Ecogeomorphic feedbacks influence elevation change across microtidal wetland settings of coastal Louisiana

arising from G. Li et al. Nature Communications https://doi.org/10.1038/s41467-024-45487-6 (2024)

Wetland dynamics in deltaic settings are highly complex due to the diversity of marsh types and geomorphic settings and varying degrees of human impacts. Therefore, understanding localized drivers of hydrodynamics and sediment transport is essential for accurately assessing vulnerability to sea-level rise. In a recent Nature Communications publication, Li et al.¹ analyzed a globally unique dataset from the Coastwide Reference Monitoring System (CRMS) in the Mississippi Delta and projected a bleak future for Louisiana’s coastal wetlands. They concluded that, under the current trajectory of rising sea levels, up to 75% of these wetlands could be lost by 2070. We suggest a re-evaluation and reinterpretation of their findings—particularly regarding wetland loss rates, inundation thresholds, and the role of ecogeomorphic feedbacks—would allow for the development of a more nuanced understanding of the mechanisms driving landscape change across the Mississippi Delta’s varied environments.

Louisiana is experiencing unprecedented wetland loss due largely to anthropogenic alterations of the Mississippi River and direct impacts such as development, levee/canal construction, and subsurface fluid withdrawal^2,3,4. Sea-level rise (SLR) and subsidence combine to exacerbate these direct human impacts. As wetlands become increasingly fragmented, they become more vulnerable to erosion. We find several designations of “Drowning” or “Safe” presented in Li et al.¹ based on marsh elevation change and water level trajectories within the CRMS network to be tenuous. We call into question some of these classifications in light of evidence that most recent wetland loss in the Mississippi River delta plain (from analysis of the same sites analyzed by Li et al.¹) is not due to drowning, but rather to lateral edge erosion⁵ (Fig. 1). Of CRMS stations experiencing persistent wetland loss, only 10% experienced drowning while 82% were eroding laterally⁵. Importantly, this land loss mechanism can occur regardless of SLR due to fetch and wind-wave energy⁶. Additionally, most sites that experienced persistent land loss were gaining in elevation over the same period. Thus, there is the potential to miscategorize sites as stable based on elevation trends when they are losing land rapidly through edge erosion. Several of the sites that Li et al.¹ classify as “Safe” exhibit the largest land loss rates according to [5] (e.g., CRMS site 0164 Fig. 1c). As surface elevation change (SEC) of these wetlands is large due to remobilization of sediment from marsh edge erosion^5,7, a more site-specific comparison of land loss, elevation change, and water levels could result in revised projections.

Fig. 1: Land loss data and geomorphic environment are imperative for assessing stability in Louisiana’s wetlands.

Example Coastwide Reference Monitoring System (CRMS) sites shown here reveal Li et al.¹ classification of marsh projections needs reconsideration. a, d, g CRMS sites viewed in Google Earth. b, e, h Land change classification within 1 km² of CRMS site from 2005–2021, where the red box denotes 0.01 km² surrounding site (see ref. ¹⁶ for full legend). Wetland surface (SET) and water level variation at: c CRMS 0513, a completely stable swamp environment that Li et al.¹ classify as “unsafe” and “drowning complete”; f CRMS 0541, a laterally eroding but super-elevated saltmarsh on the south shore of Vermilion Bay that Li et al.¹ classify as “unsafe” and “drowning projected”; and i CRMS 0164, a heavily fragmented and eroding tidal saltmarsh in lower Barataria Bay that Li et al.¹ classify as “safe” and within “dynamic equilibrium”.

Louisiana coastal wetlands encompass several diverse environments, including freshwater swamps, fluvio-deltaic influenced marshes, and tidal marshes of varying salinity. Li et al.¹ do not distinguish low-lying forested wetlands (i.e., swamps) that are typically inundated over long time periods (months to years), from coastal marshes dominated by herbaceous plants that require more regular drainage⁸. Deltaic swamps encompass roughly 15% of the CRMS sites, and their hydrodynamics are dissimilar to those of marshes. Swamp tree species such as bald cypress (Taxodium distichum) and water tupelo (Nyssa aquatica) can survive extended periods (>6 months or longer) of continuous flooding. As such, designations of “Drowning complete” based solely on SEC trends relative to water-level and inundation trends (such as CRMS site 0513, Fig. 1a) may be overly simplistic and could be refined with re-evaluation. Saltwater intrusion may be a more important loss mechanism for swamps than excessive inundation⁹, however many swamp environments in Louisiana show little to no land-area change in recent decades (e.g., CRMS site 0513; Fig. 1a).

Beyond the diverse array of plant community types and hydrodynamics across Louisiana’s coastal wetlands, broadscale geomorphic context also varies throughout the deltaic plain. Southeast Louisiana contains a mixture of prograding distributary wetlands and abandoned transgressing wetlands, while the geologically distinct Chenier Plain encompasses wetlands which have been hydrologically impounded and largely isolated from tidal exchange with the coastal ocean. A more geomorphic-oriented perspective would allow for refined interpretation of the wetland response to relative SLR presented in Li et al.¹ (their Figures 2, 3, & 4a) for each of these environments. For example, marshes in the Chenier Plain are undergoing widespread drowning due to human manipulation of hydrology throughout the region, which is exacerbated by storms and SLR¹⁰. Li et al.¹ concede that marsh drowning in this region is due to man-made impoundments, yet coalesce these sites into their analyses and conclude that ecogeomorphic feedbacks that allow marshes to adjust their elevation through sedimentation and organic matter accumulation are not at play in coastal Louisiana. For these feedbacks to occur, marshes need to experience a natural hydrologic regime with access to available sediment. Therefore, Chenier Plain marshes should not be included in these analyses. Furthermore, we suggest many of the sites classified as “Drowning projected” or even “Safe” be reconsidered. For example, CRMS 0541 is classified as “Drowning projected”, yet this saltmarsh – located on the south shore of Vermilion Bay – is very high with respect to water level fluctuations (Fig. 1b). Marshes that are high in the tidal frame or even supratidal are not uncommon, as wetlands can aggrade with river flooding and storm surges (approaching the height of floodwaters), in addition to transgressing on top of elevated structures such as natural levees as they subside¹¹. It is generally accepted that vertical accretion rates in saltmarshes with adequate sediment supply are inversely tied to elevation, primarily due to hydroperiod and ecogeomorphic feedbacks^12,13. Marsh Equilibrium Models^14,15 are inherently only utilized within the tidal frame, i.e., they assume that vegetation productivity and sediment trapping are related to inundation by tides/storm surges and thus elevation. Therefore, at CRMS 0541 and other sites that may similarly be super-elevated with respect to tidal level fluctuations, a low SEC on the platform is to be expected (Fig. 1b); with continued relative SLR, the marsh platform at this site is expected to become inundated more frequently by tides, and the SEC is expected to increase as the sediment supply and organic production respond. Thus, several “Give up drowning projected” sites categorized by Li et al.¹ should be classified as “Keep-up dynamic equilibrium”.

The summation of these comments serves to contradict Li et al.’s¹ conclusion that ecogeomorphic feedbacks are not at play in Louisiana wetlands, and that sites with smaller relative SLR and larger tidal range are more appropriate for future research. Furthermore, site-specific investigations into the mechanisms responsible for marsh drowning (e.g., relative SLR, hydrologic impoundment) could be informative as they require different wetland mitigation approaches. Management strategies such as marsh creation projects (which mitigate marshes already lost) and sediment diversions (which will restore sediment delivery in Louisiana’s remaining marshes) use this information to create sustainable wetlands. Louisiana offers the ultimate “canary in the coal mine” scenario for wetlands undergoing accelerated SLR. Having large datasets provides an incredible opportunity to analyze trends and patterns, particularly in coastal areas impacted by climate change. However, an inherent understanding of site variability and regulatory mechanisms is imperative, along with acknowledging coastal wetland responses to increased flooding are complex, even at small spatial scales. While careful consideration and inclusion of other factors such as edge erosion, vegetation adaptations to prolonged inundation, and geomorphic context into assessments of landscape resilience can be tedious, their inclusion is critical to arriving at a clearer understanding of the mechanisms that drive landscape change.