Hydro-ecoregions, a natural framework for water management

Grouping rivers into coherent geographical units is an approach offering a better understanding of the responses of aquatic ecosystems to disturbance.

June 18, 2003 -- Grouping rivers into coherent geographical units is an approach offering a better understanding of the responses of aquatic ecosystems to disturbance. Hydro-ecoregions are a new tool for implementing the European water framework directive aimed at achieving .good ecological status. for aquatic environments.

The way in which a river functions depends on the nature of the terrain through which it flows, the relief, climate and human activity in the catchment. For this reason, there is wide environmental diversity in aquatic systems.

Beneficial action in one place will not necessarily work elsewhere, pollution discharged into a river in Normandy will not have the same impact as in a mountain stream. In an attempt to overcome this difficulty in understanding the behavior of ecosystems, Jean Gabriel Wasson and his team have looked for the factors by which rivers can be grouped together according to type.

They defined homogeneous units which could be mapped, regions with the same general characteristics and sensitivity to disturbing factors. The result is a simple management tool, a hydro-ecoregional map of France, in the spirit of the framework directive in force since December 2000.

The directive aims at achieving good ecological status. for all aquatic environments within fifteen years. Good status is defined as being only a slight deviation from the reference conditions, those of a river in natural equilibrium.

In order to break France down into functional units, Jean Gabriel Wasson made use of experience previously acquired on the Loire river basin, so as to focus immediately on the proper criteria for delineating homogeneous regions. In this way, it was possible to produce the hydro-ecoregional map in less than a year after the environment ministry's order to proceed.

The biological identity of these regions was then validated from aquatic invertebrate populations and the scientists checked that the biological quality indicators under natural conditions vary significantly with the regional context. Indicator reference values are being benchmarked for the whole country in order to suggest acceptable limits on the good status criterion. These reference values vary from one region to another because they are tailored to the specific conditions in each environment.

Logical solution

France is undoubtedly the most diverse European country in terms of geography and climate, and a regional approach beneficially simplifies the task of understanding it. The work focused on factors controlling environmental functioning; by means of geological, topographic and climate data, the factors governing physical, chemical and biological characteristics of aquatic environments have been mapped. Major changes in landscape features provide the boundaries between units in most cases.

Detailed local contrasts were ignored, because rivers integrate all the characteristics of their catchments. Three geological types, four relief types and three climate categories were determined. Combining these features resulted in 22 hydro- ecological regions, of which ten account for 80% of the national land area.

The completed map is already available to river basin managers and provides a basis for the river typology required by the framework directive. It explains the leading structures of aquatic diatom and invertebrate populations. This approach is entirely compatible with the use of biological descriptors, which can henceforth be chosen with reference to the regional context.

International method

These results were promptly circulated to the relevant public agencies. Setting reference values makes it possible to proceed with the biological baseline assessment. The method was adapted in Belgium (Flanders), and comparable regional approaches are used in Spain, Germany, Austria, and in the USA.

The hydro- ecoregion concept has been applied in a very different context, the Bolivian Amazon basin, where dividing up the area allowed river characteristics to be described. Modeling is currently under way to find, in each region, statistical relationships between anthropogenic pressure on the river and its actual impact on the ecosystem. These models will help decide what measures are needed to conserve or restore these environments.

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