Epilithic metabolism of dissolved organic carbon in boreal forest rivers

Abstract

The utilisation of colloidal and dissolved organic carbon (CDOC) by boreal river epilithon was investigated. The technique of microcalorimetry was used to determine the metabolic response of the epilithon in terms of both absolute levels and utilisation of material of apparent molecular weight (AMW) above and below 1000. Short-term perfusions were used to measure absolute levels of metabolic heat output for comparison with other studies, and long-term perfusions were used to study the relative utilisation of different molecular weight fractions. The results indicated low levels of metabolic heat output from boreal river epilithon with a shorter lag period in the utilisation of CDOC than we have found in a temperature river system. Long-term perfusions indicated a considerable increase in metabolic heat output on removal of high-molecular-weight material. We discuss these results in terms of a ‘competitive access model’ whereby slowly metabolised high-molecular-weight compounds saturate adsorption sites on the epilithon surface, resulting in a low metabolic heat output. When these compounds are removed, ‘labile’ low-molecular-weight molecules may more readily adsorb to or diffuse into the epilithon matrix.