Nitrification Rates in the Baltic Sea: Comparison of Three Isotope Techniques

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Abstract
Simultaneous measurements of nitrification in the Baltic Sea were made at 10- to 30-m intervals in the months of June and November by three isotope techniques: [ 15 N]nitrate dilution, N-serve sensitive [ 14 C]bicarbonate incorporation, and [ 15 N]ammonium oxidation to nitrite and nitrate. Nitrification rates of 1 to 280 nmol liter −1 day −1 were recorded, and each method showed that the highest rates of nitrification occurred below the halocline. Even in the presence of ammonium, dark incubations of mixed layer (above ca. 50 m) waters never yielded nitrification rates exceeding 45 nmol liter −1 day −1 . The rates measured by the ammonium oxidation method were two- to sevenfold greater than those obtained by 14 C incorporation or 15 N dilution. The merits of each technique are discussed, and it is suggested that the [ 15 N]ammonium oxidation method should be used in conjunction with the [ 14 C]bicarbonate incorporation method.