Microbial organic-nitrogen transformations along the Swale–Ouse river system, Northern England

Organic-nitrogen transformations (extracellular aminopeptidase activity and bacterial assimilation of amino acids) were measured in river water, at 1 or 2-monthly intervals from May 1995 to May 1996, at six sites 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 bacterioplankton and phytoplankton chlorophyll a. These were usually straight-line increases rather than being stepwise in immediate response to sewage-works discharges. Rates of downstream increase, per unit length of river, were greatest in summer. Aminopeptidase activity and amino-acid assimilation varied seasonally, with highest values in summer. Correlation coefficients and forced-entry multiple-regression analysis suggested that organic-nitrogen transformations were potentially controlled by both microbial abundance and environmental variables, especially temperature and flow. The inorganic nitrogen regime apparently had little effect. The ratios, Vmax for aminopeptidase activity/Vmax for amino-acid uptake, and turnover rate for aminopeptidase activity/turnover rate for amino-acid assimilation, were used as indicators of the balance between 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 summer months. Since, however, ammonium-N was abundant at the downstream sites, there might have been a switch, with change in distance downstream and season, in the relative importance of amino acids and ammonium as nitrogen source. The specific growth rate (SGR) of planktonic bacteria from upstream sites increased with addition of leucine or glucose, thus organic nitrogen or carbon may have been limiting. For a downstream site, however, only leucine promoted increased SGR, hence organic-N might have been limiting. Addition of NH4-N did not increase SGR hence inorganic nitrogen appeared not to be of equal value to bacterioplankton. Rates of organic-nitrogen transformations in the River Wiske and Cod Beck resembled those in the middle and lower main river, as did bacterioplankton and phytoplankton abundance, inorganic-nitrogen concentrations and the composition of the aquatic macrophyte community. Thus, these tributaries, although they were small watercourses, were very different from the headstream of the main river, essentially being enriched lowland streams.