Many inland waters exhibit complete or partial desiccation, or have vanished due to global change, exposing sediments to the atmosphere. Yet, data on carbon dioxide (CO 2) emissions from these sediments are too scarce to upscale emissions for global estimates or to understand their fundamental drivers. Here, we present the results of a global survey covering 196 dry inland waters across diverse ecosystem types and climate zones. We show that their CO 2 emissions share fundamental drivers and constitute a substantial fraction of the carbon cycled by inland waters. CO 2 emissions were consistent across ecosystem types and climate zones, with local characteristics explaining much of the variability. Accounting for such emissions increases global estimates of carbon emissions from inland waters by 6% (~0.12 Pg C y −1). Our results indicate that emissions from dry inland waters represent a significant and likely increasing component of the inland waters carbon cycle.
Iron that precipitates under aerobic conditions in natural aquatic systems scavenges dissolved organic matter (DOM) from solution. Subterranean estuaries (STEs) are of major importance for land–ocean biogeochemical fluxes. Their specific redox boundaries, coined the “iron curtain” due to the abundance of precipitated iron(III) (oxy)hydroxides, are hot spots for the removal and redissolution of iron, associated nutrients, and DOM. We used ultra-high-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry to molecularly characterize the iron-coagulating fractions of 32 groundwater and seawater DOM samples along a salinity gradient from a shallow STE on Spiekeroog Island, North Sea, Germany, and linked our findings to trace metal and nutrient concentrations. We found systematic iron coagulation of large (>450 Da), oxygen-rich, and highly aromatic DOM molecules of terrestrial origin. The extent of coagulation increased with growing terrestrial influence along the salinity gradient. Our study is the first to show that the iron curtain may be capable of retaining terrigenous DOM fractions in marine sediments. We hypothesize that the iron curtain serves as an inorganic modulator for the supply of DOM from groundwaters to the sea, and that the STE has the potential to act as a temporal storage or even sink for terrigenous aromatic DOM compounds.
The magnitude of diffusive carbon dioxide (CO2) and methane (CH4) emission from man-made reservoirs is uncertain because the spatial variability generally is not well-represented. Here, we examine the spatial variability and its drivers for partial pressure, gas-exchange velocity (k), and diffusive flux of CO2 and CH4 in three tropical reservoirs using spatially resolved measurements of both gas concentrations and k. We observed high spatial variability in CO2 and CH4 concentrations and flux within all three reservoirs, with river inflow areas generally displaying elevated CH4 concentrations. Conversely, areas close to the dam are generally characterized by low concentrations and are therefore not likely to be representative for the whole system. A large share (44–83%) of the within-reservoir variability of gas concentration was explained by dissolved oxygen, pH, chlorophyll, water depth, and within-reservoir location. High spatial variability in k was observed, and kCH4 was persistently higher (on average, 2.5 times more) than kCO2. Not accounting for the within-reservoir variability in concentrations and k may lead to up to 80% underestimation of whole-system diffusive emission of CO2 and CH4. Our findings provide valuable information on how to develop field-sampling strategies to reliably capture the spatial heterogeneity of diffusive carbon fluxes from reservoirs.
The potent greenhouse gas methane (CH 4 ) is readily emitted from tropical reservoirs, often via ebullition (bubbles). This highly stochastic emission pathway varies in space and time, however, hampering efforts to accurately assess total CH 4 emissions from water bodies. We systematically studied both the spatial and temporal scales of ebullition variability in a river inflow bay of a tropical Brazilian reservoir. We conducted multiple highly resolved spatial surveys of CH 4 ebullition using a hydroacoustic approach supplemented with bubble traps over a 12-month and a 2-week timescale to evaluate which scale of variation was more important. To quantify the spatial and temporal variability of CH 4 ebullition, we used the quartile coefficients of dispersion at each point in space and time and compared their frequency distributions across the various temporal and spatial scales. We found that CH 4 ebullition varied more temporally than spatially and that the intra-annual variability was stronger than daily variability within 2 weeks. We also found that CH 4 ebullition was positively related to water temperature increase and pressure decrease, but no consistent relationship with water column depth or sediment characteristics was found, further highlighting that temporal drivers of emissions were stronger than spatial drivers. Annual estimates of CH 4 ebullition from our study area may vary by 75-174% if ebullition is not resolved in time and space, but at a minimum we recommend conducting spatially resolved measurements at least once during each major hydrologic season in tropical regions (i.e., in dry and rainy season when water levels are falling and rising, respectively).
The highest CH 4 production rates can be found in anoxic inland water surface sediments however no model quantifies CH 4 production following fresh particular organic matter (POM) deposition on anoxic sediments. This limits our capability of modeling CH 4 emissions from inland waters to the atmosphere. To generate such a model, we quantified how the POM supply rate and POM reactivity control CH 4 production in anoxic surface sediment, by amending sediment at different frequencies with different quantities of aquatic and terrestrial POM. From the modeled CH 4 production, we derived parameters related to the kinetics and the extent of CH 4 production. We show that the extent of CH 4 production can be well predicted by the quality (i.e., C/N ratio) and the quantity of POM supplied to an anoxic sediment. In particular, within the range of sedimentation rates that can be found in aquatic systems, we show that CH 4 production increases linearly with the quantity of phytoplankton-derived and terrestrially derived POM. A high frequency of POM addition, which is a common situation in natural systems, resulted in higher peaks in CH 4 production rates. This suggests that relationships derived from earlier incubation experiments that added POM only once, may result in underestimation of sediment CH 4 production. Our results quantitatively couple CH 4 production in anoxic surface sediment to POM sedimentation flux, and are therefore useful for the further development of mechanistic models of inland water CH 4 emission.
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