ASSIGNMENT

Assignment

Assignment

Introduction

According to the requirements of the European Water Framework Directive (WFD) (EU Commission 2000), Member States have to start biological programmes to monitor the ecological status of rivers and streams by 2006. The WFD stipulates the application of methods that are based on stream type-specific conditions of the biotic communities, which must be compared to their (near-)natural reference conditions as a benchmark for the ecological status assessment.

Study sites, materials and methods

Selection of sampling sites

In total, 57 sites at two stream types in the catchments of the Werra river in middle-eastern Germany and the Ruhr river in western Germany were sampled (Fig. 1). All sites are located in the Central Highlands (Ecoregion 9; Illies 1978) covering an altitude range from 100 to 600 m a.s.l. For both catchment areas sampling covered small- and medium-sized streams.

Fig. 1: Ecoregions and location of study areas (grey line).

A pre-selection of sampling sites was undertaken to cover a degradation gradient from reference to heavily degraded sites with focus on land use (urban areas, agricultural land use and forested areas) as the main pressure. The initial pre-selection process was based on information derived from ATKIS2 land cover data. Additional information on morphological stream status and stream reaches was compiled using data from earlier studies, monitoring reports, and data on habitat quality, such as the German River Habitat Survey.

The two investigated stream types are brooks of the Central Highlands with a catchment area of 10-100 km2 and streams of the Central Highlands with a catchment area of 100-1000 km2. Reference status was assigned to sites where the catchment was covered by natural forest or at least typical riparian vegetation was present, and hydromorphological pressure was absent. As we intended to focus on morphological degradation, sampling sites were chosen, where streams were not affected by organic pollution. Therefore, sites with sewer overflows <500 m upstream were excluded.

Stream types

Headwater streams of the Central Highlands (German Stream Type 5: Small coarse substrate-dominated siliceous highland river) are characterised by mid- to large-sized siliceous rocks, high current velocities and a slight tendency to low pH values. In reference condition, the natural riparian vegetation of these brooks consists of alder (Alnus glutinosa (L.) GAERTN.). Typical macroinvertebrates are the stonefly Perla marginata (Panzer, 1799), the mayfly Ecdyonurus venosus (Fabricius, 1775) or the caddies fly Micrasema longulum McLachlan, 1876. The dominating functional feeding groups according to Cummins and Klug (1979) are grazers and shredders (Pottgiesser & Sommerhäuser 2004).

The medium-sized streams (German Stream Type 9: Mid-sized fine to coarse substrate dominated siliceous highland rivers) show similar abiotic characteristics to stream type 5. Bed sediments consist of large rocks and boulders, current velocities are high, riffles and pools offer different habitats for the stream biota. Typical macroinvertebrates are the mayfly Baetis lutheri Müller-liebenau, 1967 and Ecdyonurus insignis (Eaton, 1870) and the caddies fly Micrasema setiferum (Pictet, 1834). Macrophyte abundance is higher than in headwater streams. Therefore, some species associated with macrophytes occur, e.g. the beetle Esolus parallelepipedus müller, 1806. Hydromorphological degradation is generally indicated by absence of riparian vegetation and low habitat diversity ...