Microtidal Salt Marshes Research Articles

Long‐Term Monitoring of Coupled Vegetation and Elevation Changes in Response to Sea Level Rise in a Microtidal Salt Marsh

AbstractTight interplays between physical and biotic processes in tidal salt marshes lead to self‐organization of halophytic vegetation into recurrent zonation patterns developed across elevation gradients. Despite its importance for marsh ecomorphodynamics, however, the response of vegetation zonation to changing environmental forcings remains difficult to predict, mostly because of lacking long‐term field observations of vegetation evolution in the face of changing rates of sea level rise and marsh vertical accretion. Here we present novel data of coupled marsh elevation‐vegetation distribution collected in the microtidal Venice Lagoon (Italy) over nearly two decades. Our results suggest that: (a) despite increasing absolute marsh elevations (i.e., above a fixed datum), vertical accretion rates across most of the studied marsh were not high enough to compensate for relative sea‐level rise (RSLR), thus leading to a progressive marsh drowning; (b) accretion rates ranging 1.7–4.3 mm/year are overall lower than the measured RSLR rate (4.4 mm/year) and strongly site‐specific. Accretion rates vary largely at sites within distances of a few tens of meters, being controlled by local elevation and sediment availability from eroding marsh edges; (c) vegetation responds species‐specifically to changes in environmental forcings by modifying species‐preferential elevation ranges. For the first time, we observe the consistency of a sequential vegetation‐species zonation with increasing marsh elevations over 20 years. We suggest this is the signature of vegetation resilience to changes in external forcings. Our results highlight a strong coupling between geomorphological and ecological dynamics and call for spatially distributed marsh monitoring and spatially explicit biomorphodynamic models of marsh evolution.

Runnels mitigate marsh drowning in microtidal salt marshes.

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.

Revisiting 228Th as a tool for determining sedimentation and mass accumulation rates

The use of 228Th has seen limited application for determining sedimentation and mass accumulation rates in coastal and marine environments. Recent analytical advances have enabled rapid, precise measurements of particle-bound 228Th using a radium delayed coincidence counting system (RaDeCC). Herein we review the 228Th cycle in the marine environment and revisit the historical use of 228Th as a tracer for determining sediment vertical accretion and mass accumulation rates in light of new measurement techniques. Case studies comparing accumulation rates from 228Th and 210Pb are presented for a micro-tidal salt marsh and a marginal sea environment. 228Th and 210Pb have been previously measured in mangrove, deltaic, continental shelf and ocean basin environments, and a literature synthesis reveals that 228Th (measured via alpha or gamma spectrometry) derived accumulation rates are generally equal to or greater than estimates derived from 210Pb, reflecting different integration periods. Use of 228Th is well-suited for shallow (<15 cm) cores over decadal timescales. Application is limited to relatively homogenous sediment profiles with minor variations in grain size and minimal bioturbation. When appropriate conditions are met, complimentary use of 228Th and 210Pb can demonstrate that the upper layers of a core are undisturbed and can improve spatial coverage in mapping accumulation rates due to the higher sample throughput for sediment 228Th.

Onset of runaway fragmentation of salt marshes

Summary Salt marshes are valuable but vulnerable coastal ecosystems that adapt to relative sea level rise (RSLR) by accumulating organic matter and inorganic sediment. The natural limit of these processes defines a threshold rate of RSLR beyond which marshes drown, resulting in ponding and conversion to open waters. We develop a simplified formulation for sediment transport across marshes to show that pond formation leads to runaway marsh fragmentation, a process characterized by a self-similar hierarchy of pond sizes with power-law distributions. We find the threshold for marsh fragmentation scales primarily with tidal range and that sediment supply is only relevant where tides are sufficient to transport sediment to the marsh interior. Thus the RSLR threshold is controlled by organic accretion in microtidal marshes regardless of the suspended sediment concentration at the marsh edge. This explains the observed fragmentation of microtidal marshes and suggests a tipping point for widespread marsh loss.

Pore water exchange‐driven inorganic carbon export from intertidal salt marshes

AbstractRespiration in intertidal salt marshes generates dissolved inorganic carbon (DIC) that is exported to the coastal ocean by tidal exchange with the marsh platform. Understanding the link between physical drivers of water exchange and chemical flux is a key to constraining coastal wetland contributions to regional carbon budgets. The spatial and temporal (seasonal, annual) variability of marsh pore water exchange and DIC export was assessed from a microtidal salt marsh (Sage Lot Pond, Massachusetts). Spatial variability was constrained from224Ra :228Th disequilibria across two hydrologic units within the marsh sediments. Disequilibrium between the more soluble224Ra and its sediment‐bound parent228Th reveals significant pore water exchange in the upper 5 cm of the marsh surface (0–36 L m−2d−1) that is most intense in low marsh elevation zones, driven by tidal overtopping. Surficial sediment DIC transport ranges from 0.0 to 0.7 g C m−2d−1. The sub‐surface sediment horizon intersected by mean low tide was disproportionately impacted by tidal pumping (20–80 L m−2d−1) and supplied a seasonal DIC flux of 1.7–5.4 g C m−2d−1. Export exceeded 10 g C m−2d−1for another marsh unit, demonstrating that fluxes can vary substantially across salt marshes under similar conditions within the same estuary. Seasonal and annual variability in marsh pore water exchange, constrained from tidal time‐series of radium isotopes, was driven in part by variability in mean sea level. Rising sea levels will further inundate high marsh elevation zones, which may lead to greater DIC export.

Role of Tidal Wetland Stability in Lateral Fluxes of Particulate Organic Matter and Carbon

AbstractTidal wetland fluxes of particulate organic matter and carbon (POM, POC) are important terms in global budgets but remain poorly constrained. Given the link between sediment fluxes and wetland stability, POM and POC fluxes should also be related to stability. We measured POM and POC fluxes in eight microtidal salt marsh channels, with net POM fluxes ranging between −121 ± 33 (export) and 102 ± 28 (import) g OM·m−2·year−1 and net POC fluxes ranging between −52 ± 14 and 43 ± 12 g C·m−2·year−1. A regression employing two measures of stability, the unvegetated‐vegetated marsh ratio (UVVR) and elevation, explained &gt;95% of the variation in net fluxes. The regression indicates that marshes with lower elevation and UVVR import POM and POC while higher elevation marshes with high UVVR export POM and POC. We applied these relationships to marsh units within Barnegat Bay, New Jersey, USA, finding a net POM import of 2,355 ± 1,570 Mg OM/year (15 ± 10 g OM·m−2·year−1) and a net POC import of 1,263 ± 632 Mg C/year (8 ± 4 g C·m−2·year−1). The magnitude of this import was similar to an estimate of POM and POC export due to edge erosion (−2,535 Mg OM/year and − 1,291 Mg C/year), suggesting that this system may be neutral from a POM and POC perspective. In terms of a net budget, a disintegrating wetland should release organic material, while a stable wetland should trap material. This study quantifies that concept and demonstrates a linkage between POM/POC flux and geomorphic stability.

Factors controlling sediment and nutrient fluxes in a small microtidal salt marsh within the Venice Lagoon

Coastal salt marshes are among the Earth's most productive ecosystems and provide a number of ecosystem services. Water quality regulation by storing, transforming and releasing nutrients, organic matter and suspended sediment is recognized as one of the most important functions of salt marshes. The self-purification capacity of intertidal ecosystems contributes to mitigating nutrient inputs, acting as a buffer zone between watersheds and coastal waters, especially in relation to climate change and the increasing frequency of impulsive extreme events. Understanding sediment and nutrient fluxes and assessing the mechanisms that control them are valuable for the preservation and future restoration of salt marshes and for enhancing their effectiveness in providing water quality regulation services.To investigate the main driving factors in microtidal environments, changes in suspended sediment and nutrient concentrations were measured during several tidal cycles in a small microtidal reconstructed salt marsh in the Venice Lagoon, which was selected as the experimental site. The tidal amplitude, nutrients and total suspended solid concentrations were measured during 6 tidal cycles between September 2011 and October 2013. The water discharge was derived by forcing the hydrodynamic MIKE-DHI numerical model with the measured tidal levels. Fluxes were assessed by coupling field data with the calculated discharges. The salt marsh filtering function was related to the inflow matter concentrations and tidal amplitude. When high suspended solid and nutrient concentrations enter the salt marsh, accumulation processes prevail on release. In contrast, in the case of low concentrations and high tidal excursion, the salt marsh functions as a nutrient and sediment source. During all campaigns, the nitrogen removal function was more effective within the intertidal vegetated areas. Sediment release was the dominant process in the outermost creek, whereas sedimentation prevailed in the inner area of the salt marsh because of the attenuation of hydrodynamic forcing during tide propagation.

Regulation of salt marsh mosquito populations by the 18.6-yr lunar-nodal cycle.

The 18.6-yr lunar-nodal cycle drives changes in tidal amplitude globally, affecting coastal habitat formation, species and communities inhabiting rocky shores, and salt marsh vegetation. However, the cycle's influence on salt marsh fauna lacked sufficient long-term data for testing its effect. We circumvented this problem by using salt marsh mosquito records obtained over a period of over four decades in two estuaries in the northeastern USA. Salt marsh mosquito habitat is near the highest tide level where the impact of the nodal cycle on flood frequency is greatest. Wavelet spectral and cross-correlation analyses revealed periodicity in salt marsh mosquito abundance that was negatively correlated with tidal amplitude. Tidal amplitude was a significant predictor of salt marsh mosquito abundance with the cycle maxima coinciding with lower mosquito populations, possibly due to access by predatory fish. However, these effects were detected only at the location with extensive salt marsh habitat and astronomical tides and were weakened or lacked significance at the location with small microtidal salt marshes and wind-driven tides. Mosquitoes can serve as proxy indicators for numerous invertebrate species on the salt marsh. These predictable cycles and their effects need to be taken into consideration when investigating, restoring, or managing intertidal communities that are also facing sea-level rise.

Spatially integrative metrics reveal hidden vulnerability of microtidal salt marshes

Salt marshes are valued for their ecosystem services, and their vulnerability is typically assessed through biotic and abiotic measurements at individual points on the landscape. However, lateral erosion can lead to rapid marsh loss as marshes build vertically. Marsh sediment budgets represent a spatially integrated measure of competing constructive and destructive forces: a sediment surplus may result in vertical growth and/or lateral expansion, while a sediment deficit may result in drowning and/or lateral contraction. Here we show that sediment budgets of eight microtidal marsh complexes consistently scale with areal unvegetated/vegetated marsh ratios (UVVR) suggesting these metrics are broadly applicable indicators of microtidal marsh vulnerability. All sites are exhibiting a sediment deficit, with half the sites having projected lifespans of less than 350 years at current rates of sea-level rise and sediment availability. These results demonstrate that open-water conversion and sediment deficits are holistic and sensitive indicators of salt marsh vulnerability.

Geomorphic Implications of Particle Movement by Water Surface Tension in a Salt Marsh

Measurements and modeling of salt marsh morphodynamics typically focus on the sediment fraction suspended in water, but another fraction moves at the air-water interface on water surface tension. We compared the geomorphic significance of this surface microlayer sediment fraction to the suspended fraction in a mesohaline, microtidal salt marsh in North Carolina. Conventional methods of suspended sediment and surface microlayer sampling were used and a method was developed to measure the surface microlayer material generated per area of benthic surface. The mass of material held in the surface microlayer was 4% to 16% of the suspended sediment mass in the water column overlying the marsh. This surface microlayer material mass was lower than the mass generated from benthic surfaces and it declined over time, indicating that the surface microlayer released sediment to the water column. Loss on ignition, carbon:nitrogen, and particle size were similar in the surface microlayer and suspended sediment, although the surface microlayer material was higher in chlorophyll a. We estimate that 19% to 100% of the suspended sediment that settled onto the marsh was lifted by surface tension during the next rising tide. This process helps explain the topography of tidal marshes and patterns of sediment accretion.

Sediment and vegetation spatial dynamics facing sea-level rise in microtidal salt marshes: Insights from an ecogeomorphic model

Modeling efforts have considerably improved our understanding on the chief processes that govern the evolution of salt marshes under climate change. Yet the spatial dynamic response of salt marshes to sea-level rise that results from the interactions between the tidal landforms of interest and the presence of bio-geomorphic features has not been addressed explicitly. Accordingly, we use a modeling framework that integrates the co-evolution of the marsh platform and the embedded tidal networks to study sea-level rise effects on spatial sediment and vegetation dynamics in microtidal salt marshes considering different ecological scenarios. The analysis unveils mechanisms that drive spatial variations in sedimentation rates in ways that increase marsh resilience to rising sea-levels. In particular, marsh survival is related to the effectiveness of transport of sediments toward the interior marshland. This study hints at additional dynamics related to the modulation of channel cross-sections affecting sediment advection in the channels and subsequent delivery in the inner marsh, which should be definitely considered in the study of marsh adaptability to sea-level rise and posterior management.

Modeling of Inundation Characteristics of a Microtidal Saltmarsh, Grand Bay National Estuarine Research Reserve, Mississippi

Ennis, B.; Peterson, M.S., and Strange, T.P., 2014. Modeling of inundation characteristics of a microtidal saltmarsh, Grand Bay National Estuarine Research Reserve, Mississippi. Intertidal saltmarsh habitats are unique productive estuarine systems in that their relative productivity is strongly influenced by the morphology of the landscape and the degree of water inundation. Despite the recognized effect that flooding in saltmarsh intertidal zones has on the natural history of resident fauna and the conservation of the habitat itself, limited data are available with regard to hydrological conditions to conduct proper assessment of these habitats. In this paper we describe a geographic information system–based model that was used to quantify the inundation characteristics of intertidal habitats within the microtidal Crooked Bayou, Grand Bay National Estuarine Research Reserves (GNDNERR), Mississippi, system. Using water levels from a nearby National Oceanic and Atmospheric Administration tide station, inundation characteristics were generated by the model for multiple sample sites and for plots within each site along a common marsh elevation gradient. The derived characteristics generated by the model were used in a principal components analysis to create factor scores depicting flooding conditions. These factor scores were then used as the means of comparison of cumulative inundation characteristics between sites and the plots within sites. Overall, inundation characteristics were found to vary significantly between sampling sites (Kruskal-Wallis, χ2 = 26.13, p < 0.001) and among the plots (Friedman's, χ2 = 19.27, p < 0.001) across the elevation gradient. In general, although the inundation potential of the sampling plots within each site significantly decreased as the marsh elevation gradient increased, there were marked differences among and within sites because of the considerable microtopography patterns we identified that exemplify the microtidal, low-lying intertidal landscape of GNDNERR.

Elevation and Plant Community Distribution in a Microtidal Salt Marsh of the Western Gulf of Mexico

The relationship between elevation and the natural occurrence of several species of emergent vascular plants was examined in a microtidal deltaic marsh system near Corpus Christi, on the southwestern Texas coast. Vegetation percent cover was estimated quarterly from November 2003 to November 2006 in 374 permanent plots located in 9 sites. Soil elevation was measured in each plot using sub-centimeter real-time kinematic GPS. Cluster analyses determined that there were two significantly (P < 0.05) different sets of co-occurring species, one comprising Borrichia frutescens, Salicornia virginica and Distichlis spicata and the other including Batis maritima, Salicornia bigelovii and Lycium carolinianum. Analysis of species groupings across an elevation gradient using non-metric multi-dimensional scaling and hierarchal cluster analysis indicated that these assemblages represent a low marsh assemblage (25–50 cm NAVD88) and a high marsh assemblage (50–65 cm NAVD88), respectively. The strong relationship between elevation and species distribution suggests that abiotic factors play a key role in determining plant distribution in this ecosystem, and indicate that even apparently small differences in elevation can be important in a microtidal system.

Description of Meloidoderita salina sp. n. (Nematoda, Sphaeronematidae) from a micro-tidal salt marsh at Mont-Saint-Michel Bay in France

Meloidoderita salina sp. n. is described and illustrated from the halophytic plant Atriplex portulacoides L. (sea purslane) growing in a micro-tidal salt marsh in the Mont-Saint-Michel Bay in France. This new species is the first member of Meloidoderita Poghossian, 1966 collected from a saline environment, and is characterized by the following features: sedentary mature females having a small swollen body with a clear posterior protuberance; slightly dorsally curved stylet, 19.9 µm long, with posteriorly sloping knobs; neck region irregular in shape and twisted; well developed secretory-excretory (S–E) pore, with markedly sclerotized S-E duct running posteriorly; prominent uterus bordered by a thick hyaline wall and filled with eggs. The adult female transforms into a cystoid. Eggs are deposited in both egg-mass and cystoid. Cystoids of Meloidoderita salina sp. n. display a unique sub-cuticular hexagonal beaded pattern. Male without stylet, pharyngeal region degenerated, S-E duct prominent, deirids small, developed testis 97.5 µm long, spicules 18.4 µm long, cloacal opening ventrally protruded, small phasmids posterior to cloaca opening and situated at 5.9 (3.2–7.7) µm from tail end, and conical tail ending in a rounded terminus marked with one (rarely two) ventrally positioned mucro. Additionally, some young malesof the new species were observed enveloped in the last J2 cuticle. Second-stage juvenile body 470 µm long, with a 16.4 µm long stylet, prominent rounded knobs set off from the shaft, hemizonid anterior and adjacent to S-E pore, small deirids located just above S-E pore level, genital primordium located at 68–77% of body length, phasmids small and located at about 19 µm from tail tip, and tail 38.7 µm long, tapering to finely pointed terminus with a finger-like projection. Phylogenetic analyses based on the nearly full length small subunit ribosomal DNA sequences of Meloidoderita salina sp. n. revealed a close relationship of the new species with Sphaeronema alni Turkina & Chizhov, 1986 and placed these two species sister to the rest of Criconematina.

Measurement of acid polysaccharides (APS) associated with microphytobenthos in salt marsh sediments

Microphytobenthos (MPB) are an important component of shallow-water habitats where there is sufficient light to support benthic photosynthesis. Production of extracellular poly- meric substances (EPS) is characteristic of the MPB, and acid polysaccharides (APS) are a major com- ponent of these EPS. A simple technique was developed to extract and measure APS concentrations in sediments. Water-extractable APS (wAPS) were extracted with ultra high purity water from lyophilized sediments. After centrifuging to remove the sediment, wAPS in the supernatant was precipitated with alcian blue, a cationic dye. The APS-alcian blue precipitate was removed by filtra- tion. The remaining alcian blue in solution was determined spectrophotometrically and was inversely proportional to the concentration of wAPS in the sediment. Chlorophyll (chl) a, carbohydrate frac- tions, and wAPS were measured in sediments from 5 contrasting sites in a microtidal salt marsh in the subtropics. wAPS concentrations, expressed as gum xanthan equivalents, ranged from 152 to 6268 mg m -2 in the upper 5 mm of sediment. The biomass (determined as chl a) of MPB at all sites was highest in the winter compared to the summer. There was a significant correlation (r = 0.675, p < 0.001, n = 107) between sediment wAPS and chl a concentrations. The ratio of wAPS to saline- extractable EPS (sEPS) was significantly higher in summer compared to winter, indicating a seasonal change in the composition of EPS. These data show that APS are a significant and variable compo- nent of the EPS pool in salt marsh sediments.

Estimating shallow subsidence in microtidal salt marshes of the southeastern United States: Kaye and Barghoorn revisited

Simultaneous measurements of vertical accretion and change in surface elevation relative to a shallow (3–5 m) subsurface datum were made in selected coastal salt marshes of Louisiana, Florida, and North Carolina to quantitatively test Kaye and Barghoorn's contention that vertical accretion is not a good surrogate for surface elevation change because of autocompaction of the substrate. Rates of subsidence of the upper 3–5 m of marsh substrate were calculated for each marsh as the difference between vertical accretion and elevation change measured with feldspar marker horizons and a sedimentation-erosion table. Surface elevation change was significantly lower than vertical accretion at each site after 2 years, indicating a significant amount of shallow subsidence had occurred, ranging from 0.45 to 4.90 cm. The highest rate of shallow subsidence occurred in the Mississippi delta. Results confirm Kaye and Barghoorn's contention that vertical accretion is not generally a good surrogate for elevation change because of processes occurring in the upper few meters of the substrate, including not only compaction but also apparently shrink-swell from water storage and/or plant production-decomposition at some sites. Indeed, surface elevation change was completely decoupled from vertical accretion at the Florida site. The assumption of a 1:1 relationship between vertical accretion and surface elevation change is too simplistic a generalization of the complex interactions between accretionary and substrate processes. Consequently, the potential for coastal marsh submergence should be expressed as an elevation deficit based on direct measures of surface elevation change rather than accretion deficits. These findings also indicate the need for greater understanding of the influence of subsurface and small-scale hydrologic processes on marsh surface elevation.