Note by Secretariat: FOR REASONS OF ECONOMY, THE DELEGATES ARE KINDLY REQUESTED TO BRING THEIR OWN COPIES OF THE DOCUMENTS TO THE MEETING

Transcription

1 HELSINKI COMMISSION HELCOM EUTRO 2/2005 Development of tools for a thematic eutrophication assessment (HELCOM EUTRO) Second Meeting Riga, Latvia, 30 June - 1 July 2005 Agenda Item 3 HELCOM EUTRO Document code: 3/7 Date: Submitted by: Denmark KATTEGAT AND THE BELT SEA REGION Note by Secretariat: FOR REASONS OF ECONOMY, THE DELEGATES ARE KINDLY REQUESTED TO BRING THEIR OWN COPIES OF THE DOCUMENTS TO THE MEETING Page 1 of 10

2 1 HELCOM EUTRO REPORT - D R A F T Kattegat and the Belt Sea Region 0. Foreword During the 1970 s and 1980 s an increasing number of algae blooms, decreasing depth limit for the bottom vegetation, wide spread oxygen deficiency and fish kills clearly indicated eutrophication problems in the Kattegat and the Belt Sea area. The measures taken in the Danish Action Plan on the Aquatic Environment (1987), the Action Plan for Sustainable Agriculture (1991), the Action Plan on the Aquatic Environment II (1998) and the Action Plan on the Aquatic Environment III (2004) constitutes a substantial effort to reduce the input of nutrients to the aquatic environment. Similar measures have been taken in Germany and Sweden. The existing environmental conditions in the Kattegat and the Belt sea area are quite well illustrated by measurements and trend assessments. The historical reference situation is more scarcely illustrated by historical data and often sampled and analysed by different methodology than used today. To be able to compare the recent conditions with the historical situation old data in combination with expert judgement and modelling are needed. The reference conditions used in this report are preliminary results and will be subject to further analyses during the intercalibration work going on in relation to the Water Framework directive. 1. Introduction Kattegat forms together with the Belt Sea Region a transitional area between the Baltic Sea and the Skagerrak. 1.1 Kattegat The Kattegat area is characterised by complex hydromorfological and hydrodynamic conditions. The outflowing low saline water from the Baltic Sea in the surface and the high saline bottom water inflow from the Skagerrak create a stable permanent stratification at water depths greater than about m and periodically stratification in the shallow western Kattegat. The average surface salinity gradient in Kattegat varies from 20 psu in the southern part to 26 psu in the northern part. The salinity of the bottom water varies between 32 and 34 psu. (Ærtebjerg et al., 1981) 1.2 The Belt Sea Region The Belt Sea Region is a shallow stratified area with a pycnocline in about 13 m depth and a complex hydrodynamic pattern. Strong in and outgoing currents in the Danish Straits and the Sound dominate the area. The average surface salinity gradient in the Belt Sea varies from 11 psu at the border to the Baltic Proper to 20 psu at the border to the Kattegat. The salinity of the bottom water varies between 18 and 32 psu (Ærtebjerg et al., 1981). Two sills one in the Sound (Drogden Sill, 8 m) and one at the

3 2 Darss Sill (18 m) between Germany and Denmark at the entrance to the Baltic Proper creates barriers for the inflow of high saline water to the Baltic Sea and intensify the stratification. 2. Reference conditions No undisturbed areas can be found in Kattegat and the Belt Sea Area. Undisturbed background conditions or reference conditions are therefore developed from historical data, expert judgement and/or modelling. 2.1 DIN, DIP, DIN:DIP and Chlorophyll a In relation to the first application of the OSPAR Comprehensive procedure the reference values for the different quality elements was estimated for the Kattegat region as indicated in table 1 (Ærtebjerg et al., 2003). The maximum chlorophyll a concentration for the growing season (spring - late summer) in offshore Skagerrak of 1.25 µg/l has been applied as reference condition for the situation in the open Kattegat although the assessment has been based on mean chlorophyll a concentrations. Table 1: Reference values for winter DIN, DIP, DIN:DIP and chlorofyll a in the Kattegat used in relation to the first application of the OSPAR Comprehensive Procedure. (Æretebjerg et al., 2003) Quality element Reference condition Elevated level Winter Dissolved Inorganic Nitrogen (DIN) 4-5 µmol/l > 25 % = >5-6 µmol/l Winter Dissolved Inorganic Phosphorus (DIP) 0.4 µmol/l > 25 % = >0,5 µmol/l Winter N:P (Redfield) 16 > 25 % = >20 Kattegat Chlorophyll a 1,25 µg/l > 25 % = >1,55 µg/l For the Kattegat and Belt Sea region historical average chlorophyll a concentrations have been calculated from historical Secchi depth measurements (Henriksen, P., 2005). The calculated concentration in the open Kattegat is 1.3 µg/l, which is similar to the value in table 1. In the Belt Sea region the calculated historical chlorophyll a concentrations are in the range of µg/l (table 2). Table 2: Average calculated "historical" 90 th percentiles of chlorophyll a concentrations in the Belt Sea region (Henriksen, P., 2005). Belt Sea region Calculated historical Chlorophyll a concentrations µg/l Elevated level Aarhus Bay 1.6 > 25% = >2.0 µg/l North of Funen 2.9 > 25% = >3.6 µg/l Northern part of the Sound 1.7 > 25% = >2.1 µg/l Fakse Bay 1.4 > 25% = >1.8 µg/l Hjelm Bay 1.6 > 25% = >2.0 µg/l

4 3 2.2 Eelgrass The depth limit of eelgrass, defined as the deepest water depth where eelgrass grows, is generally regarded as a useful bioindicator, mainly because depth limits respond predictably to eutrophication, being largely regulated by light availability (Krause-Jensen, D. et al., 2005). In the coastal areas of the Kattegat and the Belt Sea historical depth limits of eelgrass showed an average of m based on conservative estimates (Krause-Jensen, D. et al., 2005). The wide range in reference conditions indicates that variables other than water quality contribute to the regulation of depth limits e.g. exposure levels, residence time, sediment composition and sampling methodology, (Krause-Jensen et al., 2005). 3. Causative factors A recent estimate of the development of the nitrogen load from Denmark to the Kattegat and Belt Sea region per hydrological year (July-June) in the period (Figure 1) showed an increase from less than 30,000 t/year in the period to an average of ca. 93,000 t/year in the 1980 s, a more than 3-fold increase. Since then the load has decrease to an average of 55,400 t/year in the period due to the Danish Action Plans (Conley et al., submitted). The phosphorus load from Denmark to the Kattegat and Belt Sea region has decreased from 5,400 t/year in 1990 to an average of 1,700 t/year in the period DK TN load, 1000 t/year s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s Decade Figure 1. Annual nitrogen load per decade from Denmark to the Kattegat and Belt Sea region Based on annual hydrological year (July-June) nitrogen surplus in Danish agriculture , measured runoff , measured nitrogen load and population development (Conley et al., submitted). The eutrophication of the Kattegat and the Belt Sea Region is caused by different sources of nutrient loads. A nitrogen nutrient budget for the Kattegat and the Belt Sea region in the 1990 s (table 3 and fig. 2) has been established by the Danish National Environmental Research Institute (NERI) (in Ærtebjerg et al., 2003). As an average for the period Denmark contributed with approximately 1/3 of the direct and diffuse input of bio-available nitrogen to the area. Sweden and

5 4 Germany contributed each with 11 % of the input. The rest of the nutrient input came with atmospheric deposition from other countries and inflow from the Baltic Sea and the Skagerrak. Table 3: Average annual nitrogen sources to the Kattegat and Belt Sea in 1000 tons per year. (Ærtebjerg et al., 2003) Total nitrogen 1000 tons/year Bio-available nitrogen 1000 tons/year Corrected for re-circulation 1000 tons/year % contribution Denmark 1) % Sweden 1) % Germany 1) % Other countries via the atmosphere (average deposition ) % Skagerrak 2) % Baltic Sea 2) % Total % Danish Contribution 12 % 25 % 32 % 1) Average annual load in the period incl. national load via the atmosphere 2) Average gross advective transports Fig. 2: Average transports of biological active nitrogen in the Kattegat and the Belt Sea region. Loss to the bottom includes denitrification (30,000 tons) and permanent burial (47,000 tons). Periods covered see table 3. (From Ærtebjerg et al., 2003).

6 5 4. Direct effects The evaluation of the direct effects is related to the development in the phytoplankton and makrophytes. 4.1 Chlorofyll a Data for recent 90 th percentiles of chlorophyll a concentrations in the Kattegat and the Belt Sea region have been calculated from Secchi depth measurement and compared to the calculated average historical concentrations (Henriksen, P., 2005). The relation between recent and historical concentrations shows an increase in concentration between 30 % and 164 % and thus exceeds the assessment level of 25 % (table 4). Table 4: Average calculated "historical" 90 th percentiles of chlorophyll a concentrations in the Kattegat and the Belt Sea region compared with recent average calculated 90 th percentiles of chlorophyll a concentrations (Henriksen, P., 2005). Kattegat and the Belt Sea region Calculated historical Chlorophyll a concentrations µg/l Calculated recent Chlorophyll a concentrations µg/l Increase in recent concentrations % Northern Kattegat Aarhus Bay North of Funen Northern part of the Sound Fakse Bay Hjelm Bay Harmful algae blooms The pattern of dominance of phytoplankton species has changed over time, in particular in the open sea areas. The main change has been an increase in importance of dinoflagellates from the early 1980ies till the period accompanied by reduced contributions to the biomass from other non-diatom groups (Ærtebjerg et al., 2003). In some years blooms occur of potentially toxic and harmful phytoplankton species normally constituting a minor component of the phytoplankton community (table 5). Such blooms may have severe effects on other organisms or on the tourist and fishing industries (Ærtebjerg et al., 2003). 4.3 Eelgrass and macroalge Actual Eelgrass depth limit shows an average of m. Compared to the historical average depth limits of m there is no doubt that depth limits of eelgrass have declined markedly since 100 years ago (Krause-Jensen, D. et al., 2005; Boström et al., 2003; Nielsen et al., 2003). At stone reefs in the Kattegat, especially in the central Kattegat, a clear relationship between input of nitrogen and phosphorus to Kattegat and a decrease in algae cover has been established (Dahl et al.,

7 6 2003). At depths of 18 to 20 m the coverage decrease with more than 25 % when comparing an average nitrogen input to Kattegat for the years with the input of nitrogen previous to the accelerated development of agriculture and energy consumption in the society (Dahl et al., 2003). Table 5: Blooms of potentially toxic and harmful phytoplankton species in Kattegat and The belt sea region (Ærtebjerg et al., 2003). Species Years Max. Abundance or biomass Kattegat Chatonella spp. 1998, 2000, mill l -1 Chrysochromulina spp. 1988, mill l -1 Dinophysis spp l -1 Gymnodinium chlorophorum mill l -1 Karenia mikimotoi 1968, 1981, mill l -1 Nodularia spumigena 1975, 1976, µg C l -1 Prorocentrum minimum 1983, 1984, 1987, 1989, 1992, 1993, 1994, 1995, 1996, 1997, mill l -1 Pseudo-nitschia spp. 1992, 1999, µg C l -1 Belt Sea area Chrysochromulina spp. 1988, mill l -1 Dictyocha speculum 1983, 1999, mill l -1 Dinophysis spp l -1 Karenia mikimotoi 1968, 1981, mill l -1 Nodularia spumigena 1975, 1976, 1992, 1994, 1995, 1997, 1999, 2001, µg C l -1 Prorocentrum minimum 1983, 1984, 1987, 1989, 1992, 1993, 1994, 1995, 1996, 1997, mill l -1 Pseudo-nitschia spp. 1992, 1999, µg C l -1

8 7 5. Indirect effects The evaluation of the indirect effects is related to oxygen deficiency and benthic fauna. Oxygen deficiency is defined for areas with oxygen concentration < 4 mg/l and it is severe in areas with oxygen concentration < 2 mg/l. 5.1 Oxygen deficit, kills in benthic invertebrates and fish Oxygen concentrations during late summer - autumn in the Kattegat and Belt Sea from the 1970 s to late 1980s/early 1990s show significant decreases in all areas with stratified water column. No general development has been seen in the 1990 s in summer - autumn bottom water minimum oxygen concentration, but a tendency for a rise in spring minimum concentration (April - June) has been found (Hansen et al., 2000, Ærtebjerg et al., 2002, Ærtebjerg et al., 2003). For periodically stratified estuaries and coastal waters the long term trend in the average bottom water oxygen concentration during stratification in July-November can be described by TN input from land and the wind speed July-September, explaining 52% of the interannual variation in concentrations. In the open Kattegat and Belt Sea the long term trend in bottom water oxygen concentration can be described by TN input from land, advective transport of bottom water May-September and the temperature of the inflowing water from the Skagerrak during winter and spring (January-April), explaining 49% of the interannual variation (Conley et al., submitted). Oxygen deficit and kills in benthic invertebrates and fish in Kattegat and the Belt Sea region has been reported regularly since the early 1980 s (table 6). Table 6: Oxygen deficit and kills in benthic invertebrates and fish in the Kattegat and Belt Sea region Year of oxygen depletion and kills of benthic invertebrates and fish Kattegat 1981,1985,1986,1987, 1988,2000,2002 The Belt Sea Region 1981,1983,1986,1987, 1988,1990,1993,1994, 1996,1997,1998,1999, 2000, 2001,2002, 2003 References Boström, C., Baden, S. P. and Krause-Jensen, D., 2003, The seagrasses of Scandinavia and the Baltic Sea, in E.P. green and F.T. Short (eds), World Atlas of Seagrasses, California University Press, California, p Conley, D.J., Carstensen, J., Ærtebjerg, G., Christensen, P.B., Dalsgaard, T., Hansen, J.L.S. & Josefson, A.B. (submittet): Long-term changes and impacts of hypoxia in Danish coastal waters. Ecological Applications, Special issue on Coastal Eutrophication.

9 8 Dahl, K., Lundsteen, S., Helmig, S., 2003: Stenrev Havets oaser. Danmarks Miljøundersøgelser og G.E.C. Gads Forlag. Aktieselskabet af 1994, København. Hansen, J.L.S., Pedersen, B., Carstensen, J., Conley, D.J., Christiansen, T., Dahl, K., Henriksen, P., Josefson, A.B., Larsen, M., Lisbjerg, D., Lundsgaard, C., Markager, S., Rasmussen, B., Strand, J., Ærtebjerg, G., Krause-Jensen, D., Laursen, J.S., Ellermann, T., Hertel, O., Skjøth, C. A., Ovesen, N.B., Svendsen, L.M., & Pritzl, G. 2000: Marine områder - Status over miljøtilstanden i NOVA Danmarks Miljøundersøgelser. 230 p.- Faglig Rapport fra DMU nr (In Danish with an English summary). Henriksen, P., 2005: Development of reference conditions for phytoplankton in Danish Waters. In preparation. Krause-Jensen, D., Tina Maria Greve and Kurt Nielsen: Water Resources Management (2005) 19: Nielsen, K., Sømod, B., Ellegaard, C. and Krause-Jensen, D., 2003, Assessing reference conditions according to the European Water Framework Directive using modelling and analysis of historical data: An example from Randers Fjord, Denmark, Ambio 32, Ærtebjerg, G., Andersen, J., Carstensen, J., Christiansen, T., Dahl, K., Dahllöf, I., Fossing, H., Greve, T.M., Hansen, J.L.S., Henriksen, P., Josefson, A.B., Krause-Jensen, D., Larsen, M.M., Nielsen, T.G., Pedersen, B., Petersen, J.K., Risgaard-Petersen, N., Rysgaard, S., Strand, J., Ovesen, N.B., Ellermann, T., Hertel, O. & Skjøth, C. A., 2002: Marine områder Miljøtilstand og udvikling. NOVA Danmarks Miljøundersøgelser. 94 p.- Faglig Rapport fra DMU nr (In Danish with an English summary). Ærtebjerg, G., Jacobsen, T.S., Gargas, E. & Buch, E. 1981: The Belt Project. Evaluation of the physical, chemical and biological measurements. Danish EPA. 122 pp. Ærtebjerg, G., Andersen, J. H. & Hansen, O. S. (Eds), 2003: Nutrients and Eutrophication in Danish Marine Waters. A Challenge for Science and management. National Environmental research Institute. 126 pp.

10 Kattegat and the Belt Sea region Cat. I: Causative factors Tentative Reference conditions Annex 1 Assessment metrics Assessment data Score (+/-) References + comments Land based inputs (TN, TP) DK TN: 30,000 t/year >25% = 37,500 t/year 55,500 t/year + Conley et al., submittet Winter DIN 4-5 µmol/l >25% = >5-6 µmol/l 3-11 µmol/l + Ærtebjerg et al., 2003 Winter DIP N:P(:Si) ratio 0.4 µmol/l >25% = >0.5 µmol/l µmol/l + Ærtebjerg et al., 2003 Winter DIN:DIP 16 >25% = > Ærtebjerg et al., 2003 Sum for cat. I + 9 Cat. II: Direct effects Tentative Reference conditions Chlorofyll a Kattegat: 1.3 µg/l Belt Sea region: µg/l Assessment metrics Assessment data Score (+/-) References + comments > 25 % = > 1,6 µg/l >25 % = > µg/l 3.1 µg/l µg/l + + (Henriksen, P., 2005) Harmful algal blooms + Change in species composition (Ærtebjerg et al., 2003) Submerged aquatic vegetation/macroalgae Eelgrass depth limit: m Macroalgae coverage on stone reefs: 18 m: 80 % 20 m: 60 % Decrease > 25 % = Depth limit < 2.3-6,3 m Decrease > 25 % = 18 m: 60 % 20 m: 45 % Eelgrass depth limit: m Macroalgae coverage on stone reefs: 18 m: 60 % 20 m: % Sum for cat. II Krause-Jensen, D. et al., 2005 Dahl et al., 2003 Cat. III: Indirect effects Tentative Assessment metrics Assessment data Score (+/-) References + comments Reference conditions Anoxia and hypoxia <2 mg/l and < 4 mg/l Decrease in oxygen conc. + Ærtebjerg et al., 2003 (summer- autumn) Kills in benthic invertebrates & fish Frequency of kills + Ærtebjerg et al., 2003 Sum for cat. III +