Water Resources Management (2005) 19: 713–735 DOI: 10.1007/s11269-005-3020-y C  Springer 2005 Monitoring Water Quality and Assessment of Land-Based Nutrient Loadings and Cycling in Kavala Gulf GEORGIOS SYLAIOS ∗ , NIKOLAOS STAMATIS, ARGYRIS KALLIANIOTIS and PAVLOS VIDORIS Fisheries Research Institute, National Agricultural Research Foundation, Nea Peramos 640 07, Kavala, Greece ( ∗ author for correspondence, e-mail: fri@otenet.gr; Fax: +302594022222) (Received: 24 June 2003; in final form: 7 September 2004) Abstract. The results of a systematic and intensive nutrient monitoring program are reported, which took place during the period 1997–1999 in Kavala Gulf, Northern Greece, having as a general task to study the impact of the various human activities on the nutrient loadings and cycling of the aquatic environment. A nutrient budget box model was used to estimate the annual capacity of Kavala Gulf to store and transform dissolved inorganic nitrogen and phosphorus, aiming at understanding the biogeochemical processes taking place in the area of interest. Point sources of nutrients from two municipal wastewater treatment plants (Kavala and Palio Seawage Treatment Works) and a phosphoric fertilizer producing industrial facility (PFP), as well as non-point sources from agricultural activities, seem responsible for the increased concentrations of dissolved inorganic nitrogen and phosphorus. Seasonal N:P-ratios suggest that nitrogen is the limiting nutrient during the summer, autumn and winter and phosphorus during the spring. The annual nutrient budget model revealed that Kavala Gulf is presently slightly autotrophic with nitrogen fixation exceeding denitrification. Key words: coastal zone management, dissolved inorganic nutrients, Kavala Gulf, land-based sources, nutrient budget model 1. Introduction Nutrient enrichment of coastal waters has become a continually growing world- wide problem, leading to the undesirable algal growth in the water column and the development of anoxic conditions in the underlying sediments (Anderson and Taylor, 2001; Birch et al., 1986). Such problem mostly originates from the re- lease of nitrogen and phosphorus compounds from point nutrient sources, such as wastewater treatment facilities, industrial effluents and agricultural non-point in- puts (including land drainage and animal wastes) (Balls et al., 1995). On the basis of accumulated scientific evidence, nitrogen and phosphorus control has become increasingly important for the coastal water quality management. Thus, water qual- ity and ecosystem functioning are closely related and directly reflect the impact of