Suisun marsh Moyle, Peter B; Manfree, Amber D; Fiedler, Peggy L
2014., 20140418, 2014, 2014-03-26
eBook
One of California's most remarkable wetlands, Suisun Marsh is the largest tidal marsh on the West Coast and a major feature of the San Francisco Estuary. This productive and unique habitat supports ...endemic species, is a nursery for native fishes, and is a vital link for migratory waterfowl. The 6,000-year-old marsh has been affected by human activity, and humans will continue to have significant impacts on the marsh as the sea level rises and cultural values shift in the century ahead. This study includes in-depth information about the ecological and human history of Suisun Marsh, its abiotic and biotic characteristics, agents of ecological change, and alternative futures facing this ecosystem.
What controls marsh edge erosion? Houttuijn Bloemendaal, Lucila J.; FitzGerald, Duncan M.; Hughes, Zoe J. ...
Geomorphology (Amsterdam, Netherlands),
08/2021, Volume:
386
Journal Article
Peer reviewed
The survival of salt marshes depends on their ability to maintain vertical elevation and areal extent. In the lateral direction, marsh edges can expand laterally or undergo edge erosion through mass ...failure or continuous particle erosion through waves and tidal processes. In this study, we evaluate possible relationships between marsh shoreline type within the Great Marsh in Massachusetts and major geotechnical parameters along the marsh edge. We also explore if wave energy, using fetch as a proxy, affects the presence, type, and distribution of shoreline type. We mapped and classified the marsh into four categories: slumping, vertical and abrading, stable/accretionary, and bedrock or gravel, and sampled the marsh edge at 98 sites. Using over 450 measurements, we present typical ranges of values at these marsh edges for bulk density (0.10–1.43 g/cm3), organic content (0.99–55.07%), belowground biomass (0.11–36.76%), and shear strength (4.04–136.49 kPa at 20 cm into the marsh bank, and 4.04–131.03 kPa at 40 cm into the bank). We show that there are no significant differences in fetch or geotechnical properties for the different marsh edge classes. Thus, none of these parameters explain or correlate with edge erosion, even though the majority of previous edge erosion studies focus on these parameters as determinants of edge erosion. We further emphasize the heterogeneity of the marsh, as edge erosion can occur in highly exposed or sheltered areas alike with no trends in geotechnical properties, and that complex interactions between parameters not generally studied may be responsible for edge erosion.
•This large-scale field study examined the relationships among geotechnical properties, fetch, and edge erosion at over 90 sites.•Neither fetch nor the geotechnical properties of marsh banks explained the type and distribution of marsh edge erosion.•Previously identified parameters do not explain the observed system, suggesting more complex interactions are causing edge erosion.
To survive rising sea level, salt marshes must accrete vertically, migrate laterally, or undergo a combination of the two. If sufficient sediment is available for marsh accretion, the slope of the ...surrounding area is relatively flat, and edge erosion is minimal, then a marsh can theoretically maintain its areal extent through a combination of vertical accretion and upland expansion. However, in cases where sediment supply is limited and the marsh is backed by steeper slopes, it is unclear whether accretion and inland migration will be sufficient to counteract the combined effects of rising sea level and edge erosion. Given these barriers to marsh expansion, inland migration may not be a viable solution to marsh vulnerability to sea-level rise. We quantify the potential changes in areal extent under future sea-level rise scenarios for the Great Marsh in northern Massachusetts, where the marsh has a limited suspended sediment supply and relatively steep upland topography. Salt marsh is identified and classified into low or high marsh using LiDAR elevation and validated using aerial photography and vegetation surveys. We generate a simple 1D-H model using locally-measured accretion rates and their relationship to marsh elevation, to determine change in elevation and dominant plant species over time. A maximum inorganic sediment available to the marsh is prescribed for certain model scenarios to test the impact of sediment limitation. This limit is calculated based on the volumetric contribution of mineral sediment to the present marsh accretion rates. Predicted changes in marsh area over a 100-year model period are determined using the surrounding elevation gradients, calculated sediment availability, projected edge erosion, and local rates of sea-level rise (SLR). The two Representative Concentration Pathway (RCP) SLR scenarios used are based on the most recent IPCC report and also include the latest information concerning responses to ice sheet melting in Greenland and Antarctica. We find that as the rate of sea-level rise increases, the areal extent of the marsh decreases due to a lack of the suspended sediment needed to maintain marsh surface elevation and the inability of the marsh to encroach upon steep upland slopes. By comparing a model assuming constant accretion rates to one with mineral sediment-limited accretion, we find that when sediment if limited, marsh habitat conversion and loss occur earlier and more rapidly.
•Migration on to uplands is not a viable solution for marshes surrounded by steep uplands.•Systems dominated by high marsh undergoing sea level rise will rapidly convert to low marsh, delaying marsh loss but changing ecosystem services.•Coastal systems with a limited suspended sediment supply will experience earlier and more rapid habitat conversion and marsh loss.
During sea level rise, salt marshes transgress inland invading low-lying forests, agricultural fields, and suburban areas. This transgression is a complex process regulated by infrequent storms that ...flood upland ecosystems increasing soil salinity. As a result upland vegetation is replaced by halophyte marsh plants. Here we present a review of the main processes and feedbacks regulating the transition from upland ecosystems to salt marshes. The goal is to provide a process-based framework that enables the development of quantitative models for the dynamics of the marsh-upland boundary. Particular emphasis is given to the concept of ecological ratchet, combining the press disturbance of sea level rise with the pulse disturbance of storms.
•Temperate macrotidal marsh habitats are net carbon sinks.•CO2 flux measurements show the marsh netting 213gCm−2yr−1.•Core sediment samples show an average carbon accumulation rate of ...192.2gm−2yr−1.•There are significant differences in CO2 uptake between marsh plant associations.•Weather parameters all together accounted for 66% of the variability in NEE.
The study compares the amounts of carbon fixed via photosynthesis of a restored tidal marsh to the total organic carbon remaining in sediments of a natural tidal marsh and arrives at preliminary baselines for carbon sequestration and storage over time. The Eddy-covariance method (indirect method) was used to estimate marsh canopy net ecosystem exchange (NEE) and measured an annual gross primary production of 979gCm−2, while the loss through respiration was 766gCm−2, resulting in a net uptake of 213gCm−2yr−1. Time of the day, solar irradiation, air temperature, humidity and wind direction all together explained 66% of the variation in NEE. The high marsh community of Spartina patens showed NEE to be significantly higher than the low marsh community. The net ecosystem carbon balance (NECB) over long time scales was estimated by measuring the actual amount of total organic carbon contained in dated sediment cores from a natural marsh (direct method), which resulted in a carbon accumulation rate of 192.2gm−2yr−1. Changes in total organic carbon content over time in the core sample showed that 78% of organic carbon remained stored in the sediments after 130 years and only the most recalcitrant carbon (50%) remained under storage beyond 645 years. Overall the study showed that temperate macrotidal salt marshes are net sinks of carbon with potential for long term carbon storage. The marsh turned into a carbon sink at the beginning of May and switched back to being a source in late November. The average sedimentation rate estimated from the 137 CS dating (1950s to present) was 1.4mmyr−1 which is similar to accretion rates of comparable S. patens patches in the east coast. Accretion rates derived from our study are slightly lower than the 60+ year rate of sea level rise (2.6mmyr−1) recorded by tide gauge measurements in the Northeast.
Ponds are common features on salt marshes, yet it is unclear how they affect large-scale marsh evolution. We developed a spatially explicit model that combines cellular automata for pond formation, ...expansion, and drainage, and partial differential equations for elevation dynamics. We use the mesotidal Barnstable marsh (MA, USA) as a case study, for which we measured pond expansion rate by remote sensing analysis over a 41-year time span. We estimated pond formation rate by comparing observed and modeled pond size distribution, and predicted pond deepening by comparing modeled and measured pond depth. The Barnstable marsh is currently in the pond recovery regime, i.e.,every pond revegetates and recovers the necessary elevation to support plant growth after re-connecting to the channel network. This pond dynamic creates an equivalent (i.e.,spatially and temporally averaged over the whole marsh) 0.5–2 mm/yr elevation loss that needs to be supplemented by excess vertical accretion. We explore how the pond regime would change with decreased sediment supply and increased relative sea-level rise (RSLR) rate, focusing on the case in which the vegetated marsh keeps pace with RSLR. When the RSLR rate remains below the minimum unvegetated deposition rate, the pond dynamics is nearly unaltered and ponds always occupy ~10% of the marsh area. However, when RSLR rate exceeds this threshold, the ponds in the marsh interior – which receive the least amount of suspended sediment – do not recover after drainage. These ponds transition to mudflats and permanently occupy up to 30% of the marsh area depending on RSLR rate. For marshes with a small tidal range, such as the microtidal Sage Lot Pond marsh on the opposite side of the peninsula from Barnstable marsh, high RSLR rates could bring every portion of the marsh into the pond runaway regime, with the whole marsh eventually converting into mudflats. In this regime, the existing marsh would disappear within centuries to millennia depending on the RSLR rate. Because of the spatial and temporal components of marsh evolution, a single RSLR threshold value applied across the entire marsh landscape provides a limited description of the marsh vulnerability to RSLR.
Complexities of terrestrial boundaries with salt marshes in coastal lagoons affect salt marsh exposure to waves and sediments creating different potentials for marsh migration inland and seaward-edge ...erosion, and consequently, for marsh persistence. Between 2002 and 2017, migration and edge erosion were measured in three mainland geomorphic marsh types (headland, valley, hammock) and were used to assess the rate and spatial extent of marsh change for a Virginia coastal lagoon system. Treelines, shorelines, and marsh perimeters were delineated in ArcGIS at 1:600 resolution. All marsh types increased in spatial extent; increases were greatest for the valley type (0.58 ha ± 0.31 ha or + 0.32% per annum). Measured rates of migration (headland > valley > hammock) and erosion (headland > hammock > valley) for each geomorphic type were averaged and applied to obtain changes in these same marsh types at the regional scale. At this scale, valley marsh area increased (82.5 ha or 5.5 ha a
−1
) more than the other two marsh types combined. This analysis demonstrates the critical influence that geomorphic type has on lateral marsh responses to sea-level rise and that efforts to conserve or restore salt marshes are most likely to be successful when focused on valley marshes.
Tidal marshes are one of the world’s most economically valuable habitats; yet, they have experienced large and persistent declines globally. Increased knowledge of the ecosystem services delivered by ...marshes has become a powerful tool to conserve and restore them. But hesitancies regarding valuations and their application in decision-making remain. Here we draw on the literature and collective experience of participants in the “Concepts and controversies in tidal Marsh ecology revisited” workshop, November 2 and 3, 2019, Mobile, AL, to provide a concise snapshot of the current field of salt marsh ecosystem service valuation, discuss the possible risks in salt marsh valuation, and the importance of stakeholder engagement to mitigate them. We provide examples of the application of valuation in conservation-related decision-making, illustrating the growing operationalization of ecosystem services in incentivizing salt marsh conservation and restoration. Ecosystem service quantification and valuation is already playing an important role in decision-making by coastal risk managers, insurers, engineers, and policy makers. While there are legitimate criticisms of valuation techniques and remaining uncertainties in ecosystem service delivery that arise both through natural variability across space and time and through differing and shifting cultural values, our perspective is that the rise of big data, the development of valuation techniques, a growing understanding and application of environmental justice practices, and increasing interdisciplinarity to tackle these complex issues are paving the way for valuation to play a critical role in decision-making around salt marshes.
This paper quantifies the potential influence of sediment compaction on the magnitude of nineteenth and twentieth century sea-level rise, as reconstructed from salt marsh sediments. We firstly ...develop a database of the physical and compression properties of low energy intertidal and salt marsh sediments. Key compression parameters are controlled by organic content (loss on ignition), though compressibility is modulated by local-scale processes, notably the potential for desiccation of sediments. Using this database and standard geotechnical theory, we use a numerical modelling approach to generate and subsequently ‘decompact’ a range of idealised intertidal stratigraphies. We find that compression can significantly contribute to reconstructed accelerations in recent sea level, notably in transgressive stratigraphies. The magnitude of this effect can be sufficient to add between 0.1 and 0.4mmyr−1 of local sea-level rise, depending on the thickness of the stratigraphic column. In contrast, records from shallow (<0.5m) uniform-lithology stratigraphies, or shallow near-surface salt marsh deposits in regressive successions, experience negligible compaction. Spatial variations in compression could be interpreted as ‘sea-level fingerprints’ that might, in turn, be wrongly attributed to oceanic or cryospheric processes. However, consideration of existing sea-level records suggests that this is not the case and that compaction cannot be invoked as the sole cause of recent accelerations in sea level inferred from salt marsh sediments.
► Organic content and past desiccation control compressibility in salt marsh sediments. ► Compression cannot cause sharp, high-magnitude inflections in reconstructed sea level. ► Modelled compression contributions to reconstructed sea level are in the range of 0.0–0.4mmyr−1. ► Modelled transgressive successions are most prone to compression. ► Modelled uniform and regressive stratigraphies experience negligible compression.
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