Advective flows rapidly transport water, solutes, and particles into and out of permeable sand beds and significantly affects the biogeochemistry of coastal environments. In this paper, we reviewed ...the drivers of porewater and groundwater advection in permeable shelf sediments in an attempt to bridge gaps among different disciplines studying similar problems. We identified the following driving forces: (1) terrestrial hydraulic gradients, (2) seasonal changes in the aquifer level on land moving the location of the subterranean estuary, (3) wave setup and tidal pumping, (4) water level differences across permeable barriers, (5) flow- and topography-induced pressure gradients, (6) wave pumping; (7) ripple and other bed form migration, (8) fluid shear, (9) density-driven convection, (10) bioirrigation and bioturbation, (11) gas bubble upwelling, and (12) sediment compaction. While these drivers occur over spatial scales ranging from mm to km, and temporal scales ranging from seconds to years, their ultimate biogeochemical implications are very similar (i.e., they are often a source of new or recycled nutrients to seawater and transform organic carbon into inorganic carbon). Drivers 2–12 result in no net water input into the ocean. Taking all these mechanisms into account, we conservatively estimate that a volume equivalent to that of the entire ocean is filtered by permeable sediments at time scales of about 3000 years. Quantifying the relative contribution of these drivers is essential to understand the contribution of sediments to the global cycles of matter.
Riverine and atmospheric inputs are often considered as the main terrestrial sources of dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicon (DSi) in the ocean. However, the fluxes of ...nutrients via submarine groundwater discharge (SGD) often exceed riverine inputs in different local and regional scale settings. In this study, we provide a first approximation of global nutrient fluxes to the ocean via total SGD, including pore water fluxes, by combining a global compilation of nutrient concentrations in groundwater and the SGD-derived
Ra fluxes. In order to avoid overestimations in calculating SGD-derived nutrient fluxes, the endmember value of nutrients in global groundwater was chosen from saline groundwater samples (salinity >10) which showed relatively lower values over all regions. The results show that the total SGD-derived fluxes of DIN, DIP, and DSi could be approximately 1.4-, 1.6-, and 0.7-fold of the river fluxes to the global ocean (Indo-Pacific and Atlantic Oceans), respectively. Although significant portions of these SGD-derived nutrient fluxes are thought to be recycled within sediment-aquifer systems over various timescales, SGD-derived nutrient fluxes should be included in the global ocean budget in order to better understand dynamic interactions at the land-ocean interface.
Freshwaters are important sources of greenhouse gases (GHGs) to the atmosphere that may partially offset the terrestrial carbon sink. However, current emission estimates from inland waters remain ...uncertain due to data paucity in key regions with a large freshwater surface area, such as China. Here, we show that the areal fluxes of GHGs (carbon dioxide, methane, and nitrous oxide) from lakes and reservoirs in China are much larger than previous estimates. Our work summarized data from 310 lakes and 153 reservoirs, and revealed diffusive emissions of 1.56 (95% confidence interval: 1.12–2.00) Tg C-CH4/y and 25.2 (20.8–29.5) Tg C-CO2/y from reservoirs and lakes. Chinese lakes and reservoirs emit 175.0 (134.7–215.3) Tg CO2 equivalent, with 73.4% of this forcing contributed by lakes. These aquatic sources are equivalent to 14.1%–22.6% of China's estimated terrestrial carbon sink. Our results suggest a disproportionally high contribution of China's reservoirs and lakes to national and global GHGs emissions, highlighting major data gaps and the need of including more artificial and natural lakes data from developing countries like China in global GHGs budgets.
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•Much needed data for China's national lakes and reservoirs' GHG estimates are provided.•We provide magnitude and controls on GHGs emissions from China's lakes and reservoirs.•China's lake and reservoirs emit 1.56 Tg C-CH4/y and 25.2 Tg C-CO2/y.•China's lake and reservoirs disproportionately contribute to GHG fluxes.•The study advances of our standing of GHG budgets from China's inland waters.
•Driving forces on flow and transport, and chemical behavior in subterranean estuaries reviewed.•Need for better understanding of interactions between physical and biogeochemical processes.•Need to ...consider influence of real work complexities such aquifer heterogeneities.
Sustainable coastal resource management requires sound understanding of interactions between coastal unconfined aquifers and the ocean as these interactions influence the flux of chemicals to the coastal ocean and the availability of fresh groundwater resources. The importance of submarine groundwater discharge in delivering chemical fluxes to the coastal ocean and the critical role of the subterranean estuary (STE) in regulating these fluxes is well recognized. STEs are complex and dynamic systems exposed to various physical, hydrological, geological, and chemical conditions that act on disparate spatial and temporal scales. This paper provides a review of the effect of factors that influence flow and salt transport in STEs, evaluates current understanding on the interactions between these influences, and synthesizes understanding of drivers of nutrient, carbon, greenhouse gas, metal and organic contaminant fluxes to the ocean. Based on this review, key research needs are identified. While the effects of density and tides are well understood, episodic and longer-period forces as well as the interactions between multiple influences remain poorly understood. Many studies continue to focus on idealized nearshore aquifer systems and future work needs to consider real world complexities such as geological heterogeneities, and non-uniform and evolving alongshore and cross-shore morphology. There is also a significant need for multidisciplinary research to unravel the interactions between physical and biogeochemical processes in STEs, as most existing studies treat these processes in isolation. Better understanding of this complex and dynamic system can improve sustainable management of coastal water resources under the influence of anthropogenic pressures and climate change.
Mangrove forests are hot spots in the global carbon cycle, yet the fate for a majority of mangrove net primary production remains unaccounted for. The relative proportions of alkalinity and dissolved ...CO2 CO2* within the dissolved inorganic carbon (DIC) exported from mangroves is unknown, and therefore, the effect of mangrove DIC exports on coastal acidification remains unconstrained. Here we measured dissolved inorganic carbon parameters over complete tidal and diel cycles in six pristine mangrove tidal creeks covering a 26° latitudinal gradient in Australia and calculated the exchange of DIC, alkalinity, and CO2* between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol m−2 d−1 across the six systems, ranging from import of 97 mmol m−2 d−1 to an export of 85 mmol m−2 d−1. If the Australian transect is representative of global mangroves, upscaling our estimates would result in global DIC exports of 3.6 ± 1.1 Tmol C yr−1, which accounts for approximately one third of the previously unaccounted for mangrove carbon sink. Alkalinity exchange ranged between an import of 1.2 mmol m−2 d−1 and an export of 117 mmol m−2 d−1 with an estimated global export of 4.2 ± 1.3 Tmol yr−1. A net import of free CO2 was estimated (−11.4 ± 14.8 mmol m−2 d−1) and was equivalent to approximately one third of the air‐water CO2 flux (33.1 ± 6.3 mmol m−2 d−1). Overall, the effect of DIC and alkalinity exports created a measurable localized increase in coastal ocean pH. Therefore, mangroves may partially counteract coastal acidification in adjacent tropical waters.
Key Points
DIC, alkalinity, and CO2 fluxes from mangroves were determined across a latitudinal gradient
Global DIC and alkalinity export were estimated at 3.6 Tmol yr‐1 and 4.2 Tmol yr‐1, respectively
Alkalinity exports from mangroves can increase coastal ocean pH
Groundwater comprises 95% of the liquid fresh water on Earth and contains a diverse mix of dissolved organic matter (DOM) molecules which play a significant role in the global carbon cycle. ...Currently, the storage times and degradation pathways of groundwater DOM are unclear, preventing an accurate estimate of groundwater carbon sources and sinks for global carbon budgets. Here we reveal the transformations of DOM in aging groundwater using ultra-high resolution mass spectrometry combined with radiocarbon dating. Long-term anoxia and a lack of photodegradation leads to the removal of oxidised DOM and a build-up of both reduced photodegradable formulae and aerobically biolabile formulae with a strong microbial signal. This contrasts with the degradation pathway of DOM in oxic marine, river, and lake systems. Our findings suggest that processes such as groundwater extraction and subterranean groundwater discharge to oceans could result in up to 13 Tg of highly photolabile and aerobically biolabile groundwater dissolved organic carbon released to surface environments per year, where it can be rapidly degraded. These findings highlight the importance of considering groundwater DOM in global carbon budgets.
The blue carbon paradigm has evolved in recognition of the high carbon storage and sequestration potential of mangrove, saltmarsh and seagrass ecosystems. However, fluxes of the potent greenhouse ...gases CH4 and N2O, and lateral export of carbon are often overlooked within the blue carbon framework. Here, we show that the export of dissolved inorganic carbon (DIC) and alkalinity is approximately 1.7 times higher than burial as a long-term carbon sink in a subtropical mangrove system. Fluxes of methane offset burial by approximately 6%, while the nitrous oxide sink was approximately 0.5% of burial. Export of dissolved organic carbon and particulate organic carbon to the coastal zone is also significant and combined may account for an atmospheric carbon sink similar to burial. Our results indicate that the export of DIC and alkalinity results in a long-term atmospheric carbon sink and should be incorporated into the blue carbon paradigm when assessing the role of these habitats in sequestering carbon and mitigating climate change.
Groundwater may be highly enriched in dissolved carbon species, but its role as a source of carbon to coastal waters is still poorly constrained. Exports of deep and shallow groundwater-derived ...dissolved carbon species from a small subtropical estuary (Korogoro Creek, Australia, latitude −31.0478°, longitude 153.0649°) were quantified using a radium isotope mass balance model (²³³Ra and ²²⁴Ra, natural groundwater tracers) under two hydrological conditions. In addition, air-water exchange of carbon dioxide and methane in the estuary was estimated. The highest carbon inputs to the estuary were from deep fresh groundwater in the wet season. Most of the dissolved carbon delivered by groundwater and exported from the estuary to the coastal ocean was in the form of dissolved inorganic carbon (DIC; 687 mmol m⁻² estuary day⁻¹; 20 mmol m⁻² catchment day⁻¹, respectively), with a large export of alkalinity (23 mmol m⁻² catchment day⁻¹). Average water to air flux of CO₂ (869 mmol m⁻² day⁻¹) and CH₄ (26 mmol m⁻² day⁻¹) were 5- and 43-fold higher, respectively, than the average global evasion in estuaries due to the large input of CO₂- and CH₄-enriched groundwater. The groundwater discharge contribution to carbon exports from the estuary for DIC, dissolved organic carbon (DOC), alkalinity, CO₂, and CH₄ was 22, 41, 3, 75, and 100 %, respectively. The results show that CO₂ and CH₄ evasion rates from small subtropical estuaries surrounded by wetlands can be extremely high and that groundwater discharge had a major role in carbon export and evasion from the estuary and therefore should be accounted for in coastal carbon budgets.
The growing importance of resolving ecosystem carbon budgets has resulted in more studies integrating terrestrial and aquatic carbon fluxes. Although recent estimates highlight the importance of ...inland waters in global carbon budgets, the extent to which aquatic pathways contribute to the net ecosystem carbon budget (NECB) of different ecosystems remains poorly understood. Here, we provide a cross-ecosystem review of annual carbon budgets integrating terrestrial and aquatic fluxes. Large variability in the proportion of aquatic carbon offset to terrestrial net ecosystem productivity (NEP) was observed, with aquatic offsets ranging from < 1% in a boreal forest to 590% in a freshwater marsh. The total aquatic carbon flux was positively correlated with terrestrial NEP, suggesting highly productive ecosystems will have greater aquatic carbon offsets. However, due to an order of magnitude difference in the range of terrestrial NEP (~ 1000 g C m⁻² y⁻¹) compared to aquatic fluxes (~ 100 g C m⁻² y⁻¹), ecosystems with small NEP’s had greater relative aquatic carbon offsets overall in their NECB’s. Northern hemisphere peatlands and forests represented 54% of all integrated carbon budget studies collected, indicating a severe ecosystem and spatial bias. Mangroves, agricultural, and disturbed ecosystems were the most underrepresented, yet had extreme ranges in terrestrial NEP and NECB (– 638 to 1170 g C m⁻² y⁻¹). To improve our mechanistic understanding of the role of aquatic pathways in NECB’s, more site-specific integrative studies need to be undertaken across a broader range of climatic regions and ecosystem types.
•The mean pCO2 was 1474 ± 1614 µatm in the Yue River draining to the Yangtze.•Anthropogenic land use had major seasonal influence on riverine pCO2.•Annual CO2 emissions were 1.6-2.8 times greater ...than river dissolved carbon discharge.•Urban land use and nutrient concentrations were used to predict riverine pCO2.
Carbon dioxide (CO2) emissions from inland waters to the atmosphere are a pivotal component of the global carbon budget. Anthropogenic land use can influence riverine CO2 emissions, but empirical data exploring cause-effect relationships remain limited. Here, we investigated CO2 partial pressures (pCO2) and degassing in a monsoonal river (Yue River) within the Han River draining to the Yangtze in China. Almost 90% of river samples were supersaturated in CO2 with a mean ± standard deviation of 1474 ± 1614 µatm, leading to emissions of 557 - 971 mmol/m2/day from river water to the atmosphere. Annual CO2 emissions were 1.6 - 2.8 times greater than the longitudinal exports of riverine dissolved inorganic and organic carbon. pCO2 was positively correlated to anthropogenic land use (urban and farmland), and negatively correlated to forest cover. pCO2 also had significant and positive relationships with total dissolved nitrogen and total dissolved phosphorus. Stepwise multiple regression models were developed to predict pCO2. Farmland and urban land released nutrients and organic matter to the river system, driving riverine pCO2 enrichment due to enhanced respiration in these heterotrophic rivers. Overall, we show the crucial role of land use driving riverine pCO2, which should be considered in future large-scale estimates of CO2 emissions from streams. Land use change can thus modify the carbon balance of urban-river systems by enhancing river emissions, and reforestation helps carbon neutral in rivers.
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