|Can Spoonbills play a role in Integral
water management on Texel?
Paper at the 68th Eurosite Workshop
Wetland management for Spoonbills and associated waterbirds
19 22 April 2002, Texel The Netherlands
the comlete workshop report is available through Eurosite: www.eurosite.nl
Ruud Kampf1, Ben Eenkhoorn1, Edwin Foekema2, Henno van Dokkum2
1 - Waterboard Hollands Noorderkwartier (formerly Uitwaterende Sluizen), P.O. Box 850, 1440 AW Purmerend, The Netherlands,
2 - TNO Environment, Energy and Process Innovation, Department Ecological Risk Studies, P.O. Box 57, 1780 AB Den Helder, The Netherlands
The island of Texel is facing many problems and opportunities in water management. It differs considerably from the mainland, possesses great natural values, is a well-known tourist resort, remains an agricultural stronghold, and has a splendid population of Spoonbills: an island to take care of!
The island is surrounded by saline seawater. Apart from a drinking water line from the main-land, there is no external freshwater supply. Basically, it forms its own watershed: it is a small version of the mainland water system and, thus, interesting for testing the new policy and plans. Up to recently, many measures were carried out in the water system without taking "all" aspects into consideration. The agriculture wanted lower groundwater tables, leading to intrusions of saline water into the groundwater and the diminishment of the freshwater lens under the island. Nature conservationists wanted higher groundwater levels and restoration of saline groundwater in several nature areas. In addition to these, high dikes - a safeguard against the sea - also form a huge barrier for fish, the De Cocksdorp siphon fish ladder (Wintermans) having been an important step towards a more sustainable water system.
Water for Texel Master Plan
The idea arose it could be wise to tackle "all water problems" with an integrated approach of "all the aspects of water affairs". Therefore, the project Water for Texel Master Plan was started, in which all parties on the island interested in water management are taking their share.
It started with an inventory phase. The main questions were: What is the possible value of a Master Plan, and Does it really make sense, and Is it worth the effort? Some other questions: How does the water system of Texel work (surface water, groundwater, treated wastewater (effluents))? How can we increase the natural values of the surface waters on Texel by separating different flows and qualities? What are the different interests on Texel? Do the water-related organisations (agriculture, nature conservation, recreation) really want a Master plan? What do they expect from it and how much do they want to invest in improving the water system, now and in the future? The total estimated costs of the Master Plan are about 25 million Euro. The implementation began in September 2001. Several pilot projects, focusing on the improvement of diverse natural habitats were started, of which several will improve feeding habitats of Spoonbills.
Everstekoog Sewage Treatment Plant
Apart from precipitation, treated wastewater is the only freshwater source on the island. In dry periods, the wastewater is a valuable source of water, but its quality is not good enough. The water is very clear, as it originates from drinking water (plus precipitation). However, it is "dead water" from the biological point of view: it is not suitable for fish without treatment or dilution. STP Everstekoog, the main sewage treatment plant on Texel, is located in the centre of the island. Before being pumped into the Wadden Sea, the effluent from Everstekoog flows to the north towards a brackish area of great natural importance. It would be much more favourable to direct the effluent into an agricultural area south of the Everstekoog treatment plant. For this purpose, a diversion channel was constructed. Since the quality of the effluent was not good enough, a joint 4-year research project was started in 1995 by the Waterboard Uitwaterende Sluizen and the Utrecht University, aimed at improvement of the water quality in a full-scale constructed wetland. This project pointed out that the combination of an oxidation ditch with a constructed wetland consisting of a combination of open ponds, helophytes and submerged aquatic plants, is a cost-effective way to change sewage into "living" water suitable for various purposes. In 2002 a major research project has been carried out to investigate the influence of the water diversion from the north to the south.
De Cocksdorp "kwekelbaarsjes" system
During the Everstekoog research project, we observed great numbers of Daphnia spp. despite low algae concentrations in the presettling basin of the Everstekoog constructed wetland. How can these daphnids survive? Moreover, despite the high numbers of Daphnia, the presettling basin did not contain any fish. Only after a hydraulic retention time of over 2 days the water was suitable for fish, such as sticklebacks, the main food of Spoonbills breeding on the island in good numbers. The stickleback numbers appeared to be great, up to 10 ind./M2. Thus, it could be possible to design a combined "natural constructed wetland" system in order to:
This led to the development of the "kwekelbaarsjes system" shown in figure 1. Basically, the system comprises a trapped 'food~chain type' system to increase the ecological value of effluents from oxidation ditches.
Consequently, the Waterboard and the Netherlands Organisation for Applied Scientific Re search TNO started a research project to assess the feasibility of the idea. In the laboratory, we focused on the possibility of Daphnia production with activated sludge as a food source. The research showed that daphnids are indeed able to grow and reproduce on solids suspended in the effluent. The average density obtained was ca. 100 daphnids/L.
We continued the research project with experiments aimed at the pilot-scale cultivation of Daphnia in the effluent. This work is being carried out in the Everstekoog STP in four 20-M3 ponds and four 2-M3 mesocosms. The system proved to be feasible, but the process is not fully understood until now. Frequent harvesting of the daphnids promotes an increase in the total biomass yield. In 2002 we will focus on process stability of the system and plan to gain more knowledge of eco-toxicological aspects.
The project results indicate that the "kwekelbaarsjes system" near the village of De Cocksdorp will be feasible. The upgrade of the sewage treatment plant, the kwekelbaarsjes constructed wetland, in connection with the siphon fish ladder. It will lead to an innovative co-operation between engineering and nature conservation (ecological engineering). The construction of the system is already mentioned in the Water for Texel Master Plan.
Spoonbills in water management
On the island of Texel, a whole array of measures on water management are being discussed, researched, planned and taken into construction. Public will only partly recognise the effect: they will easily see higher or lower water tables, farmers will directly note the salinity of the groundwater, but it is not so easy to demonstrate the "values" of water. One of the easiest ways to demonstrate the quality of the water - and perhaps even the success of all measures taken within the Water for Texel Master Plan - could be an increase in the Spoonbill numbers on Texel, as these big, white birds are favourites of many people, both Texelaars and tourists. Consequently, a good care of Spoonbills will also benefit water management on Texel.