Suspended organic matter in fast‐flowing streams in Scotland

Abstract

Summary1. Samples of suspended microscopic particles were collected from a small stream (Shelligan Burn) over a distance of 3–6 km, in conditions of low and high flow, and from a longer stream (River Almond) over a distance of 27 km during average flow.2. In the low‐flow conditions in the Shelligan Burn the commonest material was non‐cellular detritus, with living diatoms, all originating from the bed of the stream, as the only other relatively abundant component. The concentration of some species of diatoms varied considerably in the course of the stream, others were more evenly distributed. Zoospores of Chlorophyceae were In much higher concentration in one short stretch of the stream, whereas filaments of the same group were fairly evenly distributed. Backwaters contained lower concentrations of certain components, presumably due to local sedimentation in the decreasing rate of flow. No clear differ‐ences were found between samples from pool stations and those from nearby riffles. A sample from the mouth of a small tributary contained more of certain materials originating from its denser bankside vegetation, and less moss because of its sparse‐ness on the unstable bed. Its diatom flora was different from that of the main stream, having fewer of the epilithic and epiphytic species but more of the mud‐surface ones. As there was no significant downstream increase in the volume of flow the absolute abundance of particles was relative to their concentration.3. In one flood condition in the same stream there was a downstream increase in the concentration of suspended monocotyledenous tissue and non‐cellular detritus. The absolute abundance of detritus at the most downstream station was at least forty‐five times greater than it was in the low‐flow conditions.4. In the River Almond the commonest materials in suspension were again non‐cellular detritus and diatom cells. There was a distinct downstream increase in the concentration of the diatoms and also of Chlorophyceae. The volume of flow increased downstream so that between the most upstream and downstream stations tbe absolute abundance of diatoms increased 100 times and Chlorophyceae increased 1000 times.