Spate frequency and lowland stream resilience: An experimental approach

Lowland streams are characterized by moderate flow dynamics and a soft bottomed streambed. Changes in discharge can, through changes in current velocity, disrupt the stability of substrates and affect benthic organisms. These effects may depend on the resilience of the stream. By field experimental simulations of spates in a natural and a semi-natural stream, the effects of differences in spate frequency and stream resilience were studied. The streams were split in two longitudinal sections (50 m length), whereby the water gradually was diverted through one quarter of the streambed. Each experimental spate reached a magnitude of four times the median flow and lasted 5 hours. The experiments had a BACI design. Spate frequencies were 1 in one day and 5 over a period of seven days, both in each of the streams. Both control and impacted stream sections were sampled on macroinvertebrates 3 times before, and 5 times after spate simulation. The spates caused a slight movement of fine organic material. In the natural stream no effects on the macroinvertebrates were observed. In the half-natural stream changes in number of individuals occurred as well as downstream shifts. Natural streams have a higher resilience to spates in comparison to semi-natural ones.