THE EFFECTS OF ARTIFICIAL AMMONIUM ENHANCEMENT ON RIVERINE PERIPHYTICDIATOM COMMUNITIES

An experiment to ascertain the influence of artificially enhanced ammo nium concentrations on riverine periphytic diatom communities was cond ucted during an 80 day period in winter 1988/89 (temperature: 5-degree s-C). During an initial 47 day enrichment phase, ammonium chloride at different concentrations was added to O2-saturated river water in expe rimental channels. Compared to a control, the addition of 1.2 mg NH4+- N . l-1 resulted in only slight changes in the species composition of the diatom community. The addition of 5.1 mg NH4+-N . l-1 and more (gr eater-than-or-equal-to 60 mug NH3-N . l-1) resulted in a decrease in t he biomass and a drastic change in the species composition. After the 42 day enrichment period, the diatom species Fragilaria capucina var. vaucheriae, Navicula saprophila, Navicula atomus and Gomphonema parvul um, all of which are known to be tolerant to organic pollution, were f ound with relative abundances of 44.8%, 35.6%, 2.3% and 1.5%, respecti vely. Changes in species composition of the periphytic diatom communit ies occurred within the first two weeks of the subsequent recovery pha se. After a month of recovery, all periphytic diatom communities exhib ited a high degree of structural similarity with the control.