AME 42:149-158 (2006)  -  doi:10.3354/ame042149

Comparison of nitrifier activity versus growth in the Scheldt estuary—a turbid, tidal estuary in northern Europe

Maria G. I. Andersson1,*, Natacha Brion2, Jack J. Middelburg1

1Netherlands Institute of Ecology (NIOO-KNAW), Centre for Estuarine and Marine Ecology, PO Box 140, 4400 AC Yerseke, The Netherlands
2Labo voor Analytische en Milieu Chemie, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium

ABSTRACT: Nitrifier activity and growth were measured in the Scheldt estuary over a salinity gradient. Measurements were made during all 4 seasons using 15N enriched ammonium and 14C labeled carbon incorporation. Established conversion ratios are often used to convert the growth of nitrifiers (measured as the incorporation of carbon) to nitrifying activity (i.e. oxidation of ammonium to nitrate). Our study demonstrated that the conversion of growth rates to nitrifying activity induces uncertainty because activity and growth of nitrifiers may be uncoupled. The C:N conversion ratio appears to be oxygen and temperature dependent. We advocate the use of 15N stable isotope techniques to study nitrification: this technique measures the actual activity of nitrifiers without the disadvantages involved in using inhibitors, and thus allows light inhibition to be measured.

KEY WORDS: Nitrification · Nitrifier activity · Nitrifier growth · 15N stable isotopes · 14C bicarbonate

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