The trophic state of the Baltic Sea a century ago: a model simulation study G. Schernewski * , T. Neumann Baltic Sea Research Institute Warnemu ¨nde (IOW), Seestrasse 15, D-18119 Rostock, Germany Received 29 April 2003; accepted 19 March 2004 Available online 17 August 2004 Abstract We apply a 3-D circulation model with a biogeochemical module (ERGOM) for the simulation of trophic conditions in the Baltic Sea a century ago. One aim is to provide reference or background data for nitrogen, phosphorus and chlorophyll, which is required for the implementation of the European Water Framework Directive (WFD). We assume that the situation a century ago serves this purpose well. Model input for this long-term simulation study are the regionally differentiated riverine and atmospheric nutrient loads to the Baltic Sea, which were compiled and calculated for a situation 100 years ago on the basis of various literature sources. For the mixed surface layer of the open Baltic Sea, we suggest maximum winter concentrations for dissolved phosphorus (dissolved inorganic nitrogen) of 0.23 – 0.35 (2.7 – 3.7) mmol/m 3 . Maximum chlorophyll-a concentrations are between 1.8 and 2.4 mg/m 3 . The concentrations of all parameters for different coastal waters vary in a wide range, depending on exposure to nutrient sources. Our nutrient concentrations for the situation a century ago are close to early measured data (1950 – 1960) and suggest that this data is suitable as reference data, as well. D 2004 Elsevier B.V. All rights reserved. Keywords: Water framework directive; Reference conditions; Nitrogen; Phosphorus; Chlorophyll; River loads 1. Introduction The quality evaluation and classification of the present trophic state of a coastal or marine system requires knowledge about the background or reference conditions. Reference conditions usually refer to a state as close as possible to the natural, pristine conditions, where anthropogenic impacts are more or less negligible. The deviation and determination of reference information received great attention due to the new European water framework directive (WFD). The WFD, launched in December 2001, is a very comprehensive and demanding environmental legislation of the European Union (European Parlia- ment and the Council, 2000). The aim is good or high ecological quality in all European surface waters, including coastal water bodies. The first implementa- tion step is the definition of coastal water types which are spatially defined by geographical, morphometri- cal, hydrophysical and hydrochemical factors. For every coastal water type, information on reference conditions has to be provided. In this respect, refer- ence conditions refer to a very good ecological state of 0924-7963/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2004.03.007 * Corresponding author. E-mail address: Gerald.Schernewski@io-warnemuende.de (G. Schernewski). www.elsevier.com/locate/jmarsys Journal of Marine Systems 53 (2005) 109 – 124