Study

Effects of temporary desiccation on the mobility of phosphorus and metals in sulphur-rich fens: differential responses of sediments and consequences for water table management

  • Published source details Lucassen E.C.H.E.T., Smolders A.J.P. & Roelofs J.G.M. (2005) Effects of temporary desiccation on the mobility of phosphorus and metals in sulphur-rich fens: differential responses of sediments and consequences for water table management. Wetlands Ecology and Management, 13, 135-148.

Summary

In the Netherlands, permanent damming of sulphate (SO42–)-rich surface water in order to rewet desiccated wetlands, often for nature conservation purposes, has resulted in stagnation and eutrophication of surface water. Permanent damming may prevent periodic drought during summer (as might occur in natural situations) and leads to suppressed iron-rich groundwater input and a too stimulated sulphate reduction, probably leading to depletion of reducible Fe in the sediment. A laboratory experiment was conducted to assess the importance of temporary desiccation, its differential effects on various sediment types and the consequences for water table management.

Sediments: In spring 1999, sediment samples were collected from two fens with similar vegetation (classified as Carici elongatae alnetum) along the river Meuse. Wet sediment ('Ssediment'- locations receiving Fe and sulphate-rich seepage and thus having a high reduced Fe content) and semi wet sediment (T-sediment - locations with fluctuating water tables and lower FeSx contents) were collected from Dubbroek alder carr (51º22'N, 6º05'E) and dry sediment (Dsediment - locations from desiccated alder carrs, very poor in FeSx or other reduced Fe forms) from Koelbroek alder carr (51º23'N, 6º07'E).

Sediment characteristics: Concentrations of several elements were determined, and an estimate of the oxidizable reduced S content of the sediments at the start of the experiment made.

Laboratory experiment: The experiment was carried out between March 1999 and April 2000 in a climate controlled room (air temperature 20ºC, air humidity 60%, 12 h photo-period). For each sediment type, eight aquaria (25 x 25 x 30 cm³) were filled to one-third depth with sediment and topped with water reflecting local groundwater composition. Flow was created using peristaltic pumps.

In August 1999, for each sediment type in four aquaria, water was removed to create a water table 5 cm beneath the sediment surface which was maintained for 2 months. Demineralized water was added to compensate for evaporation. The surface water in the untreated aquaria was also removed after 2 months and then all were refilled for 5 months in order to compare water chemistry development. 5 g of common duckweed Lemna minor was added to each aquarium to serve as a eutrophication bioassay. After 3 months, algae and Lemna dry weight, and total amount of phosphorous (P) taken up from the water layer (as P is the limiting nutrient in development of algae and Lemna spp. in black alder carr fens, from which these sediments were derived) was determined.

Permanent high sulphate-rich surface water tables above sediments that are indirectly affected by shallow groundwater flows, resulted in severe eutrophication. The effect of temporary desiccation on P mobilization and immobilization strongly depended on the sediment Fe and P pools combined with the sediment buffering capacity. Desiccation of sediment that is indirectly affected by shallow groundwater flows, led to a long-lasting reduction in phosphate release from the sediment because a reduced Fe pool is present, resulting in the release of Fe upon oxidation.

Formerly dry sediments that have not been influenced by groundwater for a long time do not possess such a reduced Fe pool and desiccation did not reduce P-release, resulting in eutrophication of the water upon rewetting. In sediment of seepage zones that are directly and permanently influenced by deeper groundwater, reduced Fe and calcium levels are so high that phosphate was immobilized under oxidized as well as reduced conditions.

Conclusions: These results indicate that restoration of desiccated wetlands can not be achieved by simply retaining water by damming; a means to allow temporary drops (e.g. via sluices) in water level during the summer are necessary to recharge the reducible P-binding Fe pool in large zones of the wetlands in order to prevent eutrophication.


Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at:

http://www.springerlink.com/content/gg548418tgl415u4/fulltext.pdf

 

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