Bacterial secondary production transforms organic C from the environment into new bacterial biomass. Bacterial respiration generates energy and converts assimilated organic C into COz. Two decades of research have given us a good understanding of the magnitude and regulation of bacterial production in pelagic ecosystems, but much less is known about bacterial respiration. Bacterial growth efficiency [BGE = BP/(BP + BR)] relates measurements of bactenal production and respiration.Recent reviews demonstrate a large range in BGE among and within systems; the regulation of this variance is not well understood. We made direct measurements of both BP and BR over a full seasonal cycle in the Hudson River, New York, and in a series of manipulative experiments. BGE was well correlated with BP and ranged from 0.04 to 0.66, with a majority (69%) between 0.2 and 0.5. BR and BP were correlated (r = 0.65; p < 0.0001) but BR was less variable than BP. Thus, much of the variation in BGE could be explained by the vanation in BP. The relationship (based on 24 h bioassays) between BP and BGE fit a rectdinear hyperbola [BGE = 0.10 + 0.68BP(5.21 + BP)] and explained 70% of the variation in BGE (p < 0.001). During the relatively long incubation (24 h) r e q u r e d to measure BR, conditions diverge from ambient. BP, BR and BGE all increase during this incubation period. We used the relationships between BGE and BP and BR (above) to calculate realistic ambient estimates of BGE from short-term measurements ( < l h) of BP. Based on this approach, modeled BGE for the Hudson averaged 0.16 2 0.05 (range = 0.07 to 0.23), about 50% lower than the values based on 24 h bioassays. Using thls relationship we estimate pelagic BR in the tidal, freshwater Hudson River to be between 176 and 229 g C m-' yr-'.
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...
Several researchers have proposed spectrophotometric modifications of the Winkler titrimetric method for measuring dissolved oxygen (DO). These modifications, although simple, are not widely used because of concern about accuracy, calibration, and possible sources of interference. Here we show, using natural samples from lakes and rivers as well as samples manipulated in the laboratory, that the spectrophotometric method can provide accurate and very precise measurements of DO over a wide range of concentrations (4 to ∼13 mg O2 liter−1). Further, interference from dissolved organic carbon (color) and turbidity are minor. We propose corrections for both color and turbidity, where necessary, that can be easily incorporated into the measurement design. Because of the speed and simplicity of the spectrophotometric method, it is easy to replicate measurements and thereby increase precision without greatly increasing analytical time. In 10 min of effort, we were able to achieve a coefficient of variation (CV) within one bottle of 0.09%, or 0.8% among different bottles. With n = 7 bottles, one can easily distinguish changes in DO of 0.05 mg liter−1 with this method, which makes it useful for metabolic studies in many environments. To achieve a comparable CV by conventional titration would require about 100 min of effort.
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