Epilithic and planktonic leucine aminopeptidase activity and leucine assimilation along the River Tweed, Scottish Borders.

The microbial organic-nitrogen transformation variables, extracellular leucine aminopeptidase activity, and leucine assimilation, were determined at stone surfaces and in river water at six sites along 138 km of the River Tweed and also in a major tributary, the River Teviot. Sampling was on 3 days, representative of spring, summer and autumn. The variables, except for epilithic leucine assimilation, all showed significant downstream increase on at least 2 of the sampling days. Their values tended to be lowest in spring (early April). Aminopeptidase activity, leucine assimilation, microbial abundance, and pattern of downstream increase in the Tweed were shown to be broadly similar to in the River Swale, northern England. Microbial organic-nitrogen transformations and abundance in the Teviot tended to equal or exceed those in the lower Tweed, probably because of enrichment of the tributary by sewage-works discharges.