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1. Increasing grazer density leads to linear decreases in Spartina alterniflora biomass and exponential increases in grazing pressure across a barrier island

   https://doi.org/10.3354/meps13569  

   Renzi, JJ; Silliman, BR (February 2021, Marine Ecology Progress Series)

   Researchers now recognize that top-down as well as bottom-up forces regulate salt marsh primary production. However, how top-down forces vary with grazer density is still poorly resolved. To begin to address this void, we (1) surveyed grazing intensity in short-form Spartina alterniflora across Sapelo Island, Georgia (USA), and (2) removed varying densities of grazers from 13 sites over 2 yr. Our survey revealed a non-linear relationship between snail abundance and grazing intensity, with grazing scars per stem increasing exponentially with snail density. Further, there appeared to be a threshold at ~80 snails m -2 , below which increasing snail density did not significantly increase grazing scars—potentially because snails target dead grass rather than live grass when competition with other snails is low. Increasing snail densities also exponentially reduced stem density within a plot, but only over 80 snails m -2 . Our removal experiment showed that snails linearly decreased S. alterniflora biomass across a naturally representative range of snails (0-586 snails m -2 ) and that top-down control of short-form S. alterniflora was important at multiple sites across an island, with snail removal on average increasing primary production by 164%. Our results reveal that top-down control of short-form S. alterniflora is a common process across this intensively studied island, and that grazing scars increase non-linearly with snail density, while consumer effects on biomass increase linearly. Future models based on marsh plant growth (e.g. geomorphic evolution, primary production) should incorporate both the importance and functional form of grazer control to create more accurate carbon budgets and to better understand marsh network dynamics. 

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2. Canopy Heterogeneity and Environmental Variability Drive Annual Budgets of Net Ecosystem Carbon Exchange in a Tidal Marsh

   https://doi.org/10.1029/2023JG007866  

   Hawman, P A; Cotten, D L; Mishra, D R (April 2024, Journal of Geophysical Research: Biogeosciences)

   Abstract Tidal salt marshes are important ecosystems in the global carbon cycle. Understanding their net carbon exchange with the atmosphere is required to accurately estimate their net ecosystem carbon budget (NECB). In this study, we present the interannual net ecosystem exchange (NEE) of CO₂derived from eddy covariance (EC) for aSpartina alterniflorasalt marsh. We found interannual NEE could vary up to 3‐fold and range from −58.5 ± 11.3 to −222.9 ± 12.4 g C m⁻² year⁻¹in 2016 and 2020, respectively. Further, we found that atmospheric CO₂fluxes were spatially dependent and varied across short distances. High biomass regions along tidal creek and estuary edges had up to 2‐fold higher annual NEE than lower biomass marsh interiors. In addition to the spatial variation of NEE, regions of the marsh represented by distinct canopy zonation responded to environmental drivers differently. Low elevation edges (with taller canopies) had a higher correlation with river discharge (R² = 0.61), the main freshwater input into the system, while marsh interiors (with short canopies) were better correlated with in situ precipitation (R² = 0.53). Lastly, we extrapolated interannual NEE to the wider marsh system, demonstrating the potential underestimation of annual NEE when not considering spatially explicit rates of NEE. Our work provides a basis for further research to understand the temporal and spatial dynamics of productivity in coastal wetlands, ecosystems which are at the forefront of experiencing climate change induced variability in precipitation, temperature, and sea level rise that have the potential to alter ecosystem productivity. 

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3. FLUXNET-CH ₄ Synthesis Activity: Objectives, Observations, and Future Directions

   https://doi.org/10.1175/BAMS-D-18-0268.1  

   Knox, Sara H.; Jackson, Robert B.; Poulter, Benjamin; McNicol, Gavin; Fluet-Chouinard, Etienne; Zhang, Zhen; Hugelius, Gustaf; Bousquet, Philippe; Canadell, Josep G.; Saunois, Marielle; et al (December 2019, Bulletin of the American Meteorological Society)

   This paper describes the formation of, and initial results for, a new FLUXNET coordination network for ecosystem-scale methane (CH 4 ) measurements at 60 sites globally, organized by the Global Carbon Project in partnership with other initiatives and regional flux tower networks. The objectives of the effort are presented along with an overview of the coverage of eddy covariance (EC) CH 4 flux measurements globally, initial results comparing CH 4 fluxes across the sites, and future research directions and needs. Annual estimates of net CH 4 fluxes across sites ranged from −0.2 ± 0.02 g C m –2 yr –1 for an upland forest site to 114.9 ± 13.4 g C m –2 yr –1 for an estuarine freshwater marsh, with fluxes exceeding 40 g C m –2 yr –1 at multiple sites. Average annual soil and air temperatures were found to be the strongest predictor of annual CH 4 flux across wetland sites globally. Water table position was positively correlated with annual CH 4 emissions, although only for wetland sites that were not consistently inundated throughout the year. The ratio of annual CH 4 fluxes to ecosystem respiration increased significantly with mean site temperature. Uncertainties in annual CH 4 estimates due to gap-filling and random errors were on average ±1.6 g C m –2 yr –1 at 95% confidence, with the relative error decreasing exponentially with increasing flux magnitude across sites. Through the analysis and synthesis of a growing EC CH 4 flux database, the controls on ecosystem CH 4 fluxes can be better understood, used to inform and validate Earth system models, and reconcile differences between land surface model- and atmospheric-based estimates of CH 4 emissions. 

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4. Controls on Seasonal Atmosphere‐Ecosystem Carbon Dioxide Exchanges in a Temperate Salt Marsh

   https://doi.org/10.1111/gcb.70740  

   Ruiz‐Plancarte, Jesus; Fuentes, Jose D; McGlathery, Karen J (February 2026, Global Change Biology)

   ABSTRACT Salt marshes play a vital role in the biogeochemistry of coastal zones, yet the biophysical controls on CO₂exchange with the atmosphere, or net ecosystem exchange (NEE, positive upwards) remain poorly quantified. We investigated aSpartina alternifloramonoculture salt marsh on the eastern shore of Virginia, United States, by estimating half‐hourly NEE from March 2016 to February 2017 using the eddy‐covariance method. Maximum marsh–atmosphere CO₂exchanges occurred during June and July when hourly averaged NEE values reached −10.0 ± 2.5 μmol CO₂m⁻² s⁻¹(mean ±1 standard deviation). During the most productive time of the year, a tidal inundation of 0.7 m reduced daytime CO₂assimilation and nighttime CO₂release to the atmosphere by 5.0 ± 1.2 μmol CO₂m⁻² s⁻¹and 3.0 ± 0.7 μmol CO₂m⁻² s⁻¹, respectively. Diffuse photosynthetically active radiation (PAR) conditions promoted quantum use efficiencies (α) of the ecosystem that were approximately three times greater than under direct PAR conditions (α_Cloudy = 0.012 ± 0.004 versusα_Clear = 0.004 ± 0.001 mol CO₂per (mol photons)). Under diffuse light, NEE increased more rapidly with PAR and photo‐saturation occurred at higher PAR levels compared to clear‐sky conditions. On average, under the influence of diffuse light, the assimilation of CO₂increased by 30% relative to equivalent PAR levels under direct sunlight. During March 2016 to February 2017 the marsh exchanged −269.1 ± 9.1 g of carbon per m²with the atmosphere. The findings demonstrate that tides and light quality are key regulators of carbon cycling in tidal marshes. These factors should be incorporated into models of tidal marsh biogeochemistry, particularly as both are undergoing rapid changes due to sea level rise and atmospheric warming. 

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5. Marsh vegetation and soil survey at Marsh Landing, Sapelo Island GA from July 1999.

   https://doi.org/10.6073/pasta/591b7c5c4aa8a8e37159f930c3a6a734  

   Project, Georgia_Coastal_Ecosystems LTER; Pennings, Steven C (January 2025, Environmental Data Initiative)

   Abstract: To document edaphic and vegetation patterns in a Georgia marsh, I sampled seven vegetation "zones" at Marsh Landing on Sapelo Island in July, 1999. Vegetation zones were delineated based on vegetation composition. I located 8 transects running from the Juncus zone in the high marsh down into the short S. alterniflora zone. The tall Spartina zone was not sampled. I sampled a single quadrat (0.25 m x 0.25 m) in each vegetation zone along each transect, for a total of 56 plots. I measured canopy height with a meter stick. I harvested aboveground biomass within each quadrat, sorted it to species, dried and weighed it. I measured soil water content gravimetrically by drying surface (4 cm deep) soil cores and expressing results as (water mass)/(mass of wet core). I determined porewater salinity by rehydrating dried soil cores with a known volume of deionized water, measuring the salinity of the supernatant after 48 h, and back-calculating to the volume of water originally present. I measured soil organic content as loss on ignition of dried soil cores at 450oC for 12 h. I measured relative elevation of each vegetation zone with a theodolite, with the elevation of the lowest zone arbitrarily set to zero. The resulting dataset describes how plant species composition, richness, height and biomass varies as a function of abiotic conditions in the upper part of a Georgia marsh. 

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