QUANTITATIVE ASSESSMENT OF EUTROPHICATION: A SCORING SYSTEM FOR CHARACTERISING WATER QUALITY IN COASTAL MARINE ECOSYSTEMS MICHAEL KARYDIS Department of Environmental Studies, University of the Aegean, 811 O0 Mytilini, Greece (Received: July 1995; revised: October 1995) Abstract. A scoring system based on nutrient concentrations was developed to assess coastal water quality according to the trophic level. Three nutrient data sets from eutrophic, mesotrophic and oligotrophic waters were used as the reference information for setting up a semi quantitative water quality scale (from 0 to 5) to express different nutrient loadings. The validity and sensitivity of the method was applied to a number of stations spaced out along the coastal area of Rhodes (Greece). A score for each nutrient / sampling site was calculated and the scorecard formed, was the data matrix used for numerical classification of the stations. The results showed (a) good discrimination between eutrophic, mesotrophic and oligotrophic waters (b) nitrate among the nutrients showed the maximum sensitivity in characterising pollution levels. The reference data sets used for assessing eutrophication levels ensured the objectivity of the method. The proposed method is described step-by-step and it is suggested that the method can be further adapted to describe other forms of pollution becoming a useful quantitative technique in coastal management practices. I. Introduction Eutrophication is the process of enrichment of water with nitrogen and phosphorus that stimulate primary production leading to enhanced algal growth and sometimes to phytoplankton blooms (Likens, 1972). The impact from nutrient enrichments is also reflected on the increased bacterial biomass and reduced water transparency, both parameters related to water quality (UN, 1984). Although eutrophication has been considered as the oldest problem of water quality created by mankind (Vol- lenweider, 1992) in lakes (Goldman and Home, 1983) and coastal areas (Colombo et al., 1992), the quantitative assessment of eutrophication is still a problem under investigation (Vollenweider et al., 1992). In addition to the complex bodies of data including a large number of variables (Field et al., 1982), the seasonal trends in nutrient concentrations (Pagou and Ignatiades, 1988) and the difficulty in dis- tinguishing between natural (autochthonus eutrophication) and human induced stresses in the environment (Warwick, 1993), methodological shortcomings have also to be taken into account. Relatively large analytical errors at low concentrations, mainly concerning the control sites eliminate the use of powerful statistical tools (Vounatsou and Karydis, 1991) excluding mainly univariate techniques (Zar, 1984; Austen and Warwick, 1989); In addition, different parametric values recorded in various sampling sites do not necessarily mean different levels of eutrophication and they introduce noise into Environmental Monitoring and Assessment 41: 233-246, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.