Do models of organic carbon mineralization extrapolate to warmer tropical sediments?

Freshwater sediments are important sites of organic carbon (OC) burial and mineralization. Previous studies indicate that warming can increase rates of OC mineralization, implying more CO2 release from sediments and, consequently, less OC burial, but temperatures typical of tropical ecosystems are poorly represented in the models of temperature and OC mineralization. We measured OC mineralization rates in 61 Brazilian tropical systems, including rivers, streams, lakes, coastal lagoons, and reservoirs from different regions (Pantanal, Amazonia, Atlantic Forest, and coastal areas). Oxygen consumption and dissolved inorganic carbon production in sediment core incubations were used for estimating OC mineralization rates. Multiple regression models were used to investigate the importance of temperature and other variables to predict OC mineralization. The average OC mineralization rate for all systems was 1223 6 950 mg C m22 d21. Rates increased significantly with increasing temperature and varied across system types and regions. In addition, salinity, total nitrogen, and chlorophyll a were important factors controlling OC mineralization in tropical sediments. The pattern of increasing mineralization with temperature was remarkably consistent with theoretical and empirical expectations. The explanatory power of previous temperature vs. mineralization models is confirmed and enhanced by the addition of the tropical data that substantially extended the temperature range. Sediments are recognized as important components of the carbon cycle at local and regional scales, as they are active sites of carbon storage and mineralization (Tranvik et al. 2009). In sediments of freshwater ecosystems, those processes are mainly regulated by the availability of electron acceptors (e.g., oxygen, nitrate, manganese, iron, and sulfate), mixing regimes, the quantity and quality of the organic carbon (OC), and temperature (Fenchel et al. 2012). In spite of the increasing efforts to understand the effects of each of those processes on carbon fluxes from freshwater ecosystems, uncertainties still remain, particularly concerning the potential effect of temperature (Gudasz et al. 2010). Temperature modulates many biological processes, including the metabolism of organisms (Yvon-Durocher et al. 2010). Based on models predicting the effect of temperature on metabolic processes in sediments, increasing temperature leads to higher OC mineralization rates and, consequently, less carbon burial (Gudasz et al. 2010). However, most of the studies used to develop these models cover only a limited range of temperatures (range from all studies 0uC to 25uC) and poorly represent aquatic ecosystems in tropical areas, where water temperatures often exceed 30uC (Hamilton 2010). Other studies have noted that the effect of increasing temperature on OC mineralization may not be the same in tropical and in temperate aquatic ecosystems (Pace and Prairie 2005; Yvon-Durocher and Allen 2012). However, other factors besides temperature, such as nutrient and electron acceptor availability, may also control mineralization rates (Talling