Improved methods for the measurement of dissolved and particulate organic carbon in fresh water and their application to chalk streams

Summary The development and use of an improved automatic system for the accurate measurement of dissolved organic carbon (D.O.C.) in fresh waters is described. The method, which covers the range 0‐25 mgl −1 D.O.C., is based on the oxidation of organic carbon to carbon dioxide by U.V. radiation. The carbon dioxide produced is measured by a non dispersive infra‐red gas analyser (I.R.G.A.). The I.R.G.A. is also used in the semi‐automatic measurement of particulate organic carbon (P.O.C.) by a combustion method which allows very low concentrations to be measured using samples of 200 ml or less. Results of surveys of two river systems indicate that D.O.C. concentrations of less than 2.0 mg l −1 are typical in unpolluted chalk streams. D.O.C. concentrations of streams draining acid heathland were found to be significantly higher (3‐5.5 mg l −1 ). D.O.C. concentration was found to increase rapidly during a spate, in a river draining areas of mixed underlying geology. The throughput of D.O.C. during the twenty‐five to fifty spates which occur annually in the river could amount to 20‐30% of the annual flux of D.O.C. P.O.C. concentration in chalk spring waters (0.03–0.04 mg l −1 ) were found to be higher than expected from preliminary estimates using membrane filtration techniques. The relevance of the measurement of D.O.C. and P.O.C. flux to estimates of the energy budgets of stream ecosystems is discussed and published methods for the automatic measurement of D.O.C. are reviewed.