Development of models and methods to support the establishment of Danish River Basin Management Plans Scientific documentation [Blank page] Development of models and methods to support the establishment of Danish River Basin Management Plans Scientific documentation Karen Timmermann1 (Ed.) Jesper P. A. Christensen1 Stiig Markager1 1Aarhus University, Department of Bioscience Anders C. Erichsen2 (Ed.) Flemming Møhlenberg2 Hanne Kaas2 2DHI Data sheet Title: Development of models and methods to support the establishment of Danish River Basin Management Plans Subtitle: Scientific documentation Authors, AU: Karen Timmermann1, Jesper P. A. Christensen1, Stiig Markager1 Authors, DHI: Anders C. Erichsen2, Flemming Møhlenberg2, Hanne Kaas2 Institutions: 1 Aarhus University, Department of Bioscience 2 DHI Publisher: DCE – Danish Centre for Environment and Energy & DHI © URL: http://dce.au.dk/en Year of publication: 2017 Editing completed: May, 2017 Quality assurance, AU: Marie Maar, Jacob Carstensen & Poul Nordemann Jensen Quality assurance, DHI: Hanne Kaas & Ian Sehested Hansen Linguistic QA: Anne Mette Poulsen Financial support: Danish EPA Please cite as: Erichsen AC (Ed.), Timmermann K (Ed.), Christensen JPA, Kaas H, Markager S, Møhlenberg F (2017) Development of models and methods to support the Danish River Basin Management Plans. Scientific documentation. Aarhus University, Department of Bioscience and DHI, 191 pp. Reproduction permitted provided the source is explicitly acknowledged Keywords: Statistical model, mechanistic model, Water Framework Directive, marine ecosystems Layout: Anne van Acker Drawings: Graphical Department, AU Silkeborg, the authors Front page photo: Arne Magnussen/Scanpix; Hjarnø in Horsens Fjord, 2003 Number of pages: 191 Contents 1 Prologue 5 2 Introduction 7 2.1 Eutrophication of Danish waters 7 2.2 Implementation of the Water Framework Directive in Denmark 9 3 Danish marine waters 14 3.1 WFD water bodies 14 3.2 Typology 16 4 Danish monitoring data DNAMAP (NOVANA) 19 4.1 Marine monitoring data 19 4.2 Land-based N and P loadings 20 4.3 Nutrient deposition 23 5 Overview of WFD tool development in a Danish context 24 5.1 Modelling approaches 24 5.2 Model indicators 25 6 Statistical model development 27 6.1 Application of statistical models in environmental studies 27 6.2 Development of statistical models 27 6.3 Response variables 27 6.4 Model evaluation 41 6.5 Reflections about the concept and perspective for next WFD-period 46 7 Mechanistic model development 49 7.1 Application of mechanistic models in environmental studies 49 7.2 Model description 51 7.3 Model setup 57 7.4 Skills of developed models 61 8 Model application 70 8.1 Chlorophyll-a reference and corresponding target values 70 8.2 Estimation of required nutrient reductions 86 8.3 Statistical model approach 90 8.4 Mechanistic model approach 102 8.5 Cause and effects between N loadings and indicators 119 8.6 Meta models 120 8.7 Integration of results 125 8.8 Model uncertainty and sensitivity analysis 129 9 Discussion 132 9.1 Environmental pressures 132 9.2 Indicators 137 9.3 Modelling approaches 140 9.4 Achievement of GES 141 10 Conclusion 142 11 Epilogue 143 12 References 144 Appendix A – Danish water bodies 164 Appendix B – Statistical model evaluation 168 Appendix C – Mechanistic model evaluation 190 1 Prologue The Food and Agriculture Agreement (in Danish: “Fødevare- og landbrugs- pakken”) adopted by the Danish parliament in February 2016 allocated re- sources for an international evaluation of the marine models and methods that constitute the scientific foundation for the Danish River Basin Manage- ment Plan 2015-2021 (RBMP 2015-2021). The evaluation is one of more initia- tives to ensure a best practice basis for the third Danish RBMP to be imple- mented in 2021. With the mandate to evaluate the marine model tools and methodologies applied in the RBMP 2015-2021, a panel of European experts with well-established knowledge in marine ecosystems and marine man- agement has been engaged by the government to conduct such an evalua- tion in 2017. The expert panel has been instructed to (citation from their letter of instruc- tion): “for the expert panel to perform a thorough evaluation of the marine modelling tools that form the basis for the mitigation demands for land-based nitrogen (N) runoff in the Danish River Basin Management Plans, with regards to the importance of N, as well as other relevant pressures such as phosphorous, fisheries etc. In particular, the evaluation panel has to respond to points such as: • Evaluate the use of models for determination of type specific reference values (ac- cording to WFD annex 2) for the water quality element phytoplankton (chloro- phyll). • Evaluate the use of models to determine environmental targets (Max Allowable nitrogen Input MAI) and mitigation needs to achieve good environmental status – and evaluate differences and similarities between the use of different methods and model types for coastal waters with different typology. • Evaluate the estimated nitrogen target loads and mitigation needs in the Danish River Basin Management Plans and evaluate the method for determining the Danish proportion of total mitigation needs. How is the current environmental status in Danish coastal waters determined by N runoff from Danish land areas in relation to other pressures such as N released from sediments and N loads from catchments in neighbouring countries and airborne N deposition (the Dan- ish share of the total mitigation needs related to N)?” The marine models applied in the RBMP 2015-2021 were developed by DHI, an international research and consultancy institute, and Danish Centre for Environment and Energy at Aarhus University (hereafter AU). Both organi- sations are affiliated to the Ministry of Higher Education and Science. DHI and AU have been requested to prepare scientific documentation of the models and their use in English (existing material is in Danish) to be used as basis for the evaluation, with the tasks of the expert panel in mind. Main fo- cus of this report is the models and methods developed to support the estab- lishment of Danish RBMP 2015-2021, and other pressures such as phospho- rous, fisheries etc. has been assessed according to their potential impact on the Danish RBMP as part of the final discussion (chapter 9). Hence, this report presents the scientific documentation of the developed marine models and their application in the preparation of the Danish River 5 Basin Management Plan 2015-2021, including a brief discussion of other al- ternative/potential pressures than nitrogen. Objectives and content of the marine development project The overarching objective of the model development project was to support the implementation of the EU Water Framework Directive (WFD) in Danish coastal waters by providing modelling tools and methods for calculating the maximum allowable input (MAI) of nutrients, which should not be exceeded as this would prevent maintenance/achievement of good ecological status (GES) required by the WFD. The objective was supported by the following sub-objectives: • To provide and implement a toolbox for defining and improving the Danish River Basin Management Plan 2015-2021, including development of indicators for biological quality elements and supporting elements. • To ensure optimal coverage of Danish water bodies, including areas with no or few observations. • To base the development on state-of-the-art knowledge. The current report presents the scientific documentation of the models and methods developed, illustrated by examples for selected water bodies. Allocation of roles In the project on development and application of marine models for the Danish RBMP 2015-2021, which commenced in summer 2013, DHI devel- oped models and methodologies for application of dynamic mechanistic models, while AU was responsible for the development and application of statistical models. Merging of methodologies and models into one aggregat- ed output was a joint task. Briefly described, the tasks were separated as fol- lows: • DHI contributed with the development of mechanistic hydrodynamic and biogeochemical models, the subsequent calibration and evaluation of those models as well as processing of mechanistic model results and the following input to the shared management tools. • AU contributed with the development of statistical models and their sub- sequent evaluation as well as processing of statistical model results and the following input to the shared management tools. • Both institutions contributed to the development of reference values for chlorophyll-a, preparation of methods for combining model results into specific nutrient reduction targets and corresponding MAI to ensure GES in all Danish water bodies. This separation of tasks implies that the individual chapters (chapter 6, 7, 8.3 and 8.4) have different authors and that quality assurance has been under- taken according to the rules of the responsible organisation. 6 2 Introduction 2.1 Eutrophication of Danish waters During the last half of the 20th century, coastal ecosystems worldwide have been under extensive anthropogenic and climatic pressure. Nutrient en- richment, exploitation of coastal resources, overfishing, destruction of habi- tats, chemical pollution, physical changes (Boesch 2002) all impact the eco- logical systems as well the socio-economic systems (Newton et al. 2014) and cause changes in ecosystem services and human life. Coastal eutrophication, resulting from increased nutrient input, has