Constraining Marsh Carbon Budgets Using Long-Term C Burial and Contemporary Atmospheric CO2Fluxes (original) (raw)
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Constraining Marsh Carbon Budgets Using Long-Term C Burial and Contemporary Atmospheric CO2 Fluxes
Journal of Geophysical Research: Biogeosciences
Salt marshes are sinks for atmospheric carbon dioxide that respond to environmental changes related to sea level rise and climate. Here we assess how climatic variations affect marsh-atmosphere exchange of carbon dioxide in the short term and compare it to long-term burial rates based on radiometric dating. The 5 years of atmospheric measurements show a strong interannual variation in atmospheric carbon exchange, varying from −104 to −233 g C m −2 a −1 with a mean of −179 ± 32 g C m −2 a −1. Variation in these annual sums was best explained by differences in rainfall early in the growing season. In the two years with below average rainfall in June, both net uptake and Normalized Difference Vegetation Index were less than in the other three years. Measurements in 2016 and 2017 suggest that the mechanism behind this variability may be rainfall decreasing soil salinity which has been shown to strongly control productivity. The net ecosystem carbon balance was determined as burial rate from four sediment cores using radiometric dating and was lower than the net uptake measured by eddy covariance (mean: 110 ± 13 g C m −2 a −1). The difference between these estimates was significant and may be because the atmospheric measurements do not capture lateral carbon fluxes due to tidal exchange. Overall, it was smaller than values reported in the literature for lateral fluxes and highlights the importance of investigating lateral C fluxes in future studies. Long-term annual measures of aboveground productivity reveal substantial variation from year to year (Morris & Haskin, 1990; Morris et al., 2013). This variation in marsh biomass production is most often explained by variation in drivers that affect soil salinity levels, such as anomalies in mean sea level, freshwater discharge, and rainfall (
Effects of Sea Level Induced Disturbances on High Salt Marsh Metabolism
Estuaries, 2001
Salt marshes, which provide a transition between the marine and terrestrial environments around much of the temperate world, will be the first ecosystem to feel the effects of an increased rate of sea level rise. This study examined the metabolic responses of a high salt marsh to increased inundation and wrack deposition associated with sea level rise. We measured changes in ecosystem and soil photosynthesis and respiration by analyzing carbon dioxide fluxes in the light and dark. Data from seasonal flux measurements were combined with continuously measured light and temperature data to develop a model that estimated annual production and respiration. Results suggested that increased inundation will reduce respiration rates to a greater extent than production, yielding a moderate net loss of organic carbon from the high marsh. The model also predicted a substantial loss of organic carbon from wrack-affected areas. This decreased organic carbon input may play an important role in the ability of the marsh to maintain elevation relative to sea level rise.
Estuarine, Coastal and Shelf Science, 2009
Salt marshes accrete both organic and inorganic sediments. Here we present analytical and numerical models of salt marsh sedimentation that, in addition to capturing inorganic processes, explicitly account for above-and belowground organic processes including root growth and decay of organic carbon. The analytical model is used to examine the bias introduced by organic processes into proxy records of sedimentation, namely 137 Cs and 210 Pb. We find that accretion rates estimated using 210 Pb will be less than accretion rates estimated using the 137 Cs peak in steadily accreting marshes if (1) carbon decay is significant and (2) data for 210 Pb extend below the 137 Cs peak. The numerical model expands upon the analytical model by including belowground processes such as compaction and root growth, and by explicitly tracking the evolution of aboveground biomass and its effect on sedimentation rates. Using the numerical model we explore how marsh stratigraphy responds to sediment supply and the rate of sealevel rise. It is calibrated and tested using an extensive data set of both marsh stratigraphy and measurements of vegetation dynamics in a Spartina alterniflora marsh in South Carolina, USA. We find that carbon accumulation in marshes is nonlinearly related to both the supply of inorganic sediment and the rate of sea-level rise; carbon accumulation increases with sea-level rise until sea-level rise reaches a critical rate that drowns the marsh vegetation and halts carbon accumulation. The model predicts that changes in carbon storage resulting from changing sediment supply or sea-level rise are strongly dependent on the background sediment supply: if inorganic sediment supply is reduced in an already sediment poor marsh the storage of organic carbon will increase to a far greater extent than in a sediment-rich marsh, provided that the rate of sea-level rise does not exceed a threshold. These results imply that altering sediment supply to estuaries (e.g., by damming upstream rivers or altering littoral sediment transport) could lead to significant changes in the carbon budgets of coastal salt marshes.
Response of Estuarine Marsh Vegetation to Interannual Variations in Precipitation
Estuaries, 2001
The response of deltaic emergent marsh vegetation to increases in precipitation was examined over a 14mo period at three sites in the lower Nueces Estuary in south Texas. At all three sites, significant changes in plant biomass, percent cover, and allocation of aboveground and belowground tissues were associated with more than double the rainfall in late winter and early spring 1992 compared to the previous year and the 50-yr average for this region. Water column salinities, which ranged from 10‰ to 35‰ at all three sites in 1991, remained below 10‰ through August 1992. Significant changes in marsh vegetative structure included decreases in bare space, increases in the percent cover and aboveground biomass of a relatively less salt tolerant halophytes (Borrichia frutescens), and significant increases in root:shoot ratios in B. frutescens, Batis maritima and Suaeda linearis (in Salicornia virginica root:shoot ratios decreased significantly). Higher precipitation generally led to an overall increase in the biomass of most marsh perennials, but these increases were not statistically significant. For one species, Lycium carolinianum, additional rainfall extended its growing season through August 1992, 2 mo longer than in the previous year. The expansion (؉58%) of B. frutescens at one site was also coincident with the significant loss of B. maritima, whose cover decreased nearly 20%. In an ecological context, these responses suggest that precipitation events in arid environments may be considered a major physical disturbance that can result in large changes in the composition and relative abundance of emergent vascular plants over a relatively short period. The long-term significance of these changes is unknown and demonstrates the value of ecological studies that are conducted over several years for a more complete understanding of the dynamic processes that regulate marsh productivity.
Elevated CO 2 stimulates marsh elevation gain, counterbalancing sea-level rise
Proceedings of the National Academy of Sciences of the United States of America, 2009
Tidal wetlands experiencing increased rates of sea-level rise (SLR) must increase rates of soil elevation gain to avoid permanent conversion to open water. The maximal rate of SLR that these ecosystems can tolerate depends partly on mineral sediment deposition, but the accumulation of organic matter is equally important for many wetlands. Plant productivity drives organic matter dynamics and is sensitive to global change factors, such as rising atmospheric CO2 concentration. It remains unknown how global change will influence organic mechanisms that determine future tidal wetland viability. Here, we present experimental evidence that plant response to elevated atmospheric [CO2] stimulates biogenic mechanisms of elevation gain in a brackish marsh. Elevated CO2 (ambient ؉ 340 ppm) accelerated soil elevation gain by 3.9 mm yr ؊1 in this 2-year field study, an effect mediated by stimulation of below-ground plant productivity. Further, a companion greenhouse experiment revealed that the CO2 effect was enhanced under salinity and flooding conditions likely to accompany future SLR. Our results indicate that by stimulating biogenic contributions to marsh elevation, increases in the greenhouse gas, CO2, may paradoxically aid some coastal wetlands in counterbalancing rising seas. coastal wetlands ͉ nitrogen pollution ͉ tidal marsh loss ͉ root productivity ͉ salinity
Smithsonian Contributions to the Marine Sciences, 2009
Coastal marshes must accumulate soil to keep up with rising sea levels. It is unknown how the response of these ecosystems to global change will infl uence their ability to continue to keep up with sea-level rise. Here, we describe an in situ experimental chamber approach for manipulating key environmental variables, such as atmospheric CO 2 and soil N availability, in a brackish marsh. We outfi tted each chamber with surface elevation tables (SETs) to closely monitor soil elevation change, a sensitive indicator of marsh vulnerability to sea-level rise. Further, the design facilitates measurements of ecosystem exchange of CO 2 , plant productivity, porewater chemistry, and other environmental parameters.
Journal of Geophysical Research, 2008
1] We analyzed a 5-year record of the CO 2 and energy exchange, Aboveground Net Primary Production (ANPP), maximum Leaf Area Index (LAI max ), and Enhanced Vegetation Index (EVI) for a Typha marsh in Southern California. The marsh was a net source of carbon over the study, despite high rates of ANPP. Interannual Net Ecosystem Production (NEP) variability was the largest that has been reported for any terrestrial ecosystem and was attributed to changes in maximum photosynthetic rates (GEE max ). The variation in energy and mass exchange was coupled between years; years with higher than average rates of carbon uptake were associated with lower than average sensible heat fluxes. Remotely sensed measures of surface greenness (EVI) were closely related to GEE max variation, providing further evidence of interannual variability. We were unable to attribute the fluctuations in GEE max to the direct effects of weather on ecosystem physiology, or to interannual variation in LAI max . GEE did not vary systematically with air temperature or the presence of standing water in the marsh; GEE max did not vary with LAI max between years. Rather, interannual variation in carbon exchange at the SJFM resulted from shifts in the marsh's production efficiency (the rates of gross or net CO 2 exchange per LAI) that were not caused by changes in the weather. Our findings challenge the assumptions that interannual variation of land-atmosphere exchange is universally caused by the direct effect of weather on ecosystem physiology, and that an ecosystem's physiological response to the physical environment is consistent from year-to-year.
Carbon accumulation rates are highest at young and expanding salt marsh edges
Communications earth & environment, 2022
An objective of salt marsh conservation, restoration, and creation is to reduce global carbon dioxide levels and offset emissions. This strategy hinges on measurements of salt marsh carbon accumulation rates, which vary widely creating uncertainty in monetizing carbon credits. Here, we show the 14-323 g C m −2 yr −1 range of carbon accumulation rates, derived from cores collected at seven sites in North Carolina U.S.A., results from the landward or basinward trajectory of salt marsh colonization and the intertidal space available for accretion. Rates increase with accelerating sea-level rise and are highest at young and expanding marsh edges. The highest carbon densities are near the upland, highlighting the importance of this area for building a rich stock of carbon that would be prevented by upland development. Explaining variability in carbon accumulation rates clarifies appraisal of salt marsh restoration projects and landscape conversion, in terms of mitigating greenhouse gas emissions.
Feedback of coastal marshes to climate change: Long-term phenological shifts
Ecology and Evolution
Coastal marsh carbon is an important component of the global carbon budget (Duarte, Losada, Hendriks, Mazarrasa, & Marbà, 2013). Coastal marshes have high primary production, efficiently trap suspended organic carbon when flooded, and undergo slow carbon decomposition rates under anaerobic conditions (McLeod et al., 2011; Nellemann & Corcoran, 2009). The amount of carbon stored per unit area in stable coastal marshes can be far greater than that of forests, and the carbon stored may remain for millennia, as compared to decades or centuries in forests (Nellemann & Corcoran, 2009). The marshes and their carbon stocks, however, are susceptible to the direct and indirect effects of climate change (
Elevated CO 2 stimulates marsh elevation gain, counterbalancing sea-level rise
2009
Tidal wetlands experiencing increased rates of sea-level rise (SLR) must increase rates of soil elevation gain to avoid permanent conversion to open water. The maximal rate of SLR that these ecosystems can tolerate depends partly on mineral sediment deposition, but the accumulation of organic matter is equally important for many wetlands. Plant productivity drives organic matter dynamics and is sensitive to global change factors, such as rising atmospheric CO2 concentration. It remains unknown how global change will influence organic mechanisms that determine future tidal wetland viability. Here, we present experimental evidence that plant response to elevated atmospheric [CO2] stimulates biogenic mechanisms of elevation gain in a brackish marsh. Elevated CO2 (ambient ؉ 340 ppm) accelerated soil elevation gain by 3.9 mm yr ؊1 in this 2-year field study, an effect mediated by stimulation of below-ground plant productivity. Further, a companion greenhouse experiment revealed that the CO2 effect was enhanced under salinity and flooding conditions likely to accompany future SLR. Our results indicate that by stimulating biogenic contributions to marsh elevation, increases in the greenhouse gas, CO2, may paradoxically aid some coastal wetlands in counterbalancing rising seas. coastal wetlands ͉ nitrogen pollution ͉ tidal marsh loss ͉ root productivity ͉ salinity