|
In this report the results
of a literature study and data analysis aiming at developing
Wadden Sea specific eutrophication criteria are presented. The
study was necessary to specify the trilateral Ecological Target
"to achieve a Wadden Sea which can be regarded as a
eutrophication Non-Problem Area", which has been
adopted at the 7th Trilateral Governmental Wadden Sea Conference
(Leeuwarden, NL, 1994). The work was done in close cooperation
with activities in the framework of the OSPAR Common Procedure
through which the whole OSPAR Convention Area will be designated
as either Non-Problem, Potential-Problem or Problem Area with
regard to eutrophication.
Based on available
literature a Conceptual Model was developed that links riverine
nutrient input with the nutrient cycles in the Wadden Sea. The
fundamental steps are that
(1) nitrogen presently
limits the primary production of the coastal zone and
(2) the Wadden Sea imports
organic matter from the North Sea coastal zone.
On the basis of
statistical analyses of long-term data from the Dutch Wadden Sea
it could be made plausible that nitrogen currently determines
the Wadden Sea eutrophication. It was furthermore shown that the
variability of autumn values of N remineralization products
(NH4, NO2) in both the Rhine-influenced western part and in the
North Sea-influenced eastern part of the Dutch Wadden Sea
correlated in a similar fashion with the nitrogen input into the
coastal zone via the rivers Rhine and Meuse. The autumn
remineralization in the Lower Saxonian Wadden Sea (Norderney)
showed no correlation with the Total Nitrogen input of these
rivers. However, the inter-annual autumn remineralization
pattern correlated significantly with the pattern in the eastern
Dutch Wadden Sea. On the basis of these results it is proposed
to use autumn values of N remineralization products (NH4 + NO2)
as an indicator of the eutrophication status of the Wadden Sea.
In this study the Wadden
Sea has been divided into two subareas, the Southern and the
Northern Wadden Sea. The Southern Wadden Sea has been defined as
the area between the western Dutch Wadden Sea and the Elbe
estuary. The Northern Wadden Sea has been defined as the area
between the Elbe estuary and the Skallingen peninsula. In both
the Southern and the Northern Wadden Sea eutrophication and
primary production have increased. Whereas along the Southern
Wadden Sea the variability of autumn values of N
remineralization products can be related to the variability in
nitrogen input, no such relation has been found for the Northern
Wadden Sea. Instead a possible relation between nitrate in the
coastal zone and autumn values of N remineralization products in
the Sylt-Rømø Bight was found.
Two contrasting situations
are postulated:
(1) The Southern Wadden
Sea with intense particle accumulation and a strong coupling of
productivity and remineralization with variations in nitrogen
input via Rhine and Meuse and
(2) the Northern Wadden
Sea with less intense particle accumulation, where mainly
nutrient input from the west into the German Bight and, to a
lesser extent, from Elbe river input determine primary
production in the German Bight and, consequently, the organic
matter import into the Wadden Sea.
Based on a literature
survey the parameters from the "Holistic Checklist" of
the Common Procedure were evaluated for their applicability as
eutrophication indicators for the Wadden Sea. The Causative
Factors are atmospheric and riverine nutrient input. The
effect of the increased nutrient input is best seen in changes
in the annual nutrient cycle. A Wadden Sea specific Supporting
Factor is the import of organic matter from the adjacent
coastal zone. Direct Effects of eutrophication can
be observed in all biota of the Wadden Sea. However, no clear
dose-response relation could be identified. Other factors such
as weather, temperature or more complex interactions also play
important roles in the proliferation of eutrophication effects.
This also holds true for the Indirect Effects such
as changes in zoobenthos biomass and species composition.
Based on the evaluation of
eutrophication criteria, a combination of two models is proposed
to assess the eutrophication status of the Wadden Sea. Because
of data availability, the first model was only developed for the
western Dutch Wadden Sea and is based on the causative factor
nutrient input.
The assessment of the
eutrophication status of the Wadden Sea according to Model I is
based on the relation between riverine and atmospheric nutrient
input and autumn values of ammonium plus nitrite. These values
reflect the amount of organic matter that was turned over during
the previous summer. The transition from Non-Problem Conditions
to Potential-Problem Conditions has been defined as autumn
values exceeding the background concentrations. Background
concentrations of ammonium plus nitrite were derived for the
western Dutch Wadden Sea and amount to 3±1 µM (situation in
early 1930s). According to this Model the present eutrophication
status of the western Dutch Wadden Sea is 5 times higher than
during the early 1930s.
Model II is based on the
relation between the occurrence of eutrophication phenomena and
a certain nutrient input level. The transition from Non-Problem
Conditions to Potential-Problem Conditions was set at 50% of the
eutrophication level after 1980. The transition is based on the
observation that after about 1970 the organic matter turnover in
the Wadden Sea doubled and that after 1970 also most problems
associated with Wadden Sea eutrophication occurred. According to
Model I the transition from Non-Problem Conditions to
Potential-Problem Conditions corresponds for the western Dutch
Wadden Sea to autumn NH4 + NO2 values of 8.3 µM implying this
area to be a Problem Area.
The background concentrations and the
threshold concentrations for Problem Conditions developed for
the western Dutch Wadden Sea were transposed to the other areas
of the Wadden Sea, proportional to the present day autumn values
in the subareas. In all subareas the present day autumn values
are higher than the threshold concentrations, suggesting that
the entire Wadden Sea is a Problem Area. For the Wadden Sea to
reach the status of a Potential-Problem Area a 50% reduction of
riverine nutrient loads is not sufficient. Atmospheric nitrogen
input has to be reduced as well. To reach the status of a
Non-Problem Area the riverine nutrient loads and atmospheric
nitrogen deposition have to be reduced to natural background
levels.
|
