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The requirements under the present River Basin Management Plans (RBMP) for the EU Water Framework Directive (WFD) are to reduce the total nitrogen (TN) loadings to the Hjarbæk Estuary in Denmark from the present (2015-2019) annual loading of 1900 tonnes N to 662 tonnes N in 2027. The catchment area to the estuary represents a total of 1177 km2 and the catchment is drained by four major streams. The Danish national monitoring programme has established gauging stations covering 969 km2 of the catchment area the remaining 208 km2 being ungauged areas. Modelled data on N-leaching from the root zone on agricultural fields and surface water monitoring data on N-export losses are available from the 1980’ies and onward.A detailed mapping of nitrogen (N) attenuation in the catchment have been conducted at a scale of ca. 15 km2 (ID15 sub-catchments) including mapping of both N-retention in groundwater and surface waters as well as N-delays in groundwater in Karst sub-catchments. The mapping shows large differences in N-retention in groundwater within the ID15 sub-catchment (80 %) and the same large variation is seen for N-retention in surface waters (80 %). An analysis of delays in the transport for N from fields to surface waters have shown that especially one of the four monitored catchments (Simested stream) experiences a long delay in N repsonses (> 10 years). A new portfolio of N mitigation measures to be adopted at source (e.g. catch crops, early seeding, set a side, afforestation) or during transport from field to surface water (several types of constructed wetlands, riparian buffers and restored wetlands) has been scientifically approved and made available for farmers by the Danish EPA and Agricultural Agency.The huge N-reduction needed in the Hjarbæk coastal catchment (65%) will require management efforts where farmers and authorities utilize both source oriented and transport oriented mitigation measures. These solutions should be implemented in a targeted manner guided by the local N-retention maps, as well as using all available monitoring data to pinpoint high-risk areas for N-leaching from fields and N-exports from the four sub-catchments as well as the ungauged areas. In this presentation we will showcase examples on how both targeted and collective mitigation measures can optimally be dosed in the Hjarbæk coastal catchment to reach the targets set in the RBMP 3.

The links between nitrate-nitrogen (N) leaching from agricultural fields and N measured in streams can in general terms be divided in two pathways: groundwater and a more surface-near transport (e.g. tile drains). The former with a typical slower hydrological response than the latter. Therefore, catchments with quick hydrologic pathways respond also quickly on programme of measures for N. On the other hand, catchments having bot high N-attenuation or longer N time lags makes it complicated for managers and policy makers as the response of the implemented programme of measures might both be dampened and delayed. As River Basin Management Plans (RBMPs) under the Water Framework Directive (WFD) runs in 6 years periods – such time lags might end up as an overdosing of measures. Therefore, attenuation and time lags needs to be mapped as they have major effects on the expected effects of RBMPs and its legacy for water quality. The aim of this study is to improve our understanding of N lags mapped based on 30 years of data from 160 Danish stream monitoring stations.A national wide screening for trends in annual flow-weighted total nitrogen (TN) concentrations at 163 river monitoring stations shows in most cases a downward trend (average: 30% ± 17%) during the last 30 years 1990-2019). The N-surplus has been reduced (farm gate: -44%; field: -45%) during the same period. Before 1990, the N-surplus in agriculture was increasing and started at first levelling off in the mid 1980ies. Diffuse N-sources and mostly agriculture contributed the most to TN in streams (93% ±8%) during the period 1990-2019). The reduction in the diffuse N loadings are paralleling the development of the N surplus for most Danish streams. However, in certain parts of Denmark several river monitoring stations shows a much different response, which in some cases is no response at all. Such a pattern can only be explained by N-flows in the catchments to be delayed in groundwater aquifers. Using long term data for national N-surplus a simple lag-time analysis shows that the time lags for N are long for 21 catchments (up to 20 years), medium long for N in 62 catchments and with nearly no delay for N in 80 catchments (Fig. 1). Moreover, all the stream stations experiencing long time lags are situated in the chalk and partly karstic landscapes of Denmark from the Danien period. The catchments having long delays for N shows in most cases also a very low attenuation of N in groundwater as measured N-concentrations are substantially higher than found in the streams having nearly no time lags. Therefore, we conclude that incorporation of biogeochemical and hydrologic time lag principles into water quality regulations will be necessary for providing managers and regulators with realistic expectations when implementing new policies for N.Figure 1: Map of Denmark showing catchments with short (< 20% older than 10 years), medium (20-40 % older than 10 years) and long (> 40% older than 10 years) for nitrogen in groundwater.

Intensive monitoring data from the Danish National Monitoring programme (NOVANA) from 24 smaller catchments (mean: 14 km2) was used in a two layer cross-validation to establish a model for the annual diffuse phosphorus (P) flow-weighted concentration in Danish streams. A total of 196 monitoring years with data from automatic sampling (ISCO) of water from the 24 streams were used as a training dataset. Data in the training dataset covers the period 1994-2002.Moreover, another dataset consisting of 108 agricultural mini-catchments with discrete water samples covering the period 1990-2017 was used as a control in the eight different georegions of Denmark. A total of four different models was established three models based on the intensive dataset and one model based on the larger dataset with discrete water sampling.The best model established included eight explanatory parameters and explained 53 % of the variation in the annual flow-weighted total P concentrations in the training dataset. A validation of the four different models established showed that the best model has to be bias-corrected in some of the georegions. The result of the validation shows that the models generally overestimate the total P concentrations. An overestimation of around 10-20% was to be expected as intensive automatic water sampling in streams has shown that the flow-weighted concentration of total P obtained from discrete sampling (monthly or fortnightly) is normally underestimated.The validations of the three models based on intensive dataset showed an R-square between 0.08 and 0.12. The model based on the larger data with discrete samples had an R-square (0.29).

Nitrogen (N) loads and concentrations have been successfully reduced in most Danish streams during the last 30 years. Thereby also reducing the impact of the main driver of marine eutrophication in Danish coastal waters. However, the trend in N-loads and concentrations vary substantially among the monitored streams. The understanding of this variation are of great importance and interest for the evaluation of measures implemented to combat N eutrophication and for forecasting of effects of further measures.River hydrographs can be split into base flow and quick flow components and the N concentrations in these two components can, thereafter, be calculated. The N concentration in the two components varies over time showing both longer term and seasonal variation. The quick flow component typically having a high variation reflecting present days leaching of N from fields and this strata has been significantly reduced during the last 3 decades due to a more sustainable farming practices.During base flow conditions, stream water typically holds less nitrogen due to N removal in groundwater. Reductions in agricultural nitrogen leaching over the past three decades has reduced concentrations in the quick flow component and reduced the load to ground water aquifers. As groundwater aquifers are often large with a capacity of several years of recharge, the response in base flow N-concentrations is expected to be slow compared to the response in quick flow. The low response of the N-concentrations in base flow have implications on the rate of change of the river concentrations and consequently riverine N-loads to coastal waters. In some cases, the base flow N-concentration might still be influenced by the larger N-leaching of the past (1960-1990).We have analyzed a national data set for developments in N-concentrations during base flow and quick flow. The data set covers the in country range in catchment size, land use and geology. The data set spans 29 years covering the period 1990 – 2018. In addition, measurements from a few streams monitored for a longer period have been included in the analyses

Monitoring of agricultural mini-catchments (AMC) has been part of the Danish national monitoring programme (National Monitoring Programme for Water and Nature) since 1989. Thus, nitrogen (N) concentrations and loads have been monitored in soil water, tile drains, and streams within five AMC. Moreover, extensive monitoring of N concentrations and loads in streams draining 46 mini-catchments has been conducted every year since 1989. This has resulted in two national datasets on trends in flow-weighted N concentrations relative to factors such as groundwater age and management history. We analyzed these datasets and found that the intensively monitored micro-catchments generally showed a strong signal with significant downward trends in flow-weighted N concentrations in monitored soil water (−22% to −68%), tile drains (−38% to −59%), and streams (−19% to −53%). The 46 micro-catchments monitored for N in streams also exhibited downward trends in flow-weighted N concentrations, which can mainly be ascribed to the introduction of mandatory national regulation of N in agriculture in Denmark in the mid-1980s. However, classification of the mini-catchments according to the age of the oxidized groundwater revealed significant differences in N trends between the groups of mini-catchments. Thus, the strongest downward trend in flow-weighted N concentrations was as follows: 3 years (−38%).

Land-based total nitrogen (N) loadings to Danish coastal waters have been markedly reduced since 2000. This has been achieved by general measures reducing discharges from all point sources and N leaching from farmed land supplemented with more local and targeted mitigation measures such as restoration of wetlands to increase the catchment-specific N retention. In the catchment of River Odense, restoration of wetlands has been extensive. Thus, in the major gauged catchment (485 km2) eleven wetlands (860 ha) have been restored since 2000. A comparison of data on N concentrations and loss from a gauging station in the River Odense with data from a control catchment (772 km2), in which a significantly less intensive wetland restoration programme has been undertaken, showed an excess downward trend in N, amounting to 124 t N yr−1, which can be ascribed to the intensive wetland restoration programme carried out in the River Odense catchment. In total, the N load in the River Odense has been reduced by 377 t N yr−1 (39%) since 2000. The observed downward trend is supported by monitoring data from two wetlands restored in 2001 and 2004 in the River Odense catchment.

Intensive farming has severe impacts on the chemical status of groundwater and streams and consequently on the ecological status of dependent ecosystems. Eutrophication is a widespread problem in lakes and marine waters. Common problems are hypoxia, algal blooms, fish kills, and loss of water clarity, underwater vegetation, biodiversity and recreational value. In this paper we evaluate the nitrogen (N) and phosphorus (P) concentrations of groundwater and surface water in a coastal catchment, the loadings and sources of N and P, and their effect on the ecological status of an estuary. We calculate the necessary reductions in N and P loadings to the estuary for obtaining a good ecological status, which we define based on the number of days with N and P limitation, and the corresponding stream and groundwater threshold values assuming two different management options. The calculations are performed by the combined use of empirical models and a physically based 3-D integrated hydrological model of the whole catchment. The assessment of the ecological status indicates that the N and P loads to the investigated estuary should be reduced to levels corresponding to 52 and 56% of the current loads, respectively, to restore good ecological status. Model estimates show that threshold total N (TN) concentrations should be in the range of 2.9 to 3.1 mg l−1 in inlet freshwater (streams) to Horsens estuary and 6.0 to 9.3 mg l−1 in shallow aerobic groundwater (∼ 27–41 mg l−1 of nitrate), depending on the management measures implemented in the catchment. The situation for total P (TP) is more complex, but data indicate that groundwater threshold values are not needed. The stream threshold value for TP to Horsens estuary for the selected management options is 0.084 mg l−1. Regional climate models project increasing winter precipitation and runoff in the investigated region resulting in increasing runoff and nutrient loads to the Horsens estuary and many other coastal waters if present land use and farming practices continue. Hence, lower threshold values are required in many coastal catchments in the future to ensure good status of water bodies and ecosystems.

Loading of nitrogen (N) from diffuse sources to Danish marine waters was reduced by 41% in the period 1990–2012 by a suite of general measures. However, further reductions in N loading are required if the ambitious quality goals of the EU Water Framework Directive are to be achieved. Measures will be more effective if they are implemented in N loss hot spots or risk areas. Additionally, the highly variable N reduction in groundwater and surface waters needs to be taken into account as this strongly influences the resulting effect of mitigation measures. The objectives of this study were to develop and apply an N risk tool to the entire agricultural land area in Denmark. The purpose of the tool is to identify high risk areas, i.e. areas which contribute disproportionately much to diffuse N losses to the marine recipient, and to suggest cost-effective measures to reduce losses from risk areas. In the N risk mapping part of the tool, we combined a modelled root zone N leaching with a catchment-specific N reduction factor which in combination determines the N load to the marine recipient. N leaching was calculated using detailed information of agricultural management from national databases as well as datasets of percolation and soil parameters as input parameters to an empirical N leaching model. The developed N risk tool showed that the Danish agricultural area is distributed almost evenly between N retention classes (N retention being divided into classes of 80% N retention). Hot spots for marine N loading are, however, mainly located in catchments with N retention less than 80%. Hot spots for N leaching from the root zone are mainly located in the western part of Jylland, while hot spots for marine N loading are found in all regions and especially in small catchments close to the coast. In a case study for the catchment of the Danish Odense Fjord estuary, we demonstrated how the tool facilitates cost-effective implementation of measures and how measures can be combined to reach a given reduction target for an estuary.

Since the late 1980s a wide range of regulatory measures, addressing both point sources and diffuse N pollution from agricultural land, have been implemented in estuary catchments to reduce the land based nitrogen (N) load of the Danish aquatic environment. Monitoring has been undertaken of key agro-environmental N indicators for different parts of the DPSIR concept (Driving forces, Pressures, State, Impact and Responses for N) in 10 catchments (covering 35% of the Danish land area) and their estuaries during the period 1990–2009. The N load to these 10 estuaries has been reduced by 39% in total, mostly (28%) by reducing the N loss from diffuse sources. For all the 10 linked catchments and estuaries decreasing trends have been reported for various indicators, amounting to 40–53% for the catchment N surplus, ca. 30% for the efficiency of use of cattle slurry N and ca. 40% for pig slurry N, 18–55% for the mean flow-weighted total N concentrations in inlet water to estuaries, and 24–62% for total N concentrations in the upper and middle reaches of the estuaries. In eight catchments and their estuaries we found a direct response (

We investigated leaching of dissolved phosphorus (P) from 45 tile-drains representing animal husbandry farms in all regions of Denmark. Leaching of P via tile-drains exhibits a high degree of spatial heterogeneity with a low concentration in the majority of tile-drains and few tile-drains (15% in our investigation) having high to very high concentration of dissolved P. The share of dissolved organic P (DOP) was high (up to 96%). Leaching of DOP has hitherto been a somewhat overlooked P loss pathway in Danish soils and the mechanisms of mobilization and transport of DOP needs more investigation. We found a high correlation between Olsen-P and water extractable P. Water extractable P is regarded as an indicator of risk of loss of dissolved P. Our findings indicate that Olsen-P, which is measured routinely in Danish agricultural soils, may be a useful proxy for the P leaching potential of soils. However, we found no straight-forward correlation between leaching potential of the top soil layer (expressed as either degree of P saturation, Olsen-P or water extractable P) and the measured concentration of dissolved P in the tile-drain. This underlines that not only the source of P but also the P loss pathway must be taken into account when evaluating the risk of P loss.

Documentation of the effects of different mitigation measures adopted at different scales to reduce phosphorus (P) loadings to surface waters is needed to help catchment managers select the best management practices. Water quality monitoring data from the outlets of two paired catchments (the river Odense catchment versus a neighbouring control catchment) on the island of Funen, Denmark, showed significantly different trends in annual flow-weighted P concentrations during the period 2000–2013. A significant downward trend in flow-weighted particulate P (PP) concentrations (0.051 mg P L−1) and loss (0.155 kg P ha−1) was detected for the river Odense catchment, whereas a similar trend did not emerge in the control catchment. The observed differences in PP reductions may be due to wetlands acting as P sinks since wetland restoration activities have been much more comprehensive in the river Odense catchment (1.8 ha wetlands km−2) than in the control catchment (0.5 ha wetland km−2). The excess downward trend in total P and PP in the river Odense catchment (5,600 kg P and 3,700 kg P) is corroborated by extrapolating the results from a mass-balance study and 10 years of in situ measurements of P storage (3,700 kg P and 15,000 kg P).

The Western Baltic Sea is affected by eutrophication and receives nutrients from local land-based sources, atmospheric deposition and by advection from the neighbouring North Sea and Baltic Sea. In the present study, we evaluated the importance of local (Danish) versus external (Baltic Sea–North Sea) nutrient loads for surface Chl a- concentrations, total areal primary production and the vertical distributions of primary production in the W. Baltic Sea using the 3D coupled HBM-ERGOM model. This was assessed by improving descriptions of nutrient loads and primary production in the model and by conducting scenarios of different nutrient loads according to (i) the eutrophication level in 1990, (ii) the new Danish Water Plan (DWP) and iii) DWP+ the Baltic Sea Action Plan (BSAP2). The model results showed that local nutrient reductions have a profound effect on Chl a concentrations and primary production in the coastal areas of the Western Kattegat and the Belt Sea with the highest impact

In the 1980s, Danish coastal waters suffered from eutrophication and several nutrient management plans have been implemented during the years to improve ecological status. This study aims at giving a holistic ecosystem perspective on 25 years of mitigation measures. We report trends of nutrient inputs and the responses to these in various chemical and biological components. Nutrient inputs from land were reduced by ~50 % for nitrogen (N) and 56 % for phosphorus (P) since 1990. These reductions resulted in significant and parallel declines in nutrient concentrations, and initiated a shift in the dominance of primary producers towards reduced phytoplankton biomass (chlorophyll a concentration) and increased cover of macroalgae in deeper waters. In the last 5 years, eelgrass meadows have also expanded towards deeper waters, in response to improving water clarity. An expected improvement of bottom water oxygen conditions has not been observed, presumably because more frequent stratification and higher water temperatures have counteracted the expected positive effects of reduced nutrient inputs. The biomass of the benthic macrofauna decreased as expected, but it was composed of a drastic decline of filter feeders paralleled by a more moderate increase of deposit feeders. This shift was most likely induced by increasing stratification. The reduced benthic filtration along with the limited eelgrass cover probably kept relatively more particles in suspension, which can explain why improvements in the Secchi depths were modest. Overall, several ecosystem components demonstrated clear signs of improvement, suggesting that at least partial recovery is attainable. On this basis, we propose a conceptual scheme for recovery of shallow coastal ecosystems following marked reductions in nutrient inputs.

Eelgrass depth limits and water clarity in the Skive Fjord estuarine system have not improved despite nutrient input reductions of 30%. Long-term monitoring data (1989–2010) were used to investigate the underlying causes. Dissolved inorganic and organic nitrogen concentrations decreased significantly over time, whereas particulate organic nitrogen concentration, assumed to consist primarily of phytoplankton and phytoplankton detritus and calculated as a proportional factor to chlorophyll a, did not change. Total organic carbon, mostly of autochthonous origin, remained constant despite reduced nitrogen concentrations, resulting in an increasing C:N ratio of the organic material in the water column. Phytoplankton primary production also remained constant suggesting that phytoplankton growth was only limited by nitrogen to a minor degree. Alleviated grazing pressure caused by a reduction in the blue mussel standing stock and a pelagic food web dominated by jellyfish may have contributed to the constantly high phytoplankton levels. Particulate inorganic matter, likely reflecting sediment resuspension, increased over time, most probably in response to removal of blue mussels and declining eelgrass cover. The Skive Fjord estuarine system is affected by multiple pressures—nutrient enrichment, mussel dredging and climate change that must be addressed together for water clarity to improve and eelgrass to recover.

The implementation of the Water Framework Directive requires adjustment of the administrative structure for the water management in each of the Member States. Modifications of the procedur...

In Denmark, as in many other European countries, the diffuse losses of nitrogen (N) and phosphorus (P) from the rural landscape are the major causes of surface water eutrophication and groundwater pollution. The export of total N and total P from the Gjern river basin amounted to 18.2 kg ha−1 and 0.63 kg P ha−1 during June 1994 to May 1995. Diffuse losses of N and P from agricultural areas were the main nutrient source in the river basin contributing 76% and 51%, respectively, of the total export. Investigations of nutrient cycling in the Gjern river basin have revealed the importance of permanent nutrient sinks (denitrification and overbank sedimentation) and temporary nutrient storage in watercourses. Temporary retention of N and P in the watercourses thus amounted to 7.2–16.1 g N m−2 yr−1 and 3.7–8.3 g P m−2 yr−1 during low-flow periods. Deposition of P on temporarily flooded riparian areas amounted from 0.16 to 6.50 g P m−2 during single irrigation and overbank flood events, whereas denitrification of nitrate amounted on average to 7.96 kg N yr−1 per running metre watercourse in a minerotrophic fen and 1.53 kg N yr−1 per linear metre watercourse in a wet meadow. On average, annual retention of N and P in 18 Danish shallow lakes amounted to 32.5 g N m−2 yr−1 and 0.30 g P m−2 yr−1, respectively, during the period 1989–1995. The results indicate that permanent nutrient sinks and temporary nutrient storage in river systems represent an important component of river basin nutrient budgets. Model estimates of the natural retention potential of the Gjern river basin revealed an increase from 38.8 to 81.4 tonnes yr−1 and that P-retention increased from −0.80 to 0.90 tonnes yr−1 following restoration of the water courses, riparian areas and a shallow lake. Catchment management measures such as nature restoration at the river basin scale can thus help to combat diffuse nutrient pollution.

We validated an existing physically based 3D MIKE SHE groundwater resource model (DK-model) at 175 Danish gauging stations covering different catchment sizes in order to calculate monthly water runoff in the 50% ungauged part of Denmark. Model performance was in most cases good (61% of gauging stations had a Nash-Sutcliffe (NS) coefficient0.60) but nevertheless showed a large seasonal and georegion specific bias. Therefore, bias correction factors had to be developed before applying the DK-model simulations of runoff in the ungauged areas. Simulated monthly runoff from ungauged areas and the measured monthly runoff from 178 gauging stations were distributed to 2663 smaller Hydrological Units (ca. 15 km(2)) and linked with a new empirical model for flow-weighted monthly total nitrogen (TN) concentrations (R(2) = 0.43; P0.0001) developed based on 20 years of observations (1990-2009) in 83 small catchments for calculation of monthly gross diffuse TN-loads from HU's. Nitrogen retention was calculated in streams, lakes and wetlands utilising both lake specific models and rate coefficients to calculate N retention in surface water bodies. The whole model complex was linked in the DK-QN concept for simulation of monthly TN losses from point sources and diffuse sources, TN retention and resulting loadings to Danish coastal waters. The DK-QN model was validated in 118 gauged catchments and the model simulations had for25% of the observations of monthly discharge weighted TN concentrations a NS larger than 0.26. Catchment specific monthly TN-loadings were modelled with a higher performance as 50% of the catchments had a NS greater than 0.75. The model concept allows calculation of N retention in streams, lakes and wetlands and the average annual model calculated N retention amounted to 21% of the modelled gross riverine TN loadings. The average annual gross TN loading to surface freshwater in Denmark derived from diffuse sources amounted to 97 000 tonnes N (91% of gross TN loadings) which is 54% of the total estimated N-leaching from the root zone on the Danish land area (212 000 tonnes N) during the period 1990-2009.

According to the EU Water Framework Directive all freshwater bodies must obtain good ecological status by 2015. In Denmark this means that all lakes with a surface area above 5 ha (∼600 lakes) must be evaluated individually and mitigation measures must be enforced if the ecological status is below “good”. In consequence, the nutrient pressures from point and diffuse sources must be assessed based on a quantification of the nutrient loading of each lake. In this study we focus on the loading of nitrogen. Few of the 600 lakes are monitored for runoff and nitrogen loading. Therefore, a national 3D hydrological model covering all major parts of the country was used to estimate runoff from the lake catchments. The diffuse nitrogen input to each lake was simulated with an empirical nitrogen model. Where lakes are located upstream/downstream of each other, a calculation chain involving the nitrogen retention in lakes was created. A case study was selected using a large river basin including 23 lakes in ...

Nationwide monitoring of the aquatic environment was initiated in 1988 in Denmark as a means to follow the outcome of the Action Plans for nutrient pollution of the aquatic environment. Five Action Plans have been adopted by the Danish Parliament since 1985 and the nationwide monitoring programme can be used to quantify the outcome as shown by reductions in nutrient discharges from both point and non-point sources. Moreover, the empirical experience gathered from nearly 20 years of monitoring is assisting the development and calibration of models for simulation of nitrogen leaching, nitrogen removal in groundwater and surface waters and the establishment of a P-index all covering the entire land area of Denmark.

The Danish Monitoring Programme reveals that diffuse sources are currently the major source of nutrient input to the aquatic environment. Nutrient concentration and loading data for about 50 small agricultural catchments and seven undisturbed catchments reveal that agriculture is the major diffuse source, average annual nitrogen (N) and phosphorus (P) concentration and loading being respectively, 5.1- and 6.8-fold, and 2.7- and 3.9-fold greater in streams draining agricultural catchments than in streams draining undisturbed catchments during the period 1989–96. A statistical trend analysis (discharge adjusted Mann-Kendall seasonal test) applied to long-term time series (1967–96) in three streams revealed an upward trend in nitrate concentration in Skjern Å (p

The mass balance for total nitrogen (N) was studied over a four-year period in 16 shallow mainly eutrophic 1st order Danish lakes. Water was sampled in the main inlet of each lake 18–26 times annually, and from the outlets and the lake 19 times annually. Water was also sampled from minor inlets, although less frequently. N input and output were calculated using daily data on discharge (Q), the latter being obtained either from the Q/H relationship based on automatic recordings of water level (H) for the main in- and outlet, or by means of Q/Q relationships for the minor inlets. Annual mean N retention in the lakes ranged from 47 to 234 mg N m−2 d−1, and was particularly high in lakes with high N loading. Annual percentage retention (N ret −y%) ranged from 11 to 72%. Non-linear regression analysis revealed that hydraulic retention time and mean depth accounted for 75% of the variation in annual mean N ret −y% and, in combination with inlet N concentration, accounted for 84% of the variation in the in-lake N concentration. N ret % varied according to season, being higher in the second and third quarter than in the first and fourth quarter (median 18–19%). A simple model was developed for predicting monthly nitrogen retention (N ret −m) on the basis of external N loading, the lake water pool of nitrogen N pool , hydraulic loading and lake water temperature. Calibration of only two parameters on data from the randomly selected 8 out of 16 lakes rendered the model capable of accurately simulating seasonal dynamics of the in-lake N concentration and N ret −m in all 16 lakes. We conclude that with regard to shallow, eutrophic lakes with a relatively low hydraulic retention time, it is now possible to determine not only annual mean nitrogen retention, but also the seasonal variation in N ret−m . Prediction of seasonal variation in N loading of downstream N-limited coastal areas is thereby rendered much more reliable.

We review various aspects of the structure and functioning of Danish estuaries from data collected by the National Monitoring Program and from information in published sources. We present data on the physical, chemical, and biological characteristics of estuaries in Denmark, we evaluate the functioning of these systems as filters and transformers of nutrients and we evaluate the outlook for Danish estuaries in the future. Danish estuarine systems are for the most part shallow (

A prognosis, previously published together with the applied model in 1978, has been validated, and the results are presented in this paper together with a prognosis validation of an improved version. The characteristics features of the two model versions and the lake characteristics for the case study are given. The validation shows that acceptable model prognoses can be set up, but that flexible structure models probably would do a better job, as a shift in the species composition was observed, when the nutrient loading was reduced significantly.

De danske kystområder viser nu de første tegn på signifikante forbedringer i miljøtilstand efter mere end 25 års landsdækkende indsats. Vi præsenterer data fra det nationale overvågningsprogram, som startede i slutningen af 1980’erne og som fortsat omfatter tilførsler af næringsstoffer samt mål for miljøtilstand på basis af kemiske og biologiske komponenter i kystzonen. Næringsstoftilførslerne er blevet reduceret med 50% for kvælstof og 56% for fosfor siden 1990. Disse reduktioner medførte hurtigt signifikante fald i næringsstofkoncentrationerne i overfladevand i ferskvand og i de kystnære områder. Klorofylmængden aftog og dækningsgraden af makroalger steg som funktioner af de reducerede næringsstoftilførsler. Biomassen af bundfaunaen var oprindeligt domineret af filtratorer, men i takt med de observerede reduktioner i næringsstoftilførslerne aftog den samlede biomasse og i særdeleshed biomassen af filtratorerne, hvorimod biomassen af sedimentædere steg. I modsætning til disse relativt hurtige responsmønstre, har ålegræs først bredt sig mod dybere vand inden for de sidste 5 år i takt med, at vandet er blevet klarere. Iltkoncentrationerne har ikke ændret sig væsentligt fra 1981 til 2013, formentligt på grund af klimafaktorer, som har modvirket de forventede positive effekter af reducerede næringsstoftilførsler. De samlede resultater viser en række tydelige tegn på forbedringer, som dokumenterer, at en delvis genetablering af de kystnære økosystemer er mulig. Med baggrund i de indsamlede data foreslår vi en konceptuel forklaringsmodel for udviklingen i kemiske og biologiske komponenter i kystområder efter markante reduktioner i næringsstoftilførslerfra både punktkilder og diffuse kilder. I hvilket omfang de observerede ændringer er de første tegn på varige forbedringer vil blive diskuteret.