Environ Monit Assess DOI 10.1007/s10661-009-0897-6 Classification of the Caspian Sea coastal waters based on trophic index and numerical analysis Mahshid Shahrban · Amir Etemad-Shahidi Received: 8 July 2008 / Accepted: 10 March 2009 © Springer Science + Business Media B.V. 2009 Abstract In order to characterize the trophic state of the southern coastal waters of the Caspian Sea, trophic index (TRIX) as well as numerical analysis using cluster and discriminant analysis were employed in this study. Chemical and bio- logical parameters (NO 3, NO 2, NH 4 , PT, DO, and Chla) used in this study were collected seasonally from summer 1999 to spring 2000. A new trophic index developed by modification of TRIX indi- cated mesotrophic to eutrophic conditions for the Caspian Sea. Numerical analysis revealed three groups of the study area and it was found that the used methods are in good agreement. Both of them predicted poor to moderate conditions in the western part of the study area and the numerical classification predicted trophic conditions in the study area. However, TRIX was found to be a more accurate and suitable method. It performs more conservatively than the numerical classifi- cation and characterized lower classes of water quality for the stations in central and eastern parts of the study area. Keywords Trophic state · Coastal water · TRIX · Cluster · Discriminate analysis M. Shahrban · A. Etemad-Shahidi (B ) School of Civil Engineering, Iran University of Science and Technology, P.O. Box 16765-163, Narmak, Tehran, Iran e-mail: etemad@iust.ac.ir Introduction Economic development of the coastal zone often leads to adverse impact on the environmental, economic, and social value of the coast (Clark 1983). Monitoring and assessment of coastal wa- ter quality is based on the fundamental physi- cal, chemical, and biological properties of water (Clark 1996). Marine eutrophication is a coastal phenomenon resulting from a massive nutrient in- flow such as urban or industrial effluent disposal in to the sea. The negative effects of eutrophication on the environmental quality of the coast affect tourism and other economic sectors (Clark 1983). Since eutrophication has been occurred more fre- quently over recent years, words like “oligotro- phy”, “mesotrophy”, and “eutrophy” have also become more frequent in marine literature. There is a general agreement that oligotrophy means nutrient poor and eutrophy means nutrient-rich waters (Vollenweider et al. 1998). Coastal water quality issues can be studied if an accurate esti- mate of water quality is available in the form of an index for coastal waters (Swamee and Tyagi 2000). An index is a number, usually dimension- less, which expresses the relative magnitude of some complex phenomenon or condition (Gupta et al. 2003). Several quality indices have been introduced in the last years for coastal water quality. Organization for Economic Cooperation and Development index is based on mean values