Comparison of methods for determining the turnover times of dissolved organic compounds

Abstract

A comparison of two different methods was made using the tracer technique. The first test was performed by adding only one defined amont of a 14C-labelled compound to the sample, the second one by adding different amounts of the same labelled compound, followed by a kinetic approach. In a coastal area of the western Baltic Sea (Kiel Fjord) the turnover times of various compounds (glucose, glucosamine, aspartic acid, leucine, lysine and acetate) were determined. In the open Baltic Sea (especially the Bornholm-Basin) the study was confined to glucose. The variations between the results of the two methods for determining the turnover times are only small for samples from eutrophic or polluted regions. However, larger discrepancies exist with samples from oligotrophic areas. Here, turnover times obtained with the first method were consistently shorter (sometimes only half as long) than, those obtained with the second method. This difference is attributed to the fact that in areas of low bacterial activity e.g. in oligotrophic water bodies, organic-substance uptake by the entire natural population of heterotrophic micro-organisms does not follow exactly the Michaelis-Menten equation. This is likely to be due to a greater population heterogeneity resulting in an increased diversity of uptake characteristics. However, since the calculation of turnover time according to the second method is based on a modification of this equation, errors are introduced.