Short-term impact of sediment addition on plants and invertebrates in a southern California salt marsh

Abstract

The implementation and monitoring of management strategies is integral to protect coastal marshes from increased inundation and submergence under sea-level rise. Sediment addition is one such strategy in which sediment is added to marshes to raise relative elevations, decrease tidal inundation, and enhance ecosystem processes. This study looked at the plant and invertebrate community responses over 12 months following a sediment addition project on a salt marsh located in an urbanized estuary in southern California, USA. This salt marsh is experiencing local subsidence, is sediment-limited from landscape modifications, has resident protected species, and is at-risk of submergence from sea-level rise. Abiotic measurements, invertebrate cores, and plant parameters were analyzed before and after sediment application in a before-after-control-impact (BACI) design. Immediately following the sediment application, plant cover and invertebrate abundance decreased significantly, with smothering of existing vegetation communities without regrowth, presumably creating resulting harsh abiotic conditions. At six months after the sediment application treatment, Salicornia bigelovii minimally colonized the sediment application area, and Spartina foliosa spread vegetatively from the edges of the marsh; however, at 12 months following sediment application overall plant recovery was still minimal. Community composition of infaunal invertebrates shifted from a dominance of marsh-associated groups like oligochaetes and polychaetes to more terrestrial and more mobile dispersers like insect larvae. In contrast to other studies, such as those with high organic deposition, that showed vegetation and invertebrate community recovery within one year of sediment application, our results indicated a much slower recovery following a sediment addition of 32 cm which resulted in a supratidal elevation with an average of 1.62 m (NAVD88) at our sampling locations. Our results indicate that the site did not recover after one year and that recovery may take longer which illustrates the importance of long-term monitoring to fully understand restoration trajectories and inform adaptive management. Testing and monitoring sea-level rise adaptation strategies like sediment addition for salt marshes is important to prevent the loss of important coastal ecosystems.

Testing strategies to enhance transplant success under stressful conditions at a tidal marsh restoration project

“…However, the composition is extremely heterogeneous, as hillside soil was placed by the truckload (Thomsen et al 2022). Sandier soils are often correlated with lower soil moisture, higher permeability, and low organic content (Usowicz & Lipiec 2017) and have been linked to the slow establishment of functioning at other tidal marsh restoration projects (McAtee et al 2020). Thus, it is plausible that the dredged sediment resulted in a more stressful environment.…”

Increasing Salt Marsh Elevation Using Sediment Augmentation: Critical Insights from Surface Sediments and Sediment Cores

“…The monitoring timeline spanned 6 months prior to the sediment augmentation addition to 5 years post-augmentation. The goal was to uniformly place 10,322 cubic meters of dredged material, thinly spread over 4.05 ha, to achieve a 25 cm (10") of sediment thickness and to maintain a minimum of 7.6 cm (3") increase on the marsh platform 2 years after sediment addition (McAtee et al 2020). The sediment material was sourced from an adjacent subtidal area near the refuge, within Anaheim Bay.…”

“…Engineering interventions such as hay bales, straw waddles, sandbags, and geotextile fabrics were placed along the perimeter of the augmentation site to retain the sediment material (Thorne et al 2019). Dredging challenges arose when obtaining the sediment augmentation material, which appeared to have resulted in sandier grain sizes and lower organic matter compared to the original topsoil at the augmentation site (McAtee et al 2020).…”

Climate drivers and human impacts shape 35‐year trends of coastal wetland health and composition in an urban region

“…Coastal management and restoration programs have arisen in recent decades to counteract the negative impacts humans have historically had on wetlands (e.g., Callaway et al, 2011; Kennish, 1999; Stein et al, 2020; Zedler & Callaway, 1999); however, these efforts can also have direct, and sometimes unintended, consequences to plant communities, habitat distributions, and conditions within wetland and estuarine systems (Kennish, 2001). Sediment augmentation, for example, can bolster elevations in sinking marshes but has shown mixed effects to plant recovery and communities in wetlands of Louisiana, North Carolina, New York, and California, USA (McAtee et al, 2020). We can begin to uncover the comingling effects of human impacts on wetlands, in addition to environmental drivers, by tracking wetland health and restoration trajectories.…”