Large interannual CO2 and energy exchange variability in a freshwater marsh under consistent environmental conditions (original) (raw)
Related papers
Drought legacies influence the long-term carbon balance of a freshwater marsh
Journal of Geophysical Research, 2010
1] Experimental manipulations provide a powerful tool for understanding an ecosystem's response to environmental perturbation. We combined paired eddy covariance towers with an experimental manipulation of water availability to determine the response of marsh carbon balance to drought. We monitored the Net Ecosystem Exchange of CO 2 (NEE) in two ponds from 2004 to 2009 at the San Joaquin Freshwater Marsh (SJFM), and subjected one of the ponds to a yearlong drought treatment in 2007. The two ponds experienced similar flooding and environmental regimes before and after the drought, ensuring that differences between ponds were largely attributable to the 2007 drought. Drought substantially reduced surface greenness, as measured by the Enhanced Vegetation Index (EVI) and photosynthetic carbon sequestration, primarily by inhibiting leaf area development. Respiratory carbon losses were less influenced by drought than photosynthetic carbon gains. The effect of the drought lasted several years, with delayed leaf area development and peak carbon uptake rates during the subsequent year, and reduced leaf area for a couple of years. The combined effect of the drought and legacy effects created an overall loss of carbon that was equivalent to 4 years of the maximum annual carbon sequestration observed over a decade. Our results indicate that drought can have long-term impacts on ecosystem carbon balance and that future projected drought increases in Southern California will have a negative impact on marsh carbon sequestration.
Seasonal differences in the CO2 exchange of a short-hydroperiod Florida Everglades marsh
Agricultural and Forest Meteorology, 2010
Although wetlands are among the world's most productive ecosystems, little is known of long-term CO 2 exchange in tropical and subtropical wetlands. The Everglades is a highly managed wetlands complex occupying >6000 km 2 in south Florida. This ecosystem is oligotrophic, but extremely high rates of productivity have been previously reported. To evaluate CO 2 exchange and its response to seasonality (dry vs. wet season) in the Everglades, an eddy covariance tower was established in a short-hydroperiod marl marsh. Rates of net ecosystem exchange and ecosystem respiration were small year-round and declined in the wet season relative to the dry season. Inundation reduced macrophyte CO 2 uptake, substantially limiting gross ecosystem production. While light and air temperature exerted the primary controls on net ecosystem exchange and ecosystem respiration in the dry season, inundation weakened these relationships. The ecosystem shifted from a CO 2 sink in the dry season to a CO 2 source in the wet season; however, the marsh was a small carbon sink on an annual basis. Net ecosystem production, ecosystem respiration, and gross ecosystem production were −49.9, 446.1 and 496.0 g C m −2 year −1 , respectively. Unexpectedly low CO 2 flux rates and annual production distinguish the Everglades from many other wetlands. Nonetheless, impending changes in water management are likely to alter the CO 2 balance of this wetland and may increase the source strength of these extensive short-hydroperiod wetlands.
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 (
Constraining Marsh Carbon Budgets Using Long-Term C Burial and Contemporary Atmospheric CO2Fluxes
Journal Of Geophysical Research: Biogeosciences, 2018
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 (
Agricultural and Forest Meteorology, 2002
Photosynthetic carbon assimilation is dependent on the canopy-air CO 2 concentration (Ca), which is an essential driving parameter of mechanistic models built to predict plant responses to changing environmental conditions. Short-term studies have shown that crop-canopy Ca undergoes substantial diurnal and seasonal variations as compared to Ca values measured several meters above plant canopies in well mixed conditions. Whether these canopy Ca patterns measured in crops also apply to wetland plant canopies and over long periods of time remains largely unknown. The first objective of this study was to analyze the consistency of short-and long-term Ca patterns in salt marsh canopies over a 10-year period. The second objective was to assess the impact of these canopy Ca patterns on simulated ecosystem productivity. In this study, we used Ca data collected from 1990 to 2000 in salt marsh canopies of the Chesapeake Bay, Maryland. The possible effects of short-and long-term canopy Ca patterns on plant productivity were simulated with a mechanistic model specifically adapted to the Scirpus olneyi vegetation of the salt marsh ecosystem. The annual average of daytime canopy Ca of a brackish marsh community rose by a significant (P < 0.01) 1.55 mol CO 2 mol −1 year −1 between 1990 and 2000, which parallels the atmospheric records of Mauna Loa over the same period of time. Annual Ca averages displayed some variability around this trend, which were highly correlated with the year-to-year variation in aboveground plant biomass (r 2 = 0.83, P < 0.001). Therefore, our study demonstrated that aboveground plant biomass is the main driver for inter-annual canopy Ca fluctuations. Daytime canopy Ca displayed maximum values in May and September and minimum values in July, with an amplitude of about 20 mol CO 2 mol −1 . The amplitude of the diurnal Ca cycle measured at the wetland appeared similar to that of North American agricultural ecosystems, exceeding 100 mol CO 2 mol −1 in July and August and characterized by a sharp drop in the CO 2 concentration shortly after dawn. Model simulations suggested that the increase in canopy Ca from 1990 to 2000 resulted in a substantial increase in Scirpus gross plant productivity (GPP) of 0.31% per year, which confirms that plant ecosystem simulations need to consider the actual increase in canopy Ca when the simulated time period exceeds a few years. Forcing the model with the measured diurnal Ca pattern increased the simulated net plant productivity (NPP) immediately after dawn and decreased it for the rest of the daytime period, which resulted in a simulated net NPP decrease of about 5% over the growing season. Published by Elsevier Science B.V.
Carbon dioxide exchange in a high marsh on the Texas Gulf Coast: effects of freshwater availability
Agricultural and Forest Meteorology, 2004
The supply of water to the Nueces River Delta near Corpus Christi, Texas is limited by dams and channelization of the river which restrict freshwater inflow. The upper end (high marsh) of the delta frequently dries up during the summer. The marsh consists of slightly elevated islands containing emergent halophytes, and shallow ponds interconnected by narrow channels. Carbon dioxide exchange in the marsh was measured by relaxed eddy accumulation (REA) during two periods, one in 1997 that included two floods from the river followed by an extended period of drying and disappearance of standing water, and the other in 1998 that was in the midst of a drought with no standing water present. The marsh was a net CO 2 sink during periods of high water availability and low sediment salinity, and a net source when water availability was low and salinity was high. During the 1997 period, net ecosystem exchange (NEE) and gross ecosystem production (GEP) ranged from −7.3 g CO 2 m −2 per day (net gain of CO 2) and 12.3 g CO 2 m −2 per day, respectively, after flooding to +8.7 g CO 2 m −2 per day (net loss of CO 2) and 0.4 g CO 2 m −2 per day, respectively, when sediments were dry. NEE and GEP averaged 0.5 and 7.7 g CO 2 m −2 per day, respectively, during this period. Standing water, and water in pores restricted gas exchange between sediment and the atmosphere so that ecosystem respiration (R) increased as the marsh dried, with R ranging from 1.2 to 15.6 g CO 2 m −2 per day and averaging 8.2 g CO 2 m −2 per day. During the 1998 drought, NEE, GEP, and R averaged 5.8, 3.3, and 9.1 g CO 2 m −2 per day, respectively. A 27 mm rain during this period produced a 14-fold increase in GEP and a 75% reduction in R that lasted for 2 days. In 1997, NEE and its components were all significantly correlated at the 0.05 level with water availability as estimated by the cumulative difference between rainfall and evaporation, while in 1998, only NEE and GEP were significantly correlated with water availability. Results of this study indicate that the marsh NEE behaved more like that of a dryland ecosystem than a wetland because of limited freshwater inflow.
Carbon exchange in a freshwater marsh in the Sanjiang Plain, northeastern China
Agricultural and Forest Meteorology, 2011
Northern wetlands are critically important to global change because of their role in modulating atmospheric concentrations of greenhouse gases, especially CO 2 and CH 4 . At present, continuous observations for CO 2 and CH 4 fluxes from northern wetlands in Asia are still very limited. In this paper, two growing season measurements for CO 2 flux by eddy covariance technique and CH 4 flux by static chamber technique were conducted in 2004 and 2005, at a permanently inundated marsh in the Sanjiang Plain, northeastern China. The seasonal variations of CO 2 exchange and CH 4 flux and the environmental controls on them were investigated. During the growing seasons, large variations in net ecosystem CO 2 exchange (NEE) and gross ecosystem productivity (GEP) were observed with the range of −4.0 to 2.2 (where negative exchange is a gain of carbon from the atmosphere) and 0-7.6 g C m −2 d −1 , respectively. Ecosystem respiration (RE) displayed relatively smooth seasonal pattern with the range of 0.8-4.2 g C m −2 d −1 . More than 70% of the total GEP was consumed by respiration, which resulted in a net CO 2 uptake of 143 ± 9.8 and 100 ± 9.2 g C m −2 for the marsh over the growing seasons of 2004 and 2005, respectively. A significant portion of the accumulated NEE-C was lost by CH 4 emission during the growing seasons, indicating the great potential of CH 4 emission from the inundated marsh. Air temperature and leaf area index jointly affected the seasonal variation of GEP and the seasonal dynamic of RE was mainly controlled by soil temperature and leaf area index. Soil temperature also exerted the dominant influence over variation of CH 4 flux while no significant relationship was found between CH 4 emission and water table level. The close relationships between carbon fluxes and temperature can provide insights into the response of marsh carbon exchange to a changing climate. Future long term flux measurements over the freshwater marsh ecosystems are undoubtedly necessary.
Ecosystems
How aquatic primary productivity influences the carbon (C) sequestering capacity of wetlands is uncertain. We evaluated the magnitude and variability in aquatic C dynamics and compared them to net ecosystem CO2 exchange (NEE) and ecosystem respiration (Reco) rates within calcareous freshwater wetlands in Everglades National Park. We continuously recorded 30-min measurements of dissolved oxygen (DO), water level, water temperature (Twater), and photosynthetically active radiation (PAR). These measurements were coupled with ecosystem CO2 fluxes over 5 years (2012–2016) in a long-hydroperiod peat-rich, freshwater marsh and a short-hydroperiod, freshwater marl prairie. Daily net aquatic primary productivity (NAPP) rates indicated both wetlands were generally net heterotrophic. Gross aquatic primary productivity (GAPP) ranged from 0 to − 6.3 g C m−2 day−1 and aquatic respiration (RAq) from 0 to 6.13 g C m−2 day−1. Nonlinear interactions between water level, Twater, and GAPP and RAq resul...
Global Change Biology, 2017
Wetlands can influence global climate via greenhouse gas (GHG) exchange of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual openwater diffusion and ebullition fluxes of CO2, CH4, and N2O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy-covariance measurements of wholeecosystem CO2 and CH4 exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open-water and vegetated cover types. Annual open-water CO2, CH4, and N2O emissions were 915 ± 95 g C-CO2 m −2 yr −1 , 2.9 ± 0.5 g C-CH4 m −2 yr −1 , and 62 ± 17 mg N-N2O m −2 yr −1 , respectively. Diffusion dominated open-water GHG transport, accounting for >99% of CO2and N2O emissions, and ~71% of CH4 emissions. Seasonality was minor for CO2 emissions, whereas CH4 and N2O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open-water fluxes (3.5 ± 0.3 kg CO2-eq m −2 yr −1) exceeded that of vegetated zones (1.4 ± 0.4 kg CO2-eq m −2 yr −1) due to high ecosystem respiration. After scaling results to the entire wetland using object-based cover classification of remote sensing imagery, net uptake of CO2 (−1.4 ± 0.6 kt CO2-eq yr −1) did not offset CH4 emission (3.7 ± 0.03 kt CO2-eq yr −1), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO2-eq yr −1. These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.
Wetlands, 2012
Although freshwater wetlands are among the most productive ecosystems on Earth, little is known of carbon dioxide (CO 2 ) exchange in low latitude wetlands. The Everglades is an extensive, oligotrophic wetland in south Florida characterized by short-and long-hydroperiod marshes. Chamber-based CO 2 exchange measurements were made to compare the marshes and examine the roles of primary producers, seasonality, and environmental drivers in determining exchange rates. Low rates of CO 2 exchange were observed in both marshes with net ecosystem production reaching maxima of 3.77 and 4.28 μmol CO 2 m −2 s −1 in short-and long-hydroperiod marshes, respectively. Fluxes of CO 2 were affected by seasonality only in the short-hydroperiod marsh, where flux rates were significantly lower in the wet season than in the dry season. Emergent macrophytes dominated fluxes at both sites, though this was not the case for the short-hydroperiod marsh in the wet season. Water depth, a factor partly under human control, significantly affected gross ecosystem production at the short-hydroperiod marsh. As Everglades ecosystem restoration proceeds, leading to deeper water and longer hydroperiods, productivity in short-hydroperiod marshes will likely be more negatively affected than in longhydroperiod marshes. The Everglades stand in contrast to many freshwater wetlands because of ecosystem-wide low productivity rates.