Estuaries and Coasts最新文献

In salt marshes of the Southeastern USA, purple marsh crabs (Sesarma reticulatum), hereafter Sesarma, aggregate in grazing and burrowing fronts at the heads of tidal creeks, accelerating creek incision into marsh platforms. We explored the effects of this keystone grazer and sediment engineer on salt marsh sediment accumulation, hydrology, and carbon (C) and nitrogen (N) turnover using radionuclides (²¹⁰Pb and ⁷Be), total hydrolyzable amino acids (THAA), and C and N stable isotopes (δ¹³C and δ¹⁵N) in sediment from paired Sesarma-grazed and un-grazed creeks. Sesarma-grazed-creek sediments exhibited greater bioturbation and tidal inundation compared to sediments in un-grazed creeks, as indicated by larger ²¹⁰Pb and ⁷Be inventories. Total organic carbon (TOC) to total nitrogen (TN) weight ratios (C:N) were higher and δ¹⁵N values were lower in grazed-creek sediments than in un-grazed-creek sediments, suggesting Sesarma remove and assimilate N in their tissues, and excrete N with lower δ¹⁵N values into sediments. In support of this inference, the percent total carbon (TC) and percent TOC declined by nearly half, percent TN decreased by ~ 80%, and the C:N ratio exhibited a ~ threefold increase between Sesarma fore-gut and hind-gut contents. An estimated 91% of Sesarma’s diet was derived from Spartina alterniflora, the region’s dominant salt marsh plant. We found that, as Sesarma grazing fronts progress across marsh landscapes, they enhance the decay of Spartina-derived organic matter and prolong marsh tidal inundation. These findings highlight the need to better account for the effects of keystone grazers and sediment engineers, like Sesarma, in estimates of the stability and size of blue C stores in coastal wetlands.

Estuarine wetlands are characterized by high biodiversity and active fluctuations in redox-sensitive metals (RSMs). In this study, sediment samples were collected from two sites, one with and one without vegetation, in the Yellow River Estuary Wetland (YREW). Active forms of Fe, Mn, and U were extracted using Tessier’s sequential extraction method, the bacterial community was analyzed through high-throughput sequencing, and the impact of the community on the RSMs was evaluated. The results indicated that the high nutrient content generated by vegetation withering had a positive effect on bacterial biodiversity, which led to high biomass and a wide variety of species in the sediments. Redox conditions and nutrient levels were the main factors influencing bacterial community structure. Under reducing conditions, genera such as Desulfococcus and Desulfosarcina were the main bacteria mediating the reduction of active Fe and Mn. Bacteria in genera such as Desulfatiglans and Desulfotomaculum were the main bacteria mediating the reduction of active U. These bacteria may result in obvious changes in the release of Fe, Mn, and U from salt marshes to nearshore regions. Our results can help to elucidate the interactions of bacteria and RSMs.

Tidal freshwater marshes (TFMs), found in the upper tidal reaches of river estuaries, are characterized by high diversity and productivity. These ecosystems are threatened by climate change, but unlike other coastal wetlands, there is a lack of information about the impact and their response to this threat. To understand the resilience of Jug Bay TFMs to sea level rise (SLR), surface elevation change was measured in low and mid-high marsh areas along primary and secondary channels. Elevation change exhibited significant temporal and spatial variability. A marked seasonality showed higher elevation during the growing season, and episodic storms altered elevation trajectories. Spatially, elevation change was significantly affected by channel category and marsh zone. Low marsh along primary channels lost elevation (−11.57 mm year⁻¹), while the mid-high marsh gained elevation (+2.65 mm year⁻¹). In secondary channels, both low (+11.29 mm year⁻¹) and mid-high marshes (+5.43 mm year⁻¹) gained elevation. A shoreline change analysis for the Patuxent and Western Branch rivers (2007–2018) showed change rates between −0.35 and −0.90 m year⁻¹. A 2019 upland migration study indicated that most TFMs studied are not able to migrate due to steep slopes. Overall, marsh in more protected areas, along secondary channels, are more resilient, while low marsh in primary channels the most vulnerable to SLR. With low upland migration potential, studied marshes have to rely mainly on vertical elevation gain to keep up with SLR. If restoration is considered in this system, it should focus on the vulnerable low marsh zones along primary channels.

Abstract

Tidal marshes provide numerous ecosystem services, but are threatened by recent increases in global sea level rise (SLR). Marsh restoration and creation are important strategies for mitigating marsh loss, restoring ecosystem services, increasing coastal community resilience, and providing much needed habitat for threatened species. Dredged material resulting from navigation channel maintenance can provide a substrate for these restoration projects. Few studies, however, have addressed the sustainability of these marshes. The Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island, where fine-grained, nutrient-rich dredged material from upper Chesapeake Bay is being used to create > 302 ha of tidal marshes, provides a case study. The low supply of inorganic sediment is counteracted by abundant nutrients, stimulating high rates of organic matter production and elevation change. Using > 10 years of data from 39 surface elevation tables, we found that the mean low marsh rate of elevation change (7.7 ± 3.21 mm year⁻¹) was double the mean high marsh rate (3.6 ± 0.47 mm year⁻¹) and exceeded the natural reference marsh (3.0 ± 2.28 mm year⁻¹) and relative SLR (5.7 mm year⁻¹). By stimulating organic matter production, the high nutrient substrate appears to offset the low inorganic sediment inputs in mid-Chesapeake Bay. Spatial variability was correlated with initial elevation, but was also influenced by local factors that may affect sediment redistribution within the marshes.

Invasive species exert disproportionate impacts in wetlands and pose particular challenges for rare species persisting at small spatial scales. In the urbanized San Francisco Estuary (SFE), which contains 90% of California’s remaining coastal wetlands, invasive and rare species often co-occur. One narrow endemic taxon, the federally listed Suisun thistle (Cirsium hydrophilum var. hydrophilum) is restricted to two or three locations where the invasive perennial pepperweed (Lepidium latifolium) has an increasing presence. Perennial pepperweed has invaded salt, brackish, and freshwater wetlands around the SFE, leading to high management concern. In this study, we investigated how perennial pepperweed may contribute to further rarity of the Suisun thistle, by conducting a removal experiment and surveying soil-plant relationships. Removing pepperweed led to a doubling of native species relative cover and an increase in native species richness by an average of one species per plot, positive effects on Suisun thistle cover, number, and reproductive output, and shifts in soil properties. Combined with survey data inside and outside of pepperweed stands, we conclude that pepperweed competes with Suisun thistle via competition for space, nutrients, and light, interferes with the Suisun thistle’s reproductive success, and alters brackish marsh soil physicochemical characteristics to further favor pepperweed. We recommend local control of pepperweed to prevent further loss of Suisun thistle. Further, the wide range of mechanisms by which this invasion may proceed if unchecked should be considered in other settings where rare or uncommon species are at risk from invaders.

This study examines the effects of coastal upwelling on the longitudinal water density gradient within the estuary of Baía de Todos os Santos (BTS), its effect on the gravitational circulation at the estuary entrance, and the reverse effect of gravitational circulation on the coastal upwelling. This investigation was based on a 1-year dataset of observed water temperature, mean velocities, and river discharge, as well as 2 years of numerical simulation of the estuarine flow. The results show that the upwelling regulates the thermohaline field in front of the BTS, decreasing water temperature (up to 3 °C), and increasing density (up to 0.3 kg/m³), and have sufficient intensity to more than double the speed, or even establish, the gravitational circulation. It was frequently observed that the water temperature falls after an increase in the subtidal flow shear, suggesting that the estuarine gravitational circulation acts as a facilitator to the upwelling process. Numerical simulations indicate that the coastal upwelling events are also capable of reestablishing the gravitational circulation at times with weak longitudinal density gradient, a scenario that tends to become more frequent and intense in the near future due to the ongoing climate changes.

Abstract

Eelgrass (Zostera marina) meadows and boat mooring fields co-occur in nearshore, relatively sheltered embayments. Traditional chain moorings create denuded scars in eelgrass meadows due to repeated and chronic scour of the seafloor by the chain, impacting meadow contiguity and quality. This study assessed the recovery of eelgrass into mooring scars following the conversion of traditional chain moorings to floating rode conservation mooring systems (CMS) in three Massachusetts harbors. The magnitude of eelgrass recovery following the conversion of 21 moorings to floating rode CMS was contingent on the location and positively correlated with the size of the scar associated with the mooring. While most scars started to revegetate following mooring conversion, few experienced complete recoveries and had a persistent denuded halo averaging 2 m in radius around the mooring anchors 5 years post-conversion. We observed CMS gear dragging on the bottom and impacting eelgrass when it was oversized for the depth of the site, and when it was not maintained or cleaned of fouling organisms. Overall, we show that floating rode CMS can be an important tool for eelgrass conservation; however, eelgrass recoveries following mooring conversion to floating rode CMS are variable and incomplete, and challenges pertaining to proper installation and long-term maintenance must be addressed to fully realize this potential.

In recent years, the Mississippi River Deltaic Plain (MRDP) has experienced the highest rates of wetland loss in the USA. Although the process of vertical drowning has been heavily studied in coastal wetlands, less is known about the relationship between elevation change and land loss in wetlands that are experiencing lateral erosion and the contribution of erosion to land loss in the MRDP. We quantified relationships of elevation change and land change in ten submerging tidal wetlands and found that, despite significant land loss, elevation trajectories in seven of the land loss study sites were positive. Furthermore, we observed an acceleration in elevation gain preceding the conversion from vegetated marsh to open water.

To identify regional contributions of lateral erosion to land loss, we quantified the relationship of elevation change and land change in 159 tidal marsh sites in the MRDP. Approximately half the sites were persistently losing land, and 82% of these sites were vulnerable to erosion, identifying erosion as a dominant mechanism of coastal wetland loss in this region. Notably, the sites that were vulnerable to erosion were experiencing land loss while also gaining elevation, and sites with the highest land loss exhibited accelerating elevation gain. Together, these data illustrate that (1) erosion is a dominant mechanism of wetland loss in the MRDP, (2) accelerated elevation gain is an indicator of erosion, and (3) consideration of elevation change trajectories within the context of land change is critical for providing accurate coastal wetland vulnerability assessments.

Marsh environments, characterized by their flora and fauna, change laterally in response to shoreline erosion, water levels and inundation, and anthropogenic activities. The Grand Bay coastal system (USA) has undergone multiple large-scale geomorphic and hydrologic changes resulting in altered sediment supply, depositional patterns, and degraded barrier islands, leaving wetland salt marshes vulnerable to increased wave activity. Two shore-perpendicular transect sites, one along a low-activity shoreline and the other in a high activity area of the same bay-marsh complex, were sampled to investigate how the marshes within 50 m of the modern shoreline have responded to different levels of increased wave activity over the past century. Surface sediments graded finer and more organic with increased distance from the shoreline while cores generally exhibited a coarsening upwards grain-size trend; all cores contained multiple large sedimentological shifts. ²¹⁰Pb-based mass accumulation rates over the last two decades were greater than the long-term (centurial) average at each site with the fastest accumulation rates of 7.81 ± 1.58 and 7.79 ± 1.63 kg/m²/year at the sites nearest the shoreline. A shoreline change analysis of three time-slices (1848–2017, 1957–2017, 2016–2017) shows increased erosion at both sites since 1848 with modern rates of −0.95 and −0.88 m/year. Downcore sedimentology, mass accumulation rates, and shoreline change rates paired with foraminiferal biofacies and identification of local estuarine indicator species, Paratrochammina simplissima, aided in identifying paleo marsh types, their relative proximity to the shoreline, and sediment provenance. The high-energy marsh site transitioned from middle marsh to low marsh in the 1960s, and the low-energy marsh site transitioned later, at the end of the twentieth and early twenty-first century, due to its more protected location. Marsh type transition corresponds chronologically with the coarsening upwards grain-size trend observed and the degradation of Grand Batture Island; since its submergence, signatures of multiple storm event have been preserved downcore.