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
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:
- Degree of nutrient enrichment.
- Direct effects of nutrient enrichment.
- Indirect effects of nutrient enrichment.
- 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
- 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.
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
- 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
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.
The OSPAR eutrophication quality objectives (EQO-eutro) and the Danish
criteria used in this assessment
Degree of nutrient enrichment (causative factors)
|Riverine total N and total P inputs and direct
Elevated inputs and/or increased trends (compared with previous years)
|Winter DIN- and/or DIP concentrations
Elevated level(s) (defined as concentration > 50 % above salinity
related and/or region specific background concentration)
ratios and limitation
|Increased winter N/P ratio (Redfield N/P =
Elevated cf. Redfield (>25)
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)
|Phytoplankton and harmful algal blooms
|Region/area specific phytoplankton indicator species
Elevated levels (and increased duration)
|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
|Changes/kills in zoobenthos and fish kills
Kills (in relation to oxygen depletion, H2S and/or toxic
Long term changes in zoobenthos biomass and species composition
and Fish kills
|Organic Carbon/Organic Matter (in sediments)
Elevated levels (in relation to III.1) (relevant in sedimentation
||Degradation of organic matter in estuarine
|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
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.
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