The island is surrounded by saline seawater. Apart from a drinking water line from the main land, there is no external fresh water supply. Basically it forms its own watershed, it is a small version of the water system on the main land and thus interesting for testing new policy and plans . Up to recently many measures in the water system were taken without taking "all" aspects in to consideration. The agriculture wanted lower ground water tables, leading to intrusion of saline water in the ground water and diminishing of the fresh water lens under the island. Nature conservation wanted higher ground water levels and restoration of saline ground water on several nature areas. Above that high dikes, a safeguard against the sea water also forms a huge barrier for fish, the De Cocksdorp syphon fishladder (Wintermans) has been an important step towards a more sustainable water system.

Water for Texel Master Plan

The idea appeared 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 has been started, in which all parties on the island who have interest in water management are taking their share.

Everstekoog

Apart from rain, treated wastewater is the only fresh water source on the island. During dry periods the waste water is a valuable source of water, but the quality is not good enough. The water is very clear, as it originates from drinking water (plus precipitation). However from a biological point of view it is "dead water", without treatment or dilution it is not suitable for fish. The main sewage treatment plant on Texel STP Everstekoog is located in the centre of the island. The effluent from Everstekoog flowed to the north in the direction of a brackish area, with high natural values, before being pumped in to the Wadden Sea. It would be much more favourable to use the effluent in an area with high agricultural values south of the Everstekoog treatment plant. For this purpose a diversion channel has been constructed. Since the quality of the effluent was not good enough in 1995 a joint 4-year research project was carried out by the Waterboard Uitwaterende Sluizen and the Utrecht University, to improve the water quality in a full scale constructed wetland. This project has 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 in to a "living" water suitable for various purposes. In 2002 a major research project is carried out to investigate the influence of the water diversion from the north to the south.

De Cocksdorp

During the Everstekoog research project we observed high numbers of Daphnia spp.in the presettling basin of the constructed wetland Everstekoog , despite low algae concentrations. How can these daphnids survive? In the Everstekoog constructed wetland we also observed that the presettling basin did not contain any fish, despite the high numbers of Daphnia. Only after a hydraulic retention time of over 2 days the water was suitable for fish like Stickleback, the main food of Spoonbills breeding in good numbers on the island. The number of Stickleback appeared to be high, up to 10 per m2. Could it be possible to design a combined "natural constructed wetland" system:

• to enhance the natural values on the island,
• to "produce food" for fish and subsequently for birds like Spoonbills, which feed on small zooplanktivorous fish,
• to change the effluent of the sewage treatment plant into a "living water",
• to use this improved effluent as a lure flow for a fish trap to syphon fish from the sea across a high Dutch dike?

This led to the development of the "kwekelbaarsjes system", described in the figure. 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 Research TNO started a research project to assess the feasibility of the idea. In the laboratory we focused on the possibility of production of Daphnia with activated sludge as a food source. The research has shown that daphnids are indeed able to grow and reproduce on the suspended solids in the effluent. The average density obtained was ca. 100 daphnids/L.

We have continued the research project with experiments aimed on the cultivation of Daphnia on effluent on a pilot scale. This work is carried out on the Everstekoog STP in four 20 m3 ponds and four 2 m3 mesocosms. It learned that the system is feasible, but the process is not fully understood up to now. Frequent harvesting of the daphnids causes an increase in the biomass yield from the system. In 2002 we will focus our study on process stability of the system and to gain more knowledge of ecotoxicological aspects.

Results of the research are indicating that the "kwekelbaarsjes system" near the village of De Cocksdorp, in connection with the syphon fishladder, the extension of the sewage treatment plant of De Cocksdorp in conjunction with the Master Plan will be feasible. It will lead to an innovative co-operation between engineering and nature (ecological engineering), construction of the system is already mentioned in the Water for Texel Master Plan.

Spoonbills in water management