplankton cells were mostly picoplankton (<0.45 μm). Productivity in the region decreased during the summer when repeated on-shore circulation of waters is unfavorable for external sources of nutrients, resulting in limited nutrient availability for phytoplankton growth and thus, posing a risk for reduced mussel production.