which method/approach to take for the deter- nation of reference conditions is dependent on The estimation of reference conditions is crucial the condition of the sites available for a certain in any ecological assessment programme (e. g. lake type: (a) where undisturbed or nearly undis- Moss et al. , 1996). These provide the baseline turbed conditions prevail, a validated spatial from which to determine lake change with time, network is preferred; (b) if degraded conditions and are necessary to evaluate a lake's current prevail then the preferred modelling approach status or potential for change. would be based on establishing a good relation- The most recent European water legislation, ship between stress and ecological response; (c) the water framework directive (WFD), prescribes expert judgement should be used as the last resort the assessment of ecological quality of surface and be accompanied by an acceptable validation waters using an ecological quality ratio (EQR). process. The EQR is de?ned as the ratio between the The TP status of a lake is mainly determined reference value and the observed value for a by its external nutrient supply. Phosphorus export given metric. from a catchment in an undisturbed area will As part of the assessment of ecological quality, depend on the natural fertility of the local rocks many European countries are choosing to devel- or glacial deposits, and the soils derived from op a quality classi?cation scheme for total ph- them.

Since the late 1980s, shallow lakes in Europe and North America have been receiving concerted attention from limnologists, fishery biologists and lake managers. These developments relate to deterioration of water quality in the lakes due to pollution and accelerated eutrophication, caused by increased urbanization and agriculture in the lake catchments, overexploitation of fish and higher water level fluctuations. The water-quality and management problems of lakes have led to a greater need for fundamental researches on shallow lakes in order to gain a deeper insight into the structure and functioning of the food web in these waters. New knowledge relating to littoral-pelagial interactions and the sediment-water interface has facilitated the development of new lake restoration techniques.

This volume, with 41 peer-reviewed papers, comprises the proceedings of the 5th International Symposium on Shallow Lakes, which was held at Dalfsen, The Netherlands, on 5-9 June 2005. It has the theme Shallow Lakes in a Changing World and deals with water-quality issues, such as changes in lake limnology, especially those driven by eutrophication and pollution, increased nutrient loading and productivity, perennial blooms of cyanobacteria and loss of biodiversity. The general consensus is that the lakes under restoration respond by showing an increase in water clarity followed by an increase in macrophyte coverage and biomass. Consequently, there is now ground for theoretical speculations on the so-called 'alternate stable states' observed in many such lakes under restoration.