The origin, composition and downstream transport of plant material in a small chalk stream

SUMMARY. This study was a large-scale comparative in situ experiment on the fragmentation and breakdown of typical aquatic and terrestrial inputs of a chalk stream. Two similar lengths of stream were separated by screens of 5-mm mesh across the entire stream width at their upper and lower limits. The material transported downstream was collected continuously for 3 years. The order of breakdown was: <2 weeks for the emergent herbaceous macrophyte Rorippa nasturtium-aquaticum ; 2–3 months for the dominant submerged macrophyte Ranunculus calcareus and 4–6 months for the terrestrial material, mainly leaves of Salix viminalis and Fraxinus excelsior . The degree of fragmentation of material varied with its structure, composition and time of availability relative to the seasonal changes of discharge. In general, fragmentation of material increased with its time of retention at or near its sites of origin, but this could be greatly modified by the date of the major autumnal increase in discharge. Thus, in a year of early rain, terrestrial leaf material was mainly whole when transported downstream but in a year with late rain, fragments predominated. The breakdown order coincided with an increasing ratio of carbon/nitrogen in the materials. The total annual movement of plant material was between 231 and 426 kg dry weight for both sites but there was considerable variation in the quantities of the individual species present. On an areal basis, with equal inputs of aquatic and terrestrial material, aquatic material was fragmented more than terrestrial material and therefore more of the latter was transported downstream. The aquatic macrophytes in the open section of the stream retarded the progress of terrestrial allochthonous material and resulted in a substantial increase in fragmentation of this material which would have been lost to this part of the stream.