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1. Directional movement of consumer fronts associated with creek heads in salt marshes

   https://doi.org/10.1002/ecy.3447  

   Vu, Huy D. ; Pennings, Steven C. ( July 2021 , Ecology)

   Consumers often deplete local resources and aggregate along edges of remaining resources, forming “consumer fronts.” We examined the factors that promoteSesarma reticulatumcrab aggregations at saltmarsh creek heads to explain the directional but slow movement of these fronts. We also created artificial creek heads to test the hypothesis that hydrological conditions at creek heads create superior habitat for crabs. Soil temperatures were ˜11–12% cooler, hydrogen sulfide concentrations lower (0.0 vs. ˜0.58 mg/L), and dissolved oxygen concentrations twofold higher at the creek head versus the marsh platform. In the artificial creek‐head experiment, altering hydrological conditions led to lower dissolved sulfide levels, higher dissolved oxygen levels, and increased densities of crab burrows andSesarmacrabs. Moreover, the elevation of the soil surface declined rapidly at artificial creek heads versus controls, suggesting that crabs were increasing erosion. Our results suggest that abiotic conditions for crabs are better at the leading edge of the creek head than the trailing edge, explaining the directional movement of the front. Moreover, the speed at which the front propagates appears to be limited by the rate at which the creekhead erodes, rather than by crab mobility. The directional and slow movement ofSesarmafronts compared to consumer fronts of other invertebrates appears to result from the inextricable link betweenSesarmaand marsh geomorphology, whereas other consumer fronts are associated mostly with food resources.

    

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2. Sea-level rise and the emergence of a keystone grazer alter the geomorphic evolution and ecology of southeast US salt marshes

   https://doi.org/10.1073/pnas.1917869117  

   Crotty, Sinéad M. ; Ortals, Collin ; Pettengill, Thomas M. ; Shi, Luming ; Olabarrieta, Maitane ; Joyce, Matthew A. ; Altieri, Andrew H. ; Morrison, Elise ; Bianchi, Thomas S. ; Craft, Christopher ; et al ( July 2020 , Proceedings of the National Academy of Sciences)

   null (Ed.)

   Keystone species have large ecological effects relative to their abundance and have been identified in many ecosystems. However, global change is pervasively altering environmental conditions, potentially elevating new species to keystone roles. Here, we reveal that a historically innocuous grazer—the marsh crab Sesarma reticulatum —is rapidly reshaping the geomorphic evolution and ecological organization of southeastern US salt marshes now burdened by rising sea levels. Our analyses indicate that sea-level rise in recent decades has widely outpaced marsh vertical accretion, increasing tidal submergence of marsh surfaces, particularly where creeks exhibit morphologies that are unable to efficiently drain adjacent marsh platforms. In these increasingly submerged areas, cordgrass decreases belowground root:rhizome ratios, causing substrate hardness to decrease to within the optimal range for Sesarma burrowing. Together, these bio-physical changes provoke Sesarma to aggregate in high-density grazing and burrowing fronts at the heads of tidal creeks (hereafter, creekheads). Aerial-image analyses reveal that resulting “ Sesarma- grazed” creekheads increased in prevalence from 10 ± 2% to 29 ± 5% over the past <25 y and, by tripling creek-incision rates relative to nongrazed creekheads, have increased marsh-landscape drainage density by 8 to 35% across the region. Field experiments further demonstrate that Sesarma- grazed creekheads, through their removal of vegetation that otherwise obstructs predator access, enhance the vulnerability of macrobenthic invertebrates to predation and strongly reduce secondary production across adjacent marsh platforms. Thus, sea-level rise is creating conditions within which Sesarma functions as a keystone species that is driving dynamic, landscape-scale changes in salt-marsh geomorphic evolution, spatial organization, and species interactions. 

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3. High‐frequency variability of carbon dioxide fluxes in tidal water over a temperate salt marsh

   https://doi.org/10.1002/lno.12409  

   Song, Shuzhen ; Wang, Zhaohui Aleck ; Kroeger, Kevin D. ; Eagle, Meagan ; Chu, Sophie N. ; Ge, Jianzhong ( July 2023 , Limnology and Oceanography)

   Existing analyses of salt marsh carbon budgets rarely quantify carbon loss as CO₂through the air–water interface in inundated marshes. This study estimates the variability of partial pressure of CO₂(pCO₂) and air–water CO₂fluxes over summer and fall of 2014 and 2015 using high‐frequency measurements of tidal waterpCO₂in a salt marsh of the U.S. northeast region. Monthly mean CO₂effluxes varied in the range of 5.4–25.6 mmol m⁻²marsh d⁻¹(monthly median: 4.8–24.7 mmol m⁻²marsh d⁻¹) during July to November from the tidal creek and tidally‐inundated vegetated platform. The source of CO₂effluxes was partitioned between the marsh and estuary using a mixing model. The monthly mean marsh‐contributed CO₂effluxes accounted for a dominant portion (69%) of total CO₂effluxes in the inundated marsh, which was 3–23% (mean 13%) of the corresponding lateral flux rate of dissolved inorganic carbon (DIC) from marsh to estuary. Photosynthesis in tidal water substantially reduced the CO₂evasion, accounting for 1–86% (mean 31%) of potential CO₂evasion and 2–26% (mean 11%) of corresponding lateral transport DIC fluxes, indicating the important role of photosynthesis in controlling the air–water CO₂evasion in the inundated salt marsh. This study demonstrates that CO₂evasion from inundated salt marshes is a significant loss term for carbon that is fixed within marshes.

    

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4. Exchange Flows in Tributary Creeks Enhance Dispersion by Tidal Trapping

   https://doi.org/10.1007/s12237-021-00969-4  

   Garcia, Adrian Mikhail P. ; Geyer, W. Rockwell ; Randall, Noa ( July 2021 , Estuaries and Coasts)

   The North River estuary (Massachusetts, USA) is a tidal marsh creek network where tidal dispersion processes dominate the salt balance. A field study using moorings, shipboard measurements, and drone surveys was conducted to characterize and quantify tidal trapping due to tributary creeks. During flood tide, saltwater propagates up the main channel and gets “trapped” in the creeks. The creeks inherit an axial salinity gradient from the time-varying salinity at their boundary with the main channel, but it is stronger than the salinity gradient of the main channel because of relatively weaker currents. The stronger salinity gradient drives a baroclinic circulation that stratifies the creeks, while the main channel remains well-mixed. Because of the creeks’ shorter geometries, tidal currents in the creeks lead those in the main channel; therefore, the creeks never fill with the saltiest water which passes the main channel junction. This velocity phase difference is enhanced by the exchange flow in the creeks, which fast-tracks the fresher surface layer in the creeks back to the main channel. Through ebb tide, the relatively fresh creek outflows introduce a negative salinity anomaly into the main channel, where it is advected downstream by the tide. Using high-resolution measurements, we empirically determine the salinity anomaly in the main channel resulting from its exchange with the creeks to calculate a dispersion rate due to trapping. Our dispersion rate is larger than theoretical estimates that neglect the exchange flow in the creeks. Trapping contributes more than half the landward salt flux in this region.

    

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5. Responses of stomatal features and photosynthesis to porewater N enrichment and elevated atmospheric CO 2 in Phragmites australis , the common reed

   https://doi.org/10.1002/ajb2.1638  

   Garrison, Julian R. ; Caplan, Joshua S. ; Douhovnikoff, Vladimir ; Mozdzer, Thomas J. ; Logan, Barry A. ( April 2021 , American Journal of Botany)

   Biological invasions increasingly threaten native biodiversity and ecosystem services. One notable example is the common reed,Phragmites australis, which aggressively invades North American salt marshes. Elevated atmospheric CO₂and nitrogen pollution enhance its growth and facilitate invasion becauseP. australisresponds more strongly to these enrichments than do native species. We investigated how modifications to stomatal features contribute to strong photosynthetic responses to CO₂and nitrogen enrichment inP. australisby evaluating stomatal shifts under experimental conditions and relating them to maximal stomatal conductance (g_wmax) and photosynthetic rates.

   Plants were grownin situin open‐top chambers under ambient and elevated atmospheric CO₂(eCO₂) and porewater nitrogen (N_enr) in a Chesapeake Bay tidal marsh. We measured light‐saturated carbon assimilation rates (A_sat) and stomatal characteristics, from which we calculatedg_wmaxand determined whether CO₂and N_enraltered the relationship betweeng_wmaxandA_sat.

   eCO₂and N_enrenhanced bothg_wmaxandA_sat, but to differing degrees;g_wmaxwas more strongly influenced by N_enrthrough increases in stomatal density whileA_satwas more strongly stimulated by eCO₂. There was a positive relationship betweeng_wmaxandA_satthat was not modified by eCO₂or N_enr, individually or in combination.

   Changes in stomatal features co‐occur with previously described responses ofP. australisto eCO₂and N_enr. Complementary responses of stomatal length and density to these global change factors may facilitate greater stomatal conductance and carbon gain, contributing to the invasiveness of the introduced lineage.

    

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