Systematics of Modern Benthic Foraminiferal Assemblages From the Deltaic Mangrove Ecosystem of Sundarbans

The contemporary intertidal foraminifera and estuarine environment information were collected from the four sites adjoining the deltaic mangrove environment in the Sundarbans. The marsh zones of the south-western coastal region of Bangladesh were examined for modern benthic foraminifera and to expound on the relationship of the foraminiferal assemblages with the environment. Due to high inaccessibility and remoteness, the taxonomic study of foraminifera and its depositional environment remains largely overlooked in the Sundarbans of Bangladesh. This study includes a detailed survey of depositional environment of these fluvio-deltaic to shallow marine sediments. The seventeen species belonging to fourteen genera representing nine families were recorded from surficial sediment of supratidal, intertidal, and dune environment of Kotka, Jamtala, Kochikhali, and Dimer char area. In the present study, these foraminiferal assemblages are characterized calcareous and agglutinated foraminifera. The Kotka beach is recognized as Nonionina assemblage zone, Jamtala beach designated as Ammonia assemblage zone, Kochikhali as Nonionina assemblage zone and Dimer char as Rosalina-Nonionina-Nonionella assemblage zone. The deposition of foraminifera is restricted to Sundarbans’ low to high marsh zone. J. Asiat. Soc. Bangladesh, Sci. 47(2): 121-136, December 2021

Tidal marsh restoration at Blackwater National Wildlife Refuge, Maryland: A case study in thin-layer placement

Tidal marsh loss at Blackwater National Wildlife Refuge (NWR) has been a major concern of refuge managers in recent decades. The approximately 2,035 hectares (5,028 acres) of tidal marsh that have converted to open water in Blackwater NWR since 1938 (Scott et al. 2009) represent one of the most significant areas of marsh conversion within the Chesapeake Bay. In 2013, a suite of climate adaptation strategies focused on sea level rise was developed for Blackwater NWR and surrounding areas of Dorchester County by the Blackwater Climate Adaptation Project (BCAP). The BCAP is a collaboration of The Conservation Fund, Audubon Maryland-DC, and the U.S. Fish and Wildlife Service, assisted by the Maryland Department of Natural Resources (MD DNR), U.S. Geological Survey, and others. In 2016, the BCAP implemented a thin-layer placement (TLP) project at Shorter’s Wharf in Blackwater NWR on 16 hectares (40 acres) of subsiding and fragmenting tidal marsh dominated by Schoenoplectus americanus, Spartina alterniflora, and Spartina patens. The purpose of the project was to increase the 16 hectares’ (40 acres’) resiliency to climate-driven sea level rise and storm impacts. The project built up the marsh elevation by applying thin layers of sediment dredged from the adjacent Blackwater River. The sediment enhancement was designed to extend the longevity of the marsh and increase its resiliency by raising its surface elevation in relation to the tidal regime and to return the habitat to its prior high-marsh condition with S. patens dominating. The colonization of this site by saltmarsh sparrow would be an indicator of success in reaching this goal. Dredging operations in November and December 2016 placed approximately 19,900 cubic meters (26,000 cubic yards) of sediment on the project site. Post-restoration elevations obtained one year after material placement indicated that, although the target elevations were achieved in 78% of the surveyed placement area, the material was not distributed uniformly. Coarser material tended to stack up at the discharge location while the grain size declined and the slopes flattened toward the periphery of the discharge area. In 2017, natural vegetation had regenerated through the placed sediment with vigorous regrowth of S. americanus and S. alterniflora . This regrowth was supplemented with hand-planting of more than 200,000 plugs of S. patens. Vegetation monitoring is ongoing to determine the plant composition evolution within the placement site. Pre-dredge and post-dredge bathymetric surveys reveal 70% accretion nearly two years after dredging within the borrow area footprint.

RGB Indices and Canopy Height Modelling for Mapping Tidal Marsh Biomass from a Small Unmanned Aerial System

Coastal tidal marshes are essential ecosystems for both economic and ecological reasons. They necessitate regular monitoring as the effects of climate change begin to be manifested in changes to marsh vegetation healthiness. Small unmanned aerial systems (sUAS) build upon previously established remote sensing techniques to monitor a variety of vegetation health metrics, including biomass, with improved flexibility and affordability of data acquisition. The goal of this study was to establish the use of RGB-based vegetation indices for mapping and monitoring tidal marsh vegetation (i.e., Spartina alterniflora) biomass. Flights over tidal marsh study sites were conducted using a multi-spectral camera on a quadcopter sUAS near vegetation peak growth. A number of RGB indices were extracted to build a non-linear biomass model. A canopy height model was developed using sUAS-derived digital surface models and LiDAR-derived digital terrain models to assess its contribution to the biomass model. Results found that the distance-based RGB indices outperformed the regular radio-based indices in coastal marshes. The best-performing biomass models used the triangular greenness index (TGI; R2 = 0.39) and excess green index (ExG; R2 = 0.376). The estimated biomass revealed high biomass predictions at the fertilized marsh plots in the Long-Term Research in Environmental Biology (LTREB) project at the study site. The sUAS-extracted canopy height was not statistically significant in biomass estimation but showed similar explanatory power to other studies. Due to the lack of biomass samples in the inner estuary, the proposed biomass model in low marsh does not perform as well as the high marsh that is close to shore and accessible for biomass sampling. Further research of low marsh is required to better understand the best conditions for S. alterniflora biomass estimation using sUAS as an on-demand, personal remote sensing tool.

Plant-Mediated Rhizosphere Oxygenation in the Native Invasive Salt Marsh Grass Elymus athericus

In the last decades, the spread of Elymus athericus has caused significant changes to the plant community composition and ecosystem services of European marshes. The distribution of E. athericus was typically limited by soil conditions characteristic for high marshes, such as low flooding frequency and high soil aeration. However, recently the spread of E. athericus has begun to also include low-marsh environments. A high-marsh ecotype and a low-marsh ecotype of E. athericus have been described, where the latter possess habitat-specific phenotypic traits facilitating a better adaption for inhabiting low-marsh areas. In this study, planar optodes were applied to investigate plant-mediated sediment oxygenation in E. athericus, which is a characteristic trait for marsh plants inhabiting frequently flooded environments. Under waterlogged conditions, oxygen (O2) was translocated from aboveground sources to the roots, where it leaked out into the surrounding sediment generating oxic root zones below the sediment surface. Oxic root zones were clearly visible in the optode images, and no differences were found in the O2-leaking capacity between ecotypes. Concentration profiles measured perpendicular to the roots revealed that the radius of the oxic root zones ranged from 0.5 to 2.6 mm measured from the root surface to the bulk anoxic sediment. The variation of oxic root zones was monitored over three consecutive light–dark cycles (12 h/12 h). The O2 concentration of the oxic root zones was markedly reduced in darkness, yet the sediment still remained oxic in the immediate vicinity of the roots. Increased stomatal conductance improving the access to atmospheric O2 as well as photosynthetic O2 production are likely factors facilitating the improved rhizosphere oxygenation during light exposure of the aboveground biomass. E. athericus’ capacity to oxygenate its rhizosphere is an inheritable trait that may facilitate its spread into low-marsh areas. Furthermore, this trait makes E. athericus a highly competitive species in marshes facing the effects of accelerated sea-level rise, where waterlogged sediment conditions could become increasingly pronounced.

Evolutionary responses of a dominant plant along a successional gradient in a salt-marsh system

The ecological responses of plant populations along a successional gradient have been intensively examined; however, the evolutionary responses received much less attention. Here, I explored genetic changes of key phenotypic traits of a dominant clonal plant (Elytrigia atherica) along a saltmarsh successional gradient by collecting samples along the successional gradient in the high and low marsh and growing them in a common environment (greenhouse). Additionally, to explore whether changes in traits are driven by abiotic (e.g. clay thickness) and biotic (e.g. grazing intensity) variables along the successional gradient, I measured these two variables in the field. I found that clay thickness (a proxy of total nitrogen) increased along the successional gradient both in the high and low marsh; grazing intensity from hares (the most important herbivores) decreased along the successional gradient in the high marsh but did not change in the low marsh. Meanwhile, I found that growth in number of leaves and ramets decreased, while rhizome length increased, along the successional gradient for E. atherica collected from the high marsh. Opposite trends were found for E. atherica collected from the low marsh. Results suggest that, in the high marsh, herbivores may overrule nutrients to drive trait changes. That is, at the early successional stages, E. atherica had higher growth in number of leaves and ramets to compensate for high-intensity grazing. In the low marsh, nutrients may be the dominant driver for trait changes. That is, at the late successional stages, E. atherica had higher growth in number of leaves and ramets but shorter rhizomes to maximize its expansion under the favorable conditions (higher nutrient availability). Results suggest that ecologically important abiotic and biotic variables such as nutrients and herbivores may also have a substantial evolutionary impact on plant populations.

Top-down and sideways: Herbivory and cross-ecosystem connectivity shape restoration success at the salt marsh-upland ecotone

Wetland restoration provides remarkable opportunities to understand vegetation dynamics and to inform success of future projects through rigorous restoration experiments. Salt marsh restoration typically focuses on physical factors such as sediment dynamics and elevation. Despite many demonstrations of strong top-down effects on salt marshes, the potential for consumers to affect salt marsh restoration projects has rarely been quantified. Recently, major restoration projects at the Elkhorn Slough National Estuarine Research Reserve in central California, USA provided an opportunity to examine how herbivory influences restoration success. We quantified the strength of consumer effects by comparing caged to uncaged plantings, and compared effects among plant species and sites. We used camera traps to detect which herbivores were most common and how their abundance varied spatially. Beyond characterizing consumer effects, we also tested management strategies for reducing negative effects of herbivory at the restoration sites, including caging, mowing, and acoustic playbacks of predator sounds. We found extremely strong consumer effects at sites with extensive stands of exotic forbs upland of the high marsh; uncaged restoration plants suffered heavy herbivory and high mortality, while most caged plants survived. Brush rabbits (Sylvilagus bachmani) were by far the most frequent consumers of these high marsh plants. Our work thus provides the first evidence of mammal consumers affecting salt marsh restoration success. Mowing of tall exotic forb cover adjacent to the marsh at one restoration site greatly reduced consumption, and nearly all monitored plantings survived at a second restoration site where construction had temporarily eliminated upland cover. Playbacks of predator sounds did not significantly affect restoration plantings, but restoration efforts in marsh communities vulnerable to terrestrial herbivory may benefit from concurrent restoration of predator communities in the upland habitats surrounding the marsh. A landscape approach is thus critical for recognizing linkages between terrestrial and marine vegetation.

Evolutionary Responses of A Dominant Plant Along a Successional Gradient in A Salt-Marsh System

The ecological responses of plant populations along the successional gradient have been intensively examined; however, the evolutionary responses remain to be elucidated. Here, I explored genetic changes of key phenotypic traits of a dominant plant along a successional gradient, and whether these changes were induced by abiotic and biotic variables. I measured key abiotic (e.g. clay thickness) and biotic variables (e.g. herbivore density) along the successional gradient in the high and low marsh in a Wadden Sea saltmarsh. Also, I collected samples of Elytrigia atherica, grew them in the greenhouse, and measured key functional traits. I found that clay thickness (a proxy of total nitrogen) increased along the successional gradient both in the high and low marsh; herbivore density from hares (the most important herbivores) decreased along the successional gradient in the high marsh. Also, I found that growth in number of leaves and ramets decreased, while rhizome length increased, along the successional gradient for E. atherica collected from the high marsh. Opposite trends were found for E. atherica collected from the low marsh. Results suggest that, in the high marsh, herbivores may overrule nutrients to drive trait changes. That is, at early successional stages, E. atherica had higher growth in number of leaves and ramets to compensate for high-density grazing. In the low marsh, nutrients were the dominant driver for trait changes. These results suggest that ecologically important abiotic and biotic variables such as nutrients and herbivores may also have a substantial evolutionary impact on plant populations.

Plant genotype determines biomass response to flooding frequency in tidal wetlands

The persistence of tidal wetland ecosystems like salt marshes is threatened by human interventions and climate change. In particular, the threat of accelerated sea level rise (SLR) has increasingly gained the attention of the scientific community recently. However, studies investigating the effect of SLR on plants and vertical marsh accretion are usually restricted to the species or community level and do not consider phenotypic plasticity or genetic diversity. To investigate the response of genotypes within the same salt-marsh species to SLR, we used two known genotypes of Elymus athericus (Link) Kerguélen (low-marsh and high-marsh genotypes). In a factorial marsh organ experiment we exposed both genotypes to different flooding frequencies and quantified plant growth parameters. With increasing flooding frequency, the low-marsh genotype showed higher aboveground biomass production compared to the high-marsh genotype. Additionally, the low-marsh genotype generally formed longer rhizomes, shoots and leaves, regardless of flooding frequency. Belowground biomass of both genotypes decreased with increasing flooding frequency. We conclude that the low-marsh genotype is better adapted to higher flooding frequencies through its ability to allocate resources from below- to aboveground biomass. Given the strong control of plant biomass production on salt-marsh accretion, we argue that these findings yield important implications for our understanding of ecosystem resilience to SLR as well as plant species distribution in salt marshes.

Long-term invasion dynamics of Spartina increase vegetation diversity and geomorphological resistance of salt marshes against sea level rise

The cordgrass Spartina anglica C.E. Hubbard (Poaceae) is an invasive transformer in many salt marsh ecosystems worldwide. Relatively little is known about the capacity of Spartina to accelerate salt marsh succession and to protect salt marshes against sea level rise. We analyzed long-term changes in vegetation and elevation in mainland salt marshes of the European Wadden Sea in Schleswig-Holstein, Germany, to estimate the impact of non-native Spartina on the geomorphological resistance of salt marshes to sea level rise and on changes in species diversity. From 1989 to 2019, the Spartina-zone shifted and expanded upwards to elevations of the high marsh zone and Spartina increased in frequency in several salt marsh vegetation communities. At sites where Spartina dominated the vegetation already three decades ago, elevation and species diversity increased with a higher rate compared to sites lacking Spartina. The median change rates reached for elevation MHT +8.6 versus +1.5 mm per year, for species richness +3 versus $$\pm$$ ± 0 species per three decades, and for evenness +0.04 versus −0.08 per three decades, regarding plots with versus without former Spartina dominance, respectively. Invasion of salt marshes by Spartina and its continued, long-term presence were associated with increased elevation and species diversity in the face of sea level rise.

Does invasive common reed in coastal salt marshes impact dabbling duck food availability?

Common reed, Phragmites australis , a non-native perennial grass, is considered a nuisance species to land managers and wildlife biologists. Common reed thrives in areas with reduced soil salinities, increased nitrogen availability, and anthropogenic shoreline development. The expansion of non-native common reed into tidal wetlands of North America detrimentally affects native wildlife by altering resource utilization, modifying trophic structures, and changing disturbance regimes. Thus, it also has the potential to drastically affect dabbling duck energetic carrying capacity in salt marsh ecosystems. We assessed whether invaded monocultures of common reed in dabbling duck habitat could alter the availability of invertebrate and seed foods for the mallard [ Anas platyrhynchos ], American black duck [ Anas rubripes ], green-winged teal [ Anas crecca ], northern shoveler [ Spatula clypeata ], and northern pintail [ Anas acuta ] as compared to wetland type (mudflat, low marsh, high marsh, and impoundments). We compared food and energy availability in >90% common reed monocultures to non-common reed invaded saltmarshes in five study areas in Edwin B. Forsythe National Wildlife Refuge, New Jersey, 2015–16 . To estimate wetland specific food energy supply, we collected sediment core samples, fixed with formalin, washed, dried, sorted, and weighed for seeds and invertebrates. We multiplied biomass (g) by True Metabolizable Energy values to estimate species-specific dabbling duck food energy availability. We further estimated wetland specific energetic carrying capacity (duck-energy-days) based on known species-specific energetic demands. We determined that duck-energy-days/ha were greater for dabbling ducks in wetlands invaded with common reed because they contained more consumable seed energy and less consumable invertebrate energy. However, future research should explore how accessible these foods are if common reed grass is too dense. To aid in restoration efforts once common reed is removed by control efforts, our results indicate a robust seed bank exists in the soil strata thus increasing salt marsh seed biodiversity.