The Downstream Transport of Fine Material and the Organic Matter Balance for a Section of a Small Chalk Stream in Southern England

Abstract

(1) The seasonal changes in concentration of fine particulate material being transported downstream are described and quantified for three sites along a small chalk stream in Dorset. (2) The mean monthly concentration of organic matter (dry wt) was 1-2 mg 1-1 in the summer, but rising during the high discharges of autumn and winter to 10-12 mg 1-l. Aquatic plants were important in promoting the settlement and seasonal accumulation of suspended material as banks of sediment. (3) An accumulation of organic material was predicted by summing the differences between the monthly total of import and export for a section of stream between two sampling sites over 2 years. The stream-bed, however, was found to be clear of material shortly after the start of autumn rains in both years. This difference between the actual and the predicted quantities of organic material could be accounted for by losses of the organic material by respiration, measured by the rate of 02 consumption (0. 15-0*46 g m-2 h-1); these respiration rates are similar to those previously reported for streams of thistype. (4) Tentative organic matter budgets were prepared for two sections of stream for two seasons. Fine material was found to dominate these budgets. The amounts of dissolved organic material were also large, but were only estimated. The contributions ofl large suspended plant material resulting from in situ aquatic plant production and of large terrestrial inputs were small in proportion.