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Abstract

SUMMARY 1. Radiolabelled (¹⁴C) amino acids were used to investigate the influence of sediment size as well as dissolved free amino acid (DFAA) concentration and composition on immobilization and mineralization of DFAAs by biofilms from a first-order stream. 2. Over time (240 min), biofilms on stony substrata immobilized a DFAA mixture more effectively than those on sandy substrata, but proportional mineralization of immobilized DFAAs was higher for sandy substrata (36 v 20%). 3. Using stony substrata, the DFAA mixture was immobilized more rapidly than glycine alone at 'near-natural' amino acid concentrations (c. 37 µg l^-1) as well as enriched concentrations (1 and 100 mg l^-1. Instantaneous rates of glycine immobilization and mineralization were not saturated at glycine enrichments of up to 980 mg l^-1. 4. With both the amino acid mixture and glycine alone, proportional mineralization of the immobilized amino acids increased on enrichment to 1 mg l^-1(DFAA mixture: from 25 to 37%; glycine alone: from 50 to 54%), but then fell on further enrichment to 100 mg l^-1(DFAA mixture: 11%; glycine alone: 7%). 5. Results are discussed in terms of the potential trophic utility of immobolized DFAAs as well as the apparent roles of biotic and immobilization machanisms. Immobilization and mineralization responses to variables investigated in this study give an insight into potential variability of carbon immobilization and retention in stream-bed sediments. This is fundamental to an understanding of how dissolved organic carbon may become available to higher trophic levels.