Letter pubs.acs.org/journal/estlcu
Impact of Hurricane Sandy on CH4 Released from Vegetated and Unvegetated Wetland Microsites David S. Pal, Matthew C. Reid, and Peter R. Jaffé* Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *
ABSTRACT: Hurricane Sandy was one of the largest tropical storms to pass over the Atlantic basin, causing destruction along its path as it made landfall in Jamaica, Cuba, the Bahamas, and the United States [Tropical Cyclone Report: Hurricane Sandy; National Weather Service, National Oceanic and Atmospheric Administration, 2012 (http://www.nhc.noaa.gov/data/tcr/ AL182012_Sandy)]. Hurricane Sandy passed over the Meadowlands in the midst of our multiyear study on marsh CH4 dynamics, providing a unique opportunity to evaluate the effects of hurricanes on CH4 cycling within an estuarine marsh. We modeled dissolved CH4 distributions in wetland sediments from 2011 (Reid, M. C.; Tripathee, R.; Shäfer, K. V. R.; Jaffé, P. R. Tidal Marsh Methane dynamics: Difference in seasonal lags in emissions driven by storage in vegetated versus unvegetated sediments. J. Geophys. Res.: Biogeosci. 2013, 118, 1802−1813) to 2013 and estimated that Hurricane Sandy did not degas vegetated soils but degassed between 45 and 75% of the dissolved CH4 in unvegetated sediments. Hurricanes may regularly affect coastal wetland CH4 emissions globally, but because these wetland sediments do not store substantial dissolved CH4 late in the year, the degassing of these sediments by Hurricane Sandy did not play an important role in the annual carbon emissions from this marsh.
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INTRODUCTION In an era when policymakers are looking for simple ways to mitigate and reverse the effects of human activities on the climate, wetland scientists have presented a simple option to assist with these problems, blue carbon, sequestration of carbon in coastal wetland ecosystems. Wetland sediments sequester between 21 and 473 g of C m−2 year−13,4 from the atmosphere, fixing carbon into biomass (microbial, sediment, and plant matter). Despite the potential for salt marshes to sequester large amounts of blue carbon, there is also the potential for these areas to release significant amounts of CH4. Wetlands account for the largest natural source of release of CH4 into the atmosphere, and specifically, coastal wetlands emit approximately 40−160 Tg of CH4 year−1, despite their relatively small footprint.5−7 Understanding and modeling the controlling factors for CH4 production and emissions from wetlands is a major research priority.3,8−13 The most important controls on CH4 production are temperature, sediment redox potential, and sediment organic carbon. Diffusive/advective, ebullitive, plant-mediated, and oxidative loss mechanisms balance CH4 production to control emissions from sediments. Diffusive/advective losses are from exchange of pore water within sediments (e.g., by tidal or lateral groundwater flows). Ebullitive emissions are the losses due to bubbling, and oxidative losses account for CH4 that is consumed by methanotrophs in the oxygenated layers of sediments before being released into the atmosphere. The sum © 2014 American Chemical Society
of these three pathways is estimated to be between 15 and 70%14,15 of total CH4 emissions. Plant-mediated volatilization accounts for the remaining losses from sediments, between 30 and 85% with some estimates as high as 100% of the total coastal wetland CH4 emissions.14,15 These are multiple wellestablished CH4 emissions pathways from wetlands, but there are no studies examining the effect of hurricanes as an emissions trigger.16 For example, Chabreck et al. explain how vegetation in marshes may be affected by hurricanes but do not examine below-ground nutrients or dissolved gases.17 On October 29th, 2012, Hurricane Sandy made landfall in New Jersey and passed through the tidal marshes of the Hackensack Meadowlands. Hurricane Sandy was characterized by sustained winds of >100 km h−1, with gusts up to 160 km h−1, and storm surges along the coast of New Jersey and New York up to 3 m above normal high-tide levels.1 This paper explores the effects of the storm on the degassing of CH4 pools from a marsh within the Meadowlands. This paper presents a time series of dissolved CH 4 distributions in vegetated and unvegetated sediments between March 2011 and October 2013. The data from March 2011 to June 2012 have been published previously in ref 2. An Received: Revised: Accepted: Published: 372
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exponential temperature-response model was applied to the reservoir data to explain interannual variation and separate the effects of Hurricane Sandy from “natural” variations on the CH4 reservoirs in the sediments. Although the data presented here are specific for the marsh studied, conclusions can be generalized to the effect of hurricanes on gas emissions in coastal marshes.
where CH4,t represents the below-ground dissolved CH4 reservoir/pool size (micromoles per square meter) at time t. The sediment temperature, Tt+lag, is the temperature measured from peeper-sampled water approximately 1 month earlier for vegetated sediments and 2 months earlier for unvegetated sediments. For November 2012, the closest sample point, June 2012, was used. The procedure for determining lag time, α and β, and reservoir calculations is given in detail in ref 2. To separate the effects of Hurricane Sandy from normal interannual variation, we calibrated this temperature-based model to the data up to and including June 2012. We confirmed the differences in pre- and post-Sandy methane reservoirs by performing a t test on the residuals between average peeper measurements and model predictions (p < 0.01). The model and complete data set are plotted in Figure 1 of the Supporting Information.
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SITE DESCRIPTION The Marsh Resource Meadowlands Mitigation Bank (MRMMB) (40.82°N, 74.05°W) is a tidal brackish marsh along the Hackensack River Estuary in northern New Jersey. The marsh is connected through Newark Bay to the Hackensack River by way of a series of tidal creeks. The MRMMB has an area of 83 ha, with a distribution of approximately 30% unvegetated and 70% vegetated cover, according to aerial images from 2007. Spartina alternif lora covers approximately 85−90% of the vegetated low marsh zone, with the remaining vegetated area covered by Phragmites australis. The vegetated and unvegetated microsites remain in the intertidal zone, and the vegetated sediment surface is approximately 40 cm higher in elevation than the unvegetated sediment surface. This site was restored in 1999 to remove invasive P. australis stands and restore tidal hydrology. Pore water salinity ranges between 3.0 and 10.8 ppt, depending on depth and time of year.
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RESULTS AND DISCUSSION Seasonal trends and interannual variations in CH4 distributions are shown in Figure 1. In general, the sizes of the CH4 pools
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MATERIALS AND METHODS Pore Water Dialysis Samplers. Distributions of dissolved CH4 and other geochemical constituents were measured using pore water dialysis samplers, or “peepers”, at 3 cm increments to a depth of 60 cm.18,19 The peepers could be sampled repeatedly without disturbing the sediments. These peepers need to equilibrate with the surrounding pore water for more than 1 month to ensure accurate measurements. For the sake of accuracy, peepers were allowed to equilibrate for 6 weeks between samplings. This allowed for each peeper to be measured at the same location and at consistent depths over multiple years. Two identical peepers were installed within 1 m of each other in both microsites in December 2010. Peeper sampling procedures and proper equilibration time are described in detail in refs 2 and 18. In 2011 and early 2012, peepers were sampled at low tide approximately once every 6 weeks to characterize temporal changes in CH4 dynamics.2 Beginning in June 2012, the peepers were sampled seasonally with the goal of capturing seasonal and interannual variations in CH4 profiles and reservoirs. Dissolved Gas Analysis. Dissolved CH4 concentrations were measured using a gas chromatograph (GC-2014, Shimadzu, Kyoto, Japan) equipped with a flame ionization detector and a 2m Porapak Q column (Supelco) supplied with He carrier gas. Dissolved gases were analyzed after pore water had been shaken in a N2 headspace for at least 10 min.5 CH4 Reservoir Modeling. CH4 reservoirs were calculated by numerically integrating measured CH4 concentrations from peepers in 3 cm increments from the surface to 60 cm. An empirical temperature-response model was used to describe below-ground CH4 reservoirs as an exponential function of sediment temperature.5,6 CH4, t = exp(α + βTt + lag)
Figure 1. Integrated methane pools from March 2011 to November 2013 sampled every 6 weeks or seasonally from each microsite. Circles (unvegetated) and X’s (vegetated) are data points collected, while lines represent microsite averages. After January 2012, there was only one active peeper in the unvegetated sediments, and the line follows the path of that peeper.
increase from spring through summer and then decrease gradually through the fall and winter. Throughout the year, the average unvegetated CH4 pool is larger than the average vegetated CH4 pools and remains later into the year because the root systems in the vegetated sediments vent the CH4. The mass of dissolved CH4 remains higher in the unvegetated sediments than in the vegetated sediments through the fall and winter, a difference that has been attributed to the storage of CH4 in sediment gas bubbles beneath unvegetated sediments.6 To emphasize the effects of Hurricane Sandy on late fall and winter CH4 dynamics, Figure 2 shows the range of selected CH4 reservoirs from the vegetated and unvegetated sediments in November 2011, 2012, 2013 and January 2012, 2013. Comparing November 2011 and January 2012 with November 2012 and January 2013, we see that there are substantial differences in both the range and average of CH4 reservoirs. One year after the hurricane, the November 2013 CH4 reservoirs fall between the reservoir measured in 2011 and 2012.
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decrease to a small fraction of the minimal reservoir size in the unvegetated area. This non-zero CH4 reservoir indicates the storage capacity of unvegetated sediments. Hurricane Sandy made landfall on October 29, 2012, midway through a multiyear sampling campaign of tidal marsh CH4 dynamics. Through this sampling campaign, we can compare CH4 reservoirs in vegetated and unvegetated sediments before, immediately following, and after the hurricane. In both sediment types, the November and January reservoirs immediately following Hurricane Sandy were smaller than would be expected from measures during prior years. On the basis of our modeling results, the differences in the vegetated sediments can be explained by interannual temperature variations, while the unvegetated sediments show the potential degassing effects of Hurricane Sandy. On the basis of the relative difference between modeled CH4 reservoirs and measured reservoirs in November 2012, 45−75% of belowground CH4 was removed from the top 60 cm of unvegetated sediments because of the hurricane. This accounts for 30 kg of CH4 ha−1 released to the atmosphere. Previous studies indicate that the increased water levels and wind speeds from hurricanes can change water flow paths and the export of nutrients from the wetland.20−22 These changes in flow patterns can destabilize soil columns and change soil storage capacity. This explanation would explain why the vegetated sediments, which have roots to assist sediment column stabilization, were relatively unaffected by the hurricane, as opposed to unvegetated sediments that have no root stabilization. We hypothesize that increased water flow flushed dissolved gases and other constituents through the unvegetated sediment and out to the Hackensack River. Pore water measurements revealed distinct trends in the dissolved CH4 profiles from the top 60 cm of different vegetation microsites that were not influenced by Hurricane Sandy. In the vegetated sediment, the CH4 profiles were characteristic of a traditional redoxocline of sulfate reduction− methanogenesis competition because the low-tide water table remains below the sediment surface.2 This is a result of tidaldriven redox oscillations in upper sediments.3 In contrast, the unvegetated sediment pore water profiles show elevated CH4 concentrations and depleted sulfate levels throughout the sediment column and less evidence of the sulfate−methanogenesis competition. The higher concentrations of CH4 in pore water samples from unvegetated sediments, as compared with those in vegetated sediments, demonstrate the inefficient venting of sediments without plant roots.2 It is also important to consider the timing of the hurricane and the wetland footprint on the scale of the impact. The peak CH4 reservoirs occur between August and October, and if Hurricane Sandy had hit during the peak of the CH4 reservoir’s mass, the amount of CH4 released would have doubled. If a hurricane were to hit a vegetated area earlier in the hurricane season, when there is more CH4 stored below ground, it is possible that a fraction of this CH4 would have also been released. Furthermore, we would expect to find proportionally larger amounts of dissolved CH4 below 60 cm. This would increase the expected reservoir size by a factor of 2−5, depending on the depth of the active soil. If the 90 million acres of U.S. wetlands along the Atlantic Ocean23 were affected throughout a single hurricane season, an additional 0.4−5 Tg of CH4 could be released into the atmosphere. Although larger data sets would provide more confidence for the estimates presented here, our conclusions are based on a
Figure 2. Top and bottom lines of the modified box and whisker plots represent measured methane reservoirs from individual peepers, while the middle lines represent the average of those measurements. Only November and January samples are shown in this figure to highlight the samples taken immediately following Hurricane Sandy. For the complete model data set and comparison, see the Supporting Information. The black X’s represent the modeled methane reservoirs for these time points based on the temperature-response model (eq 1). Panel a presents the measurements and model results from the vegetated sediments and panel b presents the measurements and model results from the unvegetated sediments. Note the different y-axis scales in panels a and b.
Our model was successfully calibrated to eight sampling points previous to Hurricane Sandy in the vegetated sediments with a 1 month soil temperature lag (α = 1.727, β = 0.131, R2 = 0.96) and in the unvegetated sediments (α = 4.898, β = 0.039, R2 = 0.82) with a 2 month soil temperature lag. These data show that the model accurately predicts the November 2012 and January 2013 CH4 reservoirs in the vegetated sediments based on temperature variation and show a significant decrease from the predicted CH4 reservoirs in unvegetated sediments in November 2012 and January 2013 (p < 0.01; see the Supporting Information). The relative difference between modeled and measured data in the unvegetated sediments in 2013 indicates that the effects of the hurricane may be longlasting, and these sediments take more than 1 year to return to “normal” reservoir trends. The March 2011 to June 2012 year of sampling represents a baseline year of CH4 dynamics for this wetland (Figure 1). The sizes of CH4 reservoirs in both zones increase through the summer with a peak near saturation in August/September and then decrease (vegetated sediments) or remain semiconstant (unvegetated sediments) through the fall. In the unvegetated sediments, the CH4 pool persists through the majority of the winter and decreases in size through spring. Normally, in the fall, the sizes of CH4 reservoirs in the vegetated sediments 374
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(15) Li, T.; Huang, Y.; Zhang, W.; Song, C. CH4MODwetland: A Biogeophysical Model for Simulating Methane Emissions from Natural Wetlands. Ecol. Modell. 2010, 221, 666−680. (16) Michener, W.; Blood, R.; Bildstein, K.; Brinson, M. M.; Gardner, L. R. Climate Change, Hurricanes and Tropical Storms, and Rising Sea Level in Coastal Wetlands. Ecological Applications 1997, 7, 770−801. (17) Chabreck, R. H.; Palmisano, A. W. The Effects of Hurricane Camille on the Marshes of the Mississippi River Delta. Ecology 1973, 54, 1118. (18) Macdonald, L.; Paull, J. S.; Jaffé, P. R. Enhanced Semipermanent Dialysis Samplers for Long-term Environmental Monitoring in Saturated Sediments. Environ. Monit. Assess. 2012, 185, 3613−3624. (19) Hesslein, R. An in Situ Sampler for Close Interval Pore Water Studies. Limnol. Oceanogr. 1976, 21, 912−914. (20) Morton, R.; Barras, J. Hurricane Impacts on Coastal Wetlands: A Half-Century Record of Storm-Generated Features from Southern Louisiana. J. Coastal Res. 2011, 27, 27−43. (21) Deng, Y.; Solo-Gabriele, H. M.; Laas, M.; Leonard, L.; Childers, D.; He, G.; Engel, V. Impacts of Hurricanes on Surface Water Flow within a Wetland. J. Hydrol. (Amsterdam, Neth.) 2010, 392, 164−173. (22) Kang, W.; Trefry, J. H. Retrospective Analysis of the Impacts of Major Hurricanes on Sediments in the Lower Everglades and Florida Bay. Environmental Geology 2010, 44, 771−780. (23) Status and trends of wetlands in the coastal watersheds of the Eastern United States 1998 to 2004. National Oceanic and Atmospheric Administration, National Marine Fisheries Service, and U.S. Department of the Interior, Fish and Wildlife Service, 2008.
unique set of measurements that indicate that hurricanes have an important effect on the degassing of CH4 from salt marshes.
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ASSOCIATED CONTENT
S Supporting Information *
Eight pre-Sandy calibration points versus the calibrated model. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: jaff
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was funded by National Science Foundation Collaborative Research Rapid Award 1311796. REFERENCES
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