Downstream change in leucine aminopeptidase activity and leucine assimilation by epilithic microbiota along the River Swale, northern England

Parallel determinations of epilithic extracellular leucine aminopeptidase a ctivity and leucine assimilation were made at five sites along 112 km of th e River Swale and also in two tributaries, the River Wiske and Cod Beck. Ep ilithic leucine aminopeptidase activity along the Swale increased with dist ance downstream; this increase was gradual, rather than stepwise in respons e to specific sewage-works outfalls. Epilithic leucine assimilation, in con trast, did not consistently increase along the river. Epilithic leucine ami nopeptidase activity and leucine assimilation were both potentially control led by epilithic microbial variables (bacterial abundance and chlorophyll a ) while leucine aminopeptidase activity was also strongly related to water- quality variables, especially temperature, pH and conductivity. Epilithic l eucine aminopeptidase activity and leucine assimilation were coupled, but t he magnitude of aminopeptidase activity was always substantially greater th an that of leucine assimilation. Arguments are presented, however, which su ggest that this did not necessarily indicate the constant availability of e xcess leucine, and by inference amino-acid nitrogen, to epilithic bacteria. Values of epilithic leucine aminopeptidase activity and leucine assimilati on, expressed relative to rates in overlying water, suggested that most act ivity and assimilation was epilithic rather than planktonic, although the p lanktonic contribution was proportionately greater at the deeper, more down stream, sites. In the tributaries, River Wiske and Cod Beck, values of epil ithic leucine aminopeptidase activity and epilithic microbial abundance, as well as those of many water-quality variables, resembled values in the mid dle and lower Swale. Thus, these tributaries were essentially lowland, enri ched watercourses being very different from the headstreams of the main riv er. (C) 2000 Elsevier Science B.V. All rights reserved.