Ecological characterization of surface waters in the province of Overijssel, The Netherlands

Nowadays many surface waters in The Netherlands tend to become ecologically uniform with the same mediocre quality. A differentiated approach to water management is necessary to stop this process of impoverishment of aquatic ecosystems. In The Netherlands the provincial authorities represent the appropriate level at which this differentiated approach to water management can be put into practice. Such an approach has been realized by the Department of Water Management of the Province of Overijssel. As a part of this approach the project 'Ecological characterization of surface waters in the province of Overijssel (EKOO)' was formulated in 1981.The aims of the EKOO-project are to develop a regional ecological characterization of surface waters based on macrofauna composition and to reach a better understanding of the variety and the structure of the macrofauna communities present in the waters of the province of Overijssel. The project thus provides knowledge of aquatic ecosystems on a regional scale and a basis for the development of water management policies. An additional aim of this study is to develop the typological approach used in water management.The typological approach used can be interpreted as an integration of the zonal concept and the continuum concept. Taxon combinations will be described as types. Within a limited range of environmental conditions, these types are representative of communities, but at the same time they together form a continuum. Due to hierarchical relations between major environmental variables (master factors) and evolutionary and historical factors, a type can only be described within a biogeographical region.The study was designed as a qualitative survey but organized as much as possible along quantitative lines. Macrofauna composition (taxon composition and abundance) was chosen as a basic parameter. About 70 variables that were considered physically, chemically or biologically relevant, were measured at each sampling site. In total 664 sites were sampled, distributed over about twenty physico- geographical water types. These twenty types include all the major environmental variables relevant to this region; they overlap in abiotic features. The physico-geographical water typology was used as a practical tool for carrying out the survey, but was not used in obtaining the ecological typology. The ecological typology was derived from the collected biotic and abiotic data alone.A study on the reproducibility of the standard macrofauna sample was carried out. The standard pond net macrofauna sampling technique used in The Netherlands appears to present only a semi-quantitative picture of the more common taxa. With this technique only about 55% of all the taxa present at a site at the moment of sampling were collected. However, when the standard pond net was used for a regional typological study it appeared that seasonal differences as well as inconsistencies due to sample technique were of little significance compared with differences between types. It was concluded that the reproducibility of a macrofauna sample is sufficient for typological purposes.For data analysis, the twenty physico-geographical water types were combined into five main categories. The abiotic and biotic data were processed for these five main categories. Later, all the data were processed together to obtain an ecological water typology for the province of Overijssel. Multivariate analysis techniques are appropriate in data analysis for typologi~-al purposes. Different multivariate analysis techniques (cluster analysis and canonical ordination) were used to derive and describe site groups in terms of taxon composition and mean environmental conditions. Sites intermediate between groups were manually relocated by using information from other sources (e.g. literature) about the ecology of the constituent taxa. The resulting site groups were termed cenotypes.Six cenotypes were distinguished among helocrene springs. The main differences between the cenotypes were related to hydrology (mainly duration of drought) and acidity. Furthermore, the nutrient content and/or the load of organic material differed between related cenotypes. Each cenotype contained its own microhabitat group(s). The microhabitat groups, except for two, were associated with the duration of the drought period. The other two groups were associated with the spring source and the spring stream, respectively. The natural reference situation for helocrene springs probably resembles some of the actual helocrene springs investigated. The most important human activities causing disturbance are those which cause changes in the chemical composition of the groundwater. The management of springs should be directed at this factor.Eleven cenotypes were distinguished among the streams. The main differences between the cenotypes were related to dimensions, "stream-character", duration of drought and the load of organic material. All the streams were more or less influenced by human activities. Stream regulation, especially, has caused a dramatic change in the taxon composition. Only about 2% of the total length of streams is still more or less natural in 'stream-character' and its corresponding community. These 2% are only preserved because of their geographical position on the steepest slopes. In general, efforts at improvement of the ecological character should be directed at the physical and hydraulic conditions.Eleven cenotypes were distinguished among the ditches. The main differences between the cenotypes were related to dimensions, duration of drought, acidity and current. Furthermore, the nutrient content and/or load of organic material differed between related cenotypes . It is illustrated that the taxon combination found in a ditch reflects a stage of succession in space (profile structure) and time (maturity). Therefore, an overlap in taxon combination between cenotypes occurred (continuum). Ditches are artificial ecosystems which mainly occur in cultivated areas (which implies eutrophication) and depend on regular human interference (cleaning, dredging). The ecological management of ditches should be based upon the relationbetween profile structure, succession stage and human interference.Eleven cenotypes were distinguished among the rivers, canals and large lakes. They showed great overlap in taxa (mostly opportunists) and in environmental circumstances. Increasing dimensions of the line-shaped more or less running waters go together with an increasing drainage area. These large-sized water bodies function as collectors of nutrients, organic material and toxicants. This results in chronic stress and probably the disappearance of most taxa occurring originally. The chronic stress also overrules the natural master factors of current and dimensions. Only a few taxa are still characteristic for the reach of a river or the gradient in size of canals.Nine cenotypes were distinguished among the ponds and small lakes. The main differences between the cenotypes were related to duration of drought, acidity, morphology and nutrient load. In particular, the four cenotypes within the group of stagnant, pH-neutral ponds/lakes showed an overlap in taxon compostion. These cenotypes represent a web-shaped continuum dominated by dimensions (relation of width to depth), nutrient load, and bottom composition (especially mesotrophic peat). The most important processess induced by men are acidification, eutrophication, and changes in the original hydrology.Cenotypes of different main physico-geographical watertypes can be very similar. Therefore they should be combined or rearranged. This is done by processing all abiotic and biotic data together. Again, cluster analysis and canonical ordination were applied. After each ordination along two axes, the distinctive cenotypes were removed and the remaining sites were reordinated. Through this progressive removal of groups of sites, finally, 42 cenotypes were distinguished. Some notes on the ecology of the typifying taxa and the most important environmental variables were made for each cenotype.The mutual relations between the cenotypes are shown in a hierarchical dendrogram based on biological similarity as well as in a web of cenotypes (Figure 11.1).The hierarchical dendrogram shows, among others, that a group of cenotypes related to middle and lower reaches of regulated streams, small rivers, ditches and some of the medium-sized, more or less stagnant waters, - all hypertrophic, mesosaprobic environments -, has a fair number of the macrofauna in common. Apparantly, human activity (e.g. by regulation of streams, discharge of wastes and agricultural activity in the watershed) leads to a decreasing role of the factor current in running waters and the factors dimensions (in fact shape and depth) and bottom type in the stagnant waters and leads to an impoverishment of the macrofauna.The web of cenotypes illustrates the mutual position of the cenotypes, the transitions between the cenotypes (the continuum) and the major environmental gradients. The four most dominant factors are 'stream-character', acidity, duration of drought, and dimensions.To evaluate a given water body, even subjectively, it is necessary to compare it with other water bodies in different states. This scale of evaluation needs a reference water. The reference water is not necessarily an endpoint of succession nor a pristine situation, but it should at least represent a situation that can be used to indicate the desired direction of improvement. In choosing the reference communities for water management one must focus on communities present in waters where major environmental conditions are less disturbed. Such waters are the undisturbed streams, the oligotrophic moorland pools, and the mesotrophic old meanders cut off from streams.The regional ecological typology presented (Figure 11.1) offers a tool for establishing the developmental direction from a community observed in the field towards the reference community. It is also a tool to determine the reference community for a particular water body. Because the factors in the web of cenotypes are a result of a descriptive study, the web cannot be used without caution as a predictive tool; the factors are purely indicative. The typology and associated ecological concepts are a tool to help solving water management problems. It is a basis, among others, for monitoring and assessment of waterquality, it indicates potential capacities Of surface waters and it provides guidelines for management and restoration of surface waters. May it be serve to the advantage of the environment around us.