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

Abstract

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 CO 2 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 m 22 d 21 . 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 a...

Organic carbon burial efficiency in a subtropical hydroelectric reservoir

“…Moraes were similar to the rates reported for three other reservoirs in the Brazilian Cerrado (mean 66 g C m −2 yr −1 ; Cardoso et al, 2014). The range of CH 4 fluxes from the Mascarenhas de Moraes sediments (3-23 g C m −2 yr −1 ) was low when compared to a set of temperate lakes (0.2-46 g C m −2 yr −1 , Sobek et al, 2009).…”

“…A similar approach has been used in natural boreal lakes, most of which showed mean OCBE of 5-∼ 40 % (only one lake had OCBE of 62 %; Ferland et al, 2014). The high sedimentation rates in some areas of the subtropical Mascarenhas de Moraes reservoir resulted in a higher mean OCBE than in the boreal lakes, despite the negative effect of temperature on OC burial (Gudasz et al, 2010;Cardoso et al, 2014). The boreal lakes' assessment suggested that spatially resolved mean OCBE can be predicted based on lake area and shape, with small lakes acting as more efficient carbon sinks than large ones (Ferland et al, 2014).…”

“…Other variables potentially affecting OCBE (e.g., distance to the dam, OC content and OC mineralization rates) were included as well. Temperature, which is also clearly important (Gudasz et al, 2010;Cardoso et al, 2014) was not included in these analysis, since variation in bottom temperatures among sampling sites was small (23 to 25 • C) and related to daily conditions (sampling was performed within 10 days) rather than to site-specific conditions.…”

“…Finally, the combination of the usually long water residence time, the high transparency due to the settling of suspended sediment and the high nutrient availability favors planktonic primary production in reservoirs (Bayne et al, 1983;Rangel et al, 2012). Terrestrial-derived and aquatic-derived OCs occur in different proportions along reservoirs' length and may be mineralized/buried at different rates (Gudasz et al, 2012;Cardoso et al, 2014). Moreover, the hydrology of hydroelectric reservoirs, marked by the transition between high stream velocities at the river inflow and low velocities near the dam, often leads to remarkable limnological gradients, e.g., water physical and chemical properties, sedimentation and productivity (Thornton, 1990;Armengol et al, 1999).…”

Globally significant greenhouse-gas emissions from African inland waters

“…Although benthic respiration was not measured during our study, using the average value from a recent data synthesis for tropical rivers and streams (50.8 mmolC m −2 d −1 ; ref. 28), we find that pelagic and benthic R combined still account for only <14% of the FCO 2 . Furthermore, these estimates do not account for pelagic and benthic aquatic primary production, which would decrease the dissolved CO 2 concentration.…”