NH4+ utilization and regeneration rates in freshwater lakes determined by GC-MS of derivatised dihydroindophenol.
A new gas chromatographic-mass spectrometric method was established that is applicable for the determination of NH4+ utilization and regeneration rates in freshwater. Hollow-fibre modules were used to stop the biogenic nitrogen-fluxes by separating the particulate from the dissolved matter. Incubations were performed in Tedlar bags (polyvinylfluoride), which enabled repeated sample removals through Teflon tubes, making the calculation of nitrogen-fluxes in accordance to Blackburn and Caperon much more reliable. The Berthelot reaction was performed with ammonium and a fragment ion (base peak) of tris-(trifluoroacetyl) 4,4'-dihydroxydiphenylamine was used to determine the at% excess 15N by gas chromatography-mass spectrometry. Nitrogen-flux measurements were made in the epilimnion of the deep, stratified, mesotrophic Lake Zürich, in which the cyanobacterium Planktothrix rubescens was the dominating photoautotrophic micro-organism. The size fraction <20 microm that consisted of heterotrophic bacterioplankton and nanoflagellates, and photoautotrophic pico- and nanoplankton accounted only for a minor part of the ammonium utilization (<25%) and regeneration (< or =25%) rates, whereas the size fraction >20 microm which primarily consisted of Planktothrix rubescens was responsible for the major part. In the eutrophic Lake Au, which is connected to Lake Zürich through a canal, utilization and regeneration rates as high as 700 and 482 nM h(-1) were measured.
Köster
O
O
Institute of Plant Biology, University of Zürich, Kilchberg, Switzerland.
Jüttner
F
F
D000818:Animals; D005618:Fresh Water / Q000737:chemistry* / Q000382:microbiology; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods; D007202:Indicators and Reagents / Q000737:chemistry*; D007215:Indophenol / Q000031:analogs & derivatives* / Q000737:chemistry; D009587:Nitrogen Isotopes / Q000032:analysis; D010933:Plankton / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D015203:Reproducibility of Results; D013997:Time Factors
D007202:Indicators and Reagents; D009587:Nitrogen Isotopes; D000644:Quaternary Ammonium Compounds; D007215:Indophenol