The Wadden Sea is a unique natural intertidal ecosystem of global importance, stretching approximately 750 km from Den Helder in The Netherlands, past the rivers and estuaries in Germany to Esbjerg in Denmark. The Wadden Sea region acts as a nursery areas for a variety of fish species, harbours a suite of distinctive animals such as migratory birds and seals, but is also home to humans, from tourists to people living and working in the area. Because of its geological and ecological significance, the UNESCO nominated it as a world heritage site in 2009. Despite its widely acknowledged natural and recreational value, the Wadden Sea is still influenced and modified by human use. Several national, European and international laws, directives, treaties and agreements are set to place to balance the protection of natural values with the livelihoods of man. On the EU level, the Water Framework Directive (WFD) aims at reaching ‘Good Ecological Status (GES)’ of surface waters by 2015, whilst the Marine Strategy Framework Directive (MSFD) requires ‘Good Environmental Status (GEnS)’ to protect the resources (‘ecological goods and services’) of the marine environment, to be achieved by 2020.
Beyond WFS und MFSD, the Trilateral (The Netherlands, Germany, Denmark) Cooperation for the Protection of the Wadden Sea implemented a number of transnational management tools reflecting the outstanding unique values of this area. So-called “Quality Status Reports” (e.g. Marençic & De Vlas, 2009) summarise the present state of the entire Wadden Sea in five-year intervals. They are based on a common set of parameters as described in the Trilateral Monitoring and Assessment Programme (TMAP; Bakker et al. 1998). There are a number of shortcomings in the present practice to react properly, in a timely manner, and with sufficient flexibility. The assessments are based on local data, often with sampling frequencies much too low to capture relevant variation and individual events.
Although the present assessments highlight regional differences, causes for regional differences in the trends are still unclear. As a result of this lack of information, assessments of the trends with respect to management regulations or climate change can be based on subjective, ad-hoc assumptions instead of scientific foundations. A proper understanding of the quality of Wadden Sea waters, for example, requires inclusion into the monitoring programmes of those coastal waters of the North Sea that exchange water with the Wadden Sea proper (Van Beusekom et al. 2012). Updates of concepts and information take place with a time-delay of several years. Necessary basic and comprehensive information can only be generated by compiling data from different and most up-to-date observational and modelling sources.
Phytoplankton biomass – A poor predictor for coastal productivity
Primary productivity is mainly determined by the biomass of microscopic algae in the water column (phytoplankton) and on the tidal flats (microphytobenthos), growing conditions (e.g. light and nutrients) and the productivity of these algae under these environmental conditions. With respect to microscopic algae, monitoring parameters required for indexing GES (WFD) and GEnS (MSFD) include phytoplankton biomass and species composition with a minimum operational monitoring frequency in coastal waters of once every six months (Zampoukas et al. 2012). As for many European waters, phytoplankton is sampled by various organisations using various methods, and hence information is fragmented and data access is limited (Hering et al. 2010).
Furthermore, phytoplankton biomass is a very poor predictor of primary production due to the high turnover times of microscopic algae. Therefore, the OSPAR Working Group on Food webs is investigating the possibility of adding primary production as an indicator of productivity of marine waters (Kromkamp et al. in prep.). Present parameters do not acknowledge the significant role of microphytobenthos, the production of which may add more than 50% of the total primary productivity of shallow coastal seas such as the Wadden Sea (Philippart & Cadée, 2000; Kromkamp et al. 2008). High frequency measurements of phytoplankton biomass at strategic sites revealed that the monitoring should be much more frequent than twice a year to be able to determine possible consequences of human activities, including restoration (Riethmüller, 2009), i.e. at least 2x per month during the growing season (pers. comm. J.C. Kromkamp).
The WaLTER monitoring action plan for primary productivity aims to integrate monitoring already in place across different pieces of legislation and best practices. Future efforts strongly benefit from the investments made in Earth Observation data. Biomass of phytoplankton (indexed as chlorophyll-a) in turbid waters such as the Wadden Sea can be approached by MERIS Satellite Imagery, but only for older data as this satellite is no longer operational. The biomass of microphytobenthos can presently be estimated to some degree by means of the Normalised Differential Vegetation Index (NDVI) from Aqua MODIS Satellite Imagery (Compton et al. 2013). From mid-2015 onwards, biomass of phytoplankton and microphytobenthos can be derived from satellite information provided by Sentinel 3 (WaLTER RS report (Davasuuren et al.2013).
Relationships between productivity and biomass (P:B ratio) of phytoplankton and microphytobenthos will be determined by the innovative automated Fast Repetition Rate Fluorometry (FRRF) sensor as developed in the FP7 project PROTOOL, and presently being deployed in Belgian, Dutch and German coastal waters. The advantage of the FRRF technique is that, if adopted by the EU community as a standard, it allows comparison between production estimates without a methodological bias. In addition, it limits the use of the expensive and time-consuming 14C-technique, which is becoming increasingly difficult to apply as a result of more stringent health and safety regulations.
Within such an integrated monitoring system, patterns in the variation of P:B ratios over time would be described by deploying fixed monitoring platforms at coastal observatories in the Wadden Sea. Spatial patterns of this relationship will be determined by portable monitoring platforms during joint cruises throughout the entire Wadden Sea. Automated sensors on the fixed and portable platforms would additionally measure core parameters of the WFD and MSFD such as pH, salinity, temperature, transparency and oxygen concentrations. The unprecedented high temporal resolution would aid in determining minimum monitoring frequencies. The fixed platforms will function as calibration (and validation) stations for Earth Observation data.
Earth Observation data can be accessed freely through space agencies or via specific websites such as the Environmental Marine Information System from the Joint Research Centre. Field data and data products (e.g. geospatial ‘pictures’ of primary production in the Wadden Sea) will be made accessible via the data and information portal of Wadden Sea Long-Term Ecosystem Research, an emerging LTER EU network, and is also foreseen for the COSYNA data portal (pers. comm. R. Riethmüller).
Primary productivity is in many aspects a key parameter in the functioning of any ecosystem, being the basis for the food web up to its highest trophic levels, represented in the Wadden Sea by fish, birds and seals among others. In the Wadden Sea, substantial progress in combing state-of-the-art in-situ time-series with spatial coverage by satellite imagery and numerical modelling is required to channel scientifically sound data and information into management services. It is highly ambitious as it is highly controlled by the import of nutrients from many adjacent compartments: rivers, the North Sea and the atmosphere, and by the significantly varying light conditions in an intertidal environment.
At the same time, progress in observation (Kromkamp et al. 2008, Kromkamp et al. in prep.), modelling and data management techniques in recent years, as demonstrated world-wide by a large number of integrated Coastal Ocean Observing Systems (Riethmüller et al. 2009) assure successful implementation. Special care should be taken to ensure that data from existing monitoring programmes can be included and that the new methods employed can be harmonised with hitherto practice.
Multi-year data already available (e.g. from COSYNA project at HZG and the IN PLACE project at NIOZ) could be used to set up a monitoring programme, whereas new observations could be used to validate the new Earth Observation data. Primary productivity may hence serve as a blueprint for the inclusion of further key parameters relevant for sustained coastal zone management, such as the protection of natural values (e.g. carrying capacity for birds and seals) and for ecosystem services (e.g. fisheries and aquaculture).
The Wadden Sea is not a homogeneous area and large regional differences exist (e.g. Van Beusekom et al. 2012). A prerequisite to an international monitoring plan is understanding the factors responsible for the regional differences. Once understood, this will enable the designation of representative tidal basins that serve as models for wider ranges.
The Wadden Sea productivity approach should be considered as a world-wide lighthouse example for shallow coastal ecosystems, which can be used throughout networks of turbid marine and transitional waters, such as the LTER Europe Network, the World Heritage Network and the Man and Biosphere Network. Common transnational monitoring surveys will help in sharing and minimising the costs and also ensure that data acquisition is done in a similar and comparable manner, thus reaching a comparable assessment and classification of coastal waters, even if connected areas fall under different EU legislations.
The PP monitoring service will channel the fragmented scientific data and information on productivity of the Wadden Sea into a service enabling management of this coastal ecosystem. In order for this service to be used intensively for information and management of its status, as intended by WFD and MSFD legislation, the service should not only be scientifically sound and cost-effective, but also transparent, widely applicable, and far and foremost tailored to end-users.
In the near future, decisions have to be made on adaptation measures to climate change and sea level rise, such as setting quota for coastal fisheries and building large infrastructural works to deal with extensive rainfall. To inform decision makers on possible consequences of such costly management regulations for ecosystem services (e.g. carrying capacity), the information on productivity should be coupled to other relevant information on ecological values (e.g. on the distribution of fishes, birds and seals) and socio-economic aspects (e.g. on income from tourism).
The expected impact of the full service is facilitation of evidence-based environmental policy-making, which will result in more effective and cost-efficient political decisions with regard to integrated coastal zone management.
The optimal design of a monitoring network for primary productivity of pelagic and benthic microalgae in marine coastal waters, is a combination of field measurements, automatic sampling devices and satellite-derived information.
Permanent field stations
The monitoring network would require continuous delivery of near-real-time data from permanent and easy accessible (= less costly to maintain) stations at various subtidal and intertidal locations in the Wadden Sea. Automated data would be validated by means of water samples (approximately 40x per year). If similar instruments were used for all permanent field stations, validation could be restricted to one or two locations. In order to catch the similarities and differences in high-resolution temporal dynamics of pelagic and benthic primary production within the (Dutch) Wadden Sea, two stations per tidal inlet would be sufficient (if combined with other measurements), one in the intertidal and one on the mudflats. Depending on the number of parameters, setting up and maintaining sampling stations would cost approximately 50 to 250 K Euro per year per station.
The field surveys supply information on the spatial variation in PP estimates of pelagic and benthic microalgae and deliver ground-truth information for the Earth Observation data. In order to catch the similarities and differences in high-resolution spatial dynamics of pelagic and benthic primary production within the (Dutch) Wadden Sea, one scientific field survey period per year would be sufficient (if combined with other measurements), to sample both the intertidal and the mudflats. Depending on the number of parameters and the number of sampling stations (determined by the requested scale of precision), field surveys would cost approximately 100 to 250 K Euro per year.
Earth Observation data
Computation and delivery of optical remote sensing data based on the most up-to-date algorithms. Depending on the number of parameters and the number of appropriate satellite images (determined by weather conditions among other factors), mapping of primary productivity and explanatory variables based on EO information would cost approximately 50 to 100 K Euro per year.
Harmonisation and analyses of different data sets. Depending on the number of parameters and the extent of the various data sets (permanent field stations, annual field surveys and EO data), mapping of primary productivity and explanatory variables based upon all information combined would cost approximately 50 to 100 K Euro per year.
Assuming permanent sampling stations in 2 tidal inlets (subtidal & intertidal) and one trilateral field survey per year, the total costs would be approximately 400 to 1200 K Euro per year.