Individual study: Restoration of the charophyte community by reduction of external nutrient loading at Botshol, Utrecht, the Netherlands
Simons J., Ohm M. & Daalder R. (1992) Restoration of Botshol (The Netherlands) by reduction of external nutrient load: recovery of the Characean community. Aquatic Ecology, 25, 287-294
The Botshol nature reserve (Utrecht, The Netherlands), consists of two shallow lakes (average depth 1.5 m), ditches and Phragmites reed beds originating from old peat excavations. Up to 1960 Botshol had clear-water and supported a rich Charophyte (stonewort) community (six species including the nationally rare Nitellopsis obtusa and Chara hispida, dominant in many areas). Since the 1960s water quality deteriorated as a result of increased nutrient loading, primarily via agricultural run-off. Only two stonewort species (Chara globularis and C.connivens) were recorded between1980-1988. In 1988, restoration measures were attempted aimed at reducing nutrient in puts and restoring the submerged plant community.
Three methods of nutrient reduction at Botshol (52º15'N, 4º26'E) were carried out in late 1988/spring 1989:
1. Massive reduction of phosphorus in the incoming water by adding FeCl3;
2. Hydrological isolation from agricultural areas;
3. Water management to maintain a constant water level (formerly an unnatural situation with high summer and low winter levels).
Plant monitoring: Transects were established and surveyed (dredging and snorkeling from a rowing boat) in 1987 (prior to initiation of the restoration measures) to 1991, along which the submerged plant community was surveyed. Only two stonewort species (Chara globularis and C.connivens) were recorded between 1980 and 1988. Holly-leaved naiad Najas marina persisted in some areas, and the aquatic moss Fontinalis antipyretica and the filamentous alga Vaucheria dichotoma were common. Recovery of the charopyte community (and other submerged plants) was subsequently monitored (results up to 1991 are summarised).
Nutrient enrichment, hence the process of eutrophication, was effectively halted by the restoration measures, resulting in amongst other things, lower phosphorus concentrations and higher water transparency (for further details see: www.conservationevidence.com/ViewEntry.asp?ID=1429).
Soon after initiation of these measures the charophyte community increased greatly in abundance and the number of species also increased. During the summer of 1990, and especially of 1991, a spectacular growth of C.connivens occurred, often with C.hispida in association. Water was very clear above the stonewort beds. Other, less abundant Chara species were C.aculeolata, C.aspera, C.contraria and C.globularis. N.obtusa reappeared in a small area of one of the lakes (Kleine Wije) but soon disappeared, and also in a small isolated pool where it persisted. A reason for a shift in dominance from N.obtusa to C.connivens may be the higher chloride content (one consequence of the restoration measures). Holly-leaved naiad re-established good stands at several localities, F.antipyretica was much reduced and V.dichotoma disappeared.
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