NAPSEA Review of currently used indicators, direct and indirect effects and nutrient targets

www.napsea.eu Work Package 4 Ecosystem Health 18-03-2024 DELIVERABLE 4.1 REVIEW OF CURRENTLY USED INDICATORS, DIRECT AND INDIRECT EFFECTS AND NUTRIENT TARGETS Page 2 of 24 Deliverable 4.1 Grant Agreement number 101060418 Project title NAPSEA: the effectiveness of Nitrogen And Phosphorus load reduction measures from Source to sEA, considering the effects of climate change Project DOI Deliverable title Review of currently used indicators, direct and indirect effects and nutrient targets Deliverable number D 4.1 Deliverable version concept 1 Contractual date of delivery November 1, 2023 Actual date of delivery March 26, 2024 Document status Concept Document version 1.0 Online access Yes Diffusion Public Nature of deliverable Report Work Package WP 4 Partner responsible Rijkswaterstaat Contributing Partners Umweltbundesamt (UBA), Hereon, Deltares Author(s) Lisette Enserink, Sandra Plette, Justus van Beusekom, Wera Leujak, Andreas Gericke, Theo Prins Editor van der Heijden, L.H. Approved by van der Heijden, L.H. Project Officer Christel Millet / Blanca Saez Lacave Abstract The deliverable investigates threshold values for eutrophication indicators in various water bodies, focusing on the limnic-marine gradient in specific regions. It highlights inconsistencies in assessment methods, notably reference conditions and threshold values, between directives and countries, emphasizing the need for understanding these differences to develop alternative assessment methods for ecological boundaries. Keywords eutrophication, threshold values, reference conditions, coherence, water framework directive, marine strategy framework directive, OSPAR, chlorophyll a, nitrogen Page 3 of 24 Deliverable 4.1 Contents 1. ACRONYMS ....................................................................................................................................................... 4 2. EXECUTIVE SUMMARY .................................................................................................................................... 5 3. INTRODUCTION ................................................................................................................................................. 6 4. METHODOLOGY ................................................................................................................................................ 6 5. EUTROPHICATION REFERENCE AND THRESHOLD VALUES ...................................................................... 8 5.1 OSPAR ......................................................................................................................................................... 8 5.2 MSFD ........................................................................................................................................................... 9 5.3 WFD ........................................................................................................................................................... 10 6. COMPARISON OF THRESHOLD VALUES IN THE CATCHMENTS OF THREE CASE STUDIES ................. 12 6.1 Introduction: the catchments ....................................................................................................................... 12 6.2 The Rhine catchment .................................................................................................................................. 13 6.3 The Elbe Catchment ................................................................................................................................... 14 6.4 The Wadden Sea ........................................................................................................................................ 15 6.5 Comparison of threshold values ................................................................................................................. 15 7. DISCUSSION .................................................................................................................................................... 18 8. REFERENCES .................................................................................................................................................. 20 ANNEX .................................................................................................................................................................. 24 Page 4 of 24 Deliverable 4.1 1. ACRONYMS Abbreviation Explanation DIN Dissolved Inorganic Nitrogen EQRS Ecological Quality Ratio Standardized (between 0 and 1) MSFD European Marine Strategy Framework Directive OSPAR Convention on the protection of the marine environment of the North-East Atlantic QSR Quality Status Report TN Total Nitrogen TP Total Phosphorus WFD European Water Framework Directive Page 5 of 24 Deliverable 4.1 2. EXECUTIVE SUMMARY This deliverable (D 4.1) investigates the currently used threshold values for the assessment of eutrophication indicators in marine, transitional and freshwaters. It also investigates the rationales and methods to derive these threshold values, as an acceptable deviation from environmental conditions that are less impacted by human activity. The deliverable focuses on the limnic-marine gradient, specifically in the Rhine and Elbe catchments, the Dutch and German parts of the Wadden Sea, and the areas to the North of the Wadden Islands, i.e. the plumes of the rivers Rhine, Ems and Elbe. Three indicators are considered, i.e. concentrations of chlorophyll a, dissolved inorganic nitrogen (DIN), and total nitrogen (TN). These indicators were chosen because they are to some extent applied across the entire continuum from freshwater to the sea and by both Germany and the Netherlands. The analysis shows that in general the threshold values decrease from source to sea, which is to be expected considering the dilution of nutrients released from land-based sources. However, inconsistencies appear, due to the different assessment methods used under OSPAR/Marine Strategy Framework Directive (MSFD) and the Water Framework Directive (WFD), that also consider waterbody-specific ecological conditions. In addition, the implementation of the WFD is to some extent country-specific, despite WFD intercalibration efforts. The deliverable presents a detailed description of these differences and how these evolved. This detailed understanding is a prerequisite for the next step under this work package, i.e. to develop alternative assessment methods to define safe ecological boundaries for the Wadden Sea and adjacent waters. Page 6 of 24 Deliverable 4.1 3. INTRODUCTION The NAPSEA project investigates the effectiveness of measures aimed at reducing Nitrogen and Phosphorus loads from Source to sEA considering the effects of climate change (NAPSEA acronym). Its core objectives revolve around supporting national and local authorities in identifying potent strategies to mitigate nutrient loads and creating political support for their implementation. Employing a holistic approach, the project encompasses governance and policies, nutrient pathways and measures, and ecosystem well-being. Geographically, emphasis is placed on the Wadden Sea catchment area, with detailed case studies focusing on the Rhine, Elbe, Hunze, and the Wadden Sea itself. NAPSEA functions as a platform to highlight implementation practices that are socially acceptable, sustainable, and efficient. Furthermore, it considers the influence of climate change and the supplementary advantages of measures targeting the reduction of greenhouse gas emissions. This report reviews the literature and available data on the currently used eutrophication indicators, including nutrient concentrations, direct and indirect eutrophication effects, as well as threshold values that are used in OSPAR and MSFD assessments, and in the WFD River Basin Management Plans. 4. METHODOLOGY One of the tasks of WP 4, that will focus on ecosystem health, is defining safe ecological boundaries for different types of ecosystems along the continuum from catchment to coast. This deliverable (D 4.1) intends to show and explain the evolution of threshold values along this continuum for two parameters that appear in both WFD and OSPAR/MSFD assessments: nitrogen concentration (as winter DIN and/or annual or summer total nitrogen (TN)) and chlorophyll a concentration. This continuum crosses country borders (DE-NL) as well as legal frameworks (WFD-OSPAR/MSFD), which challenges comparability of assessment outcomes. One of these challenges relates to how threshold values have been defined and which philosophy or narrative has been used. Furthermore, the expression of eutrophication effects, as a response to increased nutrient loads, depends on local physical and biological characteristics such as light climate and the presence of filter feeders. Our analysis contributes to a better understanding of the observed discontinuities from source to sea. The analysis focuses on the limnic-marine gradient, with the Wadden Sea as the ultimate receiving water body. This research partly builds on the work performed in the Interreg V A project “Wasserqualität – Waterkwaliteit” (Rönn et al., 2023; see Figure 1). However, the analysis also considers the areas to both the seaward and the landward sides of the WFD coastal water bodies (N-type), i.e. the river plumes as defined by OSPAR and WFD transitional and inland water bodies. Page 7 of 24 Deliverable 4.1 Figure 1. WFD-typology of coastal water bodies of the Netherlands and Germany, including the Wadden Sea, with current chlorophyll a threshold (EC 2018) for high/good and good/moderate boundaries, indicated as 90-percentile of chlorophyll a concentration (µg/l) of the growing season (March-September) over a six-year period. In: Rönn et al. (2023). Ecological indicators such as phytoplankton composition, macroalgae and angiosperms, are used in WFD assessments of ecological status and OSPAR/MSFD assessments of biodiversity status including pelagic habitats. However, comparison of these indicators across the limnic-marine gradient is complex, since these are type or area-specific, including different taxa. Due to time constraints, we excluded them from our analysis. Nonetheless, these indicators are addressed in the upcoming case studies. In the present deliverable, we focus on indicators of the good environmental/ecological status as proposed in the frameworks of the WFD, MSFD, and OSPAR. In the case studies, we will address safe ecological limits from a local perspective. Both viewpoints will then be synthesized to a coherent view on safe ecological limits in the riversea continuum (cf. Gericke et al., 2024). In a final stage of WP4, we will discuss whether the proposed management goals to bring the nutrient loads to levels that achieve the good environmental/ecological status (according to the WFD, MSFD and OSPAR) enable the safe ecological limits as proposed within the NAPSEA project. The information collected for this analysis is summarized in the Table in Annex I: NAPSEA Task 4.1 inventory of eutrophication indicator threshold values. The indicators selected for further analysis under this deliverable are presented in Table 1. Page 8 of 24 Deliverable 4.1 Table 1. Eutrophication indicators used across policy frameworks and countries. For these indicators the evolution across the limnic-marine border is analyzed. Policy Water type Country Indicator DIN Winter TN Summer TN annual Chlorophyll a Summer OSPAR/ MSFD river plume NL x x DE x x x WFD coastal NL x x DE x x x transitional NL x x DE x x river NL x DE x lake NL x x 5. EUTROPHICATION REFERENCE AND THRESHOLD VALUES In this chapter, the reference and threshold values for the eutrophication assessments used in OSPAR and MSFD assessments as well as in the WFD river basin management plans are described in more detail. 5.1 OSPAR OSPAR’s Quality Status Report (QSR) 2023 for the first time presented a eutrophication assessment which is coherent across country borders (OSPAR, 2023a). This is the fourth application of the Common Procedure (COMP4). Previously, OSPAR has assessed eutrophication based on national assessment areas and disparate approaches lacking a transparent and comparable basis. A more harmonized approach has now been achieved through development of ecologically relevant assessment areas defined by oceanographic criteria rather than international boundaries, allowing for consistent assessments across exclusive economic zones and acknowledging that eutrophication is a transboundary problem. Thresholds that were specific for those harmonized assessment areas and eutrophication parameters have been derived primarily from an ensemble modeling approach to determine pre-eutrophic conditions. Common assessment areas and harmonized thresholds have enabled, for the first time, an objective and comparable assessment of the eutrophication status of the whole OSPAR Maritime Area. This establishes a level playing field for managing eutrophication and a solid basis for deriving OSPAR nutrient reduction targets as a prerequisite for targeted and successful regional eutrophication management (Devlin et al., 2023). Indicators The indicators involved in the OSPAR assessment of eutrophication are: • Winter dissolved inorganic nitrogen (DIN) • Winter dissolved inorganic phosphorus (DIP) • Total nitrogen (TN) • Total phosphorus (TP) • Growing season chlorophyll a • Oxygen close to the seafloor • Secchi depth The indicators in italics are common to all OSPAR contracting parties, the others are reported by Germany and Denmark, but not by the Netherlands. Page 9 of 24 Deliverable 4.1 Reference and threshold values OSPAR agreed to use the pre-eutrophic conditions around the year 1900 as a reference for winter DIN and DIP and for chlorophyll a concentrations. These conditions were modelled using information on land use, human population size and wastewater treatment. A set of (partly overlapping) eco-hydrodynamic models estimated the distribution of resulting nutrient inputs in the sea and the chlorophyll concentrations as they would have occurred around 1900 (OSPAR, 2022). In order to allow for natural variability, and in the absence of more specific information, the assessment level was defined as the concentration 50% above the salinity-related and/or areaspecific background concentration in the first application of the Common Procedure (OSPAR, 2003). The threshold values are area-specific, taking into account e.g. the dilution of river water flowing into the sea. Many areas are shared by neighboring countries, however some smaller ones, e.g. river plumes, do not cross national borders. Figure 2 shows the so-called COMP4 assessment areas relevant for this deliverable, including the outcomes of the assessments of Winter DIN and chlorophyll a. The WFD (coastal) water bodies were not assessed by OSPAR. Figure 2. COMP4 assessment areas and the results of the COMP4 assessment of winter DIN (left panel) and chlorophyll a (right panel). The colors refer to EQRS classes, see legend on top of the Figure (OSPAR, 2023a). 5.2 MSFD The OSPAR COMP4 assessments are used for the 2024 MSFD Article 8 reporting by most OSPAR contracting parties that are also EU member states, including Germany and the Netherlands and rely on the same indicators as listed above and the same threshold values. Germany Germany reports the coastal waters including the Wadden Sea as a part of the MSFD area and recalculates the eutrophication status using OSPAR’s COMP4 assessment rules. The indicator assessments follow the WFD methodology and are using WFD thresholds. Netherlands Under the MSFD the Netherlands report the status of the coastal and open sea waters to the seaward side of the ‘basiskustlijn’. Hence, the Netherlands do not report the Wadden Sea under the MSFD and use the WFD 2021 assessment for the coastal water bodies Ems-Dollart, Waddencoast, Dutch coast (‘Hollandse kust’), Northern Deltacoast (‘Noordelijke Deltakust’) and Zeeland coast (‘Zeeuwse kust’). The OSPAR COMP4 results are used for the areas to the seaward side of the coastal WFD water bodies (Figure 3). Page 16 of 24 Deliverable 4.1 freshwaters, nitrogen is not expressed as DIN but as the inorganic N components except nitrate (NH4-N, NH3-N, and NO2-N). In Dutch fresh waters nitrogen is only expressed as TN (see Figure 7). However, the threshold values decrease from land to sea, with higher values for transitional waters. For the Rhine, the lowest value is in the Rhine Plume, which is to be expected given the dilution of river water flowing into the sea. For the Elbe Plume, the threshold value is slightly higher than the threshold value for the water body “Helgoland”. Looking at the outcomes of the area-specific assessments, the transitional and coastal waters in the Elbe catchment, including the Elbe Plume are all assessed as ‘not good'. For the Rhine catchment, the coastal waters to the North of the Dutch Wadden Islands (water body ‘Waddenkust’) are assessed as ‘good', while the Rhine Plume adjacent to this area is considered ‘not good'. Figure 6. Threshold values and states for dissolved inorganic N (DIN) of different types of water bodies belonging to rivers Rhine (left) and Elbe (right). The two river plumes (marine areas, crosses) are linked to OSPAR (MSFD), the remaining water bodies along the main rivers (arms) to the WFD. The reference points (x=0 m) are located at the German-Dutch border (r. Rhine) and the limnic-marine border (r. Elbe). The x value approximates the flow distance along the linear water bodies (rivers) extended by the shortest Euclidean distance between the endpoints of the river network and the centroid of the other water bodies. Points with a black dot refer to the Wadden Sea. The threshold values refer to the average winter DIN between December and February (OSPAR, NL) and November and February (DE). The state of German WFD water bodies refers to nitrogen. Total nitrogen (TN) Figure 7 shows the changes in threshold values for TN along the river Rhine and Elbe. Since TN is only used for fresh WFD water bodies in the Netherlands, comparison across the limnic-marine boundary of the Rhine catchment is not possible. In German freshwaters, nitrogen is expressed as separate components (NH4-N, NH3N and NO2-N). However, Germany uses a TN threshold value for the national (Rhine) and limnic-marine borders and reports TN for transitional, coastal and marine (OSPAR) water bodies/assessment areas. For the Elbe catchment a decrease in threshold values is shown from land to sea, which is consistent with the dilution of river water when it flows into the sea. The outcomes of the assessments are consistently ‘not good’ for the Elbe catchment, while a mixed pattern is seen in the Rhine catchment. In Dutch transitional and coastal waters nitrogen is expressed as winter DIN (see Figure 6) and therefore cannot be compared to the freshwater threshold values. Page 17 of 24 Deliverable 4.1 Figure 7. Threshold values and states for total N (TN) of different types of water bodies belonging to rivers Rhine (left) and Elbe (right). The two river plumes (marine areas, crosses) are linked to OSPAR (MSFD), the remaining water bodies along the main rivers (arms) to the WFD. The reference points (x=0 m) are located at the German-Dutch border (r. Rhine) and the limnicmarine border (r. Elbe). The x value approximates the flow distance along the linear water bodies (rivers) extended by the shortest Euclidean distance between the endpoints of the river network and the centroid of the other water bodies. Points with a black dot refer to the Wadden Sea. The threshold values refer to the average annual TN. The state of German WFD water bodies refers to nitrogen. Chlorophyll a Figure 8 shows the changes in threshold values for chlorophyll a along the river Rhine and Elbe. The indicator chlorophyll a is used across most of the river catchments, enabling comparison across the limnic-marine border. As an exception this indicator is not applied to the Dutch R-type water bodies, which creates a gap between the German-Dutch border and the more downstream water bodies in the Netherlands (Figure 8, left panel). In the case of the Rhine, the threshold values do not decrease consistently from land to sea as a number of (heavily modified) lakes have higher threshold values, probably due to their high retention of nutrients. Furthermore, the threshold value for the Rhine Plume is higher than the adjacent (landwards) WFD coastal water bodies. This is not the case for the Elbe, where OSPAR threshold values were adjusted to ensure a decrease from coastal WFD water bodies to the Elbe Plume. Regarding the outcome of the assessments, the more upstream part of the German Rhine is in a ‘good’ state, while the more downstream part is ‘not good’ anymore. The German water bodies along river Rhine have the same threshold value. From the German-Dutch border, good and not good status both occur and there is no distinct pattern across the limnic-marine border. For the Elbe catchment, most of the areas are assessed as being in ‘not good’ status, although some, more downstream freshwater water bodies are 'good'. Page 18 of 24 Deliverable 4.1 Figure 8. Threshold values for chlorophyll a and phytoplankton states of different types of water bodies belonging to rivers Rhine (left) and Elbe (right). The two river plumes (marine areas, crosses) are linked to OSPAR (MSFD), the remaining water bodies along the main rivers (arms) to the WFD. The reference points (x=0 m) are located at the German-Dutch border (r. Rhine) and the limnic-marine border (r. Elbe). The x value approximates the flow distance along the linear water bodies (rivers) extended by the shortest Euclidean distance between the endpoints of the river network and the centroid of the other water bodies. Points with a black dot refer to the Wadden Sea. The threshold values refer to the growing season between March and September (OSPAR, NL, transitional and coastal areas in DE) or October (rivers in DE) based on 90percentiles divided by two to approximate the seasonal mean value. 7. DISCUSSION Although a large set of indicators for the assessment of eutrophication status and the status of pelagic or benthic habitats is available across the legal assessment frameworks WFD and OSPAR/MSFD, only a limited selection is comparable across these frameworks and across the German-Dutch border. The only parameter that allows such a comparison is chlorophyll a, although this indicator is not applied to the Dutch R-type water bodies. The narratives behind the threshold values also vary. OSPAR uses modelled reference values around the year 1900 with an acceptable deviation of +50%, while for the WFD different approaches are used in Germany and the Netherlands. These are not necessarily the same as the ‘1900’ reference plus 50% and vary across water body types, see column G in the Excel matrix presented in the Annex. Therefore, expecting a consistent gradient of threshold values from land to sea is difficult. This lack of harmonization seriously hampers further analyses. This problem was also encountered by Poikane et al. (2019) who reviewed nutrient criteria for surface waters under the European WFD and came to similar conclusions. They suggest that further development of nutrient criteria should be based on relationships between ecological status and nutrient concentrations, taking into account the need for comparability between different water categories, water body types within these categories, and countries. One of the problems in setting thresholds is related to the underlying ecological problem. Depending on which problem is encountered, different goals can be formulated to mitigate the problems (Boers et al., 1995; see Table 2). Page 19 of 24 Deliverable 4.1 Table 2: A list of possible environmental goals in the coastal North Sea and target concentrations of TN in the river Rhine to encounter these environmental problems (from Boers et al., 1995) Objective Rhine, mg/l N North Sea (coastal area), mg/l N Coastal waters Natural concentration 0.6 0.34 50% biomass reduction in Spring 1.8 0.6 25% reduction of annual mean biomass 3.0 No oxygen depletion in stratified parts 3.0 Max. biomass of Phaeocystis < 5 µg/l 1.8 N-limited growth 1.8 N:P < 7 g/g Lake IJssel No dominance of blue-green algae 1.4 River Rhine Natural N:P ratio (0.15 mg/l TP) 1.9 N-limited algal growth 1.0 50% reduction of emissions 2.7-3.0 The comparison of the threshold values shows a variety of patterns, as described in section 5.6, although a general decreasing trend is seen from land to sea. The variability in patterns is partly related to the encountered environmental problems. For instance, in the inner Elbe estuary, chlorophyll a levels are good, but oxygen levels are not good. This is linked to extremely high phytoplankton biomass in the riverine part of the Elbe, low primary production in the inner estuary but ongoing high levels of grazing leading to severe oxygen depletion. In other words, we have to be aware of the environmental bottlenecks in the land-river-ocean continuum leading to a discontinuity in the gradients of certain environmental indicators. Different narratives in setting threshold values may also add to the observed discontinuities. As an example, we show recent values of chlorophyll a (summer values; May-September) from the Wadden Sea (Figure 9). The selected stations are all in or near the tidal inlet. Large differences exist between the northern and southern part of the Wadden Sea, probably due to a stronger import of organic matter and nutrients from the coastal zone to the southern Wadden Sea (NL, Lower Saxony (DE-NI) in DE) than in the northern Wadden Sea (SchleswigHolstein (DE-SH) in DE, DK; see van Beusekom et al., 2019). The status of water bodies in the Elbe catchment (DE-SH) is generally 'not good', while for the Rhine a more mixed status is found. This contrasts with the ecological status being that seagrass recovered in the northern Wadden Sea, which is influenced by the Elbe, to pre-eutrophication levels but not in the southern Wadden Sea, which is influenced by the Rhine. The green bar in Figure 9 shows the range of chlorophyll a levels prevailing when seagrass recovery accelerated. Both in the western Dutch Wadden Sea (WDWS in Figure 9) and in the Lower Saxonian Wadden Sea between the Ems and Jade (EJWS), chlorophyll a levels approach these conditions. This is in line with first signs of recovery, but additional reduction measures are needed for a full recovery of seagrass. Details will be elaborated a.o. in the case study Wadden Sea. Conclusion and Outlook The review of threshold values highlights the inconsistency of (inter)national policies. Furthermore, the review highlights apparent discontinuities in the indicators and threshold values. These discontinuities occur partly due to the fact that environmental indicators and assessments reflect the local conditions whereas overarching (downstream) factors are not always taken into account. One of the remaining challenges in the project will be to further develop an integrated view on eutrophication including both terrestrial, limnic, estuarine and marine aspects. As example we will use our case studies. Elements considered are the conditions, including nutrient loads and chlorophyll a concentrations, that enable seagrass recovery and suppress oxygen problems in the Elbe estuary. Page 20 of 24 Deliverable 4.1 Figure 9: Present average summer chlorophyll a values (May – September; 2008 – 2016). The values in the grey box at the bottom show the good-moderate boundary by assuming that the 90-percentile is twice the average. The green box denotes the range of chlorophyll a values (3.8 – 8.2 µg/l) prevailing in the northern Wadden Sea when seagrass return accelerated (van Katwijk et al., in revision). Note that the threshold values apply to the entire growing season (March – September). 8. REFERENCES Baptist, H.J.M., and Jagtman, E. 1997. Watersysteemverkenningen 1996. De AMOEBES van de zoute wateren. Rijksinstituut voor Kust en Zee: Den Haag. RIKZ 97.027, 149 pp. Behrendt, H., Bach, M., Kunkel, R. Opitz, D., Pagenkopf, W.-G., Scholz, G., and Wendland, F. 2003. Nutrient Emissions into River Basins of Germany on the Basis of a Harmonized Procedure. Texte 82/2003. 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In: Typologie, Bewertung, Management von Oberflächengewässern. Stand der Forschung zur Umsetzung der EG-Wasserrahmenrichtlinie, Feld et al. (Eds.). Limnologie Aktuell 11. Stuttgart: Schweizerbart, 46–62. Mischke, U., Venohr, M., and Behrendt, H. 2011. Using Phytoplankton to Assess the Trophic Status of German Rivers. International Review of Hydrobiology 96(5): 578–98. https://doi.org/10.1002/iroh.201111304 Mischke, U., Riedmüller, U., and Hoehn, E. 2022. Verfahrensanleitung für die Bewertung von planktondominierten Flüssen und Strömen mit Phytoplankton gemäß EG-Wasserrahmenrichtlinie. Page 22 of 24 Deliverable 4.1 PhytoFluss Online Version 5.1.x. Stand 30. November 2022. 31 pp., https://www.gewaesser-bewertungberechnung.de/files/downloads/phytofluss/Verfahrensanleitung_PhytoFluss_Version_5.1.pdf OSPAR 2003. 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G., Salas Herrero, F., Pitt, J.-A., Jarvie, H. P., Claussen, U., Leujak, W., Lyche Solheim, A., Teixeira, H., and Phillips, G. 2019. Nutrient criteria for surface waters under the European Water Framework Directive: Current state-of-the-art, challenges and future outlook. Science of the Total Environment 695: 133888. https://doi.org/10.1016/j.scitotenv.2019.133888 Prins, T., Troost, T.A., and Birk, S. 2017. Phytoplankton in NEA 3/4 coastal waters - WFD Class boundary values for chlorophyll-a. Deltares report for Rijkswaterstaat. https://kennisbank.deltares.nl/repos/11200888_000_0002.pdf Rolauffs P., Hering, D., Mischke, U., Gutowski, A., Hofmann, G., Halle, M., and Vogl, R. 2020. 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Low discharge intensifies nitrogen retention in rivers–a case study in the Elbe River. Science of the Total Environment 904: 166740. https://doi.org/10.1016/j.scitotenv.2023.166740 STOWA 2020. Referenties en maatlatten voor natuurlijke watertypen voor de Kaderrichtlijn Water 2021-2027, versie juni 2020, report 2018-49, STOWA: Amersfoort, 489 pp. https://www.stowa.nl/publicaties/referenties-en-maatlatten-voor-natuurlijke-watertypen-voor-dekaderrichtlijn-water-2021 Topu, D., Brockmann, U., and Claussen, U. 2006. Assessments of the eutrophication status in the German Wadden Sea, based on background concentrations of nutrients and chlorophyll. NERI Technical Report 573: 53-72. Available at: https://www2.dmu.dk/1_viden/2_Publikationer/3_fagrapporter/rapporter/FR573_Proceeding_Part_2.pdf van Beusekom, J. E. E., Carstensen, J., Dolch, T., Grage, A., Hofmeister, R., Lenhart, H., Kerimoglu, O., Kolbe, K., Pätsch, J., Rick, J., Rönn, L., and Ruiter, H. 2019. Wadden Sea Eutrophication: Long-Term Trends and Regional Differences. Frontiers in Marine Science 6. https://doi.org/10.3389/fmars.2019.00370 van den Berg, M. (Ed.) 2004. Achtergrondrapportage referenties en maatlatten fytoplankton, unpublished report, 41 pp. van den Berg, M., and Pot, R. (Eds.) 2007. Achtergronddocument referenties en maatlatten fytoplankton en behoeve van de Kaderrichtlijn Water. unpublished report, 61 pp. Available at: https://www.vliz.be/imisdocs/publications/ocrd/148357.pdf Page 23 of 24 Deliverable 4.1 van der Molen, D.T., and Pot, R. 2007. Referenties en maatlatten voor overgangsen kustwateren ten behoeve van de Kaderrichtlijn Water 2015-2021. Update februari 2007. STOWA, Digitale, verbeterde versie vam STOWA Rapport nr 2004-44, 50 pp. van Katwijk, M.M., van Beusekom, J.E.E., Folmer, E., Kolbe, K., de Jong, D., and Dolch, T. (in revision). Seagrass recovery trajectories and recovery potential in relation to nutrient reduction. Wiedner, C., and Schlief, J. (Eds.) 2016: Positionspapier des Projekts NITROLIMIT – Stickstofflimitation in Binnengewässern – Ist Stickstoffreduktion ökologisch sinnvoll und wirtschaftlich vertretbar? https://opus4.kobv.de/opus4-btu/frontdoor/index/index/docId/4019) Wulffraat, K.J., Smit, T., Groskamp, H., and de Vries, A. 1993. De belasting van de Noordzee met verontreinigende stoffen 1980-1990. Rijkswaterstaat: The Hague, Dienst Getijdewateren, Rapport DGW93.037, 152 pp. Page 24 of 24 Deliverable 4.1 ANNEX • The basis of the inventory of eutrophication indicators and related reference conditions and threshold values is summarized in a matrix containing: • Tables for German and Dutch assessment areas/water bodies under OSPAR/MSFD and the WFD: OSPAR_MSFD (NL and DE together), WFD NL Marine waters (K and O-types), WFD DE Marine waters (N and T-types), WFD NL Fresh waters (R and M-types), WFD DE Freshwater (R-type); • A Readme page containing legends/explanations and references. • Each Table presents for relevant assessment areas/water bodies: • Description of the indicator • Season taken into consideration • Unit (e.g. mg/l) and how it is derived (e.g. annual mean or P90) • Threshold value narrative • Threshold value for each assessment area/water body • Assessment outcome for each assessment area/water body • The matrix is available on the NAPSEA website: https://napsea.eu/wp-content/uploads/2024/03/NAPSEA-task-4.1-inventory-indicator-TVs_forpublication.xlsx