Dissolved organic carbon (DOC) is one of the largest biologically available sources of organic C in aquatic ecosystems, and its dynamics have implications for local and global C cycling. Usually, DOC biodegradation is assessed in laboratory incubations using native microbial communities as inoculum. The use of native inocula might hamper the comparability of biodegradation rates across systems and obstructs the identification of other controls on DOC biodegradation. Here, we propose the use of the Standardized Bacterial Inoculum (SBI) in experiments of freshwater DOC degradation that would allow for the assessment of the potential degradability of DOC and identification of the drivers of degradation without interferences from the diverse metabolic capabilities of native communities. The SBI is composed of six bacterial strains that grow easily under laboratory conditions and showed better DOC degradation performance than the single strains separately. The SBI performance was tested on simple C sources, humic acids, and natural organic matter from a range of freshwater ecosystems. The fraction and rates of C‐substrates degradation by the SBI was consistently replicated under oxic and anoxic conditions. Moreover, DOC degradation rates by the SBI were equivalent to those of the native communities. Finally, the SBI consumed up to 31% of natural DOC from a variety of lakes, rivers, and peatlands over a 28‐d period. Overall, the use of the SBI in future DOC degradation experiments will improve comparability among studies and will allow for the separation of effects caused by intrinsic DOC properties from those caused by heterogeneous microbial assemblages.

The accumulation rate of authigenic barite (BaSO 4 ) in marine sediments is a promising proxy for reconstructing marine export production, but many aspects of barite precipitation and dissolution in the water column remain unknown. Here, we collected, imaged, and quantitatively analyzed 5481 barite microcrystals in bottle casts from the Eastern Pacific water column to gain a better understanding of in situ barite dynamics. Barite crystal abundance increases rapidly between the surface and 500 m in depth and then declines to predominantly low abundances below~1000 m. The falloff in barite abundance between the oxygen minimum zone (OMZ) and the ocean interior suggests 60% AE 20% loss of barite by dissolution, nearly all of which is complete by water depths of 1000 m. However, there are occasional samples, as deep as 1250 m, with unusually high barite abundance that may represent marine snow deposition events. We found that microcrystals associated with organic matter substrates were smaller and less solid than free crystals, which suggests ongoing barite precipitation toward larger, more regularly shaped microcrystals within organic matter aggregates. Trends in barite microcrystal size with depth suggest that organic matter aggregates also play a role in shielding barite microcrystals from dissolution. In addition, our extensive data set raises new questions regarding marine barite nucleation and spatial heterogeneity. By helping bridge the gap between hypothesized barite dynamics and in situ observations of barite microcrystals, this study advances our understanding of water column Ba processes and the utility of sediment barite as an export production proxy.

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<p><strong>Abstract.</strong> Reservoirs are a prominent feature of the current global hydrological landscape, and their sediments are the site of extensive organic carbon burial. Meanwhile, reservoirs frequently go dry due to drought and/or water management decisions. Nonetheless, the fate of organic carbon buried in reservoir sediments upon drying is largely unknown. Here, we conducted a 45-day-long laboratory incubation of sediment cores collected from a western Mediterranean reservoir to investigate carbon dynamics in drying sediment. Drying sediment cores emitted more CO<sub>2</sub> over the course of the incubation than sediment cores incubated with overlaying water (206.7 ± 47.9 vs. 69.2 ± 18.1 mmol CO2 m<sup>−2</sup> day<sup>−1</sup>, mean ± SE). Organic carbon content at the end of the incubation was lower in drying cores, which suggests that this higher CO<sub>2</sub> efflux was due to organic carbon mineralization. However, the apparent rate of organic C reduction in the drying sediments (568.6 ± 247.2 mmol CO<sub>2</sub> m<sup>−2</sup> day<sup>−1</sup>, mean ± SE) was higher than C emission. Meanwhile, sediment cores collected from a reservoir area that had already been exposed for 2+ years displayed net CO<sub>2</sub> influx from the atmosphere to the sediment (−136.0 ± 27.5 mmol CO<sub>2</sub> m<sup>−2</sup> day<sup>−1</sup>, mean ± SE) during the incubation period. Sediment mineralogy suggests that this CO<sub>2</sub> influx was caused by a relative increase in calcium carbonate chemical weathering. Thus, we found that while organic carbon decomposition in newly dry reservoir sediment causes measurable organic carbon loss and carbon gas emissions to the atmosphere, other processes can offset these emissions on short time frames and compromise the use of carbon emissions as a proxy for organic carbon mineralization in drying sediments.</p>