MICROBIAL ORGANIC-NITROGEN TRANSFORMATIONS ALONG THE SWALE-OUSE RIVERSYSTEM, NORTHERN ENGLAND

Organic-nitrogen transformations (extracellular aminopeptidase activit y and bacterial assimilation of amino acids) were measured in river wa ter, at 1 or 2-monthly intervals from May 1995 to May 1996, at six sit es along 148 km of the Swale-Ouse and at sites on two tributaries, the River Wiske and Cod Beck. The aminopeptidase activity and amino-acid assimilation increased with distance downstream, as did bacterioplankt on and phytoplankton chlorophyll a. These were usually straight-line i ncreases rather than being stepwise in immediate response to sewage-wo rks discharges. Rates of downstream increase, per unit length of river , were greatest in summer. Aminopeptidase activity and amino-acid assi milation varied seasonally, with highest values in summer. Correlation coefficients and forced-entry multiple-regression analysis suggested that organic-nitrogen transformations were potentially controlled by b oth microbial abundance and environmental variables, especially temper ature and flow. The inorganic nitrogen regime apparently had little ef fect. The ratios, V-max for aminopeptidase activity/V-max for amino-ac id uptake, and turnover rate for aminopeptidase activity/turnover rate for amino-acid assimilation, were used as indicators of the balance b etween supply and demand of organic nitrogen to bacterioplankton. Low values of these ratios, potentially indicating limited supply of amino -acid nitrogen, were recorded at the more downstream sites in the summ er months. Since, however, ammonium-N was abundant at the downstream s ites, there might have been a switch, with change in distance downstre am and season, in the relative importance of amino acids and ammonium as nitrogen source. The specific growth rate (SGR) of planktonic bacte ria from upstream sites increased with addition of leucine or glucose, thus organic nitrogen or carbon may have been limiting. For a downstr eam site, however, only leucine promoted increased SGR, hence organic- N might have been limiting. Addition of NH4-N did not increase SGR hen ce inorganic nitrogen appeared not to be of equal value to bacteriopla nkton. Rates of organic-nitrogen transformations in the River Wiske an d Cod Beck resembled those in the middle and lower main river, as did bacterioplankton and phytoplankton abundance, inorganic-nitrogen conce ntrations and the composition of the aquatic macrophyte community. Thu s, these tributaries, although they were small watercourses, were very different from the headstream of the main river, essentially being en riched lowland streams. (C) 1998 Elsevier Science B.V.