UNCORRECTED PROOF ECOMOD 5270 1–15 Please cite this article in press as: Trolle, D., et al., The Water Framework Directive: Setting the phosphorus loading target for a deep lake in Denmark using the 1D lake ecosystem model DYRESM–CAEDYM. Ecol. Model. (2008), doi:10.1016/j.ecolmodel.2008.08.005 ARTICLE IN PRESS ECOMOD 5270 1–15 ecological modelling xxx ( 2 0 0 8 ) xxx–xxx available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ecolmodel The Water Framework Directive: Setting the phosphorus loading target for a deep lake in Denmark using the 1D lake ecosystem model DYRESM–CAEDYM 1 2 3 Dennis Trolle a,b,* , Henrik Skovgaard c , Erik Jeppesen a,d,* 1 a National Environmental Research Institute, University of Aarhus, Department of Freshwater Ecology, Vejlsøvej 25, PO Box 314, 8600 Silkeborg, Denmark 4 5 b Centre for Biodiversity and Ecology Research, Department of Biological Sciences, School of Science and Engineering, The University of Waikato, Private Bag 3105, 3240 Hamilton, New Zealand 6 7 c Environmental Centre Aarhus, Ministry of the Environment, Lyseng Allé 1, 8270 Højbjerg, Denmark 8 d Institute of Plant Biology, University of Aarhus, Ole Worms Allé, Building 1135, 8000 Århus C, Denmark 9 10 article info 11 12 Article history: 13 Received 26 May 2008 14 Received in revised form 27 July 2008 15 16 Accepted 21 August 2008 17 18 Keywords: 19 Lake Ravn 20 Phosphorus dynamics 21 External loading reduction 22 Lake ecosystem model 23 Water Framework Directive 24 abstract We used the one-dimensional DYRESM–CAEDYM model to elucidate and quantify the influ- ence of the external phosphorus loading on ecosystem dynamics in moderately deep Lake Ravn, which is situated in an agricultural landscape in Denmark. Model simulations were used to quantify the extent to which the external phosphorus loading must be reduced to meet upcoming lake ecological quality requirements according to the European Union Water Framework Directive (WFD). The model generally showed good agreement with observed data for temperature and oxygen from the epilimnion and hypolimnion during the calibration period (7 years) as well as the validation period (5 years); although peaks of oxygen concentrations in epilimnion during late spring often were underestimated. Phos- phate and total phosphorus (TP) concentrations were generally well reproduced in both the epilimnion and hypolimnion, though hypolimnetic phosphorus was occasionally under- estimated in late summer. There was also good agreement between monitored data and modelled biomass of diatoms and dinoflagellates as well as the zooplankton biomass of cladocerans and calanoid copepods, although the timing of biomass peaks occasionally deviated slightly from observations. Root-mean-square-errors (RMSE), used to quantify the model error, were overall similar for the calibration and the validation period. Simulations of scenarios with a reduced external TP loading suggest that a substantial reduction (40–50%) of the TP loading is required if phytoplankton biomass is to drop to a level sufficiently low to meet the proposed WFD requirements (summer average <6.5 g chlorophyll a l -1 ). The predicted outcomes of considerable loading reductions should, however, be treated with some caution, as the conceptual model in this study could not fully account for the changes in trophic structure occurring at radically reduced TP loading. This particu- larly applies to changes in the fish stock, which may have extensive cascading effects via increased zooplankton grazing on the phytoplankton when external TP loading is reduced. This would most likely lead to higher transparency than that predicted by the model. ∗ Corresponding authors at: National Environmental Research Institute, University of Aarhus, Department of Freshwater Ecology, Vejlsøvej 25, PO Box 314, 8600 Silkeborg, Denmark. Q1 E-mail addresses: dennistrolle@gmail.com (D. Trolle), ej@dmu.dk (E. Jeppesen). 0304-3800/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2008.08.005