Explore the vast range of titles available.

Artificial canals may function differently than the natural coastal wetlands, floodplains, and estuaries they often replace. Here, we assess the impact of canal estate development on saline groundwater exchange (tidal pumping) and associated nutrient fluxes. Time series observations of short-lived radium isotopes and dissolved nutrients were performed in a canal estate and a nearby mangrove creek in subtropical Australia. A mass balance model based on ²²³Ra (1.3 ± 0.4 and 3.4 ± 0.9 cm day⁻¹ in the mangrove and canal, respectively) and ²²⁴Ra (2.8 ± 3.0 and 5.4 ± 4.6 cm day⁻¹) revealed tidally driven groundwater exchange rates were ~ 2-fold greater in the canal. Lateral fluxes of total dissolved nitrogen (TDN) from the nearby estuary into the canal estate were comparable with the mangrove creek (8.4 and 9.1 mmol m⁻² day⁻¹ in the mangrove and canal, respectively). Groundwater flows into the canal released ~ 5-fold more TDN than the mangrove. As expected, mangroves appear to be more efficient at retaining groundwater-derived nitrogen than vegetation-stripped, sandy canals. Overall, this study demonstrates that land reclamation for canal estate development not only drives losses of ecosystem services, but also modifies groundwater and related nutrient exchange with coastal surface waters.

Abundant crab burrows in carbon-rich, muddy salt marsh soils act as preferential water flow conduits, potentially enhancing carbon transport across the soil–water interface. With increasing recognition of blue carbon systems (salt marshes, mangroves, and seagrass) as hotspots of soil carbon sequestration, it is important to understand drivers of soil carbon cycling and fluxes. We conducted field observations and flow modeling to assess how crab burrows drive carbon exchange over time scales of minutes to weeks in an intertidal marsh in South Carolina. Results showed that continuous advective porewater exchange between the crab burrows and the surrounding soil matrix occurs because of tidally driven hydraulic gradients. The concentrations of dissolved inorganic (DIC) and organic (DOC) carbon in crab burrow porewater differ with that in the surrounding soil matrix, implying a diffusive C flux in the low-permeability marsh soil. Gas-phase concentrations of CO₂ in ∼ 300 crab burrows were approximately six times greater than ambient air. The estimated total C export rate via porewater exchange (1.0 ± 0.7 g C m−2 d−1) was much greater than via passive diffusion transport (6.7 ± 2 mg C m−2 d−1) and gas-phase CO₂ release (0.93 mg C m−2 d−1). The burrow-related carbon export was comparable to the regional salt marsh DIC export, groundwater-derived DIC export, and the net primary production previously estimated using ecosystem-scale approaches. These insights reveal how crab burrows modify blue carbon sequestration in salt marshes and contribute to coastal carbon budgets.

Salt marshes can sequester large amounts of carbon in sediments, but the relation between carbon storage and exportation remains poorly understood. Groundwater exchange can flush sediment carbon to surface waters and potentially reduce storage. In this study, we estimated groundwater fluxes and associated carbon fluxes using a radon (222Rn) mass balance and sediment carbon burial rates using lead (210Pb) in a pristine salt marsh (North Inlet, SC, USA). We used δ13C to trace carbon origins. We found that groundwater releases large amounts of carbon to the open ocean. These groundwater fluxes have the potential to export 7.2 ± 5.5 g m−2 of dissolved inorganic carbon (DIC), 0.2 ± 0.2 g m−2 of dissolved organic carbon (DOC) and 0.7 ± 0.5 g m−2 of carbon dioxide (CO2) per day. The fluxes exceed the average surface water CO2 emissions (0.6 ± 0.2 g m−2 day−1) and the average sediment carbon burial rates (0.17 ± 0.09 g m−2 day−1). The δ13C results suggest that groundwater carbon originated from salt marsh soils, while the sediment carbon source is derived from salt marsh vegetation. We propose that the impact of salt marshes in carbon cycling depends not only on their capacity to bury carbon in sediments, but also on their high potential to export carbon to the ocean via groundwater pathways.

Greenhouse gas (GHG) emissions from freshwater streams are poorly quantified in sub-tropical climates, especially in the southern hemisphere where land use is rapidly changing. Here, we examined the distribution, potential drivers, and emissions of carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) from eleven Australian freshwater streams with varying catchment land uses yet similar hydrology, geomorphology, and climate. These sub-tropical streams were a source of CO2 (74 ± 39 mmol m−2 day−1), CH4 (0.04 ± 0.06 mmol m−2 day−1), and N2O (4.01 ± 5.98 µmol m−2 day−1) to the atmosphere. CO2 accounted for ~ 97% of all CO2-equivalent emissions with CH4 (~ 1.5%) and N2O (~ 1.5%) playing a minor role. Episodic rainfall events drove changes in stream GHG due to the release of soil NOx (nitrate + nitrite) and dissolved organic carbon (DOC). Groundwater discharge as traced by radon (222Rn, a natural groundwater tracer) was not an apparent source of CO2 and CH4, but was a source of N2O in both agricultural and forest catchments. Land use played a subtle role on greenhouse gas dynamics. CO2 and CH4 increased with catchment forest cover during the wet period, while N2O and CH4 increased with agricultural catchment area during the dry period. Overall, this study showed how DOC and NOx, land use, and rainfall events interact to drive spatial and temporal dynamics of GHG emissions in sub-tropical streams using multiple linear regression modelling. Increasing intensive agricultural land use will likely decrease regional CO2 and CH4 emissions, but increase N2O.

Coastal waterways can be significant sources of the potent greenhouse gas nitrous oxide (N₂O) due to nitrogen inputs and eutrophication. Here, we quantify groundwater derived N₂O inputs and atmospheric emissions within a modified urban embayment (Sydney Harbour, Australia). Overall, we found low N₂O saturation (91–171%) and air–water fluxes (−2.2 to 24.6 μmol m−2 d−1). Concentrations were highest in upstream brackish areas and a commercial/industrial subembayment. Dissolved inorganic nitrogen concentrations were low and inversely correlated to N₂O throughout the harbor. N₂O surface water dynamics were apparently driven by saline submarine groundwater discharge, as quantified by the radioisotope tracer radon-222. Groundwater discharge was highest within the embayments and mangrove-lined upper estuary. While groundwater was a net N₂O source to surface waters, two upstream sub-embayments featured groundwater N₂O concentrations lower than surface water, suggesting a sink driven by surface waters recirculating in intertidal sediments. Surface-water N₂O was undersaturated within one upstream embayment, likely due to N₂O consumption within sediments. Contrastingly, the downstream embayments featured higher groundwater N₂O and accounted for 45% ± 21% of the groundwater N₂O flux. Sydney Harbour was a net source of N₂O to the atmosphere (mean 0.6 ± 0.3 μmol m−2 d−1) with larger N₂O fluxes occurring from relatively small areas. N₂O emissions (expressed in CO₂ equilivents) were equivalent to 17% of CO₂ emission estimates from previous studies. The low N₂O emissions in Sydney Harbour contrast with other modified estuaries which often emit higher N₂O fluxes due to larger nitrogen inputs.

Abundant crab burrows in carbon‐rich, muddy salt marsh soils act as preferential water flow conduits, potentially enhancing carbon transport across the soil–water interface. With increasing recognition of blue carbon systems (salt marshes, mangroves, and seagrass) as hotspots of soil carbon sequestration, it is important to understand drivers of soil carbon cycling and fluxes. We conducted field observations and flow modeling to assess how crab burrows drive carbon exchange over time scales of minutes to weeks in an intertidal marsh in South Carolina. Results showed that continuous advective porewater exchange between the crab burrows and the surrounding soil matrix occurs because of tidally driven hydraulic gradients. The concentrations of dissolved inorganic (DIC) and organic (DOC) carbon in crab burrow porewater differ with that in the surrounding soil matrix, implying a diffusive C flux in the low‐permeability marsh soil. Gas‐phase concentrations of CO 2 in ∼ 300 crab burrows were approximately six times greater than ambient air. The estimated total C export rate via porewater exchange (1.0 ± 0.7 g C m −2 d −1 ) was much greater than via passive diffusion transport (6.7 ± 2 mg C m −2 d −1 ) and gas‐phase CO 2 release (0.93 mg C m −2 d −1 ). The burrow‐related carbon export was comparable to the regional salt marsh DIC export, groundwater‐derived DIC export, and the net primary production previously estimated using ecosystem‐scale approaches. These insights reveal how crab burrows modify blue carbon sequestration in salt marshes and contribute to coastal carbon budgets.