Geheimen diepe plassen ontsluierd

Intense sand and gravel mining has created numerous man-made lakes around the world in the past century. These small lake systems (1-50 ha) are usually hydrologically isolated, often deep (6 – 40 meters) and stratified during summer and in cold winters. Our study area is located in the catchment area of the rivers Meuse and Rhine, in the southern part of the densely populated Netherlands, the province of Noord-Brabant. Due to their small size, these deep man-made lakes are usually not included in the regular monitoring campaigns, such as monitoring required for the European Water Framework Directive. Therefore, not much is known about their ecological functioning. During two summers, we measured the macrophyte diversity and a range of physio-chemical and biological parameters including pH and phosphate concentration in the water column in 51 deep man-made lakes. Comparing these deep lakes to the surrounding shallow water bodies, these deep systems add a good water quality and ecological quality to the total landscape. Man-made lakes are often described as dead and empty underwater deserts, but out results show this is not always the case. To preserve the good quality waters, care should be taken when selecting deep man-made lakes for storing e.g. dredging material. We urge water managers to not only judge the quality of the deep man-made lakes by the biological quality of the surface water, but include deep water quality as a key parameter to determine their suitability for storing materials.