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  Technical scientific assessment  
  Nutrient inputs  
  Nutrient concentrations, ratios and limitation  
  Phytoplankton and harmful algal blooms  
  Oxygen depletion  
  Degradation of organic matter in estuarine sediments  
  Submerged aquatic vegetation  
  Softbottom macrobenthos  
  Fish kills  
  Map of Danish marine waters  
Box 2 presents the OSPAR eutrophication quality objectives (ECO-eutro) & the Danish criteria used in this assessment.


Box 3 presents background or reference conditions.

Technical-scientific assessment

This assessment is based on data from the Nationwide Aquatic Monitoring Programme (1988–1997) and the National Aquatic Monitoring and Assessment Programme (1998–2003) (See Danish EPA 2000 for details). The data used for this assessment covers the period 1989–2001 (in some cases only 1989–2000). In a limited number of analyses other data have been included for support.

The national monitoring programme is run in collaboration between the Danish Counties and NERI. The counties are responsible for the activities within the coastal waters. NERI carries out most of the monitoring of the open marine waters, co-ordinates the programme, runs the national marine database and produces annual reports on the state of the marine environment.

Marine eutrophication is an international problem and can only be solved by co-ordinated national and international efforts. The same principle applies for assessment of eutrophication status of marine waters receiving and transporting nutrients from different countries. Therefore, it is important that Denmark and the neighbouring countries (Germany, Norway, and Sweden) have almost harmonised ways of assessing the eutrophication status of common waters. Such common understanding of principles and criteria are discussed and agreed within OSPAR and HELCOM.

The OSPAR Strategy to Combat Eutrophication is together with the monitoring activities among the main drivers of the assessment process (OSPAR 1998). The strategy has the aim of identifying the eutrophication status of all parts of the convention area by the year 2002. The Common Procedure for the Identification of the Eutrophication Status of the Maritime Area is a main element of the strategy. The Common Procedure includes a checklist of qualitative assessment criteria to be used when assessing eutrophication status. In addition, a set of quantitative criteria has been developed in order to assist a harmonised assessment (OSPAR 2001). These assessment criteria fall into the following four categories:

  1. Degree of nutrient enrichment.
  2. Direct effects of nutrient enrichment.

  3. Indirect effects of nutrient enrichment.

  4. Other possible effects of nutrient enrichment.

The harmonised assessment criteria and the assessment levels are summarised in Box 2.

Assessments of eutrophication status of the marine environment should be based on knowledge of how the situation would be without any anthropogenic influence. Two factors are important when assessment criteria are developed:

  • What are the reference conditions for the given parameter?
  • What are an acceptable deviation from reference condition?

The principle of assessing the actual state of the environment in relation to the reference conditions is important. However, the critical factor is not the definition of background conditions but the decision on acceptable deviation from background values. The OSPAR assessment criteria acknowledge this and give the countries the opportunity to establish national, regional or even site-specific criteria.

Recent work in relation to the national implementation of the Water Framework Directive and the Habitat Directive has focused on reference conditions and classification of ecological quality for eelgrass and macroalgae and the parameters controlling the growth of submerged aquatic vegetation (chlorophyll, Secchi depth, runoff, nutrient concentrations, depth etc.). The on-going work with regard to classification of ecological quality shows:

  • The acceptable deviation should be 15-20%, cf. Henriksen et al. 2001.

  • A generally acceptable deviation of 25% will result in a limited number of false positive situations when compared to the existing Danish assessments principles, cf. Krause-Jensen et al. (subm.).

  • The Swedish assessment criteria (Swedish EPA 2000) for macroalgae will be met almost every year, even in wet years with high anthropogenic inputs of nutrient, to the marine environment, cf. Henriksen et al. 2001.

  • A 50% deviation from reference conditions for macrovegetation at reefs in Kattegat suggests that the reefs are not subject to eutrophication at all, cf. Henriksen et al. 2001.

The OSPAR eutrophication assessment criteria have therefore been adjusted at a national level. The Danish assessment criteria are:

  • The acceptable deviation for winter DIN and DIP concentrations is 25%, cf. Henriksen et al. 2001.
  • The acceptable deviation for maximum and mean chlorophyll a is 25%, cf. Henriksen et al. 2001. The growing season covers the period March – October.
  • The acceptable deviation for macrophytes including macroalgae is 25%, cf. Krause-Jensen et al. (subm.) and Henriksen et al. 2001.
  • The Danish oxygen depletion criteria are: severe acute oxygen depletion: 0-2 mg O2 l-1, oxygen depletion: 2-4 mg O2 l-1. These values have been national assessment criteria since the mid-1980s. The OSPAR criteria are: < 2 mg l-1 and 2-6 mg l-1. These criteria are not applicable for Danish marine waters, where the oxygen concentrations due to natural reasons may be 5-6 mg l-1. Such values are region specific and due to strong stratifi-cation in summer and autumn, which prevents the saline, cold bottom waters from being mixed with the brackish and oxygenated surface waters. The Danish assessment criteria match the natural effect criteria. Fish will try to avoid waters with oxygen concentrations below 4 mg l-1. Fish and benthic invertebrates can only survive concentrations below 2 mg l-1 for a limited time.

Box 2 summarises the OSPAR eutrophication quality objectives (EQO-eutro) and the Danish criteria used in this assessment.

Background data describing reference conditions when anthropogenic inputs of nutrients were at natural levels are scarce. Hence descriptions of ecological structure and function before the enrichment took place is subject to an element of uncertainty. Box 3 summarises the current understanding of the reference conditions in Danish marine waters.

The implementation of the EU Water Framework Directive and the development of Ecological Quality Objectives (EQOs) defining the ecological status of all European coastal waters will represent a major step forward. The descriptions of ecological status will be based on commonly agreed definitions of reference conditions. The basis for assessing the ecological status will change from expert judgements to operational and numeric quality classes.

An EU funded R&D project “Characterisation of the Baltic Sea Ecosystem: Dynamics and Function of Coastal Types” (CHARM) will be a major contribution to this in the Baltic Sea/Kattegat Area. One of the products of the CHARM project, which runs for the years 2002-2004, is descriptions of reference conditions for nutrients, phytoplankton, submerged aquatic vegetation and macrozoobenthos.

Box 2.
The OSPAR eutrophication quality objectives (EQO-eutro) and the Danish criteria used in this assessment
OSPAR EAC Danish EQO-eutro
I: Degree of nutrient enrichment (causative factors)
Riverine total N and total P inputs and direct discharges (RID)
Elevated inputs and/or increased trends (compared with previous years)
25 %
Nutrient inputs
Winter DIN- and/or DIP concentrations
Elevated level(s) (defined as concentration > 50 % above salinity related and/or region specific background concentration)
25 %
Nutrient concentrations, ratios and limitation
Increased winter N/P ratio (Redfield N/P = 16)
Elevated cf. Redfield (>25)
Nutrient limitations
II: Direct effects of nutrient enrichment (growing season)
Maximum and mean Chlorophyll a concentration in March–October
Elevated level (defined as concentration > 50 % above spatial (offshore) / historical background concentrations)
25 %
Phytoplankton and harmful algal blooms
Region/area specific phytoplankton indicator species
Elevated levels (and increased duration)
Nutrient ratios
Macrophytes including macroalgae (region specific)
Shift from long-lived to short-lived nuisance species (e.g. Ulva)
Submerged aquatic vegetation
III: Indirect effects of nutrient enrichment (growing season)
Degree of oxygen depletion
Decreased levels (<2 mg O2 l-1: acute toxicity;
2-6 mg O2 l-1: deficiency)
< 2 and 4 mg l-1 Oxygen depletion
Changes/kills in zoobenthos and fish kills
Kills (in relation to oxygen depletion, H2S and/or toxic algae)
Long term changes in zoobenthos biomass and species composition
Softbottom macrobenthos
and Fish kills
Organic Carbon/Organic Matter (in sediments)
Elevated levels (in relation to III.1) (relevant in sedimentation areas)
Degradation of organic matter in estuarine sediments
IV: Other possible effects of nutrient enrichment (growing season)
Algal toxins (DSP/PSP mussel infection events)
Incidence (related to Category II.2)
Algal toxins in mussels

Box 3
Background or reference conditions
Background winter concentrations for DIN in the Kattegat, the Skagerrak and the North Sea and in the Wadden Sea has within OSPAR provisionally been estimated to 4-5, 10, and 6.5 µmol l-1, respectively. Winter DIP concentrations have been estimated to 0.4, 0.6 and 0.5 µmol l-1, respectively.
Phytoplankton biomass and production
Chlorophyll a background concentration for offshore areas in the Skagerrak has within OSPAR been estimated to <1.25 µg l-1, and for the North Sea coast to 2-10 µg l-1. The phytoplankton primary production in the Kattegat and Belt Sea has increased from 80-100 mg C m-2 year-1 in the 1950s – 1960s to 120-290 mg C m-2 year-1 in the 1970s – 1980s (Ærtebjerg Nielsen et al. 1981; Richardson & Christoffersen 1991; Heilmann et al. 1994, Richardson & Heilmann 1995).
From scattered measurements of oxygen concentrations in Danish waters from the period 1902–1975 and more systematic measurements since then it seems that the major decrease took place from the 1960s to the late 1980s. In the Kiel Bight the bottom water oxygen concentration in July-August decreased from about 8 mg O2 l-1 in the late 1950s to about 4 mg O2 l-1 in the late 1980s (Babenerd 1991). Trend analysis of bottom water oxygen concentrations in late summer – autumn in the Kattegat and Belt Sea area generally shows a decrease of 1.5-2.2 mg O2 l-1 during the period from mid 1970s to about 1990 (Ærtebjerg et al. 1998).
In 1900, eelgrass was widely distributed in Danish coastal waters, and covered approximately 6726 km2 or 1/7 of all Danish marine waters. In the 1930s, the world wide wasting disease substantially reduced eelgrass populations, especially in north-west Denmark. In 1941, eelgrass covered only 7% of the formerly vegetated areas, and occurred only in the southern, most brackish waters and in the low saline inner parts of Danish estuaries. Analyses of aerial photos from the period 1945–1990s, reveal an initial time lag of more than a decade before substantial re-colonisation of the shallow eelgrass populations began. The photos also show that large populations had recovered in the 1960s. Today eelgrass again occurs along most Danish coasts but has not reached the former area extension. Comparisons of eelgrass area distribution in two large regions, the Sound and Limfjorden, in 1900 and in the 1990s, suggest that the present distribution area of eelgrass in Danish coastal waters constitutes approximately 20-25% of that in 1900. Reduction in area distribution is partly attributed to loss of deep populations. In 1900 colonisation depths averaged 5-6 m in estuaries and 7-8 m in open waters, while in the 1990s the colonisation depths were about halved to 2-3 m in the estuaries and 4-5 m in open waters.
There have been changes of macrozoobenthos communities in the Kattegat area over the last 100 years. The reference material is mainly the large-scale mapping performed by C.G.J. Petersen at the end of the 19th and the beginning of the 20th century. Comparisons in the 1980s and 1990s indicate that biomass, and then probably secondary production, has increased with at least a factor of 2. Main contribution to the increase in deeper waters is from the suspension-feeding brittle star Amphiura filiformis and some polychaetes, whereas some amphipod crustaceans have decreased in importance. In shallower waters it is likely that biomass of bivalves have increased as has been documented from the eastern and southern Baltic Sea. Local reductions of benthic faunal biomass due to hypoxia seems to have occurred at times in recent decades both in the southern Kattegat, the Belt seas and some Danish estuaries.


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Danish Environmental Protection Agency & National Environmental Research Institute • updated: