Bepalen van het maximaal en het goed ecologisch potentieel, alsook de huidige toestand, voor een aantal Vlaamse (gewestelijke) waterlichamen die vergelijkbaar zijn met de categorie meren – deel 1

This report deals with the identification of good ecological potential, and ecological status assessment for the European Water Framework Directive of artificial and heavily modified water bodies classified as being most comparable to natural alkaline, stratified freshwater lakes (regional water type Aw) in Flanders, Belgium. As it is the first lake-oriented study of this kind undertaken in the region, and member state, general concept development as well as methodological aspects are addressed. Also, a case study of two gravel pits connected to the Common Meuse River, near Maaseik in the province of Limburg, is documented.

In order to develop a general framework to facilitate further monitoring obligations, an overview is presented of the major hydromorphological pressures resulting from specified uses affecting alkaline, stratified freshwater bodies in Flanders. These pertain, e.g., to changes in morphology, water level, substrate, land use, management and recreation. For artificial and heavily modified water bodies, the impacts of connection to a river or stream, e.g. on nutrient conditions or water level, can also be related to hydromorphological change to some extent. The presumed effects of such pressures on the status assessment of the biological quality elements, phytoplankton, phytobenthos, macrophytes, macroinvertebrates and fish, by methods proposed in Flanders is discussed. For various pressures and elements, the good ecological potential cannot (yet) be discerned from the expectations at good ecological status in natural conditions, due to insufficient knowledge and quantification of responses, interactions with other variables or lack of sensitivity of the assessment. However, guidelines can be given for some, e.g., for restraints imposed upon aquatic macrophytes by water level fluctuations and certain morphological changes.

The general framework for alkaline, stratified freshwater lakes is applied to derive the good ecological potential for both gravel pits, taking specified uses, planned morphological developments and possible mitigation measures into account. Good ecological potential in this case allows for some estimated increase of nutrient concentrations (i.e. TP) above the proposed standards for natural lakes by riverine inputs – determined by the nutrient standards for the river type – and its presumed effects on chlorophyll a concentrations, and the consequent change of the area/depth distribution of submerged vegetation due to decreased transparency. This deviation is constrained, however, in order to safeguard good ecological functioning of the lake. Local modifications are also made for expectations with regard to the occurrence and the growth-form diversity of aquatic macrophytes and the shore-line vegetation in relation to physical alterations.

A major part of the report deals with the description and assessment of hydromorphology, physical-chemical conditions and the different biological quality elements in the gravel pits, completed with a survey of their zooplankton populations, based on detailed observations in 2006-2007. Based on these observations, previously suggested monitoring and assessment methods are improved upon, i.c. for phytoplankton and phytobenthos. The results show that good ecological potential is not attained in either case. Besides poor water quality, leading to cyanobacterial blooms, intense boat traffic, waterskiing and ongoing gravel works, the abundance of the invasive alien species, Elodea nuttallii, impedes reaching this goal.

Finally, sugestions are made concerning future monitoring and possible mitigation measures forboth water bodies, as well as further improvement of biological status assessment of lakes in general. Also, a number of issues are identified that need to be resolved further to allow a more precise and reliable estimation of good ecological potential.