Concrete slab ford crossing – An anthropogenic factor modifying aquatic invertebrate communities

Abstract

Unimproved fords provide an easy and inexpensive method of stream crossing. However, vehicles compact the stream bottom and flatten the banks. One simple and relatively permanent solution to this problem is the use of concrete slabs, which can hinder the movement of fish and invertebrates, especially in the presence of low water levels. Although the impact of fords on the stream invertebrate fauna seems clear, there is little available data on this topic, as most reports instead focus on water quality. The impact of a concrete slab ford on the invertebrate fauna of a foothill stream was examined which led to the hypothesis that the benthic invertebrates inhabiting the ford and stream sections immediately upstream and downstream of the crossing would differ significantly from those at a non-transformed natural site. Results revealed that the invertebrate fauna in the vicinity of the ford was altered over relatively long stream sections. The ford disrupted the natural balance between deposition and erosion, with the former dominating above the ford and the latter dominating below the crossing. In small streams, where there are often several similar crossings, this effect would likely be multiplied. The ford's interference in the stream's natural balance between deposition and erosion caused a change in the grain size of the substrate and altered the available organic matter, which directly affected the qualitative and quantitative composition of the macrozoobenthos. As the habitat becomes more homogeneous, the community of invertebrates becomes less diverse. Our results can support management decisions for erosion control.