Carrying capacity and development of the Wadden Sea
The discussion presented below originates from a scientific workshop on carrying capacity and development of the Wadden Sea. The Wadden Sea is subject to strict nature-conservation requirements under the European Natura 2000 framework. These requirements are based on conservation objectives established under the Birds and Habitats Directives. Proposed activities must be assessed in relation to their effects on these conservation objectives. Carrying-capacity considerations may support such assessments, but do not replace the applicable legal requirements. Without taking these requirements as its starting point, the workshop explored more freely how carrying capacity, environmental limits and adaptive management could be conceptualized for a highly dynamic coastal system such as the Wadden Sea.
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Carrying capacity in a dynamic ecosystem
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The Wadden Sea is both a highly protected natural area and a region subject to substantial human use and development pressure. Assessing the effects of new activities is particularly difficult because the ecosystem is highly dynamic and its environmental baseline changes through natural variability, climate change and other long-term developments. This complicates the distinction between natural changes and effects attributable to human activities. Moreover, the intensive use of the coastal zone can restrict possibilities for alternative locations or compensating measures. The workshop explored how carrying-capacity considerations and adaptive management could contribute to dealing with these issues.
Carrying-capacity assessment can contribute to Wadden Sea management by identifying the processes or resources that constrain a particular activity and by defining indicators that show whether limiting conditions are being approached. In some cases these constraints can be estimated from physical or ecological processes; in others, especially where different ecological, social and economic interests are involved, the choice of acceptable conditions or threshold values requires a broader decision-making process.
Adaptive management
The application of adaptive management is discussed for some of the human activities that may occur in the Wadden Sea.
Extraction of natural gas
Extraction of natural gas approximately 4 km below the surface of the Dutch Wadden Sea is conducted from the mainland at the locations Moddergat, Lauwersoog and Vierhuizen. The licensing procedure established in 2007[1] for this gas extraction project is an example of adaptive management, prescribing conditions for process control and monitoring. The resulting subsidence of the seabed touches on the geophysical component of the Wadden Sea habitat directly. The geophysical response can be predicted with available models, which can be validated and improved with existing monitoring techniques. However, the response of biological components is less well known and predictable if the combined rate of sea-level rise and subsidence exceeds the rate at which the basin can adjust through net sedimentation. The licensing therefore is based on a maximum allowable subsidence, in order to prevent potential ecological effects. An important condition is that gas extraction develops gradually, so that subsidence can be monitored and the extraction rate adjusted before critical limits are approached. Further favorable factors are: the existence of (a) a long time series of geophysical monitoring and modelling of subsidence as related to existing gas extraction schemes (b) understanding of net sedimentation rates, (c) a long time series of biological and sedimentation monitoring measurements since the 1980s. Gas extraction could be reduced or stopped if monitoring revealed unexpected changes attributable to the extraction.
Note: Legislation of 2024 excluded new gas extraction under the Wadden Sea.
Aquaculture
Aquaculture is becoming an increasingly important source of proteins and income worldwide. In the case of the Wadden Sea, it might be an innovation for mussel fisheries. A rough expert estimate suggests that 100.000 tons of mussels could be produced in an area of 15 km2 if deep tidal gullies are used. Mussel culture can be seen as a supplement or alternative to mussel fisheries in the Wadden Sea. Such replacement is in line with the global trend from capture fishery to aquaculture. It was suggested that mussel culture might also mitigate some trends such as the decrease in plaice recruitment and increase in Pacific Oyster abundance that are expected to occur as a result of global warming. However, such suggestions might be considered controversial as effects on the Wadden Sea ecosystem have not been evaluated in detail.
Large-scale mussel culture competes with other suspension feeders for available primary production and requires suitable space. Assessing its ecological carrying capacity therefore requires consideration of food availability, hydrodynamic exchange and effects on other ecosystem components. Ecological modelling can support such assessment, although uncertainty remains and the precautionary principle applies where significant effects cannot be excluded.
Eutrophication
Measures to reduce nutrient loads over the last few decades have been successful. A resulting decrease in phytoplankton biomass is observed, as is the recovery of seagrass in the Northern part of the Wadden Sea. However, present species dynamics show a significant deviation from (historic) reference conditions. Eutrophication-related objectives under OSPAR and the Water Framework Directive have not yet been fully achieved throughout the Wadden Sea. Changes in nutrient loading alter primary production and food availability and can therefore change the carrying capacity for populations at higher trophic levels. Because of nonlinear ecosystem interactions, these responses are difficult to predict quantitatively. Nutrient management therefore requires continued monitoring and adaptation to observed ecosystem responses.
Carrying capacity in the decision-making process
Because the Wadden Sea is highly dynamic, management needs to account for changing environmental conditions and remaining uncertainties. Carrying-capacity assessment can support adaptive management of a particular activity by identifying limiting processes, relevant indicators and conditions under which intervention may be required.
Despite extensive research on the Wadden Sea, important knowledge gaps remain. Monitoring and periodic reassessment are therefore essential where ecosystem responses cannot be predicted with confidence. Stakeholder knowledge and practical experience can contribute to understanding pressures, evaluating management options and identifying feasible measures. Decisions must at the same time comply with the objectives and requirements of the applicable legislation.
References
The article is the outcome of a workshop among a group of experts involved in research, policy and management of coastal zones from Denmark, Germany, The Netherlands and the UK including Flemming Møhlenberg (DHI), David Prandle, Fransiscus Colijn, Clivia Haese (GKKS), Folkert de Jong (CCWS Secretary), Justus van Beusekom, Ragnhild and Harald Asmus (AWI-Sylt ), Michael Assouline (Corila), Joop Marquenie (NAM), Myra Kremer (DG Water), Florence Bloemkolk (Rijkswaterstaat WD), Geert van Wirdum (TNO), Maria van Leeuwe (Waddenvereniging), Anna Kroon (TU Delft), Klaas Deen (Waddenacademie), Jan-Theo Ijnsen (Rijkswaterstaat N Nederland), Lian Zigterman (WUR), Johan Krol (Natuurcentrum Ameland), Bert van der Valk, Albert Oost, Patricia Schouten, Claudette Spiteri (Deltares), Joost Stronkhorst (chairman). The workshop was held within the framework of a European network project ENCORA from 29-31 October 2008 at Ameland, The Netherlands.
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References
- ↑ Raad van State 2007. Uitspraak gaswinning Waddenzee (in Dutch) https://www.raadvanstate.nl/uitspraken/@24161/200606028-1/
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