skip to main content


Title: Are Elevation and Open‐Water Conversion of Salt Marshes Connected?
Abstract

Salt marsh assessments focus on vertical metrics such as accretion or lateral metrics such as open‐water conversion, without exploration of how the dimensions are related. We exploited a novel geospatial data set to explore how elevation is related to the unvegetated‐vegetated marsh ratio (UVVR), a lateral metric, across individual marsh “units” within four estuarine‐marsh systems. We find that elevation scales consistently with the UVVR across systems, with lower elevation units demonstrating more open‐water conversion and higher UVVRs. A normalized elevation‐UVVR relationship converges across systems near the system‐mean elevation and a UVVR of 0.1, a critical threshold identified by prior studies. This indicates that open‐water conversion becomes a dominant lateral instability process at a relatively conservative elevation threshold. We then integrate the UVVR and elevation to yield lifespan estimates, which demonstrate that higher elevation marshes are more resilient to internal deterioration, with an order‐of‐magnitude longer lifespan than predicted for lower elevation marshes.

 
more » « less
Award ID(s):
1637630 1832221
NSF-PAR ID:
10452161
Author(s) / Creator(s):
 ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geophysical Research Letters
Volume:
47
Issue:
3
ISSN:
0094-8276
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. As a symptom of accelerated sea level rise and historic impacts to tidal hydrology from agricultural and mosquito control activities, coastal marshes in the Northeastern U.S. are experiencing conversion to open water through edge loss, widening and headward erosion of tidal channels, and the formation and expansion of interior ponds. These interior ponds often form in high elevation marsh, confounding the notion applied in predictive modeling that salt marshes convert to open water when elevation falls below a critical surface inundation threshold. The installation of tidal channel extension features, or runnels, is a technique that has been implemented to reduce water levels and permit vegetation reestablishment in drowning coastal marshes, although there are limited data available to recommend its advisability. We report on 5 years of vegetation and hydrologic monitoring of two locations where a total of 600-m of shallow (0.15–0.30-m in diameter and depth) runnels were installed in 2015 and 2016 to enhance drainage, in the Pettaquamscutt River Estuary, in southern Rhode Island, United States. Results from this Before-After Control-Impact (BACI) designed study found that runnel installation successfully promoted plant recolonization, although runnels did not consistently promote increases in high marsh species presence or diversity. Runnels reduced the groundwater table (by 0.07–0.12 m), and at one location, the groundwater table experienced a 2-fold increase in the fraction of the in-channel tidal range that was observed in the marsh water table. We suggest that restoration of tidal hydrology through runnel installation holds promise as a tool to encourage revegetation and extend the lifespan of drowning coastal marshes where interior ponds are expanding. In addition, our study highlights the importance of considering the rising groundwater table as an important factor in marsh drowning due to expanding interior ponds found on the marsh platform.

     
    more » « less
  2. Abstract

    Sea level rise (SLR) is threatening coastal marshes, leading to large‐scale marsh loss in several micro‐tidal systems. Early recognition of marsh vulnerability to SLR is critical in these systems to aid managers to take appropriate restoration or mitigation measures. However, it is not clear if current marsh vulnerability indicators correctly assess long‐term stability of the marsh system. In this study, two indicators of marsh stress were studied: (i) the skewness of the marsh elevation distribution, and (ii) the abundance of codominant species in mixtures. We combined high‐precision elevation measurements (GPS), LiDAR imagery, vegetation surveys and water level measurements to study these indicators in an organogenic micro‐tidal system (Blackwater River, Maryland, USA), where large‐scale historical conversion from marshes to shallow ponds resulted in a gradient of increasing marsh loss. The two indicators reveal increasingly stressed marshes along the marsh loss gradient, but suggest that the field site with the most marsh loss seems to experience less stress. For the latter site, previous research indicates that wind waves generated on interior marsh ponds contribute to lateral erosion of surrounding marsh edges and hence marsh loss. The eroded marsh sediment might temporarily provide the remaining marshes with the necessary sediment to keep up with relative SLR. However, this is only a short‐term alleviation, as lateral marsh edge erosion and sediment export lead to severe marsh loss in the long term. Our findings indicate that marsh elevation skewness and the abundance of codominant species in mixtures can be used to supplement existing marsh stress indicators, but that additional indices such as fetch length and the sediment budget should be included to account for lateral marsh erosion and sediment export and to correctly assess long‐term stability of micro‐tidal marshes. © 2020 John Wiley & Sons, Ltd.

     
    more » « less
  3. Abstract

    Wetlands in the Mississippi River Delta are rapidly degrading. Sea level rise and low sediment supply are widely recognized as the two main factors contributing to land‐to‐water conversion. To determine what marsh areas are more resilient, it is fundamental to identify the drivers that regulate marsh accretion and degradation. In this study, a combination of field data and aerial images is used to determine these drivers in Terrebonne Bay, Louisiana, USA. We find that accretion and degradation patterns depend on whether the marsh is located inland in a sheltered area or facing open water. In the first case, the distance to the nearby channel is important, because during flooding of the marsh platform more sediment is deposited in the proximity of channel banks. The accretion rates of marshes facing open water are high and correlate to fetch, a proxy for the ability of waves to resuspend bottom sediment. These areas are more resilient to sea level rise, but waves are also the main mechanism of degradation, as these marshes tend to degrade by edge erosion. Consequently, we propose a bimodal evolution trajectory of the marshes in Terrebonne Bay: marshes close to the bay and facing open water accrete rapidly but are affected by lateral erosion due to waves, whereas sheltered marshes accrete slowly and degrade in large swathes due to insufficient sediment supply.

     
    more » « less
  4. Abstract

    Intertidal marshes develop between uplands and mudflats, and develop vegetation zonation, via biogeomorphic feedbacks. Is the spatial configuration of vegetation and channels also biogeomorphically organized at the intermediate, marsh‐scale? We used high‐resolution aerial photographs and a decision‐tree procedure to categorize marsh vegetation patterns and channel geometries for 113 tidal marshes in San Francisco Bay estuary and assessed these patterns' relations to site characteristics. Interpretation was further informed by generalized linear mixed models using pattern‐quantifying metrics from object‐based image analysis to predict vegetation and channel pattern complexity. Vegetation pattern complexity was significantly related to marsh salinity but independent of marsh age and elevation. Channel complexity was significantly related to marsh age but independent of salinity and elevation. Vegetation pattern complexity and channel complexity were significantly related, forming two prevalent biogeomorphic states: complex versus simple vegetation‐and‐channel configurations. That this correspondence held across marsh ages (decades to millennia) and at both high and low marsh elevations suggests the following: (1) marshes of shared physiography can exhibit highly variable ecosystem structures; (2) young marshes are not necessarily simple nor necessarily develop vegetation complexity with age and elevation; (3) Bay marshes should continue to exhibit both simple/complex configurations in the future despite a likely shift toward low marshes; (4) salt marshes may tend to occupy two alternative stable states characterized by linked complexity in vegetation and channel organization. This final point may help fill the gap at the marsh scale between biogeomorphic models explaining marsh occurrence at larger coastal and smaller vegetation patch scales. Copyright © 2016 John Wiley & Sons, Ltd.

     
    more » « less
  5. Abstract

    Expansion of drainage networks through the headward erosion of tidal creeks is an eco‐geomorphologic response of salt marshes to accelerated sea‐level rise (SLR). This response can counter the negative impacts of an elevation deficit by increasing drainage and encouraging plant health, thereby reducing potential for submergence and marsh platform loss. In the wetlands of Cape Romain, SC, intense bioturbation near creek heads by the common marsh crabSesarma reticulatumhas been found to facilitate sediment erosion and rapid creek growth. This keystone grazer has been recently observed to have increasing influence on landscape evolution throughout the southeast US coast. Here, we compare measurements taken at Sapelo Island, GA, with those previously collected at Cape Romain, to confirm that eco‐geomorphic feedbacks facilitating creek growth at each location are similar, and to compare these processes under differing background conditions. We use sediment cores, precise elevation measurements and historical imagery to compare substrate properties, elevation within the tidal frame, creek growth rates and drainage morphology at both sites. Our results show identical processes; however, the higher elevation of the marsh at Sapelo Island leads to shallower and shorter periods of tidal inundation, explaining the greater soil strength and lower belowground biomass compared with the marsh at Cape Romain. The smaller tidal range at the site in Cape Romain compared with Sapelo Island translates to a proportionally shallower depth of tidal creeks, which therefore requires less erosion to produce headward creek extension. These effects are likely to have contributed to slower growth rates of tidal creeks at Sapelo Island during the past several decades of SLR. Our findings highlight the similarities in process but differences in rates in how marshes are responding to climate‐related stress.

     
    more » « less