Kinetic model for aerobic mineralization of labile and refractory dissolved organic matter: an experimental approach

In aquatic systems, microbiota composition is strongly related to dissolved oxygen (DO) availability, which modulates the rates of biochemical reactions on decomposition. DO is a key variable that defines the prevalence of metabolic pathways in the well-oxygenated aquatic ecosystems. In this context, this study proposes a kinetic model to describe oxygen consumption due to the degradation of labile and refractory dissolved organic carbon (DOC) in aquatic ecosystems, including stoichiometric ratios of oxygen to carbon for each defined pathway (O/C). The values used for parameterizing the model were derived from in vitro mineralization experiments; for this, DOC samples were collected at three sites in the Barra Bonita Reservoir (São Paulo State, Brazil). The model was developed considering two pathways for mineralization, the oxidation of labile and refractory DOC. The mineralization of labile and refractory DOC follows distinct pathways, with different reaction times (average half-life values: 1 and 267 days, respectively). On average, the stoichiometric O/C ratios (1.81 and 2.13 for refractory and labile carbon, respectively) obtained were lower than expected, given the C-compound oxidation as a reference (2.67). In addition to the reaction rate values, the O/C values found in this study provide the orders of magnitude and the amplitudes of these relationships. Knowledge of these parameters and the use of this proposed model can contribute to a better understanding of oxygen consumption processes in aquatic environments, providing accurate information on oxygen balance to the calibration of water quality models.