In this study, Istanbul, which is the most crowded city in Turkey, was selected as the study area and the chlorophyll-a distribution was monitored using GIS technologies and in situ measurements at the southwestern coast of the Istanbul metropolitan area, which would be expected to be influenced by nutrient run offs from the city. Five sampling stations located at the south- western coast of the city were selected for investigating the spatial and temporal chlorophyll-a distribution in the region. Monitoring stations were selected at locations that were either highly urbanized or heavily deteriorated. Sampling in these stations was carried out monthly between January 2008 and December 2010. The parameters of air and sea surface temperature were measured in the selected stations in situ.