Restoration of Botshol (The Netherlands) by reduction of external nutrient load: the effects on physico-chemical conditions, plankton and sessile diatoms

  • Published source details Rip W.J., Everards K. & Houwers A. (1992) Restoration of Botshol (The Netherlands) by reduction of external nutrient load: the effects on physico-chemical conditions, plankton and sessile diatoms. Aquatic Ecology, 25, 275-286.


The Botshol nature reserve (Utrecht, the Netherlands), consists of two shallow lakes (average depth 1.5 m), ditches and Phragmites reed beds, originated from old peat excavations. Up to 1960 Botshol was a clear-water stonewort Charophyte lake system. Since the 1960s water quality deteriorated as a result of increased nutrient loading. In 1988, several restoration measures were attempted aimed at reducing the loading of phosphorus; phosphorous is the usual choice of nutrient as techniques to remove it are fairly easy.

Three methods of nutrient reduction in Botshol Three (52º15'N, 4º26'E) were carried out in 1988:

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).

Water analysis: In 1987, one year before the restoration, monitoring commenced; results up to 1991 are presented. Total phosphorus, dissolved phosphorus, total nitrogen, nitrate, ammonium, chloride and chlorophyll-a, were determined in the surface water at four sampling locations (one in each lake; one at the sedimentation area used for phosphorus elimination; and the fourth where two agricultural areas had their outlet of surplus water prior to restoration). A Secchi-disc was used as a measure of underwater light.

Plankton and sessile diatoms: Phytoplankton and zooplankton samples were collected through the study period. Reed samples were taken from which sessile diatoms were removed and identified.

The combined restoration measures resulted in an 80% reduction of the external phosphorus loading (0.6 to 0.12 g/m2/y) over the study period and significant reductions of phosphorus concentrations at all sampling locations; after restoration initiation total phosphorus was reduced by 50% within a few months.

Water clarity improved; except for some deep zones, Secchi disk transparency reached the bottom throughout the whole area; summer average of chlorophyll-a decreased from 11 to 7.5 µg/l. Phytoplankton also declined, with distinct changes in community structure over time.

In 1988, the two sampling stations in the lakes differed in their zooplankton concentrations. Station II had up to 5-times more zooplankton than station I. After restoration measures, the zooplankton decreased greatly at station II (particularly Rotifera), less so at station I; it is probable that zooplankton declined as a result of increasing chloride and/or food limitation (due to a reduction of the nutrient loading less food is available).

Most sessile diatoms were of a slightly brackish water types; dominating taxa were Achnanthes minutissima, Cocconeis placentula and Oiatoma ssp. The changes in hydrology and nutrient loading were not reflected in the composition of these taxa, perhaps as they have a wide ecological tolerance. Also, the samples were collected close to the water's surface where light penetrated even prior to restoration, therefore the increase in water clarity would have little effect on the majority of the clear water species.

In the last three months of 1990, a Prymnesium parvum algal bloom caused a fish kill, but despite this the restoration of the lakes is considered successful to date.

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