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Abstract

The catchment area of the Breitenbach (Hesse, Germany) consists mainly of Bunter sandstone. The stream bed sediment consists of sandstone plates but also of sand and silt deposits with varying amounts of organic matter. The grain size pattern was influenced primarily by discharge fluctuations. During periods of low flow sedimentation occurred, filling up the interstitial spaces with clay and organic matter, resulting in an interruption of the exchange of sediment and stream water. Consequently a physicochemical discontinuity between stream water and interstitial water developed at a few mm depth, below which contents of dissolved organic carbon (DOC) and soluble reactive phosphorus (SRP) increased compared with the surface water. pH decreased, but alkalinity increased in the sediment water; sulphur and nitrogen were reduced. However, only slight changes in concentration were measured. Sandy sediments with high contents of organic matter but without oxygen below the surface can only be exploited by a few specialized organisms, e.g. Ptychoptera paludosa , that receive oxygen from the stream water or even from the atmosphere. Nearly the entire macrofaunal community of the Breitenbach was thus restricted to the uppermost parts of the sediment and may be seriously influenced by unpredictable flood events.