Calculating transboundary nutrient fluxes in the North Sea and adjacent sea areas

Calculating transboundary nutrient fluxes in the North Sea and adjacent sea areas 1. Introduction This document provides material to underpin discussion on the determination of transboundary nutrient fluxes (transport) in the context of assessments of marine eutrophication carried out for OSPAR. It should be read in conjunction with the OSPAR Term of reference for intersessional work on eutrophication and more specifically a related Work Programme given in the EUC(2) Summary Record Annex 8 (products C1 and C2 of the Terms of Reference for eutrophication modeling). Documents C1 and C2 are included here as an Appendix and the complete document is available from the Cefas web site (www.cefas.co.uk/eutmod). This document sets the proposed work into context with regard to OSPAR and provides a draft specification for carrying out the proposed work. In addition, it identifies some of the key questions to be addressed at the meeting and these are highlighted in shaded text boxes in the document. These questions will be used to structure the discussion at the meeting. An amended version of this document is expected to form the basis of a set of recommendations to OSPAR for subsequent work on transboundary nutrient transport (TBNT). 2. Context For the past two decades marine eutrophication has been one of the major issues tackled by OSPAR. The OSPAR commission has agreed a Common Procedure (CP) as the first step in a strategy to combat eutrophication. The CP aims to identify those areas in the maritime region, where nutrient inputs are likely, directly or indirectly, to cause eutrophication, and to achieve a substantial reduction in the inputs of phosphorus and nitrogen into these areas. Eutrophication in the North Sea may arise as result of excessive inputs of anthropogenic nutrient loads that are discharged into the sea mainly from the large rivers. In addition to the river loads, atmospheric deposition plays a (limited) role and there are potentially significant point sources arising from the discharges from Waste Water Treatment Plants. Point sources are generally not quantified for most countries bordering the North Sea whilst diffuse sources have largely been neglected. A further contribution of nutrients to national maritime regions results from the transboundary transport of nutrient. These latter inputs are potentially important for downstream areas with coastal currents, and (temporarily) offshore depositional areas receiving nutrient inputs from adjacent marine areas. Identification and quantification of source(s) of these transboundary inputs is critical in order to be able to identify and take cost-effective measures to combat eutrophication in the OSPAR maritime area. For example, for the Dutch Continental Shelf the river Rhine is the largest, anthropogenicallyinfluenced source, but to a lesser extent the rivers Meuse, Scheldt and Ems also play a role. For these rivers the nutrient loads are not only determined by emissions in the Dutch sector of the catchment area, but also by emissions in the upstream areas of these rivers. Additionally, the nutrient loads to the Dutch Continental Shelf are also influenced by transport from the Belgian coastal zone and possibly by discharges from UK rivers. Other relevant sources include the background loads delivered through the Channel and from the Atlantic Ocean. In their turn nutrient transports from the Dutch Continental Shelf influence the German Bight. As a consequence of various transport mechanisms, multiple sources and other contributions it is not straight forward to directly link effects (e.g. assessment as a Problem Area) in maritime areas to cause in term of the individual nutrient sources. The identification of appropriate management measures is hampered by this lack of clarity and requires further work in order to identify all potential sources/contributions and the flows of the nutrients. This work programme describes how models can be used to estimate the flows of nutrients through the North Sea and to determine the relative contributions of the major sources to these flows. It is anticipated that participants will carry out a work programme, agreed at this meeting, using their own ecological model systems. 3. Objectives 1

1. to quantify the relative contributions of the various nutrient sources, with their natural and anthropogenic fractions, to transboundary transports and the consequences on ecosystem function and behavior; 2. to construct nutrient budgets for a number of predefined areas. 4. Approach To carry out the work the participants will carry out a number of defined simulations using dynamical ecological hydrodynamical models. The range of nutrient sources to be quantified are: 1. the major rivers; 2. the North Atlantic ocean and the Channel boundaries; 3. the atmosphere. 5. Definitions 5.1. Sources In the framework of this work programme sources are the different discharges into the North Sea. For the southern North Sea the important loads are from: the Channel and the northern North Sea Belgian coast Dutch rivers German rivers British rivers For the northern North Sea: Scottish rivers Danish coast Norwegian coast And for both Atmospheric deposition Substantial WWTP outflows have to be taken into account. In general diffuse sources are neglected. Question for discussion: Sources:- for which major rivers will nutrient sources be followed? 5.2. Transboundary fluxes and predefined areas Fluxes across a boundary are called transboundary fluxes. Such boundaries may be national maritime boundaries or boundaries of predefined areas. These fluxes will be measured as water volumes per unit time (Sv) or amounts of nutrients, expressed as weight per time, e.g. mol/d or mol/y. Targeted measures that may be required to reduce nutrient inputs require information on the national origin of the nutrients and on the source. Further information on the relative contribution of nutrients arising from anthropogenic and natural sources is also required. The areas of interest for work on TBNT are the national maritime areas designated as problem areas following the application of the OSPAR CP and where transboundary nutrient transport has been cited as a causative factor in the assessment. Prime candidates are (to be completed by participants): German Bight, Oystergrounds. Question for discussion: Do we agree with the proposed subdivisions of the North Sea (see Fig.1 )? Which predefined areas (e.g. one box per country) will be selected for determination of fluxes. 2

Fig. 1. Target areas for the inter-comparison of the CP assessment. Red lines and blue dotted lines delineate target areas, green lines are the national marine boundaries. The blue shaded (highlighted) areas are the designated target areas for the nutrient reduction scenario testing for the second workshop in September 2007. The non-shaded areas are potentially available for calculation of fluxes but only a subset of these will be selected following discussion at the workshop. The target areas delineated in red lines have been discussed previously with the contact person for modelling of each country involved, the ones delineated with blue dotted lines are a first proposal. Are the proposed position of transects acceptable; Should we use an idealized national boundaries for easier implementation in models. 5.3. Variables to be simulated The minimum requirement is for nitrogen, phosphorus, and phytoplankton biomass (expressed as chlorophyll), and primary production to be prognostic variables in the model. It is also recommended to include benthic variables, as prognostic variables or as a closure function. 3

Question for discussion: Specification of the form of the nutrients (dissolved, particulate, organic or total). Particulate nutrient transport how important, what measurements do we have, how do we represent it in our models? 5.4. Simulation period(s) As simulation period(s) a set of years that differ sufficiently in weather conditions (dry/wet years) is proposed. Probably (a year within) the period 1996-2003 is suitable (e.g. 1988 and 2001 were wet, 2003 dry, 1995 had an anomalous bloom), because of the availability of data. For the whole period realistic meteorological forcing is available. 2002 is the year used in the second workshop in September 2007. Is 2002 a suitable year for the calculation of the transboundary nutrient transports or do we prefer another year? 5.5. Data Data for 2002 have been made available on the web site (ftp://ftp.ifm.unihamburg.de/outgoing/lenhart/ospar). These data comprise: 1. Calibration and validation 2. River loads 3. Boundary conditions 4. Meteorological data 5. Atmospheric data For detailed description of the data, see the Data Description file. In the case of a different year being selected, new data will have to be made available. Is the data as used for this September 2007 workshop fit for the future purpose of calculating fluxes? We already have a unique data set what further data might we require to ensure comparable model results. 5.6. Detailed Approach Each participant is asked to carry out several model runs. A run with the hydrodynamical and transport model is required to estimate the water flux through a defined boundary, together with a conservative tracer to allow calculation of the fraction of this water which originates as river input. 1. A run with the coupled hydrodynamic-ecosystem model in the standard set-up in order to calculate: a. the nutrient dynamics, b. the nutrient loads, transported over specified boundaries, c. the nutrient budgets in the defined areas. 2. As 1 with a tagged variable system (e.g., Ménesguen, 2006; Wijsman, 2003) that allows tracking of specific nutrients from specific rivers through the nutrient cycle to calculate: a. the nutrient loads from a specified river, transported over specified boundaries, b. the proportions of the nutrient budgets in the defined areas originating from a specified rivers. 4

3. Different nutrient reduction scenario runs (including a scenario without anthropogenic influences) to investigate the effects of nutrient reductions on transboundary transports. 5.7. Anticipated Results The study should result in a description of: the relative contribution of the different sources to the water mass of the relevant areas; the relative contribution of the different sources to the nutrient concentrations and -loads of the relevant areas; the relative contribution of the different sources to phytoplankton blooms (preferably their share in primary production, their share in biomass) in the areas chosen. In addition to an annual mean it is important to present the results in more detail during the growing season (March-September). Will the proposed approach deliver results in a form suitable for the policy driven requirements of OSPAR and the contracting parties? What else to we want to determine during this exercise? For example do we want to: Identify (interannual) variability in TBNT by meteorological /climatological conditions Compare flux results from different models for the same location and compare to get some idea of the variability Estimate recovery capacity/time of areas designated as problem areas (the predefined areas) Set up 'transfer functions' between source and destination areas? Calculate age of tracer entering different areas. 6. Plenary discussion and agreement on work programme The questions raised above need to be addressed and conclusions reached to enable planning for the future work on transboundary nutrient transport. Questions to be addressed: Manner of presentation of results (e.g. highly resolved time series, annual means, as graphs, as maps, as video?) Effort for participants could be quite large we may decide to ask some groups to do more than others A methodology for progressing the work needs to be agreed and also a view on whether a further workshop is the best option for bringing together results from the planned work and fully addressing the OSPAR TORs It is important that following our discussions we can agree: A specification for work on calculating transboundary nutrient fluxes (What we will do!) A timetable for implementation (When we will do it!) Where required identification of who (which group) will progress any key tasks required in order to progress the agreed work programme (Who will do it!) References Ménesguen, A. & P. Cugier, 2006. A new numerical technique for tracking chemical species in a multi-source coastal ecosystem applied to nitrogen causing Ulva blooms in the Bay of Brest (France). Limnol. Oceanogr. 51: 591-601. Wijsman, J., H. Los, J. Van Beek, 2003. Filtering capacity of an estuary for nutrients. WL report Z2836. 51pp. 5

Appendix 1 Excerpt from EUC(2) Summary Record Annex 8 C. Activities relating to transboundary nutrient fluxes Planning group of the ICG-EMO for this task are: Dr David Mills (convenor/uk), Dr Hermann Lenhart (DE), Dr Alain Ménesguen (FR) and Dr Hanneke Baretta-Bekker (NL). This activity should be considered as one package and initiating work is a priority activity. C1. To provide specification for the application of models with regard to using transboundary nutrient fluxes in an assessment context. C2. A work programme should set out the elements and timeframe for the intersessional work on transboundary nutrient fluxes based on the outline appended to this document. This includes the following activities: C2.1. With regard to intercomparison studies, it seems to be premature at this stage to consider an OSPAR workshop; this would be an option at a later stage when the work on transboundary transport has progressed and protocols for model application has been developed. C2.2. Identification of locations for estimation of transport flows (for example model area boundaries). C2.3. The development of protocols for the application of models to the estimatation of transport rates (calibration, validation, assembly of shared datasets for model use) is essential for the model comparison and reliability of model results. This needs to address a range of technical activities such as: a. initial application of hydrodynamic model with conservative tracer; b. subsequent application of coupled hydrodynamic-ecological model; c. develop an agreed method for describing uncertainty in model derived estimates of transport rates; d. provide a full and detailed specification for model application, treatment and presentation of results; e. calculate the relative contribution of specific riverine nutrient sources; f. consider the relative contribution of atmospheric inputs in relation to estimates of transboundary transport fluxes; g. consider the calculation of nutrient budget for defined areas; h. whether a workshop should be held to review the results. Outline for the development of a work programme for work on transboundary transport of nutrients 1. It is proposed to develop a work programme in a similar manner to the previous workshop through the efforts of a steering group. The membership will include the previous participants from France (A. Ménesguen), Germany (H. Lenhart), the Netherlands (H. Baretta) and the United Kingdom (D. Mills). However, the team may be extended depending on levels of interest and specific planning needs. 2. The previous workshop on eutrophication modelling demonstrated the value of bringing modellers together to discuss results. On the basis of this experience and the potentially contentious nature of the findings a workshop will play an important role in reaching consensus about how to estimate transboundary nutrient transport using models. On that basis, plans will be developed to facilitate such an intercomparison and this document will outline the approach to be taken. 3. As a first step in defining the work programme a clear definition of transboundary transport needs to be agreed. Its units and its manner of presentation (e.g. highly resolved time series, annual means) need to be specified in such a way that it provides the most useful information for (OSPAR) eutrophication assessment purposes. Following this step the location of sections for estimation of transboundary transport rates (fluxes) needs to be agreed. These cannot simply be imposed but 6

consultation with likely participants is required. While the primary focus will be on transport of anthropogenically derived nutrients it will be important to consider the relative transboundary transport of nutrients from natural sources, including that arising from natural background concentrations. 4. Development of protocols will be the next step in the preparatory phase. A method for model application will be described including proposals for calibration and validation. As with the prior workshop opportunities to gather and make available the generic data for forcing and boundary conditions will be identified, and a plan for implementation will be developed. The application method will also take into account the likely need for specification of scenario testing in order to identify the relative contribution of different sources of natural and anthropogenic inputs to any computed fluxes. 5. A stepwise approach to model applications is likely to be proposed. Step 1 will require application of a hydrodynamic model to estimate water flux through a defined boundary. A specified conservative tracer will be used to allow estimation of the proportion of this water which originates as river input. This will allow calculation of anthropogenic nutrient transboundary transport. Step 2 will be to apply the coupled hydrodynamic-ecosystem model in order to calculate the transport of nutrients under different nutrient reduction scenarios. This approach would allow investigation of the effects of the nutrient reductions on transboundary transport. A final step 3 could be to use a tagged variable system that allows tracking of specific nutrients from specific rivers through the nitrogen cycle until it crosses a boundary. These steps involve applications of increasing complexity and it is possible that not all participants will have the time or capability to implement each one. Therefore, some flexibility in the requirements for participation in the workshop will be required. 6. A key issue to be addressed is the specification of the form of nutrient to be modelled i.e. whether dissolved, particulate, organic or total. 7. Whilst it is recognised that determining the level of uncertainty in any estimate of transboundary transport flux is important, it is likely to prove difficult in practice not least because of the paucity of observations to compare with model results. It may be possible to use results from different models for the same location and compare fluxes get some idea of the variability in predicted transport rates. 8. An important outcome from the previous OSPAR workshop on eutrophication modelling was a recognition of the need for detailed description of the treatment of model results and their presentation in order to aid interpretation of results. An appropriate scheme for treatment and presentation of result will be developed. 9. A critical part of the model application will be to determine the relative importance of specific riverine inputs and also the relative contribution of natural oceanic sources of nutrients. This information is required in order to assess the contribution of particular contracting parties of riverderived nutrients to fluxes of nutrients across national boundaries. The relative contribution of atmospheric sources will also need to be taken into account. 7