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A key value of saltmarshes is their role in providing trophic subsidies, notably in the form of prey pulses of crab zoeae (CZ). No studies to date, however, have investigated quantitatively the patterns between crab zoeae pulses and saltmarsh tidal connectivity in tropical estuaries. In this study, CZ densities were examined over successive tides and months using a zooplankton sampling pump to examine links between tidal fluctuations and pulses of CZ in a tropical Australian estuary with a complex of tidal saltmarshes, mangroves, and unvegetated flats. CZ densities were linked to spatially explicit information on tidal wetland inundation that was derived from Unattended-Aerial-Vehicle (UAV) Structure from motion (SfM) photogrammetry and two-dimensional hydrodynamic modelling. The study found that: (1) tidal connectivity is a key trigger to prey pulse export; (2) while tidal connectivity was critical in the export of CZ, not all tidal connections resulted in meaningful ecological connectivity; and (3) succulent saltmarsh was one of the tidal wetland types contributing to the export of CZ. Surprisingly, we reveal that not all succulent saltmarshes were uniformly participating in CZ export. These findings highlight the significance of quantitative eco-hydrological approaches to assess saltmarsh and tidal connectivity values. This study supports the need for management and restoration approaches to integrate a contextual understanding of the synergies between hydrology, ecology, and habitat heterogeneity. These data emphasise the need to move beyond generalisations that "similar" habitat type share similar ecological functions and should be managed equally. • Tidal connectivity is a key trigger to the export of pulses of intertidal crab zoeae. • Tropical succulent saltmarsh inundation is contributing to crab zoea pulses. • Not all connections and saltmarsh patches were important in supporting crab zoea export. • The importance of contextual-based understanding of saltmarsh function is highlighted. [ABSTRACT FROM AUTHOR]

In this study, we compared the CH 4 and N 2 O fluxes from a tidal brackish Cyperus malaccensis marsh ecosystem and nearby shrimp ponds, converted from C. malaccensis marsh in the last 3–4 years, in the Min River estuary of southeast China over the aquaculture period of the year. Significant differences in CH 4 and N 2 O fluxes were observed in space (between brackish marsh and shrimp ponds) and in time (between sampling occasions that were distributed over the aquaculture period). CH 4 fluxes from the shrimp ponds were on an average 10-fold higher than from the brackish marsh. N 2 O emissions, on the other hand, were lower from the shrimp pond (25% of the emissions from the brackish marsh). Accessory data indicates that these patterns were primarily linked to water level variability and temperature (all fluxes), sediment porewater sulfate concentrations (CH 4 flux) and total nitrogen concentrations (N 2 O flux). Our research demonstrates that the coastal marsh ecosystem converted to aquaculture ponds considerably alter emissions of CH 4 and N 2 O and provides input to the global discussion on how to account for emissions from various types of flooded land in greenhouse gas inventories. [ABSTRACT FROM AUTHOR]

A field study was conducted in man-made ditches in a tidal saltmarsh in Lewes, Delaware, USA. Ditches are prevalent throughout tidal marshes along the Atlantic US coast, and influence hydrodynamics and sediment transport. The field study focused on measuring near-bed velocity, shear stress, sediment concentration, and bed level variability at 5 stations over a 3-week period. Velocities in the ditch (2–5 m wide, 1 m deep) peaked between 0.4 and 0.6 m/s and were slightly ebb dominated. Velocity and shear stress were maximum during a storm event, with peak shear stresses of 2 N/m 2 . Bed levels were estimated from acoustic amplitude return of a downward-looking velocity profiler. The bed level in the ditch at the landward locations increased ∼ 0.03 m over 3 weeks, while there was ∼ 0.01 m bed level decrease at the most seaward site suggesting a net import of sediment into the channel. At all sites, erosion ( ∼ 0.005–0.015 m) occurred during the accelerating phase of the flood tide, and accretion of a similar magnitude occurred during the decelerating phase of the ebb tide. This erosion-deposition sequence resulted in small net changes in bed level at the end of each tidal cycle. The intratidal behavior of the bed level was simulated using erosion and deposition flux equations based on shear stress, critical shear stress, and suspended sediment concentration. Erosion was predicted well with RMS errors on the order of 2 ⋅ 10 −3 m. The bed level during the deposition phase could not be reproduced using the simple approach. Model inaccuracies for deposition were attributed to advection and variations in fall velocity due to flocculation that were not modeled due to lack of ground-truth observations. [ABSTRACT FROM AUTHOR]

Modeling studies of estuary circulation show great sensitivity to the water exchange into and out of adjacent marshes, yet there is significant uncertainty in resolving the processes governing marsh surface flow. The objective of this study is to measure the estuary channel-to-saltmarsh pressure gradient and to guide parameterization for how it affects the surface flow in the high marsh. Current meters and high-resolution pressure transducers were deployed along a transect perpendicular to the nearby Little Ogeechee River in a saltmarsh adjacent to Rose Dhu Island near Savannah, Georgia, USA. The vertical elevations of the transducers were surveyed with static GPS to yield high accuracy water surface elevation data. It is found that water level differences between the Little Ogeechee River and neighboring saltmarsh are up to 15 cm and pressure gradients are up to 0.0017 m of water surface elevation change per m of linear distance during rising and falling tides. The resulting Little-Ogeechee-River-to-saltmarsh pressure gradient substantially affects tidal velocities at all current meter locations. At the velocity measurement station located closest to the Little Ogeechee River bank, the tidal velocity is nearly perpendicular to the bank. At this location, surface flow is effectively modeled as a balance between the pressure gradient force and the drag force due to marsh vegetation and bottom stress using the Darcy–Weisbach/Lindner's equations developed for flow-through-vegetation analysis in open channel flow. The study thus provides a direct connection between the pressure gradient and surface flow velocity in the high marsh, thereby overcoming a long-standing barrier in directly relating flow-through-saltmarsh studies to flow-through-vegetation studies in the open channel flow literature. [ABSTRACT FROM AUTHOR]

Accelerated sea-level rise (SLR) poses the threat of salt marsh submergence, especially in marshes that are relatively low-lying. At the same time, restoration efforts are producing new low-lying marshes, many of which are thriving and avoiding submergence. To understand the causes of these different fates, we studied two Long Island Sound marshes: one that is experiencing submergence and mudflat expansion, and one that is undergoing successful restoration. We examined sedimentation using a variety of methods, each of which captures different time periods and different aspects of marsh elevation change: surface-elevation tables, marker horizons, sediment cores, and sediment traps. We also studied marsh hydrology, productivity, respiration, nutrient content, and suspended sediment. We found that, despite the expansion of mudflat in the submerging marsh, the areas that remain vegetated have been gaining elevation at roughly the rate of SLR over the last 10 years. However, this elevation gain was only possible thanks to an increase in belowground volume, which may be a temporary response to waterlogging. In addition, accretion rates in the first half of the twentieth century were much lower than current rates, so century-scale accretion in the submerging marsh was lower than SLR. In contrast, at the restored marsh, accretion rates are now averaging about 10 mm yr −1 (several times the rate of SLR), much higher than before restoration. The main cause of the different trajectories at the two marshes appeared to be the availability of suspended sediment, which was much higher in the restored marsh. We considered and rejected alternative hypotheses, including differences in tidal flooding, plant productivity, and nutrient loading. In the submerging marsh, suspended and deposited sediment had relatively high organic content, which may be a useful indicator of sediment starvation. [ABSTRACT FROM AUTHOR]

Coastal wetlands are vulnerable to sea-level rise (SLR) but are also valued for their potential to provide effective nature-based solutions to climate change mitigation and adaptation. Ecological benefits from these ecosystems can be constrained under urban settings by anthropogenic disturbances and pressures, so restoration activities are promoted as a management approach. Here we report on the potential for restoration of disused commercial salt extraction pans to enhance carbon (C) sequestration in the urban Swartkops Estuary, South Africa. We also considered the impact of SLR to 2100 on the distribution of estuarine habitats, the vulnerability of built infrastructure to tidal flooding, and how C sequestration is projected to change over time using the Sea-Level Affecting Marshes Model (SLAMM). Potential restoration of all salt pans (320 ha) to estuarine habitat was estimated to result in a gain of 67 850 Mg C. Establishing tidal connectivity was investigated as a potential restoration action, but most of the salt pan area was above the elevation of the current tidal range and would require excavation. Although conversion of the salt pans to estuarine habitat was predicted to occur without intervention under SLR, 44% of the original area would remain unchanged. Restoring hydrological connectivity to the estuary for these salt pans would significantly increase the extent of transitional/floodplain marsh, even under SLR to 2100. C sequestration was predicted to be 15% higher (54 614.8 Mg C) by 2100 if the salt pans could be restored, compared to if no action is taken. Overall, restoration of the salt pans has the potential to enhance C sequestration, but SLR will still cause large losses of supratidal marsh due to 'coastal squeeze' and extensive tidal flooding of developed areas by 2100 in the lower reaches of the estuary. A full-scale restoration approach for the Swartkops Estuary could use C sequestration potential to fund the project through carbon offsetting if the revenue exceeds the cost of the restoration activities, but additional social and ecological goals also need to be incorporated if the outcome is to be holistic and beneficial. [Display omitted] • Restoration of disused commercial salt extraction pans can enhance carbon storage. • With sea-level rise, up to 66% of desiccated areas will revert to natural habitat. • With restoration, an additional 54 614.8 Mg C can be gained by 2100. • Sea-Level Affecting Marshes Model used to assess sea-level rise vulnerability. • Residential areas in lower reaches will experience tidal flooding by 2100. [ABSTRACT FROM AUTHOR]