The present work is concerned with the development of methodological procedures for studying the spatial distribution of eutrophication in the marine environment. Seven ecological indices (S, number of phytoplankton species; N, total number of individuals; D(Mg), Margalef's index; D(Mn), Menhinick's index; D(Od), Odum's species per thousand individuals; H', Shannon's Diversity index; E, Evenness index), based on phytoplankton community data collected from Saronicos Gulf, Greece were used for describing the eutrophication state of the sea environment. A representation of the spatial distribution for each ecological index was developed using the Inverse Distance Weighted (IDW) interpolation method. A eutrophication scale for each index was also developed for indicating four different trophic levels in the marine environment (eutrophic, upper-mesotrophic, lower-mesotrophic, oligotrophic); categorization of the interpolated values of each index resulted in a clear illustration of these trophic levels on seven thematic maps. Two methodological procedures were finally applied for synthesizing the information of these thematic maps. The purpose was the development of a final map illustrating the spatial distribution of eutrophication in the study area. The first procedure was based on the integration of unsupervised and supervised classification methods, widely used in the field of remote sensing, while the second one on the overlay technique, simply carried out within the frame of a Geographical Information System (GIS). The hybrid unsupervised/supervised classification method can assess the optimal number of classes in which data values from simultaneously considered parameters could be allocated, while the overlay technique is an additive procedure not taking into account existing trends in the dataset. The advantages and disadvantages of each procedure are further discussed.