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

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Abstract
The seasonal changes in concentration of fine particulate material being transported downstream are described and quantified for 3 sites along a small chalk stream in Dorset. The mean monthly concentration of organic matter (dry wt) was 1-2 mg l-1 in the summer but rose during the high discharges of autumn and winter to 10-12 mg l-1. Aquatic plants were important in promoting the settlement and seasonal accumulation of suspended material as banks of sediment. 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 2 sampling sites over 2 yr. The stream bed was 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 O2 consumption (0.15-0.46 g m-2 h-1); these respiration rates are similar to those previously reported for streams of this type. Tentative organic matter budgets were prepared for 2 sections of stream for 2 seasons. Fine material dominated these budgets. The amounts of dissolved organic material were also large but were only estimated. The contributions of large suspended plant material resulting from in situ aquatic plant production and of large terrestrial inputs were small in proportion.