4. Impact of human activities on water status
4.1 Groundwater
This section examines the environmental effects
that are apparent in aquifer 8 as a result of the environmental pressures described in Section 3.4.1.
The subdivision of aquifer 8 into groundwater
bodies 8-1 to 8-6 has been dealt with in Section
1.7.2. This subdivision was made on the basis of
the same chemical substances as are examined
here.
4.1.2 Groundwater chemical status
In order to be able to closely assess the chemical status of the groundwater in the individual
groundwater bodies the following three groundwater chemistry parameters have been examined:
4.1.1 Abstraction of groundwater
• 2,6-dichlorbenzamid (BAM)
• Nitrate
• Conductivity.
Based on large series of soundings, attempts have
been made to identify changes in the groundwater potential caused by human activities. Useful
data are hard to come by as soundings have usually been made in the waterworks abstraction
wells. This can lead to pronounced local lowering of the water table, which is not representative
of changes in the general water table. Moreover,
there are problems with determining the reference level by the sounding such that systematic
errors arise.
Changes in water level have been detected in
some wells. These changes can be attributed to
variation in the climate, including the precipitation. In connection with the present work it
has not been possible to identify changes in the
groundwater potential that are attributable to
human activities. Neither has it been possible to
identify an increasing water table despite the fact
that groundwater abstraction by waterworks has
decreased by 30% over the past 10 years,
The method chosen is that recommended
and described in the Guidance Document on
groundwater (GD2.8), with the groundwater
chemical status being described on the basis of
the arithmetic mean of the content in the individual groundwater bodies together with the
respective confidence intervals. The status for
2,6-dichlorbenzamid, nitrate and conductivity
is shown in Table 4.1.1. The arithmetic mean
and CL(AM) is shown for each component in
groundwater body 8-1 to 8-6. CL(AM) is the sum
of the arithmetic mean and the 95% confidence
interval. Assuming that the data are normally
distributed, 95% of the measured values will thus
lie below the CL(AM).
The data for BAM show that subdivision of
aquifer 8 into several groundwater bodies was
the right thing to do in that the average concentration in groundwater body 8-3 is considerably
higher than in the other groundwater bodies. If
the whole aquifer had been considered to be a
2,6-dichlorbenzamide
(µg/l)
Groundwater body
8-1
Arithmetic mean
CL(AM)
No data
Nitrate
(mg/l)
Arithmetic mean
Conductivity
(µS/cm)
CL(AM)
78.5
Arithmetic mean
CL(AM)
1 650
8-2
0.02
0.04
5.85
11.73
752.07
823.17
8-3
0.68
1.83
1.54
3.4
756.55
1 149.08
8-4
0.04
0.08
8.89
14.55
610.53
667.95
8-5
0.01
0.01
0.32
0.47
697.76
760.53
8-6
0.02
0.04
9.79
24.22
692.79
774.29
Table 4.1.1
Groundwater status
with respect to 2,6dichlorbenzamid, nitrate and conductivity
for each of the groundwater bodies of aquifer
8. Both the mean and
the CL(AM) (mean +
95% confidence interval) are given.
Odense
PRB
Odense Pilot River Basin
97
4.1 Groundwater
single groundwater body this would have been
masked, and the BAM concentration would have
been high for the whole aquifer. The same applies
for nitrate, where the concentration is higher in
groundwater bodies 8-2, 8-4 and 8-6 than in the
other groundwater bodies. With conductivity,
only groundwater bodies 8-1 and 8-3 differ from
the remaining bodies in aquifer 8.
It transpires that the groundwater bodies
that have been identified on the basis of a raised
conductivity (Table 1.7.10) also do have a higher
conductivity (Table 4.1.1). The same applies for
BAM and especially for nitrate. Based on the
groundwater chemical status of the individual
groundwater bodies it can be concluded that the
description of status is in agreement with the subdivision of aquifer 8 into groundwater bodies.
Chemical stratification of the groundwater
bodies
From the available chemical data it is not possible to identify any chemical stratification of the
groundwater bodies. The reason for this is partly
the relatively flimsy data material, and partly
that the groundwater bodies are restricted with
respect to their vertical distribution.
Trend in groundwater chemical status
Based on the nitrate data, attempts have been
made to identify any trend in groundwater chemical status. It has not been possible to identify any
trend, again due to the flimsy data material, including the lack of large unbroken time series.
4.1.3 Objective compliance and risk
of future lack of compliance
The current objectives for the two selected substances, 2,6-dichlorbenzamid and nitrate, are
described in Section 1.7.3.
The characterization of the chemical status
shows that with respect to nitrate, all the groundwater bodies meet the objective. With groundwater bodies 8-2, 8-4 and especially 8-6, however,
there is the future risk that they will not meet the
current objective for nitrate (maximum 25 mg/l)
if the concentration increases. From the existing
data there is presently no evidence to indicate
that the nitrate concentration is increasing in
these three groundwater bodies, though.
As far as concerns 2,6-dichlorbenzamid, the
characterization shows that groundwater bodies 8-2, 8-3, 8-4 and 8-6 do not meet the current
objectives. No 2,6-dichlorbenzamid has been
detected in groundwater body 8-5, and it has
not been analysed for in groundwater body 81. 2,6-dichlorbenzamid has been detected in an
aquifer beneath groundwater body 8-1, though.
It is therefore concluded that despite the fact that
groundwater body 8-1 has not been analysed for
2,6-dichlorbenzamid, it does not meet the objective for this substance.
The extent of 2,6-dichlorbenzamid contamination is expected to increase in future as the subsurface groundwater gradually moves deeper. It
is thus expected that the groundwater bodies will
not be able to meet the objectives with respect to
pesticides and their metabolites in 2015. Neither
is it expected that it will be possible to meet this
environmental objective with the technological
solutions currently available. In view of land use
and the natural protection above the groundwater bodies, moreover, it is concluded that groundwater body 8-5 will not become contaminated
with 2,6-dichlorbenzamid.
4.2 Watercourses
4.2.1 Physical pressures
Anthropogenic pressure on the watercourses
in Fyn County really started around 5 000
years ago with the clearing of the woodland for
agricultural use, thereby rendering many of the
watercourses unshaded. Moreover, from around
the 13th Century some of the watercourses were
dammed for mill operation. This – together with
the establishment of dams for meadow irrigation
in the 19th Century – disrupted the continuity
of the watercourses, thereby hindering fish such
as trout and eel from their natural migration between fresh water and sea water. In newer times
other obstructions have arisen too, for example
culverts under roads.
The appearance of the watercourses has also
been changed through other forms of regulation.
Many naturally meandering or sinuous watercourses have been straightened and the beds excavated, and in places the slope has been neutralized by the construction of falls. These activities
really started in the 18th Century and accelerated
up through the 19th Century until the last major
regulation project in 1960, which encompassed a
considerable part of the River Odense (Riis et al.,
1999). The aim of such regulation was to ensure
improved drainage and thereby to increase the
Odense
98
PRB
Odense Pilot River Basin
4.2 Watercourses
possibilities to use the adjoining land for agricultural production. At the same time, rapid removal of the water was further ensured in many
places through intensive maintenance in the form
of clearance of all aquatic vegetation as well as
vegetation along the banks and borders of the watercourses and dredging of the bed substratum.
The regulation, the changed physical conditions (with soft, unstable bed and rapid draining away of the water), the restricted hydraulic
interaction with the immediate surroundings
(see Section 4.4) and the continued disturbances
contributed – together with pollution – to the
disappearance of many sensitive plant and animal
species from the watercourses. A few species have
not just disappeared locally, but are no longer
found in Fyn County at all (see for example Riis
et al., 1999).
Even though numerous improvements have
been made to the physical conditions over the
past 15 years, including actual restoration and
the introduction of more environment-friendly
maintenance, there is no doubt that the physical
conditions are often a hindrance to attainment of
good watercourse ecological status.
Water flow in the watercourses of Fyn County
generally varies relatively much during the
course of the year in that only a small part of the
water derives from deep groundwater. By far the
majority of the discharge is accounted for by subsurface groundwater, drainage water or surface
runoff (see Section 1.3). Both plants and animals
are affected by – but adapted to – these natural
fluctuations in water flow. This applies not least
to those species associated with regularly summer-dry watercourses. Human intervention in
the natural water cycle causes further stress to
the organisms, however.
Each year in Fyn County, approx. 38 million
m3 of groundwater are abstracted for the drinking water supply. In addition, approx. 11 million
m3 are abstracted for industrial purposes, crop
irrigation, etc. (see also Section 4.1). The amount
of water abstracted corresponds to approx. 25%
of the mean summer runoff in the watercourses
of Fyn County, or more than half of the amount
of water that flows in the watercourses in dry
summers. A considerable part of the abstracted
drinking water is “returned” to the watercourses
in the form of treated wastewater, but not necessarily in the area from where it was abstracted.
Large amounts of groundwater are abstracted
within Odense River Basin (Figure 4.2.1), in
some catchments amounting to over 50% of
the median minimum water flow in the associated watercourses. For example, groundwater to
Figure 4.2.1
Relative impact (in
%) of groundwater
abstraction on median
minimum water flow
in the various catchments within Odense
River Basin. The relative impact of groundwater abstraction is
calculated as the total
abstraction multiplied
by 0.6, which is the
factor whereby abstraction is expected to affect groundwater input
to the watercourses.
Median minimum water flow is without any
input of wastewater
from treatment plants
and is reported as
“output” from a given
catchment.
N
0
5
10 km
supply Odense City is abstracted from wells at
Borreby in the catchment of Holmehave Brook.
This water is discharged as treated wastewater
in the lower part of the River Odense. As a
consequence, parts of the Holmehave Brook
system lose a large proportion of their natural
water flow, which in summer can result in some
reaches drying out completely. The Stavis and
Lunde watercourse systems also “lose” water
due to groundwater abstraction, albeit that this
is partly compensated for in the Lunde Stream
through the input of treated wastewater from the
catchment of Stavis Stream.
Despite the major impact that it still has, total abstraction of groundwater in Fyn County
has generally fallen by approx. 35% since 1979,
among other reasons due to the introduction
of green taxes. In several cases, groundwater
abstraction has nevertheless led to the loss of
species (macroinvertebrates) from watercourses
in Fyn County, especially in the upper ends of
the watercourses.
The lowermost reaches of the River Odense
are subject to a rather special form of physical
pressure, namely large amounts of saline cooling water discharges from the combined heat
and power plant Fynsværket. The cooling water
is abstracted from Odense Canal, which is in
direct contact with Odense Fjord. The cooling
water discharge is so considerable that much of
the warm, salty water penetrates like a “wedge”
>50
25-50
15-25
0-15
No data
Odense
PRB
Odense Pilot River Basin
99
4.2 Watercourses
at least 2.5 km upstream from the outlet of the
river. The salt water lies at the bottom such that
the fresh water flows over it. As a consequence,
the river bed is devoid of any actual watercourse
fauna. However, watercourse fauna exists in the
subsurface vegetation along the banks of the
watercourse, except downstream of the cooling
water outlet, where the salinity is too great.
4.2.2 Impact of pollutant loading
Human activities, especially during the past 100
years, have caused pollution of the watercourses
in Fyn County. These conditions are described
in detail and documented in Fyn County (2001a).
The present section therefore only briefly examines the main factors that have been and are of
greatest significance for watercourse environmental state.
Until just over 40 years ago, badly treated
wastewater from dairies, abattoirs, towns, etc.
comprised a major source of organic matter
Municipal/private watercourses
Environmental status 1984-2002
100
Apportioned by fauna class (%)
Figure 4.2.2
Environmental status assessed from the
macroinvertebrate
fauna in major (county) and minor (municipal and private)
watercourses in Fyn
County in the period
1984–2002.
P: Pesticide-affected;
FC: Fauna class.
80
60
40
20
0 ll
84
Other (desiccation etc.)
l
ll
86
l
ll
88
l
ll
90
l
ll l ll
92 94
Year
l
ll
96
l
ll
98
l
ll
00
l
ll
02
l
ll
00
l
ll
02
P
FC 1-3
County watercourses
Environmental status 1984-2002
FC 4
FC 5
100
Apportioned by fauna class (%)
FC 6-7
80
60
40
20
0 ll
84
l
ll
86
l
ll
88
l
ll
90
l
ll l ll
92 94
Year
l
ll
96
l
ll
98
input. Until just about 15 years ago, moreover,
highly polluting discharges of silage juice, liquid
manure and manure seepage from agricultural
holdings were common. As a result of these discharges the flora and macroinvertebrate fauna
became highly impoverished, and the natural
trout population disappeared from the majority
of watercourses.
Since the end of the 1980s, the environmental
state of watercourses in Fyn County (including
those in Odense River Basin) has improved considerably, however, especially that of the major
watercourses (Figure 4.2.2).
Today the main sources of readily degradable
organic matter pollution (measured as BOD5) are
wastewater from public wastewater treatment
plants and stormwater outfalls in the towns and
wastewater from sparsely built-up areas. Approx. 85% of the population in the region – and
somewhat more in Odense River Basin – inhabit
towns. Wastewater treatment has been highly
centralized over the past 25 years, however, and
the wastewater treatment plants have been modernized (over 99% of the wastewater is treated
biologically) in order to effectively remove both
the organic matter and the nitrogen/phosphorus
compounds. Thus over 95% of the organic matter and phosphorus is removed, while the treatment efficiency for nitrogen is over 85%. As a
consequence, especially the main watercourses
that receive urban wastewater have become much
cleaner. The BOD5 content of these watercourses
has thus decreased to an average of approx. 2
mg/l within the past 20 years. The minor watercourses in Odense River Basin are affected by
poorly treated wastewater from a large proportion of the approx. 6 900 properties located in
the sparsely built-up areas. The BOD5 content of
these watercourses is therefore somewhat higher
than in the larger watercourses. The macroinvertebrate fauna in the minor watercourses is markedly affected by the enhanced organic matter
content (see Fyn County, 2001c).
Ammonium-(and ammonia-)nitrogen and
phosphorus also largely derive from wastewater.
As with BOD5, however, the concentration of
these two substances in the larger watercourses
has decreased markedly over the past 20 years.
The present phosphorus content is not solely attributable to discharges from urban wastewater
treatment plants and sparsely built up areas, but
also to input from arable land. Thus despite the
above-mentioned considerable reduction, the
phosphorus concentration is considerably higher
than the natural concentration in watercourses
in Fyn County (and Denmark).
With ammonium-(and ammonia-)nitrogen in
contrast, the concentrations reached nowadays
Odense
100
PRB
Odense Pilot River Basin
4.2 Watercourses
are only very rarely critical for fish, and are of
no significance to the macroinvertebrates. The
phosphorus content is of no significance to the
macroinvertebrates and fish either, and is unlikely to be of any significance to higher plants,
which obtain a considerable proportion of their
nutrients from the watercourse sediment. In contrast, the presence of large amounts of filamentous green algae (especially Cladophora), which
can be harmful to other watercourse organisms,
is primarily determined by the raised concentrations of dissolved phosphorus (Fyn County,
1997). Such blooms of filamentous algae are often
hindered by the macroinvertebrates grazing the
algae while these are still “small”, however.
The nitrate-nitrogen concentration is markedly enhanced compared with the natural level
in streams in Fyn County (and Denmark). This
is primarily attributable to leaching from arable
land. The nitrate-nitrogen concentration has
decreased by approx. 20–25% over the past 15
years or so, however. Despite the enhanced levels, nitrate-nitrogen probably has no significant
impact on the occurrence and amount of plants,
macroinvertebrates and fish in the watercourses.
A large number of different pesticides have
been detected in the watercourses in Fyn
County, including the River Odense. As many
as 40% of the 100 or so substances that have been
analysed for have thus been detected with varying frequency and in concentrations of up to 11
µg/l. The greatest proportion of the pesticides
detected are herbicides, which also comprises the
largest group of pesticides. Given the relatively
low concentrations in which they are present,
the substances detected are unlikely to pose a
major threat to life in the watercourses. For example, aquatic plants are unlikely to be harmed.
The opposite is the case, though, with a number
of pyrethroid insecticides that have not been
analysed for because they are expected to occur
in concentrations considerably below the detection limit with normal analyses. In a number of
documented cases, these substances have caused
considerable mortality among insects and crustaceans in particular, and in some cases also fish.
The assessment is that over the past 10 years,
these substances have caused considerable damage to the macroinvertebrate fauna in up to 14%
of the watercourses in Fyn County. The acute
effects have reduced considerably in recent years,
though, among other reasons due to a campaign
focused on the users of the pesticides. The input
of pesticides primarily occurs via runoff from locations used to fill and clean spraying equipment
in agriculture, via runoff from market gardens,
through leaching from fields in surface runoff or
drainage water, and to some extent via the dis-
charge of wastewater. From the environmental
point of view, the first two sources are undoubtedly the most important.
Wastewater is an important source of other
hazardous substances present in the watercourses
even though they are removed to some extent before reaching the watercourses, all depending on
their biodegradability and the type of treatment
plant. Another major source of these substances
is stormwater outfalls. A large number of different substances have been detected, among other
places in the lower part of the River Odense. The
levels are not very high, though, and only PAH
compounds occur in slightly elevated concentrations relative to the national criteria. Investigations of the River Odense do not indicate that
PAH compounds, for example, have any significant effect on macroinvertebrates. However,
there is the risk of reproductive disturbances in
fish caused by hazardous substances with endocrine disrupting properties, although this has not
been investigated in the River Odense.
4.2.3 Objective compliance and risk
of future lack of compliance
This section describes the present state of compliance with the objectives stipulated in the Regional Plan 2001–2013 and Action Plan on the
Aquatic Environment II, and assesses the risk
of failure to meet the provisional objectives established pursuant to the WFD by the year 2015
with the measures hitherto adopted.
In 2002, the current objectives have been met
at approx. 50% of the monitoring stations in
Odense River Basin and in the watercourses of
Fyn County as a whole (the criteria are apparent
from Table 1.4.9). This is also the case for the
open parts of the watercourses assigned a quality
objective in the two test areas, the main course
of the River Odense and the Ryds Stream catchment (see Section 1.4). The degree of compliance
with the objectives roughly corresponds to the
average situation in Denmark (Skriver, 2002). If
instead one applies the criterion that the objective has to be met for five consecutive years (see
the grounds for this in Section 1.4), the percentage of watercourse stations in Fyn County that
meet the objectives stipulated in the Regional
Plan is much smaller – only 25%. Irrespective
of the choice of assessment period, the degree of
objective compliance is greatest for watercourses
assigned quality objective A and least for watercourses assigned quality objective B3 (see Table
1.4.9, where these codes are explained). That so
few watercourses assigned quality objective B3
Odense
PRB
Odense Pilot River Basin
101
4.2 Watercourses
Table 4.2.1
Overview of water
bodies in the main
course of the River
Odense and Ryds
Stream catchment indicating typology (provisional system), heavy
modification (HM, provisional designation),
reference conditions
assessed from the macroinvertebrates (fauna
class, FC; provisional
classification), current
status assessed from the
macroinvertebrates
(FC), ecological status
assessed from the macroinvertebrates and
physical conditions
(provisional, based
on FC and modified
Aarhus Index), current
objective pursuant to
the Regional Plan currently in force, future
objective (provisional
assessment) pursuant to
the Water Framework
Directive (WFD), and
expected compliance
with the objective
(provisional assessment ) in 2015. The
WFD objectives are
abbreviated H: High;
G: Good; M: Moderate;
P: Poor and B: Bad.
* Indicates that the
reach is encompassed
by the Habitats Directive, such that special
requirements can be
expected concerning
safeguarding certain
species or habitats. **
Indicates that the objective for the watercourse
was first set during an
inspection in spring
2003. Expected compliance with the objective has been assessed
with respect to both
improved water quality in connection with
planned wastewater
treatment in sparsely
built-up areas (WWT)
and improved physical conditions (IPC).
Expected compliance
is indicated by +, and
non-compliance by –.
WB No. Type
Provisional
HM
Ref.
status
(FC)
Current status
(FC)
Provis. ecol. status
(FC/Phys.index)
Expected
provis. compliance
(WWT/IPC)
O1
O2
O3
(O4)
O5
O6
O7
O8
O9
O10
O11
O12
O13
O14
O15
O16
1
1
1
(Lake)
2
2
3
3
3
3
3
3
3
3
3
3
+
+
+
+
+
+
+
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
4
7
Unknown
3-4
5-6
5-6
Unknown
5-7
5-7
5-7
3-4
5-6
3-5
4-6
4
M/P
H/H
? / ?M
(B)
P-M/P-B
G-H/P-G
G - H / M - B (pre)
? / ?P
G-H/G
(G - H / M - P)
G-H/G-H
P - M / ?P
G-H/G
P - G / ?P - M
M-H/M-G
M / ?P
B1/B2
B1/B2
B1/B2
(A1)
B1/B2 - B3
B1/B2
B1/B2
B1/B2
B1/B2
B1/B2
B1/B2
B1/B2
B1/B2
B1/B2
B3
B3
H*
H*
H*
(H) *
G*
H*
H*
G*
H*
H*
H*
G*
H*
G*
H
G
+/+/+
+/(- / )
-/+/+ / + (Rest.)
+/+/+
+/+/+
-/-/-/-/-/-
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13
1
1
1
1
1
1
1
1
1
1
2
2
2
+
+
+
+
+
+
+
+
+
7
7
5
7
7
7
7
7
7
7
7
7
7
4 + rør
5 + rør
Unknown + pipe
Piped
Piped
4 - 5 + pipe
4 - 6 + pipe
4 - 5 + pipe
Unknown + pipe
5
5-7
6-7
4
M / M + pipe
G / P - ?G + pipe
? / ?P + pipe
Piped = B
Piped = B
M - G / M - B + pipe
M - H / M - G + pipe
M - G / M - ?G + pipe
? / ?P + pipe
G/M-G
G-H/M-G
H/M-G
M/P
B1/B2
A
B3
B1/B2
B3
B1/B2 **
B3
B1/B2
B1/B2
B1/B2
B1/B2
G
H
G
G
G
G
H
G
G
H
H
H
G
+/+/-/-/-/+/+/-/-/+/+/+/-/-
comply with the objective is sometimes due to
the fact that the physical conditions are so bad
as to preclude attainment of fauna class 5, even
though the watercourse water is clean. Moreover,
objective compliance is clearly greatest for the
large watercourses, corresponding to the median
fauna class being one grade higher than in the
smaller watercourses. The difference is greatest if
one examines objective compliance over a period
of five years. The degree of objective compliance
has increased over the past 15–20 years, not least
since 2000.
The future environmental objective pursuant to the WFD is that all watercourses should
achieve good (or better) ecological status by 2015
at the latest. Moreover, the Directive stipulates
that the present status must not be allowed to
deteriorate. Based on present knowledge of the
biological and physical conditions (see Section
1.4) a provisional assessment has been given of
the ecological status of the watercourse reaches
in the two test areas (Table 4.2.1), as well as an assessment of the possibilities for complying with
the WFD environmental objectives by 2015 (Table 4.2.1). The objectives have been established
on the basis of the criteria in Table 1.4.10 and the
condition that the existing state (assessed from
the fauna class) may not deteriorate. Moreover,
the objective has been set to high ecological
status in the whole of Rislebækken Brook out
of consideration for Lake Arreskov, which is
presently assigned the highest quality objective
A1 (see Section 4.3). The possibilities for meeting
the WFD’s objectives within the specified time
frame have been assessed on the basis of the plans
hitherto adopted and in force and assuming that
the environmental measures (improved wastewater treatment) in the sparsely built-up areas are
implemented as planned. It is apparent that with
the present initiatives, only 4 out of 28 watercourse reaches (14%) will fully comply with the
provisional objectives established pursuant to the
WFD, and that it is particularly the poor physical conditions and discharges of wastewater from
stormwater outfalls that limit the possibilities for
compliance with the objectives for watercourses
(assessment of the significance of watercourse
water for lakes and coastal waters is not encompassed here; see Sections 4.3 and 4.5). It should
be noted, moreover, that it can take up to 5–10
years before the effect of the implemented environmental improvements have a serious impact
on the biological conditions in the watercourses.
Odense
102
Current
Provis.
objective objective
(WFD)
PRB
Odense Pilot River Basin
4.3 Lakes
4.3 Lakes
4.3.1 Physical pressures
4.3.2 Impact of pollutant loading
Over the years, many lakes have been exposed to
various physical pressures in the form of water
level regulation, reclamation etc. In some places,
moreover, water abstraction has reduced water
flow to the lakes. Conversely, new lakes have
arisen through damming and excavation.
Nutrients
Over the years, the majority of the lakes have
been subjected to extremely high nutrient loading. Some of the lakes served as recipients for
poorly treated urban wastewater, and many are
affected by nutrient loading from agricultural
sources and wastewater from sparsely built-up
areas. This has led to a marked increase in algal
growth, blooms of potentially toxic blue-green
algae, the shading-out of submerged macrophytes, and the impoverishment of the lake fauna
(macroinvertebrates, fish and birds).
Within the past six years, investigations encompassing fish, submerged macrophytes, phytoplankton, zooplankton and physico-chemical
variables have been carried out in six major lakes
(15–317 ha) (see Table 4.3.1). The lakes are all of
the type “alkaline, clearwater, shallow freshwater lakes”, except Lake Søby, the mean depth
of which is 3.6 m, and which must therefore be
considered as deep, cf. Section 1.5.3.
The investigations show that the lakes have
large algal populations, often dominated by
blue-green algae, absent or poorly developed
submerged macrophyte vegetation, high nutrient concentrations and poor Secchi depth. In
addition, the fish populations are generally too
dominated by roach and bream, with too few
piscivores such as perch and pike.
Based on the provisional proposals for a lake
typology (National Environmental Research
Institute, in prep.) all the lakes will belong to
the class “poor ecological status” cf. Section 1.5.
The chief determinant of lake state is the phosphorus availability, and based on the phosphorus
concentration alone the classification of some of
the lakes is a little better, namely “moderate” to
“poor” ecological status. One of the reasons for
this might be that following a reduction in nutrient loading, the state of some of the lakes is just
slowly improving, and that some of the biological
conditions – for example the distribution of submerged macrophyte vegetation – lag behind. Another problem is that the nitrogen concentration
in the lakes is typically too high. None of the six
lakes are presently able to meet the criteria for
good ecological status.
Less comprehensive investigations have also
been made of 30 minor lakes. Nutrient concentrations were enhanced and the submerged
Water level regulation
The water level in most lakes can be regulated by
a retaining weir in the outlet. The impact of this
on lake ecological state is so small that it is not
considered to have any significant impact, however. In certain cases, water level regulation can
be used to improve the state of the lake through
enhanced wash-out of phosphorus during the
summer period.
Water abstraction
Pursuant to the Regional Plan, water abstraction
interests are prioritized as follows: 1) The public
water supply, 2) Maintenance of environmentally
acceptable water flow in watercourses and water
exchange in lakes, and 3) Other purposes such
as abstraction for industrial, agricultural and
recreational use.
Abstraction of surface water is thus severely
restricted, and to the greatest extent possible
groundwater abstraction must not have negative
effects on watercourses and lakes.
The overall assessment is that groundwater
abstraction does not presently have any serious
impact on the lakes’ water balance. Current
work with integrated models for groundwater
and surface water will be able to shed more light
on this issue.
Reclamation
Many water bodies – especially the smaller ones
– have disappeared since the end of the 19th Century as a result of drainage and lowering of the
water level. In the catchment of Lake Arreskov,
for example, the number of lakes decreased by
76% from 276 in 1890 to 65 in 1992.
Larger lakes have also disappeared, for example
Lake Næsbyhoved, which was drained and partially excavated as a harbour in Odense in 1863.
See also Section 1.1.
The former – and now reclaimed – lakes are
not included in this report unless they still exist
as wetlands.
Odense
PRB
Odense Pilot River Basin
103
4.3 Lakes
macrophyte vegetation was markedly reduced in
nearly all of these lakes (90%).
Based on the above-mentioned investigations it
must be concluded that by far the majority of the
lakes in Odense River Basin are affected by nutrients to such a degree that they do not meet the
objective of a natural and diverse flora and fauna
or the criteria for good ecological status.
Heavy metals
Water phase
In 1998 and 2001, the surface water concentrations of seven heavy metals were analysed in
Lake Arreskov. All concentrations were below
the limit levels for surface waters, cf. Ministry of
Environment and Energy (1996).
Sediment
The content of heavy metals in the surface sediment of the lakes is moderate, and in most of the
eight lakes investigated in Odense River Basin
within the normal range for lakes that have not
Table 4.3.1
Biological and physical conditions in six
lakes i Odense River
Basin (1997–2002).
The plankton values
are means for the summer period. The water
chemistry values are
medians for the same
period. The ecological class determined
according to the National Environmental
Research Institute (in
prep.) is also indicated.
Lake:
received wastewater containing heavy metals,
cf. Danish Environmental Protection Agency
(1983a). The sediment content of chromium is
markedly raised in Lake Brahetrolleborg Slotssø,
though, as this lake formerly received wastewater
from a tannery.
The moderate heavy metal concentrations
detected in the water phase and sediment of
the lakes reflect the fact that, apart from Lake
Brahetrolleborg Slotssø, they do not or have not
received wastewater containing high levels of
heavy metals.
Pesticides
In 2001, water from Lake Arreskov was analysed
for 47 pesticides/pesticide residues. Four pesticides and four pesticide residues were detected.
The substance most frequently detected was 2,6dichlorbenzamid (BAM), which was detected in
all samples. The following substances were also
detected: Hydroxyatrazine, AMPA, hydroxysimazine, glyphosat, TCA (trichloro acetic acid),
Arreskov
Dallund
Nr. Søby
Søbo
Year of investigation
2002
1998
1997
2001
2001
1998
Area (ha)
317
15
18
69
18
21
Mean depth (m)
1.9
1.9
3.1
2.3
0.5
3.6
236
98
275
474
266
300
4.709
4.606
4.914
5.566
12.442
4.032
11
3
0
2
0
0
Coverage (%)
11.4
<1
0
<1
0
<1
Chlorophyll a (µg/l)
35.8
30
70
55
239
29.7
Blue-green algal biomass
3
(mm /l)
34.1
2.73
7.59
2.82
3.26
0.058
7.5
5.75
5.79
3.53
17.18
3.75
Zooplankton:phytoplankton ratio
0.21
1.27
0.34
0.57
0.63
0.74
Water chemistry
Total P (mg/l)
0,15
0.07
0.174
0.075
0.28
0.074
Total N (mg/l)
1.586
0.997
2.21
1.33
3.47
1.19
SS (mg/l)
13.21
11.87
12.7
7.29
61.15
5.67
pH
8.63
8.05
8.74
8.48
9.53
8.25
Secchi depth (m)
1.10
1.00
0.94
0.78
0.24
1.55
Based on phosphorus content
Poor
Moderate
Poor
Moderate
Bad
Poor
With inclusion of biological parameters
Bad
Bad
Bad
Bad
Bad
Bad
Fish
CPUE No.
CPUE Wt (kg)
Vegetation
Phytoplankton
Zooplankton
Number of species
3
Biomass (mm /l)
Langesø Nørresø
Ecological status:
Odense
104
PRB
Odense Pilot River Basin
4.3 Lakes
Lake
Type
cf. Table
1.5.5
Reference status
Total P
(mg/l)
Arreskov
Brændegård
Sortesø
St. Øresø
Nørresø
Fjordmarken
Søbo
Langesø
Nr. Søby
Dallund
Fjellerup
Brahetrolleborg Slotssø
Notes:
1)
2)
3)
4)
5)
6)
12
12
11
12
12
?
13
12
12
12
12
12
5)
0.044
1)
0.015
1)
0.015
1)
0.015
2)
0.030
1)
0.015?
1)
0.008
2)
0.135?
1)
0.015
2)
0.020
1)
0.015
1)
0.015
Current ecological
status
Chl. a
(µg/l)
6)
39
1)
4
1)
4
1)
4
?
1)
4
1)
4
?
1)
4
?
1)
4
1)
4
Total P
(mg/l)
0.189
1.816
0.164
0.043
0.073
0.087
0.172
0.283
0.059
0.208
0.868
Current
objective
Provisional future objective
Chl. a
(µg/l)
100
45
210
8
60
58
119
237
34
109
94
A1
A1
A1
A1
A1
A1
B
B
B
B
B
B
High
High
High
High
High
High
Good
Good
Good
Good
Good
Good
Total P
<(mg/l)
Chl. a
<(µg/l)
0,050?
3)
?
4)
0,025
4)
0,025
0,040?
0,025?
4)
0,013
0,135?
4)
0,050
4)
0,050
4)
0,050
3)
?
42?
3)
?
4)
7
4)
7
10?
7?
4)
7
?
4)
13
4)
13
4)
13
3)
?
6)
Expected
compliance
with provisional future
objective
Reason for
failure to
meet objective
No
?
No
No
No
?
No
No
No
Perhaps
No
?
a, b
a, b
b
a, c
a, b
?
a, b
a, b
a, b
a, b
a, b
a, b
Table 4.3.2
Cf. Table 1.5.6
Overview of lake type,
Assessed on the basis of paleolimnological studies
P content and algal biomass naturally elevated due to the presence of a cormorant colony in Lake Brændegård expected concentration of phosphorus and
Cf. National Environmental Research Institute (in prep.)
chlorophyll a in the
Model calculations, cf. Fyn County (2003a)
Model calculations based on total P, cf. Jensen et al. (1997)
surface water in refer-
Reason for failure to meet objective:
a) N and P loading from agriculture
b) P release from sediment
c) Wastewater from sparsely built-up areas
terbutylazin and simazine. The concentrations in
which the substances were detected were all low,
and none exceeded the limit value for drinking
water of 0.1 µg/l.
Other hazardous substances
In autumn 2002, the surface sediment of three
lakes (Arreskov, Langesø and Store Øresø) was
investigated for the following groups of compounds: PAHs, PCBs, chlorinated pesticides,
chlorobenzenes, chlorophenols, plasticizes, Ptriesters, nonlyphenols, bisphenol-A, LAS, phenols, and brominated flame retardants.
Although the results have not yet been assessed, it does not seem that the sediment content
of these substances is high.
4.3.3 Objective compliance and risk
of future lack of compliance
The present assessment is that only one lake,
Lake Store Øresø, currently meets the objective
of a natural and diverse flora and fauna. It has a
low phosphorus concentration, a low algal abundance and a widespread submerged macrophyte
vegetation. It is uncertain, though, whether this
lake will be able to meet the more specific provisional criteria for good ecological status pursuant
to the WFD. In addition, a number of the cleanest gravel quarry lakes will probably be able to
meet the current objective, and perhaps also the
criteria for good ecological status/good ecological potential.
The remaining lakes are all to some extent affected by former and present-day nutrient loading, and therefore do not meet their current objective. Some as yet uninvestigated isolated lakes
may exist that meet the objective, however.
The state of those lakes that were previously
polluted by large amounts of urban wastewater
is slowly improving. Thus the submerged macrophyte vegetation has returned in some of these
– but is only widespread in Lake Arreskov.
A precondition for improvement in lake state
is that nitrogen and phosphorus loading from agricultural sources and wastewater from sparsely
built-up areas are adequately reduced. During the
coming years, phosphorus loading from wastewater can be expected to decrease considerably
in line with the introduction of requirements for
improved phosphorus removal from wastewater
from properties in the catchments of the larger
lakes. This is hardly likely to be sufficient to
ensure that the lakes will meet the objective,
though, and it will therefore be necessary to also
take action to reduce phosphorus and nitrogen
loading from agricultural sources. The trend in
the lakes will depend on the measures imple-
ence conditions (mean
for the summer period
1.5–30.9), current concentration of phosphorus and chlorophyll a
in the surface water
(summer mean), current objective, provisional future objective
with proposed requirements as to phosphorus
and chlorophyll a
concentrations in the
surface water (summer
mean), expected compliance with objective
and reason for failure
to meet the objective
for 12 lakes in Odense
River Basin.
Odense
PRB
Odense Pilot River Basin
105
4.4 Wetlands
mented in this area.
Another factor is that lakes react very slowly
to reductions in loading, among other reasons
due to nutrients accumulated in the sediment.
It is therefore hardly likely that more than just
a few of the lakes can meet the objective of good
ecological status by 2015.
By way of example, on the basis of current
knowledge about the lakes and measures already
planned an assessment has been made of the
extent to which 12 of the large lakes in Odense
River Basin will meet the provisional future objectives by 2015 (Table 4.3.2). The planned measures are first and foremost improved treatment
of the wastewater from sparsely built-up areas.
It is apparent that the majority of lakes cannot
be expected to meet the objective with the
hitherto adopted measures. Only a single lake
might possibly meet the objective. With regard
to the WFD’s general objective of at least good
ecological status by 2015, there are possibly two
out of the 12 lakes that can comply. As regards
the remaining lakes in Odense River Basin, it is
hardly likely that more than just a very few of
them can meet the objective of good ecological
status by 2015.
White water-lily
( Nymphaea alba)
.
4.4 Wetlands
4.4.1 Physical pressures
The physical pressures on the wetlands in the
form of regulation of watercourses, drainage/
ditching of wetlands and the nearby surroundings, abstraction of groundwater, filling-in and
peat mining have had a considerable impact on
the function of the wetlands as nutrient sinks and
on their natural state.
From the second half of the 19th Century, a
pronounced desire arose from agriculture and society in general for better drainage of the fields.
Minor regulations of the River Odense and its
tributaries were carried out on several occasions
during that period. With adoption of the Land
Reclamation Act in 1940, major regulation of
watercourses was initiated. As a consequence,
many wetlands have partially or completely
disappeared. An example is the regulation of the
River Odense, which took place over the period
1941–1960. This resulted in the cultivation of the
lowland areas along the watercourses. It has not
been possible to maintain this cultivation on all
of this land right up to the present time, however.
For example, large areas around Ulvebækken
Brook are presently meadow and mire. A subsidence survey (Nielsen, 2002) has shown that large
parts of the lowland areas have subsided by up to
1 m, while those alongside Ulvebækken Brook
has subsided by up to 3 m. The rule of thumb
is 1 cm per year. There are several reasons for
the subsidence. When the water level is lowered
in peaty soils these dry out, thereby enabling
consolidation to take place. At the same time the
peat becomes oxidized due to drainage, whereafter decomposition increases. The new water table
in re-created wetlands will therefore have to be
adapted to the subsidence that has taken place;
otherwise, very large lakes will form in the watercourse systems themselves, which is undesirable from the point of view of fish passage.
Since 1890, the area of extensively farmed
lowland has decreased by 45% in Odense River
Basin. This reduction is typically attributable
to drainage and ditching of meadows/wetlands,
straightening and deepening of watercourses
and the pumping of water away from wetlands.
Watercourse maintenance has also influenced
the water level in the adjoining wetlands. Prior
to adoption of the new Watercourse Act in 1983,
the watercourses were maintained frequently,
and in a hard-handed manner. With the new Watercourse Act the purpose was changed such that
drainage has to be ensured while concomitantly
Photo: Bjarne Andresen, Fyn County
Odense
106
PRB
Odense Pilot River Basin
4.4 Wetlands
1930s
Figure 4.4.1
Consequences of
drainage for the river
valley along the River
Odense/Tørringe
Brook. Scenarios for
the drainage level in
the 1930s, 1960s and
in between, which are
used when re-creating
new wetland in 2003.
Lake
Swamp
Wet meadow
Dry meadow
N
2003
Open water
0
5
10 km
Swamp
Wet meadow
Humid meadow
Dry meadow
1960s
Swamp
Wet meadow
Dry meadow
Odense
PRB
Odense Pilot River Basin
107
4.4 Wetlands
taking into consideration the environmental requirements to watercourse quality.
Abstraction of groundwater for drinking water, industry and irrigation also influences the
water level in wetlands. This is particularly the
case for wetlands located in upwelling areas. A
lower water table will also diminish water flow
in the watercourses, however, which will have a
more general effect on all the wetlands along the
watercourses and hence have a great impact.
The filling-in of wetlands and peat mining in
wetlands naturally have very direct consequences for wetlands. Filling-in leads to the disappearance of the wet low-lying parts through covering
of the peat layer, usually with other soil types.
Conversely, peat mining entails removal of the
peat layer, whereby valuable plant communities
disappear and are replaced by water-filled peat
mines. Peat mining has now ceased in Fyn County, and the filling-in of wetlands has reduced
considerably in recent years.
An example of a wetland that has been drained,
but which is now being re-established under Action Plan on the Aquatic Environment II, is the
reach of the River Odense upstream of Tørringe
Brook. This reach was regulated during the period 1944–1950. Scenarios for the consequences of
drainage of the floodplain have been established
for the 1930s, the 1960s and for drainage state in
between. These are being used in connection
with the re-establishment of a new wetland in
2003 (See Figure 4.4.1).
4.4.2 Impact of pollutant loading
Pollutant loading is described in Section 3. As
regards the wetlands, this encompasses both
waterborne and airborne pollution in the form
of the nutrients nitrogen and phosphorus, organic matter, heavy metals and hazardous substances. The loading derives from waterborne
and airborne discharges and emissions from the
agricultural sector, sewage and rain water from
sparsely built-up areas, emissions from industry,
power stations, traffic, etc. Sewage from wastewater treatment plants is no longer discharged to
wetlands and lakes. All discharges from wastewater treatment plants are now led solely to watercourses and coastal waters.
Nutrient loading of wetlands influences their
natural state, especially the vegetation, but also
the aquatic animals and birds. Other animals
associated with wetlands due to the presence
of a particular type of vegetation, e.g. insects,
will also be indirectly affected if the vegetation
changes.
The waterborne loading primarily derives from
agriculture and has its greatest effect on those
wetland areas located closest to the source of
pollution. Nitrogen compounds are transformed,
especially nitrate, which is reduced to gaseous nitrogen under anaerobic conditions, such that the
load decreases with increasing distance from the
source. The extent to which the flora and fauna
will be affected depends on the magnitude of
loading and the type of wetland.
Certain pesticides will also be degraded in
wetlands, but the flora and fauna will be affected
depending on the type of pesticide. As regards
phosphorus, heavy metals and hazardous substances, these can sediment, bind or be released
depending on the redox state and biological
processes. Transport of particulate matter out of
wetlands, typically during major runoff events,
can result in the release from wetlands to downstream water bodies.
The airborne pollution distributes over the
whole surface of the wetlands, although in such a
manner that loading is highest in the parts closest
to the source. The deposition encompasses nitrogen, primarily in the form of ammonia, phosphorus, heavy metals and hazardous substances,
including pesticides.
Nutrient loading of wetlands favours the nutrient-tolerant plant species, resulting in loss of
vegetation diversity and an increase in the rate
at which the wetlands become overgrown. Thus
it is already necessary to take steps to conserve
the rare natural habitat types in the open countryside.
Internationally agreed critical loads have been
established indicating how much nitrogen loading the various habitat types can tolerate if they
are to be conserved as characteristic and varied
habitat types in the long term. For the naturally
nutrient-poor habitat types such as raised bogs,
poor fens, rich fens, heaths and some dry grasslands, the lowest critical load is 5–20 kg N/ha/yr.
The critical load for the naturally more nutrientrich habitat types such as freshwater meadows,
certain types of mire and dry grasslands is of the
order of 20–35 kg N/ha/yr. The critical load for
woodland varies between 7 and 30 kg N/ha/yr
depending on the soil conditions and type of
woodland (Bak, 2001).
The average background loading with airborne
nitrogen in Fyn County is approx. 20 kg N/ha/
yr (Bak, 2001). Due to livestock production,
though, the total nitrogen loading from the air
can be much higher than 20 kg N/ha/yr locally.
Odense
108
PRB
Odense Pilot River Basin
4.4 Wetlands
The critical load for natural and woodland habitats in Fyn County will therefore be exceeded in
many places. For example, the critical load for
the majority of the mires in Fyn County that
have been assigned a high quality objective in the
Regional Plan 2001–2013 (quality objective A or
B) is of the order 5–20 kg N/ha/yr.
Investigation of the nature quality of mires
in Fyn County has shown that Red List plant
species associated with nitrogen-poor soils are
at significantly greater risk of becoming extinct
than Red List plant species that are able to grow
on more nitrogen-rich soils (Vinther & Tranberg, 2002). At the same time, the majority of the
Danish Red List plant species are associated with
nutrient-poor habitats (Ejrnæs, 2000). Corresponding critical loads have not been established
for phosphorus.
With regard to the wetlands being re-established under Action Plan on the Aquatic Environment II, the aim is to denitrify as much nitrogen as possible and retain as much phosphorus
as possible while concomitantly re-creating some
natural habitats with natural hydrology as an
alternative to cultivated fields. Nitrogen retention amounts to 200–350 kg N/ha/yr for the 21
wetlands in Fyn County. The range is based on
the experiences from monitored re-established
wetlands and from investigated wetlands with
calculated nitrogen transformation rates. Six of
these 21 wetlands are located in Odense River
Basin, corresponding to approx. 570 ha. Of this,
approx. 220 ha of wetland will have been established by the end of 2003. This corresponds to
a nitrogen reduction of approx. 114–200 tonnes
N/yr in Odense River Basin.
Re-established wetland
at Karlsmosen 2002.
Photo: Lars Bangsgaard, Fyn County
Odense
PRB
Odense Pilot River Basin
109
4.5 Coastal waters
4.5 Coastal waters
4.5.1 Physical pressures
The physical pressures on Odense Fjord are
described in Section 1.8.4, and physically modified areas are identified in Figure 1.8.7. The main
physical pressures on the fjord are dyking and
reclamation of former areas of fjord, construction of harbours, excavation of shipping fairways
and changes in the fjord’s water exchange, salinity and temperature as a result of the intake and
discharge of cooling water by Fynsværket combined heat and power (CHP) plant.
Dyking and land reclamation
The extensive dyking and reclamation of former
segments of the fjord have considerably reduced
the capacity of the fjord and associated wetlands
to retain and reduce nutrients from the surrounding arable land. Nutrient loading will therefore
be reflected by raised nutrient concentrations in
the fjord to a greater extent than previously.
Shipping fairways – excavation and clearance
The establishment and clearance of a shipping
Total N in Odense Fjord (mg N/l)
2.5
Wastewater (kg N/ha/yr)
40
* Diffuse loading (kg N/ha/yr)
2.0
30
1.5
20
1.0
10
0.5
0
l
ll l l ll l l ll l l ll l l ll l l ll 0.0
86/87 89/90 92/93 95/96 98/99 01/02
Total N in surface water (mg N/l)
50
N load (kg N/ha/yr)
Figure 4.5.1
Trend in source-apportioned annual nitrogen
and phosphorus loading from land-based
sources together with
the annual mean
concentration in the
surface water at station
ODF17 in the outer
part of Odense Fjord.
*inclusive wastewater from sparsely built-up areas
P load (kg P/ha/yr)
3.0
Total P in Odense Fjord (mg P/l)
Wastewater (kg P/ha/yr)
0.35
0.30
* Diffuse loading (kg P/ha/yr)
2.5
0.25
2.0
0.20
1.5
0.15
1.0
0.10
0.5
0.05
0.0
l
ll l l ll l l ll l l ll l l ll l l ll 0.00
86/87 89/90 92/93 95/96 98/99 01/02
*inclusive wastewater from sparsely built-up areas
Total P in surface water (mg P/l)
3.5
fairway considerably deeper than the surrounding natural fjord bottom entails the risk of intrusion of hypoxic bottom water from the border
zone (which is frequently subject to oxygen deficit) via the fairway to the outer parts of Odense
Fjord (“imported oxygen deficit”), as well as
the development of local oxygen deficit in the
stagnant bottom water in the fairway. Sediment
dispersal during excavation of the fairway will
reduce the transparency of the fjord water and
result in enhanced release of otherwise sedimentbound organic matter, nutrients and hazardous
substances, both during and following the excavation work. Habitat conditions for the flora and
fauna will thus deteriorate, among other reasons
due to diminished light penetration of the water
column, enhanced sedimentation, enhanced
oxygen consumption in the water column due
to turnover of organic matter, and enhanced
concentrations of nutrients and hazardous substances. The methods selected to maintain the
fairway and dispose of the removed substrate,
the duration of the clearance work and the interval between fairway maintenance operations
will significantly affect the resultant pressure on
the fjord.
Cooling water
The circulation of cooling water through Fynsværket CHP plant negatively affects the Odense
Fjord system through mortality of the organisms
that are sucked into the cooling water system,
through the effects of warm, saline water on
the River Odense, through obstruction of the
upstream migration of eel and sea trout in the
River Odense and Stavis Stream, through the
effects of heat on the inner fjord and through
enhanced growth of sea lettuce in the inner
fjord. The raised temperature generally increases
phytoplankton and macrophyte production in
the area, and in warm summer periods there is
an enhanced risk of heat stress and hence of increased mortality among the seagrasses, etc. According to model calculations, the cooling water
circulation can also be interpreted as having positive effects on the fjord in that water exchange
in the inner fjord is increased, thereby entailing
lower nutrient concentrations. Furthermore,
the growth of seagrasses is stimulated, and the
phytoplankton production reduced. The latter
is relatively low already, though, and in terms of
eutrophication is of lesser importance compared
with the growth of eutrophication-dependent
macroalgae. Through modelling it has been
Odense
110
PRB
Odense Pilot River Basin
4.5 Coastal waters
Fishery
Trawling for mussels constitutes a major pressure on the areas where fishery is carried out,
among other reasons because of damage inflicted
upon the eelgrass vegetation and the benthic invertebrates. At present, the use of mussels from
Odense Fjord for human consumption is prohibited, cf. Section 4.5.2. If the consumption of
mussels from the fjord is permitted once again,
however, resumed fishery will place such great
physical pressure on the fjord as to impede the
attainment of good ecological status.
Fishery in the fjord is presently carried out
with passive gear. Restrictions are in force in
parts of the fjord (see Section 2, Figure 2.7),
among other reasons to ensure the free migration of salmonids to the River Odense and Stavis
Stream. The effects of fishery on the fish stock in
the fjord have not yet been determined.
4.5.2 Impact of pollutant loading
Odense Fjord receives nutrients and hazardous
substances from wastewater treatment plants and
individual industrial discharges, as well as from
diffuse loading, primarily from agriculture, from
its approx. 1 060 km2 catchment, which corresponds to just under one third of Fyn.
Since the beginning of the 1990s, 99.8% of all
household wastewater from the sewerage catchment is effectively treated. This has considerably
Seden Strand
30-31 August 1982
µg/l
Chlorophyll a
±SD
25
6910008
Annual mean
20
15
10
5
0
reduced phosphorus loading of the fjord, and
in summer also nitrogen loading. Nevertheless,
considerable nutrient loading, especially with
nitrogen, still takes place from the cultivated
catchment (Figure 4.5.1).
Correcting for the interannual variation in precipitation and riverine runoff leads to the conclusion that riverine nutrient loading of the coastal
waters around Fyn has decreased by 25–35% for
nitrogen and approx. 75% for phosphorus relative
to the period prior to adoption of the first Action
Plan on the Aquatic Environment in 1987; corresponding reductions have been obtained with
respect to loading of Odense Fjord.
Diffuse loading accounts for more than 80% of
the nitrogen input to Odense Fjord and 75% of
the phosphorus input; most of the remainder derives from point sources, and a minor part from
the atmosphere (see also Section 3.4). During the
summer period, though, nitrogen loading from
wastewater is of the same magnitude as diffuse
loading and significantly affects the environmental state of especially the inner fjord (Seden
Strand).
Seden Strand
2000-2001
Sea lettuce
Sea lettuce
76-100%
76-100%
51-75%
51-75%
26-50%
26-50%
Vigelsø
1-25%
Vigelsø
1-25%
Widgeon grass
51-100%
Distribution of
widgeon grass
Scattered <50%
Tornø
51-100%
Tornø
Eelgrass
Scattered
Filamentous algae
Bladder wrack
ø
N
Stige
Stige
ø
N
0
Figure 4.5.2
Trend in annual mean
concentration of chlorophyll a in the inner
part of Odense Fjord
(Seden Strand).
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
possible to quantify certain of the effects with a
satisfactory degree of certainty, while determinations of the effects of greatest significance for the
fjord, namely those on biological conditions, are
less certain.
1 km
0
1 km
Figure 4.5.3
Distribution of macrophytes (Sea lettuce,
Ulva lactuca ; Widgeon grass, Ruppia
maritima ; Eelgrass,
Zostera marina ;
Bladder wrack, Fucus
vesiculosus) and filamentous algae (mainly
the brown alga Ectocarpus siliculosus and
the green algae Cladophora sp.) in the inner
fjord (Seden Strand) in
1982 and 2000–2001.
Odense
PRB
Odense Pilot River Basin
111
4.5 Coastal waters
Figure 4.5.4
Distribution of filter
feeders (soft clams,
Mya arenaria , and the
polychaete, Nereis diversicolor) in Odense
Fjord.
N
OTTERUP
Odense
Fjord
st.3
st.1st.2
Soft clams
>60 ind/m²
<60 ind/m²
<10 ind/m²
MUNKEBO
KERTEMINDE
Seden
Strand
Polychaetes
>2 000 ind/m²
<2 000 ind/m²
ODENSE
0
1
2
3 km
Nutrients
The lower level of phosphorus loading is reflected in a significant decrease in the phosphorus concentration in the fjord water. Moreover,
the trend in nitrogen concentration is towards
decreasing concentrations in the summer. The
nitrogen concentration in the fjord is closely
related to nitrogen input. In years when riverine runoff is high, the nitrogen concentration is
relatively higher in the fjord water than in years
when riverine runoff is low, for example as in
1996–97 (Figure 4.5.1).
Despite a significant reduction in nutrient discharge, especially of phosphorus, input of both
nitrogen and phosphorus is still great. Together
with a considerable discharge of hazardous substances this makes environmental conditions in
the fjord very unstable, as is reflected in the fjord
flora and fauna.
Oxygen conditions
In the shallow inner part of the fjord, Seden
Strand, the formerly very high sea lettuce
biomass caused large fluctuations in oxygen
Figure 4.5.5
Sexual transformation
(intersex) in the female
common periwinkle
( Littorina littorea) in
Odense Fjord in the
period 1998–2002.
%
Intersex
Odense Fjord
St.1
St.2
St.3
80
60
40
20
0
1998
1999
2000
2001
2002
conditions and pH in the water, both on a daily
basis and over longer periods during the summer.
When the sea lettuce biomass decomposed, this
was frequently accompanied by oxygen deficit
and the release of hydrogen sulphide. The decreasing sea lettuce biomass due to the falling
nutrient concentrations has resulted in more stable oxygen conditions in the inner fjord. Oxygen
deficit and release of hydrogen sulphide are now
relatively rare in this area.
The outer fjord still experiences occasional oxygen deficit, primarily as a result of the intrusion
of hypoxic bottom water from the area outside
the fjord via the excavated fairway, as mentioned
above. The risk of such “imported oxygen deficit” occurring largely depends on the frequency
and duration of oxygen deficit in the border zone
outside the fjord, and hence on the general oxygen conditions in the coastal waters of Fyn.
Phytoplankton
Phytoplankton biomass in Odense Fjord expressed in terms of the chlorophyll a concentration has remained largely unchanged over the
past 25 years (Figure 4.5.2), despite the fact that
nutrient concentrations can now be so low in
periods as to limit phytoplankton growth (see
Section 1.8). Phytoplankton biomass in the fjord
is somewhat low relative to the nutrient load and
compared with other fjords in Denmark; blooms
still occur frequently, however.
One of the reasons for the low phytoplankton
biomass is that Odense Fjord is a very shallow
fjord with an unusually high content of polychaetes and mussels, which filter the phytoplankton out of the water column. These have the potential to filter the water in the inner fjord several
times daily. Other contributory factors are the
relatively short residence time (couple of weeks),
the use of fjord water for cooling by Fynsværket
CHP plant with the resultant enhanced mortality of both phytoplankton and zooplankton, and
nutrient uptake by macrophytes.
Vegetation
As a result of the reduction in nutrient loading
of the fjord, the former mass occurrences of sea
lettuce have diminished, and the rooted macrophyte vegetation has gained ground in the inner
fjord (Figure 4.5.3). Widgeon grass – and to some
extent eelgrass – have recolonized in the inner
fjord, and a more diverse macroalgal community
has established in the outer fjord, including a
large proportion of the slowly growing brown
algal species bladder wrack. Nevertheless, nutrient loading remains so high that for long periods
Odense
112
PRB
Odense Pilot River Basin
4.5 Coastal waters
of the growth season the inner fjord is still dominated by rapidly growing ephemeral macroalgae,
especially sea lettuce and filamentous algae, and
major blooms of filamentous algae are a recurring problem in the outer fjord.
The distribution of the freely floating ephemeral macroalgae varies considerably from year
to year. These shade out the submerged macrophytes, thus making it difficult to establish and
maintain a stable rooted macrophyte vegetation. Eelgrass coverage exhibits large temporal
variation relative to other areas, and at present
is generally less than 7% in the eelgrass-vegetated
depth intervals in the fjord. This is very low compared to other shallow areas, such as in the South
Fyn Archipelago (coverage >60%), and to the
estimated reference situation for Odense Fjord,
where the criterion for eelgrass coverage is >80%
in the macrophyte-vegetated depth intervals (see
section 1.8.5).
An impact of hazardous substances on the occurrence and distribution of the vegetation cannot be excluded. Preliminary studies in Odense
Fjord have demonstrated effects on eelgrass and
widgeon grass of such substances as TBT (tributyl tin), PAHs (polyaromatic hydrocarbons),
and other as yet unidentified substances in the
sediment.
polychaetes and molluscs. The same species are
found in both the inner and outer fjord, but the
density of both individuals and biomass is far
greater in the inner fjord than in the outer fjord.
Moreover, the abundance of filter feeders is by far
the greatest in the inner fjord (Figure 4.5.4).
The fauna in Odense Fjord is severely affected
by hazardous substances. The content of TBT,
PAHs and PCBs (polychlorinated biphenyls)
in common mussels is so great that limit values
recommended in the international conventions
are exceeded . The Danish Veterinary and Foods
Administration advises against consuming mussels from Odense Fjord and has prohibited commercial mussel fishery in the fjord. The TBT
content in the fjord sediment is also so high as
to cause sexual and reproductive changes in the
common periwinkle (Littorina littorea). The majority of the females closest to the source of TBT
have become masculinized, and many of the
male periwinkles have a reduced number of penis
glands, possibly diminishing their reproductive
capacity (Figure 4.5.5).
Benthic invertebrates
Odense Fjord has a diverse benthic invertebrate fauna consisting of polychaetes, molluscs,
echinoderms and crustaceans. In terms of both
number of individuals and biomass, the community is dominated by eutrophication-dependent
Pursuant to the WFD, the Article 5 characterization and analysis has to contain an assessment of
the likelihood that with the measures hitherto
adopted the water bodies will fail to meet the objective of at least good ecological status by 2015
at the latest.
Type
Provisional
objective
Expected
compliance with
objective
Reason
for noncompliance
1
G
No
N, P, HS
ODF NW
2
H
No
N, P, HS
ODF NE
2
G
No
N, P, HS
ODF Mid
2
G
No
N, P, HS
3
G
No
N, P
Water body
Inner fjord (Seden Strand)
Outer fjord
4.5.3 Objective compliance and risk
of future lack of compliance
Boundary zone
Type 1:
Type 2:
Type 3:
Inner fjord, salinity <18 PSU
Fjord, salinity 7–18 PSU, mean depth 0–3 m
Inner marine waters between Djursland/Sjællands Odde and Fyns Hoved/Røsnæs, salinity 15–20 PSU,
depth <15 m
N:
P:
HS:
Nitrogen
Phosphorus
Hazardous substances
H:
G:
High ecological status
Good ecological status
Table 4.5.1
Overview of the five
marine water bodies encompassed by
Odense River Basin:
Type, provisional objective, expected compliance with objective
and reason for failure
to meet the objective.
Odense
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Odense Pilot River Basin
113
4.5 Coastal waters
Due to the unstable nature of the biological
structure of the fjord and the impact of hazardous substances on the fjord flora and fauna, the
current quality objective for the whole fjord is
not met. Provisional objectives pursuant to the
WFD are presented for Odense Fjord in Section
1.8.6. According to these proposals, the parts of
the fjord that have been assigned the highest quality objective in the Regional Plan have to meet
the environmental objective “High ecological
status”, while the remainder of the fjord and the
border zone have to attain “Good ecological status” (see also Table 1.8.2, as well as Section 1.8.6
for a discussion of the establishment of the objectives for the areas encompassed by international
protection). A precondition for meeting the objective is the establishment and maintenance of
a stable biological structure; in areas with high
ecological status this may show “no, or only
very minor, evidence of distortion” relative to
undisturbed conditions, while in areas with good
ecological status it may “deviate only slightly”
undisturbed conditions (WFD, Annex V).
These status criteria are not presently met in
the fjord (Table 4.5.1).
Attainment of the assigned ecological status
will among other things require that nutrient
loading is reduced to a level where climate-related interannual variation in riverine nutrient
loading has no significant effect on the biological
structure of the fjord. Provisional scenario calculations made by dynamic modelling of the fjord
have shown that assuming a macrophyte criterion of max. 50% deviation from the reference
conditions as to macrophyte biomass, it will not
be possible to attain good ecological status until
nutrient loading has been reduced to less than approx. 800 tonnes N/yr and 30 tonnes P/yr. This
means that the current level of nitrogen loading
will have to be reduced by somewhere in the
order of 1 200 tonnes N/yr, and the phosphorus
loading by approx. 43 tonnes P/yr (Nielsen et al.,
2003b).
Model calculations show that the measures
hitherto implemented and adopted pursuant to
Action Plans on the Aquatic Environment I+II
together with the measures directed at wastewater from sparsely built-up areas, etc. will reduce
loading of the fjord to approx. 1 900 tonnes N/yr
and 65 tonnes P/yr assuming the same meteorological conditions as in 1998 (Nielsen et al.,
2003b). These provisional calculations thus show
that there is the likelihood that the WFD objective of good ecological status will not be met
with the measures hitherto adopted.
As far as concerns hazardous substances and
heavy metals, the WFD requires the concentrations, in the reference conditions/high ecological
status to be close to zero for hazardous substances
and not above background level for heavy metals.
With good ecological status, the concentrations
may not exceed standards set on the basis of toxicity tests etc. (cf. WFD, Annex V). These criteria are not presently met. The overall magnitude
of hazardous substance and heavy metal loading
of the fjord is not presently known (cf. Section
3.6). Analyses of the sediment have revealed very
high concentrations, though, and marked effects
on the fauna and possibly also the flora have been
demonstrated. The attainment of good ecological
status will require a considerable reduction in the
present concentration levels in the fjord.
In connection with review of Fynsværket
CHP plant’s discharge permit it was concluded
that the physical pressures on the fjord associated
with the intake and discharge of cooling water
by the plant could be one reason why it may be
difficult to meet the objective in a future loading
situation.
The effects of fishery, including possible future
trawling for mussels, could also impede attainment of good ecological status in the fjord and
the border zone outside the fjord.
Sea lettuce ( Ulva
lactuca) on the fjord
bottom..
Photo: Nanna Rask, Fyn County
Odense
114
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Odense Pilot River Basin