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26. marts 2004 er der udgivet en engelsk version (bl.a udvidet med en længere beskrivelse af overvågningsprogrammet) i NERI Technical Report no. 488: Tools to assess the conservation status of marine Annex 1 habitats in Special Areas of Conservation. Phase 1: Identification of potential indicators and available data

SAMMENFATNING Kvalitetsmålsætningerne for de danske farvande er kun opfyldt i ganske få fjorde, kystvande og åbne havområder. De få kystvande, hvor målsætningerne er opfyldt, er generelt lavvandede områder uden lagdeling af vandsøjlen og med relativt lille næringsstoftilførsel fra land. De åbne havområder, der opfylder målsætningerne, er de ikke kystnære dele af Nordsøen, Skagerrak og det nordlige og centrale Kattegat. Årsagerne til manglende opfyldelse af målsætninger angives især at være effekter af tilførsler af næringsstoffer, forekomst af iltsvind og effekter af TBT. Opfyldelse af målsætningerne - og herunder væsentlige og varige forbedringer i miljø- og naturforholdene - forudsætter, at tilførslerne af næringsstoffer, især kvælstof men også fosfor fra diffuse kilder, samt i visse farvandsområder TBT og miljøfarlige stoffer, skal reduceres yderligere. Karakteristik af året 2001 Luftens middeltemperatur i Danmark var i 2001 0,5°C højere end langtidsmiddel for perioden 1961- 90. Dermed har 12 ud af de seneste 14 år været varmere end normalt. Især var januar, juli-august og oktober varmere. Nedbøren og ferskvandsafstrømningen var i 2001 tæt på normal med den højeste nedbør i september. Det mest bemærkelsesværdige i 2001 var vindforholdene. Selvom der gennem hele året var hændelser med kraftig vind, så var middel vindhastigheden i alle måneder under eller lig med normalen, undtagen i oktober, og antallet af dage med kuling var i alle måneder væsentligt lavere end normalt. Dette påvirkede især iltforholdene i 2001 i negativ retning. Næringsstoftilførsler I 2001 udgjorde afstrømningen fra åbent land (diffuse kilder inklusiv spredt bebyggelse) 90% af den totale kvælstoftilførsel og 65% af den totale fosfortilførsel fra land til fjorde og kystvande. Resten kom fra punktkilder, enten via ferskvand eller direkte til de marine områder. Kvælstofudledningerne fra Danmark, korrigeret for år til år variationer i afstrømningen, er siden 1990 reduceret signifikant med. 35%. De ca. 21% skyldes reduktion i udvaskning fra landbrugsjorde, mens de resterende 14% skyldes reducerede punktkildeudledninger. I samme periode er fosforudledningerne reduceret 60%, hovedsageligt pga. udbygning af spildevandsrensningen. Også kvælstofdepositionen fra atmosfæren på de danske farvande synes reduceret (ikke statistisk signifikant) omkring 15% siden 1989 pga. tiltag i vesteuropæiske lande og den økonomiske afmatning i Østeuropa. Både næringsstoftilførslerne fra land og fra atmosfæren varierer betydeligt fra år til år pga. variationer i nedbør og afstrømning. Der er etableret et gennemsnitligt årsbudget for kvælstof i de indre farvande for perioden 1989-96, hvor de nødvendige data for tilførslerne fra de omgivende lande, atmosfæren og de tilstødende havområder var tilgængelige. Budgettet viser, at Danmarks andel af de samlede total-N tilførsler fra de omgivende lande, atmosfæren, Skagerrak og Østersøen i gennemsnit udgjorde 12%. Indregnes biotilgængeligheden af kvælstoffet i de forskellige kilder og recirkulering via Skagerrak, udgør Danmarks andel 25-32%. Næringsstofkoncentrationer De faldende næringsstoftilførsler afspejles i næringsstof koncentrationerne i farvandene. Kvælstofkoncentrationerne i fjorde og kystvande var i 2001 blandt de lavest observeret i perioden 1989-2001 og på samme niveau, som i de meget tørre år 1996-97, selvom afstrømningen var tæt på normal. I fjorde og kystvande er de afstrømningskorrigerede kvælstofkoncentrationer faldet statistisk signifikant efter 1997, og har i de åbne indre farvande været signifikant faldende siden 1989. 2 I fjorde og kystvande har fosforkoncentrationerne stabiliseret sig efter statistisk signifikante fald i begyndelsen af 1990'erne i forbindelse med udbygningen af spildevandsrensningen. I de åbne indre farvande er faldet mindre tydeligt, men dog signifikant. Effekter af næringsstoffer De faldende næringsstof koncentrationer har påvirket de biologiske systemer i de danske farvande. I fjorde og kystvande er vandets klarhed steget, og mængden og produktionen af planteplankton er faldet signifikant frem til midten/slutningen af 1990'erne. I de åbne indre farvande er ændringerne mindre udprægede, men vandets klarhed er større, og mængden af kiselalger og planteplanktonets produktion er lavere i 1990'erne end i 1980'erne. Generelt kan iltforholdene i 2001 karakteriseres som middel for de seneste 15-20 år. Fra 1970'erne til omkring 1990 faldt iltindholdet i bundvandet signifikant i de indre farvande. Det er påvist både for fjorde og åbne indre farvande, at den del af iltsvindet, der ikke er betinget af bundvandets opholdstid og temperarur, er relateret til kvælstoftilførslerne, jo større kvælstoftilførsel jo lavere iltkoncentration. De flerårige bundlevende organismer udviser ikke en entydig udvikling. Nogle udviklingstendenser svarer til det forventede ved faldende koncentrationer af næringsstoffer og fytoplankton. Individtætheden af bunddyr i de åbne indre farvande er faldet gennem 1990'erne, svarende til reduktionen i mængden af kiselalger og planteplanktonets produktion. Ålegræssets dækningsgrad er øget på større dybder i yderfjordene svarende til den øgede sigtdybde, og mængden af enårige, eutrofieringsbetingede makroalger er generelt reduceret på lavere vanddybder i overensstemmelse med mindre tilgængelighed af næringsstoffer. Dækningsgraden af makroalger på stenrev i Kattegat var høj i 2001 og på niveau med de tørre år 1996- 97, men ingen generel udvikling kan dog påvises i perioden 1994-2001. Imidlertid er der andre udviklingstendenser, som ikke harmonerer med det generelle billede af forbedrede miljøforhold. Dybdeudbredelsen af ålegræs er ikke øget som forventet ud fra den stigende sigtdybde, men derimod reduceret i fjordene og uændret på åbne kyster. Dette skyldes muligvis forekomsten af gentagne iltsvind neden for den nuværende dybdegrænse, eller ålegræssets generelle vanskeligheder med at genetablere sig og overleve på større dybder, så øgning af dybdeudbredelsen forsinkes. Desuden er dækningsgraden af ålegræs mindsket på lavere vanddybder i ydrefjordene og langs åbne kyster. Bunddyrsamfundene i fjorde og kystvande viser store variationer fra år til år. Dette kan skyldes forekomsten af gentagne iltsvind, der forhindrer samfundene i at stabiliseres. Resultaterne viser, at eutrofieringsproblemer stadig er udbredte i de danske farvande, især når det gælder bundlevende organismer, og især i fjorde og kystvande. Tungmetaller og miljøfremmede stoffer Koncentrationerne af tungmetaller i muslinger i danske farvande svarer generelt til 'ubetydeligt til moderat forurenet' ifølge det norske vejledende klassifikationssystem, dog med enkelte undtagelser med højere værdier. Målinger af miljøfremmede stoffer i muslinger og fisk viser, at de danske fjorde og indre farvande er forurenet med så høje koncentrationer af tributyltin (TBT) fra bundmalinger på skibe og polyaromatiske hydrokarboner (PAH) fra olieprodukter, at der må forventes at forekomme økologiske effekter heraf. Hormonforstyrrelser (imposex og intersex hos havsnegle) induceret af tributyltin (TBT) er da også udbredt, først og fremmest i kystnære områder, hvor TBT-niveauet forventeligt er højest, men i følsomme arter er der også effekter i de åbne farvande, selv i den åbne Nordsø. Generelt er koncentrationerne af polyklorerede bifenyler (PCB) og de øvrige klorerede forbindelser mindre bekymrende, men dog stadig på niveauer, hvor det ikke kan udelukkes, at de kan medføre effekter på miljøet. * The quality criteria for Danish marine waters were only fulfilled for a restricted number of the estuaries, coastal regions and open waters. The few coastal areas that meet the criteria are generally non-stratified shallow water areas with relatively restricted load from land based sources. The open sea areas that meet the criteria are the non-coastal parts of the North Sea, the Skagerrak and the Kattegat (Northern and Central). The failure to attain the criteria is primarily a result of nutrient loading, the occurrence of oxygen depletion and the effects of TBT. In order to fulfil the criteria for Danish marine waters and achieve significant and longlasting improvements in their condition, further reductions in nutrient load (in particular from diffuse sources), and for certain areas a decrease in TBT and other organic contaminants, are greatly needed. Year 2001 The annual mean temperature in 2001 was 0.5°C above normal. 12 of the last 14 years have been warmer than long-term average 1961-1990. Especially January, July, August and October were warm. Precipitation and runoff in 2001 were close to normal, with the highest precipitation in September. The most noteworthy climate incident in 2001 was the unusually low frequency of strong wind events in all months, in particular throughout winter and spring. Nutrient supply In 2001 the diffuse sources (including small settlements) made up 90% of the total nitrogen load and 65% of the total phosphorus load to the Danish estuaries and coastal waters. The nitrogen load from Denmark, corrected for inter-annual variations in runoff, has decreased significantly. About 35% in the period 1990-2001, with 21% due to the reduction in export from agricultural soils, and 14% due to the reduction in load from point sources. During the same period the phosphorus load has decreased 60%, mainly due to improved sewage treatment. There has also been a reduction in the atmospheric nitrogen deposition to open waters of about 15% since 1989. Both the loads from land and the atmosphere vary considerably from year to year due to variations in precipitation and runoff. Nutrient concentrations The decreasing nutrient load to Danish waters is reflected in their nutrient concentrations. The nitrogen concentrations in 2001 were the lowest observed during the period 1989-2001 and at the same level as the very dry years 1996 and 1997. In the open waters (the Kattegat, the Sound and the Belt Sea) the runoff corrected nitrogen concentration shows a steady decrease since 1989. In the estuaries and coastal waters, a significant decrease was observed after 1997. In the estuaries and coastal waters, the phosphorus concentrations have stabilised at a low level after significant decreases in the beginning of the 1990s. Corrected for inter annual variation in runoff, the export of nutrients from the estuaries to the open waters has decreased, for phosphorus since mid 1980s and for nitrogen since mid 1990s. The nutrient exchange with the Baltic Sea and the Skagerrak varied from year to year in the period 1999-2001. An average annual nitrogen budget for the Kattegat - Belt Sea area for the period 1989-96 shows that the Danish contribution of total nitrogen to the total supply from surrounding countries, atmosphere, Skagerrak and the Baltic Sea amounts to 12%. The Danish contribution increases to 25-32% taking the bioavailability of the nitrogen from different sources and recycling via Skagerrak into consideration. Effects of nutrient enrichment The decreasing nutrient concentrations have affected the biological systems in the Danish waters. In the estuaries and coastal waters a significant increase in the transparency and decrease in phytoplankton biomass and primary production is observed. In the open waters the effects are less pronounced. However, the transparency is higher, and the diatom biomass and the phytoplankton primary production are lower than in the 1980s. The bottom water oxygen concentration late summer and autumn in the open waters has decreased significantly from the 1970s to the late 1980s. During the 1990s, no general development is observed, neither in the open waters nor the estuarine and coastal waters, most likely due to inter-annual climatic variations. The occurrences of oxygen depletion in 2001 can be characterised as average for the latest 15-20 years. It has been shown both for estuaries and the open Kattegat and Belt Sea that the part of the oxygen deficiencies which cannot be attributed to climatic forcing (stratification, bottom water residence time, temperature) is related to the nitrogen load, the higher load the lower oxygen concentration. The long-living benthic organisms show less unambiguous development. Some trends are as expected from decreasing nutrient and phytoplankton concentrations. The abundance of macrozoobenthos has decreased during the 1990s in the Kattegat, the Sound and the Belt Sea, which would be expected from the decrease in nitrogen concentrations and in diatom biomass. The coverage of eelgrass has increased at greater depths in the outer estuaries corresponding to an increase in the transparency of the water, and the amount of short-lived nuisance macroalgae has generally decreased at low water depths, in accordance with lower nutrient availability. The coverage of macroalgae at stone reefs in the Kattegat was high in 2001 and at the same level as in the dry years 1996 and 1997, but no general development can be shown over the period 1994-2001. Other developing trends, however, do not match the general picture of an improved environmental quality. The depth distribution of eelgrass has not increased, but decreased in the estuaries and remained unchanged at open coasts. This might be due to repeated oxygen deficiency below the present depth limit, or other general difficulties for the eelgrass in re-establishment at greater depths. Beside, the coverage of eelgrass has decreased at shallower depths in the outer estuaries and at open coasts. The macrozoobenthos communities in the estuaries and coastal waters show large variations from year to year. This might be due to repeated occurrences of oxygen deficiency, preventing the communities to stabilise. The results show that eutrophication problems still exist in the Danish waters, especially concerning bottom living organisms, and especially in the estuarine and coastal waters. Heavy metals and organic contaminants Heavy metal concentrations in mussels in Danish waters in 2001 correspond generally to "insignificant to moderate polluted" according to the Norwegian classification system. Measurements of organic contaminants in mussels and fish show that the concentrations of TBT and PAH in the Danish estuaries, the Kattegat, the Sound and the Belt Sea are high, and biological effects are expected to occur. Imposex and intersex induced by TBT are still widespread in the four gastropod species investigated. The highest level of effects is observed in coastal waters, where the TBT level is expected to be highest. However, in the most sensitive species, effects are also observed in open waters, even in the open North Sea. Generally the concentrations of PCB and other chlorinated compounds cause less concern. However, their levels are still so high that negative effects on the biota cannot be excluded.

FORORD: Denne rapport er det første skridt i det udviklingsarbejde, som er en forudsætning for implementeringen af Vandrammedirektivet (EU's Direktiv om fastlæggelse af en ramme for Fællesskabets vandpolitiske foranstaltninger 2000/60/EF) i marine områder i Danmark. Miljøstyrelsen nedsatte i december 2000 en styringsgruppe for projektet bestående af Henning Karup, Jens Brøgger Jensen, Steen Pedersen (Miljøstyrelsen), Stig Helmig (Skov- og Naturstyrelsen), Nanna Rask (Fyns Amt), Bent Sømod (Århus Amt), Trine Christiansen og Kurt Nielsen (Danmarks Miljøundersøgelser). Nanna Rask har været Amtsrådsforeningens repræsentant, og Bent Sømod har arbejdet som konsulent for Danmarks Miljøundersøgelser. Rapportens anbefalinger og konklusioner er udgangspunkt for det fortsatte udviklingsarbejde, som foregår såvel nationalt som i internationale samarbejdsprojekter. Rapporten kan derfor ikke betragtes som udkast til en fremtidig vejledning for planlægning af vandkvaliteten i marine områder. INDLEDNING: Vandrammedirektivet har til formål at forebygge forurening og forringelse af den økologiske tilstand samt at danne grundlag for restaurering af vandområder med henblik på at opnå en god økologisk og kemisk tilstand. Direktivet udstikker generelle retningslinier for, hvordan et klassifikations- og målsætningssystem i vandløb, søer og kystvande samt grundvand kan opstilles på baggrund af referencetilstanden i de pågældende områder. Klassifikations- og målsætningssystemet, som udvikles med baggrund i Vandrammedirektivet, skal afløse det nuværende målsætningssystem, som har udgangspunkt i den gældende vejledning for planlægningen af vandkvaliteten i danske kystvande, Miljøstyrelsens vejledning i Recipientkvalitetsplanlægning, del 2 kystvande 1983. Miljøstyrelsens vejledning, del 2 kystvande bygger på et hovedprincip om, at forureningen skal kunne kontrolleres ved såkaldte konfliktfronter omkring forureningskilder. Konfliktfronten er den afstand fra forureningskilden, hvor forureningspåvirkningen netop kan spores. Forureningspåvirkningen fra de enkelte kilder antages at kunne måles ud fra størrelsen af arealet af det forurenede område. Vej-ledningen forudsætter også, at de danske kystområder er upåvirkede eller kun svagt påvirkede af forurening, hvorfor den ikke tager højde for den generelle forurening, der som bekendt påvirker alle danske kystområder. Vejledningen indeholder endvidere ikke kvanti-tative biologiske krav til de enkelte målsætningsklasser, sådan som det er forudsat i Vandrammedirektivet. Det nuværende planlægningssystem kan altså ikke leve op til de fælles europæiske standarder og dermed ikke anvendes som et fremtidigt planlægningsredskab under Vandrammedirektivet. Amterne har, som ansvarlige for planlægningen af vandkvaliteten, hidtil i varierende omfang anvendt vejledningens målsætningssystem. Nogle amter har ændret systemet betydeligt og udviklet egne principper for fastlæggelse af målsætninger. Andre amter har anvendt vejledningens målsætningssystem med mindre ændringer. Amterne har således anvendt meget forskellige metoder i deres plan-lægning af vandkvaliteten. Amterne har tidligere givet udtryk for, at den gældende vejledning anses generelt for at være forældet (Hansen, 1995). Miljøstyrelsen har i erkendelse af, at der ikke eksisterer et målsætningssystem for de danske kystvande, som opfylder Vandrammedirektivets krav, igangsat et udviklingsarbejde. Danmarks Miljøundersøgelser og Århus Amt har udarbejdet denne arbejdsrapport som resultatet af projektets Fase 1. ABSTRACT: Summary The report provides a characterization and analyses of the runoff in Danish rivers. The basis of the report is the archive of hydrometric data collected over the last century. The runoff is described by means of characteristic parameters, duration curves, extreme value analyses, trend analyses and mapping of runoff and water balance. The report presents an overwiev of the national hydrometric data archive pro-viding maps, tables and key data. Runoff monitoring In Denmark, runoff has been systematically registered since around 1917. Runoff is currently measured continuously at almost 400 monitoring stations distributed in the app. 64,000 km small and large streams. The total catchment of the monitoring stations constitutes app. 55% of the total area of the country. Denmark's largest stream is the river Gudenå (149 km) draining an area of more than 2,600 km2 . The river with the most water, however, is the river Skjern Å with an average discharge of app. 37,000 l/sec. Data collection For many years the water stage has been registered by means of manual readings of a water level gauge placed at many of the sta-tions, but also automatic chart recorders have been in operation since the beginning of the 20th century. Nowadays, registrations are nor-mally performed by dataloggers and occasionally with a telemetry link to the stations. At most stations, however, the actual registration is still performed by the use of float and pulley in a stilling well. Flow measurements are performed by the use of a propeller instrument. Flow velocity and river gradients Fall gradients of Danish streams fall almost exclusively within the interval 0.1-10 per thousand. The gradient tends to decrease down-stream in the river system (the closer it gets to the coast) but there are marked deviations between the various parts of the country. The streams on Bornholm, for instance, have relatively large gradients. The flow velocity in streams varies between app. 0.2 to 0.5 m s-1 and the velocity is often greater in large streams. The average stream ve-locity is app. 0.3 m s-1. Trends in runoff The annual mean runoff shows an increasing trend over the past 80 years in most of the Danish streams. This relates primarily to a corre-sponding increase in the quantity of precipitation. Some streams, however, show a falling trend caused by for example water abstrac-tion from the stream or from groundwater within the catchment. Also the annual maximum and minimum runoff show a predominantly increasing trend, but the results are not unambiguous. The indicated trends are only significant in a small proportion of the analysed time series. Lowflow Median minimum runoff is used as a key parameter in river man-agement and water resources administration. Year-to-year minimum runoff may differ considerably from the median minimum. During very dry years the level will be less than half of the median minimum and in some streams even as little as a quarter of the median mini-mum. Consequently, the determination of median minimum runoff is sensitive to the choice of length, starting and ending time of the cal-culation period. Extreme events By means of long time series combined with adjusted distribution functions, the probability of certain runoff events are estimated. In Denmark, T-year values are often used that are exceeded or reduced every T-th year on average. T could be, for example, 2, 10, 20, 50 or 100-year events and the values are used in connection with agricul-tural and draining interests, bridge structures, fish ladders and river restoration projects. Period of reference On the basis of the analyses and assessments of this report as well as earlier investigations it is recommended that the median minimum, median maximum and mean runoff be calculated on the basis of data for the period 1971-1998, and that the period is increased to a 30-year period from 2001 (1971-2000). In the long term it is recommended that the period of reference be forwarded at the beginning of each new decade. The national topic centre of hydrometry has previously recommended the period 1971-1990. By changing the period of refer-ence to 1971-1998, the level for the average median minimum runoff will be reduced by app. 4 per cent. Runoff characteristics There are considerable variations in the runoff pattern in Danish streams. Duration curves are an excellent tool for describing and analysing these variations. In some streams the runoff is very con-stant with little variation between winter and summer water flow and without significant increases in connection with for example heavy precipitation. Others have a very varying water flow with large runoff during certain periods of the year and a very marked response to precipitation events and may still almost dry up during summer. Relation between water quality index and runoff characteristics Correlation analysis of flow-ecological parameters and biotic water quality index shows that poor water quality is in general related to streams with large flow-variations. However, it is not revealed if the correlation is caused by the influence of flow-variations on the fauna, or if other factors correlate to both flow-parameters and biotic index. This could apply for organic matter. The flow-ecological parameters are closely correlated to each other, and it is uncertain which values could be most important for the river ecology. Small versus large streams Analyses of the relationship between runoff parameters that may be used as indicators for ecological conditions show that there are well-defined variations between small and large streams and between the various regions of the country. The variations are generally less sig-nificant in large streams than small ones. A level of 7 times the me-dian water flow is only exceeded between 0 and 15 times a year in large streams whereas it is exceeded between 1.5 and 5 times in small streams. The ratio between median minimum and the median water flow is between app. 0.1 and 0.4 in small streams and between app. 0.3 and 0.6 in large streams. Mean runoff in Denmark The average annual mean runoff for Denmark as a whole is 318 mm (10.1 ls-1 km-2). In the eastern part of the country it is app. 200 mm, and in the western part 400 mm per year. In central and southern parts of Jylland 600 mm are exceeded. The considerable differences in runoff are related to precipitation pattern, evaporation, regional groundwa-ter migration and abstraction. In years with sparse precipitation such as e.g. 1976 and 1996, the runoff is less than 200 mm, and in years rich in precipitation, such as e.g. 1981 and 1994, it is more than 450 mm. Extreme values - regional patterns Low-flow values vary considerably in the various parts of the coun-try. In the eastern part of the country the average level is around 0.5 l s-1 km-2, and in the western part 10 times higher. The low-flow pattern is mainly related to soil type and groundwater reservoirs. Flood val-ues also reveal large variations over the country. In regions with heavy fine-grained soils, where infiltration of precipitation is re-stricted, the 2-year flood is in the interval 50 - 100 l s-1 km-2. In regions with sand-dominated soils the 2-year flood is within the interval 20 - 40 l s-1 km-2. Hydrological regimes Cluster analysis is performed on a set of flow variables, and separate groups can be identified. The analysis shows that there is generally an unstable water flow regime on the Danish islands, whereas the conditions in streams in northern and western Jylland are more mod-erate. Water balance When considering the water balance for the country as a whole over a period of many years, there is a good fit between precipitation, evaporation and runoff. As an average for the period 1971 - 1998 the annual precipitation is 854 mm, the potential evaporation 520 mm and the runoff 322 mm. For small regions or short periods there are considerable deviations caused mainly by ground water movements across the topographic water divides, water abstraction or fluctuation in the ground water level. Summary The report provides a characterization and analyses of the runoff in Danish rivers. The basis of the report is the archive of hydrometric data collected over the last century. The runoff is described by means of characteristic parameters, duration curves, extreme value analyses, trend analyses and mapping of runoff and water balance. The report presents an overwiev of the national hydrometric data archive pro-viding maps, tables and key data. Runoff monitoring In Denmark, runoff has been systematically registered since around 1917. Runoff is currently measured continuously at almost 400 monitoring stations distributed in the app. 64,000 km small and large streams. The total catchment of the monitoring stations constitutes app. 55% of the total area of the country. Denmark's largest stream is the river Gudenå (149 km) draining an area of more than 2,600 km2 . The river with the most water, however, is the river Skjern Å with an average discharge of app. 37,000 l/sec. Data collection For many years the water stage has been registered by means of manual readings of a water level gauge placed at many of the sta-tions, but also automatic chart recorders have been in operation since the beginning of the 20th century. Nowadays, registrations are nor-mally performed by dataloggers and occasionally with a telemetry link to the stations. At most stations, however, the actual registration is still performed by the use of float and pulley in a stilling well. Flow measurements are performed by the use of a propeller instrument. Flow velocity and river gradients Fall gradients of Danish streams fall almost exclusively within the interval 0.1-10 per thousand. The gradient tends to decrease down-stream in the river system (the closer it gets to the coast) but there are marked deviations between the various parts of the country. The streams on Bornholm, for instance, have relatively large gradients. The flow velocity in streams varies between app. 0.2 to 0.5 m s-1 and the velocity is often greater in large streams. The average stream ve-locity is app. 0.3 m s-1. Trends in runoff The annual mean runoff shows an increasing trend over the past 80 years in most of the Danish streams. This relates primarily to a corre-sponding increase in the quantity of precipitation. Some streams, however, show a falling trend caused by for example water abstrac-tion from the stream or from groundwater within the catchment. Also the annual maximum and minimum runoff show a predominantly increasing trend, but the results are not unambiguous. The indicated trends are only significant in a small proportion of the analysed time series. Lowflow Median minimum runoff is used as a key parameter in river man-agement and water resources administration. Year-to-year minimum runoff may differ considerably from the median minimum. During very dry years the level will be less than half of the median minimum and in some streams even as little as a quarter of the median mini-mum. Consequently, the determination of median minimum runoff is sensitive to the choice of length, starting and ending time of the cal-culation period. Extreme events By means of long time series combined with adjusted distribution functions, the probability of certain runoff events are estimated. In Denmark, T-year values are often used that are exceeded or reduced every T-th year on average. T could be, for example, 2, 10, 20, 50 or 100-year events and the values are used in connection with agricul-tural and draining interests, bridge structures, fish ladders and river restoration projects. Period of reference On the basis of the analyses and assessments of this report as well as earlier investigations it is recommended that the median minimum, median maximum and mean runoff be calculated on the basis of data for the period 1971-1998, and that the period is increased to a 30-year period from 2001 (1971-2000). In the long term it is recommended that the period of reference be forwarded at the beginning of each new decade. The national topic centre of hydrometry has previously recommended the period 1971-1990. By changing the period of refer-ence to 1971-1998, the level for the average median minimum runoff will be reduced by app. 4 per cent. Runoff characteristics There are considerable variations in the runoff pattern in Danish streams. Duration curves are an excellent tool for describing and analysing these variations. In some streams the runoff is very con-stant with little variation between winter and summer water flow and without significant increases in connection with for example heavy precipitation. Others have a very varying water flow with large runoff during certain periods of the year and a very marked response to precipitation events and may still almost dry up during summer. Relation between water quality index and runoff characteristics Correlation analysis of flow-ecological parameters and biotic water quality index shows that poor water quality is in general related to streams with large flow-variations. However, it is not revealed if the correlation is caused by the influence of flow-variations on the fauna, or if other factors correlate to both flow-parameters and biotic index. This could apply for organic matter. The flow-ecological parameters are closely correlated to each other, and it is uncertain which values could be most important for the river ecology. Small versus large streams Analyses of the relationship between runoff parameters that may be used as indicators for ecological conditions show that there are well-defined variations between small and large streams and between the various regions of the country. The variations are generally less sig-nificant in large streams than small ones. A level of 7 times the me-dian water flow is only exceeded between 0 and 15 times a year in large streams whereas it is exceeded between 1.5 and 5 times in small streams. The ratio between median minimum and the median water flow is between app. 0.1 and 0.4 in small streams and between app. 0.3 and 0.6 in large streams. Mean runoff in Denmark The average annual mean runoff for Denmark as a whole is 318 mm (10.1 ls-1 km-2). In the eastern part of the country it is app. 200 mm, and in the western part 400 mm per year. In central and southern parts of Jylland 600 mm are exceeded. The considerable differences in runoff are related to precipitation pattern, evaporation, regional groundwa-ter migration and abstraction. In years with sparse precipitation such as e.g. 1976 and 1996, the runoff is less than 200 mm, and in years rich in precipitation, such as e.g. 1981 and 1994, it is more than 450 mm. Extreme values - regional patterns Low-flow values vary considerably in the various parts of the coun-try. In the eastern part of the country the average level is around 0.5 l s-1 km-2, and in the western part 10 times higher. The low-flow pattern is mainly related to soil type and groundwater reservoirs. Flood val-ues also reveal large variations over the country. In regions with heavy fine-grained soils, where infiltration of precipitation is re-stricted, the 2-year flood is in the interval 50 - 100 l s-1 km-2. In regions with sand-dominated soils the 2-year flood is within the interval 20 - 40 l s-1 km-2. Hydrological regimes Cluster analysis is performed on a set of flow variables, and separate groups can be identified. The analysis shows that there is generally an unstable water flow regime on the Danish islands, whereas the conditions in streams in northern and western Jylland are more mod-erate. Water balance When considering the water balance for the country as a whole over a period of many years, there is a good fit between precipitation, evaporation and runoff. As an average for the period 1971 - 1998 the annual precipitation is 854 mm, the potential evaporation 520 mm and the runoff 322 mm. For small regions or short periods there are considerable deviations caused mainly by ground water movements across the topographic water divides, water abstraction or fluctuation in the ground water level.

SAMMENFATNING Denne rapport giver en landsdækkende beskrivelse af miljøtilstanden i de danske farvande i 2000 samt en oversigt over udviklingen siden Vandmiljøplanens iværksættelse. Det marine miljø var i 2000 præget af iltsvind i de indre farvande af en udstrækning og varighed der ikke er set siden 1980erne. På flere områder er miljøtilstanden dog bedret i løbet af de sidste 10-20 år. Planteplanktonets primærproduktion og klorofylkoncentrationen er faldet, sigtdybden er steget og fosfortilførslen er reduceret kraftigt, mens tilførslen af kvælstof til de marine områder har været moderat faldende. Dog er der i perioden siden 1989 sket en reduktion af ålegræssets dybdegrænse i de indre dele af fjordene. Siden midten af 1990erne er bundfaunaens individtæthed faldet. Koncentrationen af tungmetaller svarer i de fleste områder til ubetydeligt til moderat forurenet. Koncentrationerne af de miljøfremmede stoffer TBT og PAH er så høje at der må forventes at kunne forekomme effekter heraf. Kun et fåtal af de undersøgte områder opfyldte i 2000 de fastsatte målsætninger. Opfyldelse af målsætningerne og væsentlige og varige forbedringer i miljø- og naturforholdene forudsætter at tilførslerne af næringsstoffer, især kvælstof fra diffuse kilder, samt i visse farvandsområder TBT og miljøfremmede stoffer, reduceres yderligere. * SUMMARY The year 2000 was an unusually warm year with an average temperature of 9.2oC. Particularly the winter, spring and autumn months were warmer than normal. The average rainfall for 2000 was 768 mm and thus close to the normal rainfall, but with higher rainfall than normal in the 1st and 4th quarters. The most note-worthy climate incident in 2000 was the prevailing wind from south-east in September and subsequently from south-south-west up until December combined with a lower frequency of strong wind events than normal. In general the 1990s were characterised by good water exchange and reduced salinity in the bottom water in the Baltic entrance area. In 2000 the residence time and salinity of the bottom water increased to a level com-parable to the mid-1980s. After a number of warm summers in the 1990s the maximum temperature in sur-face water has been reduced during the last couple of years. The annual mean temperature of the surface water in Kattegat has increased by approximately 1.1oC since 1970. The wind conditions during autumn 2000 caused a prolonged outflow of brackish surface water from the Baltic Sea and a strengthening of water mass stratification in the Baltic entrance area. Residence time of the bottom water was longer and renewal of bottom water happened later than normal. The salinity and tem-perature in the bottom waters remained higher than normal until late in the year. The combination of the hydrographical conditions and large nitrogen runoff from land in 1998, 1999 and the beginning of 2000 gave rise to oxygen depletion during summer and autumn in the Baltic entrance area. The oxygen depletion was unusually extended and of long duration. In the shallow estuaries and coastal areas oxygen conditions on the other hand were relatively good and oxygen depletion events were short lived. In the end of the 1980s extensive oxygen depletion events were observed whereas the 1990s were characterised by generally better oxygen conditions. Single years, for example 1995 and 2000, were outside of this pattern. Bottom water residence time and temperature can explain most of the observed variation in bottom water oxy-gen conditions. The residual variations were correlated with runoff and nitrogen loading during the previous year. Since 1989 reductions in nutrient loading and loss of nutrient pools in sediments have resulted in a positive development for nutrient concentrations in estuaries and coastal areas. The load of phosphate and total-phosphorous to marine areas, in addition to the concentrations in the water, have significantly declined since the beginning of the 1990s as a consequence of reduced loading from point sources. During the 1990s, an increase in the loading of phosphorous from diffuse loads occurred, and since the middle of the 1990s dif-fuse loading has been the largest source of phosphorous. Since 1998 the concentrations in estuaries and coastal areas have stabilised without decreasing further. In most of the estuaries there is an internal load of phosphorus from the sediment to the water. This internal load can be especially important during the sum-mer period. For four of the estuaries the internal load of phosphorus comprised 36-93% of the total phospho-rus load during the period June-August 2000. In the open waters concentrations of phosphate have varied from year to year without a tendency of either increasing or decreasing concentrations. The concentration of total-phosphorous has fallen significantly until 1999 in open waters. The development in the open water mirrors the reduction in export of total-phosphorous from the coastal areas in the period 1989 to 1999. Concentrations of inorganic nitrogen and total nitrogen have shown large inter-annual variations that reflect the variations in load. Since 1989 there has been a significant decline in both the diffuse load and the total load of nitrogen to marine areas. In the period 1989-97 there was a strong dependence between freshwater runoff from land and nitrogen concentrations in the seawater, but after 1997 nitrogen concentrations were markedly lower than expected from runoff. When corrected for freshwater runoff from land, nitrogen con-centrations have gradually fallen in the open sea areas since 1989, whereas the concentrations in estuaries and coastal areas have been very constant until 1997, after which they have fallen markedly. In estuaries nitrogen is typically taken up from the water to the sediment by benthic microalgae in the spring, but during the rest of the year nitrogen is released from the sediment to the water. This internal load of nitrogen is larg-est in June-August, where in four investigated estuaries it comprised 10-78% of the total nitrogen load. The exports of total-nitrogen from the coastal areas to the open sea area are tightly coupled to the load to estuar-ies and therefore to freshwater runoff from land. The runoff-corrected export of total-nitrogen to the open sea areas has been relatively constant since 1989, but with a tendency to a reduction in the period 1997-2000. Reductions in nutrient concentrations have had significant effects on water transparency (secchi depth) and phytoplankton, especially in estuaries and the coastal areas. Water transparency increased in the middle of the 1990s to a higher level than in the previous decade. The opposite trend was seen for primary production and chlorophyll concentration. In the 1980s primary production and chlorophyll concentration were ap-proximately 40% and 25%, respectively, higher than the average for the 1990s. In the open waters the effect was less pronounced. Water transparency was higher and primary production lower in the 1990s than at the end of the 1980s, but a decline in chlorophyll concentrations, as in the estuaries, was not found. Diatoms comprise a large portion of the phytoplankton in marine waters. Their vigorous growth and subse-quent sinking from surface waters in the spring result in loss of nutrients from surface waters and retention of the nutrients in organic matter in the sediment for a long period. The organic matter is cycled slowly and acts as a stable food source for bottom animals. The biomass of diatoms has fallen in estuarine and coastal areas from 1986 up until 1988-89. Since then the biomass has remained at a relatively constant level. In the open waters, however, the biomass of diatoms has significantly declined from 1980 to 2000. Eelgrass beds increased in some areas and decreased in others. The most notable change was a dramatic de-cline found in Augustenborg Fjord. In Limfjorden the eelgrass beds have been gradually reduced over a long period. There was a tendency of a reduction in eelgrass depth limits in the inner parts of estuaries in the period 1989-2000. Eelgrass coverage decreased significantly in the depth 0-2 m in the inner estuaries and in the near coastal waters in the same period. On the stone reefs in the open waters, the area covered by macroalgae varied in the period 1994-2000 with characteristic lower coverage in years with high runoff and large coverage in dry years with reduced runoff. Nutrient loading to the Kattegat influences the coverage of the macro algae vegetation via light absorption in the pelagic system arising from effects on chlorophyll concentrations and secchi depth in the water. Bottom fauna in the open waters has varied in both abundance and biomass since the beginning of the 1980s. The most marked changes have been in abundance that was highest in the beginning of the 1980s and mid-1990s. Since the middle of the 1990s there has been a decline in abundance. The concentration of heavy metals in sediments exceeded in almost all investigated areas the lower limits determined by OSPAR's Ecological Assessment Criteria (EAC). The upper EAC limit was surpassed for Pb in the Baltic, Cd in Roskilde Fjord and Horsens Fjord and for Hg in the Sound. Heavy metal concentrations in mussels and fish would be classified as insignificantly to moderately polluted according to the Norwegian Pollution Level (SFT). TBT and PAH were found in the Danish estuaries and open waters in concentrations where effects may be expected. Imposex and intersex caused by TBT was still widespread in the investigated species of snails, first and foremost around harbours where TBT levels are expected to be high. For the sensitive species effects were found in open waters. Concentrations of other environmental contaminants are less of a concern. How-ever, PCBs were still present at concentrations where effects on the environment may be possible. Very few of the areas and stations included in the National Monitoring Program fulfilled the ecological qual-ity objectives in 2000. The load and effects of nutrients were the most common reason given for missing ful-filment of the quality objectives. In addition, the loads and effects of TBT hindered fulfilment in many areas. A general fulfilment of ecological quality objectives and substantial improvements in the environment re-quire considerable reductions of loads of nutrients, especially nitrogen from diffuse sources. In some areas, reductions in the concentrations of TBT and environmental contaminants are required.

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