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Tidal range and hydraulic stress
The creation of habitats with reduced tidal range due to lowering the summer dike around the in- and outlet structure was successful. The in- and outlet structure effectively prevented the decrease of low tide; the chosen construction therefore is able to withstand existing hydraulic pressures. A habitat in large parts independent from the hydraulic pressures of the lower Weser River (e.g. high tidal range, increased current velocities) developed which overtakes characteristic functions of floodplain back waters.
However, a minor increase of tidal range on the project area took place since 1997 due to increases of mean high water levels in the main river as a consequence of deepening measures.
Siltation
Due to relatively high suspended matter concentrations and low energy input, tidal polders as substitution habitats for natural floodplains in estuaries are strongly affected by siltation. Sedimentation depends on both suspended matter concentration and the amount of water exchanged per tide. The polder design including a relatively high in- and outlet structure did not only aim at tidal range reduction, but also at reducing the inflowing water amounts and therefore suspended matter loads. Bathymetry recordings of the polder proved this approach to be basically successful: water volume slightly decreased between 2004 and 2011. However, figure 4 shows that the suspended matter input to the polder exceeds the output. The resulting siltation tendencies within the polder lead by trend to an ongoing decrease of shallow water areas and intertidal mudflats. However, comparisons of bathymetry show that the distribution of shallow and deep water areas created in 1997 did not basically change until 2010 (Figure 5).
Functions
So far, submersed macrophytes did not establish on the project area. The reasons are not clear yet. A possible reason could be the lower, but still existing tidal range. This frame condition may prevent especially the shallow, regularly uncovered side areas from being settled by under-water-vegetation. In the deeper areas, the light conditions are probably not sufficient. Turbidity is promoted e.g. by phytoplankton development in spring.
Zooplankton
Between 2004 and 2010, the function of the polder in terms of the Weser River changed. In 2010, considerably more zooplankton was found in the water flowing out of the polder than in 2004.
Referring to the amounts of inflowing and outflowing water, it has to be noted that the results only represent snapshots. In order to formulate a scientifically sound statement, a time series including a sequence of several tidal cycles would be necessary. The recorded zooplankton occurrences make the tidal habitat Vorder- and Hinterwerder a potentially suitable living space for fish (nursery ground).
Phytoplankton
From 2004 to 2010, the function of the polder in terms of the Weser River changed in summer and autumn. In 2010, less phytoplankton both on a species and on a community level was found in the water flowing out of the polder compared to the situation in 2004. Besides sedimentation, increased predation by zooplankton and fish larvae is estimated to be a reason.
In terms of the Weser River, this result means that there is still significant phytoplankton input coming from the polder in spring. In summer and autumn, phytoplankton is used as food resource by zooplankton and probably fish larvae. This development of the polder can be considered positive and is estimated to stabilize until sedimentation will lead to a slow reduction of water covered area and to the development of fresh water mudflats.
Benthic invertebrate fauna
By 2010, 66 taxa of benthic invertebrate fauna were proven on the project area. Compared to the limnic lower Weser River, the tidal habitat Vorder- and Hinterwerder shows a species rich benthic invertebrate fauna.
On the one hand, the benthos community consists of characteristic species of slow flowing waters including insect species (e.g. Odonata, Heteroptera, Caddisfly and Mayfly) and gastropoda (snails). These species are generally common, but no longer present at the strongly anthropogenically influenced lower Weser River. Behind this background, the polder initiates a significant diversification of species which is presently restricted to the Vorder- and Hinterwerder area.
Besides the taxa exclusively recorded in the polder, the species spectrum on the other hand recruits from a second, smaller group of current ‘Weser species’ especially including estuarine shrimps (e.g. Neomysis integer), amphipoda (e.g. Gammarus zaddachi) and bivalvia (Corbicula). Mobile crustaceans visit the polder for recreational and feeding purposes, possibly also for reproduction. Snails arrive at the polder by means of tidal currents as larvae and are able to settle.
Starting with a comparatively high number of taxa in 1998, species diversity decreased until 2004 (Figure 6). In 2010, the settlement consolidated and stabilized on the level of 1998 (50 taxa). In comparison, the benthic invertebrate species spectrum in the river section bordering on the project area consists of 15 taxa. It is not possible to predict if the number of species in the project area will continue increasing. Thinkable is an expansion of fauna typical for slow flowing waters. The establishment of typical fauna also depends on a possible establishment of submersed macrophytes.
Fish
The project area functions as feeding, recreational and reproduction ground for fish. Additionally, the polder’s function for overwintering is proven. Between 2004 and 2010, larvae or juveniles of 11 fish species were found.
In the polder, the conditions for the listed functions are presently given by e.g. comparatively well water quality, shallow and deeper subtidal areas and suitable spawning substrates (reeds) as well as low tidal range. Relatively high numbers of larvae found in 2010 show that the polder is probably used as spawning ground mainly by Cypriniformes and Perciformes. Additionally, the polder is temporarily used by immigrating larvae and juveniles of smelt, flounder and goby. The reproduction and nursery ground functions taken over by the polder are presently defective in the lower Weser River.
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Vorder- und Hinterwerder
Table of content
- 1. Measure description
- 1a. Measure description
- 1b. Monitoring
- 1c. Monitoring results
- 2. Execution of main effectiveness criteria
- 2a. Effectiveness according to development targets of measure
- 2b. Impact on ecosystem services
- 2c. Degree of synergistic effects and conflicts according to uses
- 3. Additional evaluation criteria in view of EU environmental law
- 3a. Degree of synergistic effects and conflicts according to WFD aims
- 3b. Degree of synergistic effects according to Natura 2000 aims
- 4. Crux of the matter
- 5. Literature
Additional information
for this measure:
No further information available.
for this measure:
No further information available.
Monitoring results
The monitoring results described in the following are based on HANEG 2011.Tidal range and hydraulic stress
The creation of habitats with reduced tidal range due to lowering the summer dike around the in- and outlet structure was successful. The in- and outlet structure effectively prevented the decrease of low tide; the chosen construction therefore is able to withstand existing hydraulic pressures. A habitat in large parts independent from the hydraulic pressures of the lower Weser River (e.g. high tidal range, increased current velocities) developed which overtakes characteristic functions of floodplain back waters.
However, a minor increase of tidal range on the project area took place since 1997 due to increases of mean high water levels in the main river as a consequence of deepening measures.
Siltation
Due to relatively high suspended matter concentrations and low energy input, tidal polders as substitution habitats for natural floodplains in estuaries are strongly affected by siltation. Sedimentation depends on both suspended matter concentration and the amount of water exchanged per tide. The polder design including a relatively high in- and outlet structure did not only aim at tidal range reduction, but also at reducing the inflowing water amounts and therefore suspended matter loads. Bathymetry recordings of the polder proved this approach to be basically successful: water volume slightly decreased between 2004 and 2011. However, figure 4 shows that the suspended matter input to the polder exceeds the output. The resulting siltation tendencies within the polder lead by trend to an ongoing decrease of shallow water areas and intertidal mudflats. However, comparisons of bathymetry show that the distribution of shallow and deep water areas created in 1997 did not basically change until 2010 (Figure 5).
Functions
So far, submersed macrophytes did not establish on the project area. The reasons are not clear yet. A possible reason could be the lower, but still existing tidal range. This frame condition may prevent especially the shallow, regularly uncovered side areas from being settled by under-water-vegetation. In the deeper areas, the light conditions are probably not sufficient. Turbidity is promoted e.g. by phytoplankton development in spring.
Zooplankton
Between 2004 and 2010, the function of the polder in terms of the Weser River changed. In 2010, considerably more zooplankton was found in the water flowing out of the polder than in 2004.
Referring to the amounts of inflowing and outflowing water, it has to be noted that the results only represent snapshots. In order to formulate a scientifically sound statement, a time series including a sequence of several tidal cycles would be necessary. The recorded zooplankton occurrences make the tidal habitat Vorder- and Hinterwerder a potentially suitable living space for fish (nursery ground).
Phytoplankton
From 2004 to 2010, the function of the polder in terms of the Weser River changed in summer and autumn. In 2010, less phytoplankton both on a species and on a community level was found in the water flowing out of the polder compared to the situation in 2004. Besides sedimentation, increased predation by zooplankton and fish larvae is estimated to be a reason.
In terms of the Weser River, this result means that there is still significant phytoplankton input coming from the polder in spring. In summer and autumn, phytoplankton is used as food resource by zooplankton and probably fish larvae. This development of the polder can be considered positive and is estimated to stabilize until sedimentation will lead to a slow reduction of water covered area and to the development of fresh water mudflats.
Benthic invertebrate fauna
By 2010, 66 taxa of benthic invertebrate fauna were proven on the project area. Compared to the limnic lower Weser River, the tidal habitat Vorder- and Hinterwerder shows a species rich benthic invertebrate fauna.
On the one hand, the benthos community consists of characteristic species of slow flowing waters including insect species (e.g. Odonata, Heteroptera, Caddisfly and Mayfly) and gastropoda (snails). These species are generally common, but no longer present at the strongly anthropogenically influenced lower Weser River. Behind this background, the polder initiates a significant diversification of species which is presently restricted to the Vorder- and Hinterwerder area.
Besides the taxa exclusively recorded in the polder, the species spectrum on the other hand recruits from a second, smaller group of current ‘Weser species’ especially including estuarine shrimps (e.g. Neomysis integer), amphipoda (e.g. Gammarus zaddachi) and bivalvia (Corbicula). Mobile crustaceans visit the polder for recreational and feeding purposes, possibly also for reproduction. Snails arrive at the polder by means of tidal currents as larvae and are able to settle.
Starting with a comparatively high number of taxa in 1998, species diversity decreased until 2004 (Figure 6). In 2010, the settlement consolidated and stabilized on the level of 1998 (50 taxa). In comparison, the benthic invertebrate species spectrum in the river section bordering on the project area consists of 15 taxa. It is not possible to predict if the number of species in the project area will continue increasing. Thinkable is an expansion of fauna typical for slow flowing waters. The establishment of typical fauna also depends on a possible establishment of submersed macrophytes.
Fish
The project area functions as feeding, recreational and reproduction ground for fish. Additionally, the polder’s function for overwintering is proven. Between 2004 and 2010, larvae or juveniles of 11 fish species were found.
In the polder, the conditions for the listed functions are presently given by e.g. comparatively well water quality, shallow and deeper subtidal areas and suitable spawning substrates (reeds) as well as low tidal range. Relatively high numbers of larvae found in 2010 show that the polder is probably used as spawning ground mainly by Cypriniformes and Perciformes. Additionally, the polder is temporarily used by immigrating larvae and juveniles of smelt, flounder and goby. The reproduction and nursery ground functions taken over by the polder are presently defective in the lower Weser River.