Use of elemental composition to predict bioavailability of dissolved organic matter in a Georgia river

Abstract

The relationship between the bioavailability of dissolved organic matter (DOM) and its bulk chemical composition was examined on three dates at 10 sites on the Ogeechee River, a blackwater river in Georgia. Samples of riverine DOM were concentrated from filtered river water using reverse osmosis. In addition, particulate organic matter (POM), in the form of leaf litter, algae, and macrophytes, was leached with synthetic rainwater to obtain fresh DOM. Elemental composition, carboxylic acid content, and bacterial growth were measured on all samples. The results of this study indicate that fresh DOM in POM leachates is generally more bioavailable than riverine DOM. The bioavailability of riverine DOM appears to be greater under low discharge conditions and decreases with distance downstream. The bioavailability of all DOM samples is very well predicted (r2 = 0.93, n = 20) by an empirical equation of the form: bioavailability = a0 + a1(H:C) + a2(O:C) + a3(N:C). When compositional data are plotted on a van Krevelen diagram, it is evident that POM leachates and, to a lesser degree, DOM from headwater sites have compositions that differ little from simple mixtures of major components of biomass (lipids, sugars, proteins, and lignins). Father downstream, major diagenetic alteration of organic matter is evident from the compositions of DOM samples, whose H: C and O: C ratios are lower and higher, respectively, than for any possible mixture of biomass components. Bioavailability of DOM is closely related to the percentage of aliphatic carbon in a sample, and downstream decreases in bioavailability are mainly attributable to selective degradation of aliphatic carbon in riverine DOM.