De Levende Natuur nummer 2 van 2018


Vennen van Midden-Brabant


DLN 2018-2

Changes in landscape, human impact and management of moorland pools near Oisterwijk and Boxtel

H. van Dam

The complex of over 150 moorland pools in the Oisterwijk-Boxtel area is one of the most important and well-known areas  of shallow western European softwater lakes. Thirty pools with extensive historical records of chemistry and biota were resampled in 2015 and data on changes in landscape, human impact and management were retrieved from archives and interviews. Until the end of the 19th century the pools were situated in heathlands and aeolian drift sands. They formed an essential part of the agricultural landscape as well as a source of energy (peat cutting) and proteins (fish farming). The fish farming required inlet of well-buffered water from nearby small streams, so gradients between inlet water and the originally poorly buffered pool water caused a high biodiversity. In the course of the 19th century the common heathland areas were sold to private owners and partially transformed to forest of Scots pines or arable land. In the first half of the 20th century the areas with the largest nature values were included in nature reserves, who could be barely protected from the desiccation and eutrophication of the environment. In the second half of this century the pools were also acidified by atmospheric deposition of sulphur and nitrogen compounds. In the last quarter of the 20th century and the first decade of the present century measures were taken to reduce the impact of acidification and eutrophication, including sediment removal and inlet of well-buffered groundwater. However it is a nearly impossible task to compensate for the excess ammonium deposition (mainly originating from current agricultural practice), which is two to four times higher than the critical load.

Consequences of changes in seepage and inlet of surface water for the moorland pools near Oisterwijk

K. Hanhart & E. Brouwer

A diverse hydrological pattern is one of the driving factors behind the biodiversity of moorland pools near Oisterwijk and Boxtel. These pools are classified in four types. (1) Where a soil layer with a poor hydraulic conductivity is present under the pool only, the pool is fed by rain water and annual fluctuations of the water level are small. (2) If  this soil layer extends far beyond the perimeter, such pools  periodically receive moderately acid seepage water and water level fluctuations are potentially higher. (3) A third type of shallow lakes is fed by groundwater that has travelled a longer distance through the sandy soils of the Boxtel formation. Before the period of acidification, this groundwater was less acid and more rich in ferrous iron. Finally, shallow pools along the stream valleys differ from this type by receiving somewhat calcareous groundwater originating from deeper sandy formations. In some pools, the inlet of  rather nutrient poor, but calcareous stream water created additional gradients extremely rich in biodiversity. Water quality severely declined due to pollution of river water,  reduced groundwater seepage and pollution of groundwater. The understanding of the interaction between hydrology and biochemistry in these lakes is of vital importance for successful restoration of the former biodiversity.

Long term changes of surface water chemistry in shallow soft water lakes in Central Brabant

H. van Dam & E. Brouwer

Changes over the last century of surface water chemistry in thirty moorland pools near Oisterwijk and Boxtel were studied. From the first half of the 20th century until the 1970’s the median pH in eight poorly to very weakly buffered pools dropped from 5,4 to 4,2, due to acid atmospheric deposition and isolation from other surface waters. Between 1980 and 2015 in 24 similar pools the median pH increased from 4,4 to 5,4, along with sharp decreases of sulphate, ammonium and aluminium, indicating a strong recovery from acidification. However, decomposition of organic matter during denitrification and sulphate reduction induces a risk of eutrophication. The scattered data from few weakly to moderately buffered (and eutrophicated) pools indicate no changes since 1980. Strong decreases of nutrients, chlorophyll-a and turbidity occurred after removal of fish and sediments from one of these pools.

Changes of flora and vegetation of moorland pools near Oisterwijk and Boxtel

H. van Dam, E. Brouwer, D. Tempelman

Changes over the last century of the macrophyte species composition in thirty moorland pools in the southern part of The Netherlands were studied. In 2015 a field survey was conducted and data dating back to 1912 were retrieved from reports and publications. Particularly in the beginning of the 20th century shallow soft water lakes in this region were rich in gradients and harboured a rich variety of macrophytes. The high macrophyte diversity decreased from an average of 13.4 rare species in the period 1900 – ’49 to 2.2 in the 1980’s, mainly due to acidification by acid atmospheric deposition and eutrophication. In the period 2010 – ’15 the number of rare species had increased to 6.2. Most rare species are characteristic for very weakly buffered soft waters, whilst some of these species are typical for acid waters or moderately buffered fens. Some of the rare species disappeared from the area and are extinct in The Netherlands now. The recovery is not only due to the decrease of acid atmospheric deposition in the last three decades, but also to measures as dredging and artificial supply of buffered water, cleaning of the banks and cutting of trees and shrubs. The beneficial effects of management measures are visible in the whole range of acid to weakly buffered pools. Although the nutrient concentrations, particularly ammonium, in the surface water of the pools have declined considerably in the past few decades the rooted aquatic plants indicate an increase of nutrient availability.

Desmids of the moorland pools in the Kampina and near Oisterwijk

B.F. van Tooren

The area around the village Oisterwijk (Brabant, in the south of the Netherlands) and in the near by heathland area the Kampina was once famous for the high diversity of desmids (Desmidiaceae) in the moorland pools. Due to several reasons, e.g. the reduction of groundwater seepage and the increased air pollution, almost all rare species found during the first half of the 20th century, disappeared. Over the last decades the air pollution decreased and in many pools restoration measures were performed. Consequently in the nutrient poor heathland pools the number of species has increased over the past 40 years. However, rare species have not returned. On the other hand, in the slightly buffered pools, the number of species increased tremendously. Now, some of these pools again belong to the most important waters for desmids in The Netherlands, although also here few rare species returned.

Longterm changes of diatom assemblages in moorland pools near Oisterwijk and Boxtel

H. van Dam, A. Mertens & J. van der Wal

Changes over the last century of the diatom species composition in thirty moorland pools in the southern part of The Netherlands were studied. In 2015 a field survey was conducted and samples dating back to 1916 were retrieved from botanical museums. Diatom slides were prepared and counts were performed by standard methods. In total 552 taxa were recorded. The moorland pools are very rich in diatom taxa due to the long environmental gradients. The number of rare taxa in the counts declined from an average of 23 in the period 1900 – ’49 to 19 in the period 1970 – ’79 and 16 in the period 2010 – ’15. Due to acid atmospheric deposition and eutrophication (mainly from agriculture) the similarity between the pools increased between about 1920 and 1975. Thereafter the acid atmospheric deposition declined and partial recovery occurred. Particularly in the acid pools taxa from acid, but more or less eutrophic water are present. Measures to counteract eutrophication, like dredging, and artificial influx of buffered (ground)water stimulate the occurrence of rare taxa (target taxa) and diatom diversity in the pools.

Macrofauna, dragonflies and caddisflies in moorland pools in Central Brabant

D. Tempelman & M.J. Sanabria

The macrofauna, including adult caddisflies, dragonflies and mayflies, of 30 moorland pools in the province of North-Brabant was studied in 2015. The aim of the study was to obtain insight of the natural value of the moorland pools based on the occurrence of the macrofauna. Also attempts were made to compare the current natural values with that of previous times. In total, 42 species of Odonata (both larvae and adult; over half of the Dutch fauna), 60 species of Trichoptera (a third of the total Dutch fauna list), 85 species of water beetles, nine species of semi-aquatic bugs and 27 species of water bugs were found. Many rare and typical species were found. It was concluded that the 30 moorland pools are of significant importance for these fauna groups. The measures such as removing sediment have positive effects but not in all pools. A comparison with old data shows that the current value is significantly higher than that of the 80’s, when many pools were severely acidified and the natural value was poor. By contrast, several rare and typical species found in the early years of the 20th century, are now absent.

The breeding birds of moorland pools in Central Brabant

F.J.H. van Erve

 During research in 30 moorland pools in the nature reserves ‘Oisterwijkse vennen’ and ‘Kampina’  (province Noord-Brabant, The Netherlands) 18 species of breeding species were found.  Species that prefer eutrophic open water appeared to be most common. Aythya fuligula, Anas platyrhynchos, Anser anser and Branta canadensis are most numerous. Tachybaptus ruficollis is present in many more oligotrophic pools, while Anas crecca only was found in few pools. Among marshland birds Luscinia svecica and Emberiza schoeniclus are most numerous. Botaurus stellaris returned after more than twenty years of absence. In the last decades of the 20th century several species settled as new breeding birds, probably favoured by enrichment of pools as a result of atmospheric deposition. Comparison with historical recordings reveals that several species have disappeared as breeding birds, partly due to deteriation of suitable habitat, partly due to longterm significant decline.  Among them are birds on the Red List, like Sterna hirundo, Chlidonias niger, Acrocephalus arundinaceus,  Locustella luscinioides and Anas querquedula. A colony of Larus ridibundus was eliminated because of continuous negative impact on water quality. The growing populations of Anser anser and Branta canadensis are nowadays being reduced for the same reason. A great loss is stated concerning diversity and quality of the population of breeding birds in the moorland pools. 

Nature recovery in the Winkelsven

B.F. van Tooren

The shallow soft water lake Winkelsven in the south of the Kampina was supplied with buffered water by inundation of the small river the Beerze. However, when this river became polluted, small dikes were supplied in 1963 to prevent further inundation. As a consequence the water acidified. The Winkelsven was once one of the most species rich lakes in the Netherlands, with many rare species like Deschampsia setacea. Since these rare species were disappearing in 2006/2007 the lake was cleaned by removing the mud and supplied with buffered water from a nearby drinking water station. The recovery of rare species was tremendously within several years. Nowadays more than 20 Red Data List phanerogams occur in the lake again. The lake Winkelsven is also very species rich for e.g. desmids, diatoms and dragonflies.

Ups and downs in the Central moorland pools near Oisterwijk

E. Brouwer, H. van Dam, K. Hanhart & B.F. van Tooren

At the start of the 20th century, the cluster of the lakes Goorven, Witven and Van Esschenven, near the village of Oisterwijk, was one of the best examples of the diversity of Dutch moorland pools. Driving factors behind this biodiversity were 1) the inlet of moderately alkaline surface water, 2) seepage of weakly buffered or acid groundwater and 3) small scale removal of organic matter by hand. Around 1950, the inlet of surface water was stopped, because it became to nutrient rich. Despite the isolation and the removal of sludge layers, the decline of biodiversity continued. The isolated lakes acidified, a process reinforced by acid deposition. In 1995, the lakes were partially restored by renewed dredging, by pumping up groundwater and by a reduction in nitrogen deposition. Lack of vegetation structures, reduced groundwater quality and quantity, and the steep, forested shores probably are the most important factors inhibiting further recovery.

Past, present and future of the moorland pools Huisvennen (Kampina)

H. van Dam, K. Hanhart, E. Brouwer & F.J.H. van Erve

The Huisvennen moorland pools are situated in a former bog area (1 km2) in the southern part of The Netherlands. The bog was partly drained and peat has been extracted since mediaeval times. In the lowest parts the remaining pools, with fluctuating water levels, were fed with slightly buffered seepage water and became a good habitat for isoetids. In the 20th century the isoetids declined by acid atmospheric deposition, but also due to the decreased seepage of weakly buffered water as the water level in the area was increased again. Bog remnants lost their quality due to eutrophication by a gull colony. Clear effects of cutting surrounding forests, sod cutting along the shores and dredging of some pools in the last decade have not been observed until now, with the exception of the appearance of pioneer shore vegetation. The decline of acid atmospheric deposition has caused an increase of the diversity of microscopic algae and positive changes in the dragonfly fauna. Partly due to acidification native fish species have disappeared, but the introduced eastern mudminnow thrives very well and is bulk food for wetland fowl. In the past fifty years the breeding bird population declined, but the causes are not clear. In order to improve the quality of the area, at least 50% reduction of the atmospheric nitrogen deposition is required. Within the area measures to improve the hydrology can be taken, but first a choice has to be made where bog vegetations (requiring a more or less constant water level) and isoetid vegetations (requiring fluctuating water levels and slightly buffered conditions) can be developed.

Buffering the moorland pool Galgeven near Tilburg: too much of a good thing?

H. van Dam, E. Brouwer & K. Hanhart

The moorland pool Galgeven is an originally oligotrophic and very weakly buffered lake in the southern part of The Netherlands. In the 1980s it was heavily acidified (pH < 4), due to acid atmospheric deposition, and the original isoetid vegetation was replaced by acid tolerant species like Juncus bulbosus and submerged Sphagnum spp. After inlet of well buffered groundwater in the period 2006 – 2012 the pH increased to 7,4 and alkalinity to 0,55 meq/l. In 2015 Lobelia and Isoetes were (still) not present, but other species from more buffered and eutrophic variants of the Littorelletea were found, as well as several Potamogeton­-species from well buffered, eutrophic lakes. Moreover, the assemblages of desmids, diatoms, macroinvertebrates and amphibia recovered from acidification, particularly after the inlet of groundwater. However, the presence of the exotic snail Physella acuta indicates that the pool is more alkaline and eutrophic than undisturbed isolated moorland pools. In recent years Podiceps nigricollis appeared as a breeding bird, probably due to the increase of suitable food (macroinvertebrates). Possible measures to improve the habitat for species from weakly buffered waters include: partly cutting of the surrounding forest to improve the quality of the groundwater percolating to the lake, inlet of small amounts of buffered groundwater and liming of the watershed. Regular monitoring of alkalinity and biota is necessary to adjust the measures. Reintroduction of isoetids can be considered.  

Moorland pools in Central Brabant seem to survive

E. Brouwer, H. van Dam, B.F. van Tooren, K. Hanhart & F.J.H. van Erve

In 2015, the environmental conditions and species composition of 30 moorland pools near Oisterwijk and Boxtel were monitored. Also, a large amount of historical data was collected. For a broad range of species groups, the same trends were found.  As a reaction to a strong decline in acid deposition and a moderate decline of nitrogen deposition between 1975 and 2000, a long decline of biodiversity and environmental quality switched into a gradual recovery that is still progressing. In particular, a recovery in pH and buffering capacity was observed, along with some recovery of acid-sensitive species. The presence of a sludge layer inhibited recovery from eutrophication, but where this layer was removed, species from oligotrophic environments also returned. However, many species from fragile, ecologically complex situations have not returned. Further actions to promote recovery are suggested.