Tropical wetlands: seasonal hydrologic pulsing, carbon sequestration, and methane emissions
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- Mitsch, W.J., Nahlik, A., Wolski, P. et al. Wetlands Ecol Manage (2010) 18: 573. doi:10.1007/s11273-009-9164-4
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This paper summarizes the importance of climate on tropical wetlands. Regional hydrology and carbon dynamics in many of these wetlands could shift with dramatic changes in these major carbon storages if the inter-tropical convergence zone (ITCZ) were to change in its annual patterns. The importance of seasonal pulsing hydrology on many tropical wetlands, which can be caused by watershed activities, orographic features, or monsoonal pulses from the ITCZ, is illustrated by both annual and 30-year patterns of hydrology in the Okavango Delta in southern Africa. Current studies on carbon biogeochemistry in Central America are attempting to determine the rates of carbon sequestration in tropical wetlands compared to temperate wetlands and the effects of hydrologic conditions on methane generation in these wetlands. Using the same field and lab techniques, we estimated that a humid tropical wetland in Costa Rica accumulated 255 g C m−2 year−1 in the past 42 years, 80% more than a similar temperate wetland in Ohio that accumulated 142 g C m−2 year−1 over the same period. Methane emissions averaged 1,080 mg-C m−2 day−1 in a seasonally pulsed wetland in western Costa Rica, a rate higher than methane emission rates measured over the same period from humid tropic wetlands in eastern Costa Rica (120–278 mg-C m−2 day−1). Tropical wetlands are often tuned to seasonal pulses of water caused by the seasonal movement of the ITCZ and are the most likely to be have higher fire frequency and changed methane emissions and carbon oxidation if the ITCZ were to change even slightly.
KeywordsBotswanaCarbon sequestrationClimate changeCosta RicaFire ecologyInter-tropical convergence zone (ITCZ)Methane emissionsMonsoonal wetlandsOkavango DeltaPulsing hydrologyTropical swamps
Perhaps on this week in May 2007, the whole of subtropical Florida’s climate had gone mad.
Per unit area, wetlands are among the most important yet vulnerable ecosystems on the planet. They are keenly tuned to the hydrology of their climate, their watersheds, and, in some cases, their coastlines. Wetlands are one of the largest natural sources of the greenhouse gas methane (Bergamaschi et al. 2007) yet at the same time, they have the best capacity of any ecosystem to sequester and retain carbon through permanent burial (Mitsch and Gosselink 2007). Carbon storage in wetlands has another implication. Of the total storage of organic carbon in the earth’s soils, 20–30% or more may be stored in wetlands (Lal 2004, 2008) and is more vulnerable to loss back to the atmosphere as both carbon dioxide and methane if the climate warms or becomes drier.
When climate changes, wetlands are among the first ecosystems to experience the impact. If rainfall does not come on time, if droughts are prolonged by watershed changes, or if water tables drop, wetlands will dry out and sequestered carbon is sent back to the atmosphere by oxidation—either biological processes or sudden fires. Excessive precipitation can expand the areas of wetlands and possibly lead to excessive release of greenhouse gases such as methane and nitrous oxide. Dams and other water impediments to water pulses could lead to extreme anoxic conditions and even greater methane generation than that which occurs during normal river pulsing conditions (Altor and Mitsch 2006, 2008).
While some is known about the global extent and carbon dynamics of northern peatlands in the face of projected climatic changes (See e.g. Wieder and Vitt 2006, Strack 2008), relatively little is known about tropical wetlands—particularly their carbon biogeochemistry. Hydrology of large-scale tropical wetlands has been fairly well described from satellite observations (Hamilton et al. 2002; Prigent et al. 2007) as have overall methane patterns (e.g. Melack et al. 2004). Less is known, however, about the effects that climate shifts might have on tropical wetlands.
This paper first estimates the extent of wetlands in the tropics and then describes the importance of the inter-tropical convergence zone (ITCZ) in general and specifically on hydrologic conditions in southern Africa’s Okavango Delta, a large wetland that is keenly tuned to a seasonal pulse of water coming from hundreds of kilometers away. We then summarize current studies on carbon biogeochemistry in Central America where we are attempting to determine both the importance of wetlands on carbon sequestration and methane emissions and the importance of hydrology, which can be affected at either a climate or watershed scale, on these fluxes of carbon.
The extent of tropical wetlands
Estimates of extent of wetlands in the world by climatic zone (from Mitsch and Gosselink 2007)
Wetland area (× 106 km2)
Maltby and Turner (1983)
Matthews and Fung (1987)
Aselmann and Crutzen (1989)
Finlayson and Davidson (1999)
Ramsar Convention Secretariat (2004)
Lehner and Döll (2004)
Total wetland area
The intertropical convergence zone (ITCZ)
an equatorial zonal belt of low pressure near the equator where the northeast trade winds meet the southeast trade winds. As these winds converge, moist air is forced upward, resulting in a band of heavy precipitation. This band moves seasonally.
The ITCZ dominates the Earth’s tropical and subtropical climate, is the energy origin of hurricanes and typhoons, and has significant seasonal movement from the northern to the southern hemispheres. Its importance for sustaining tropical ecosystems, including monsoonal tropical wetlands, is unmistakable. Close to the equator where the ITCZ persists all year, precipitation amounts of 3,000–4,000 mm/year are common—these regions are referred to as the humid tropics. Further away, where the ITCZ seasonally moves, precipitation is in the order of 500–1,500 mm/year and subsequent river runoff has a distinct flood pulse that is vital for wetlands that have seasonal patterns of wet and dry. In these tropical/subtropical regions the inter-annual variation in rainfall is large with sometimes several consecutive very dry years that might dry up normally wet peaty areas and make them vulnerable to more (or less) methane emissions, droughts and oxidative loss of carbon with fires and microbial respiration.
The ITCZ is a cause of inter-annual variability of flow of many great tropical rivers. Tropical river flows, such as with the Ganges, Nile, Amazon, and Congo, have been shown, in turn, to be correlated with the more familiar El Nino-Southern Oscillation (ENSO) (Khan et al. 2006). Global climate model (GCM) projections as reported by IPCC (2007) suggest some of the tropics will have higher precipitation and streamflow as a result of climate change while other parts of the tropics, particularly the dry tropical regions, will have even less rainfall.
The ITCZ and seasonal pulsing in the Okavango
Two annual endowments of water replenish the delta. The first comes as a quiet flood….The second flush of water comes as the flood recedes. Local rains fall in the austral summer….Over the course of a year, the Okavango expands and contracts to the syncopated rhythms of water. This wild, pulsing heart of the Kalahari is a constant that’s constantly changing.
Frans Lanting (1993), describing the Okavango Delta dual pulses of precipitation, followed by river flooding months later.
The importance of seasonal flooding that results from the seasonal oscillation of the ITCZ is no better illustrated than with the water budget and hydroperiod of the Okavango Delta in northern Botswana in southern Africa. The Okavango Delta is a 12,000 km2 (total flooded area during average years) to 15,000 km2 (total area inundated during extremely wet years) tropical freshwater wetland/upland complex in the semi-arid Kalahari Basin of northern Botswana, Africa. Water flows from Okavango River in Angola and is trapped between two faults (Gomare and Thamalakane Faults), forming the inland delta. Very little if any water leaves by surface or groundwater flow and the Delta is nowhere near a seacoast. Water takes about 4 months to flow 250 km from Mohembo (1,000 m above sea level) to Maun (940 m asl), a very slight gradient. Ecosystems in the Okavango include non-flooded uplands, seasonally flooded floodplains (which are mostly dominated by grasses and sedges rather than woody species) and stream channels and their permanently flooded floodplain dominated by hydrophytes (Ramberg et al. 2006a, b).
Carbon sequestration in wetlands (partially from Mitsch and Gosselink 2007)
g-C m−2 year−1
General average for peatlands
General range for wetlands
Mitra et al. (2005)
Peatlands (North America)
Peatlands (Alaska and Canada)
Turunen et al. (2002)
Turunen et al. (2002)
Thoreau’s Bog, Massachusetts
Tropical papyrus wetland, Kenya
Jones and Humphries (2002)
Tropical papyrus wetlands, Uganda
Saunders et al. (2007)
Created temperate marshes, Ohio
Anderson and Mitsch (2006)
Prairie pothole wetlands, North America
Euliss et al. (2006)
Restored (semi-permanently flooded)
Tropical peatland, Indonesia
56 (for 24,000 year)
Page et al. (2004)
94 (for last 500 year)
Flow-through freshwater wetlands
Bernal and Mitsch (2008a)
Costa Rica (humid tropical)
We are currently investigating soil carbon sequestration at a number of tropical and temperate wetlands in Costa Rica using similar sampling techniques and radiometric dating of the soil cores with 137Cs measurements. Early results comparing carbon sequestration in one of the Costa Rican wetlands with a similar wetland in temperate zone Ohio, first reported by Bernal and Mitsch (2008a), are summarized here. Tropical soil cores were taken from a 112-ha wetland slough at EARTH University campus in the humid tropics of Costa Rica. That wetland is dominated by the swamp palm Raphia taedigera Mart., but it also has a diverse woody canopy and numerous herbaceous understory plants (see Mitsch et al. 2008 for site description). Soil cores were also extracted in a similar fashion from Old Woman Creek (OWC), a 56-ha freshwater flow-through temperate wetland on the Lake Erie coastline in Ohio, USA, during the same month. Two composite cores consisted of three sediment cores spaced within 40 cm were taken in each wetland. Soil core layers were analyzed by gamma spectroscopy and organic and inorganic carbon contents were determined as described by Bernal and Mitsch (2008b).
These preliminary results using identical field and laboratory techniques in tropical and temperate wetlands with similar hydrologic (slow-flowing sloughs) conditions suggest but do not prove that humid tropical wetlands sequester more carbon than do similar wetlands in the temperate zone. More samples will be taken in both Costa Rica and Ohio to validate this conclusion. The conversion of these permanently wet wetlands in the humid tropics to seasonally flooded wetlands with a shift in the ITCZ might lead to more carbon oxidation and less carbon sequestration.
Estimates of annual fluxes of methane from wetlands and other sources, Tg–CH4/year (from Mitsch and Gosselink 2007)
Recent discoveries that aerobic tropical forests emit methane (Frankenberg et al. 2005; Keppler et al. 2006) have led some to suggest that we do not know as much as we thought about wetland methane emissions in the tropics. Bergamaschi et al. (2007) speculated that as a result of these discoveries methane emission estimates from tropical wetlands may “have been overestimated.”
There is the question of how methane emissions from wetlands affect climate but a more important question might be how climate change could affect methane emissions from wetlands in the future. While these questions have been frequently asked for boreal peatlands, few studies have looked at the effects climate and hydrologic changes would have on methane emissions from tropical wetlands if, for example, the ITCZ were to shift in its seasonal movements.
Most of the methane emission studies to date have been in peatlands (bogs and fens) and freshwater marshes. Researchers in boreal wetlands found deep ponds to have higher methane flux rates than other wetland types, neutral fens to have higher rates than acid fens and bogs, and freshwater swamps and marshes to generate more methane than do coastal salt marshes and mangroves. Whalen’s (2005) estimates (Table 3) suggest that tropical and subtropical wetlands may have higher rates of methane production than originally believed. Sorrell and Boon (1992) found methane emissions in Australian billabongs of about 32–60 mg-C m−2 day−1. Delaune and Pezeshki (2003) reported methane emissions of ~7 to over 600 mg-C m−2 day−1 in subtropical Louisiana freshwater marshes with the greatest methane emissions occurred in the summer months. Hadi et al. (2005) measured methane emissions from tropical peatlands in Indonesia and found only 12–53 mg-C m−2 day−1, with the highest number from cultivated paddy fields. Shindell et al. (2004) simulated a global climate model (GCM) with double CO2 conditions and found that methane emissions rose 78%, with most of the increased from existing tropical wetlands.
The upland areas adjacent to the wetlands produced no methane emissions except for the uplands at La Selva tropical rain forest site. The mean upland methane rate there was 19 mg-C m−2 day−1 (Fig. 9). This small methane emission is most likely due to the moist rain forest landscape with its constantly wet soils where methane emissions have been found to be significant (Keppler et al. 2006).
Our preliminary results suggest that wetlands with pulsing hydrology typical of the monsoonal wet-dry cycles may have higher methane emissions than those wetlands that are continuously wet. If tropical wetlands were to transition from permanently flooded conditions and 3,000 mm/year of precipitation to seasonal flooding with 1,500 mm/year of precipitation, as could occur if the ITCZ were to shift its seasonal patterns, much higher methane emissions might result.
Fire significantly affects subtropical and seasonally flooded tropical wetlands such as the Florida Everglades and the Okefenokee Swamp in southeastern USA (see Forward), and the Okavango Delta in northern Botswana during their dry season. These fires represent a major reintroduction of carbon stored in the wetland plants and soils back into the atmosphere. If fire frequency changes in these seasonally flooded wetlands, the release of stored carbon could be much more significant than any slower releases caused by minor changes in water level.
Most fires in the Okavango take place in the two dry periods that fall between the rain pulse of summer that start in September and the seasonal flooding pulse that often reaches the delta as late as May (winter). During these two dry periods thunderstorms are uncommon and most fires are man-made (Cassidy 2003, Heinl 2005). There are several reasons for these man-made fires including: to improve the quality of grazing for wildlife and facilitate their viewing by tourists; to improve hunting; to improve access for fishing; to improve in the quality of thatching grass, reeds and papyrus that all are commonly utilized by villagers; and to construct fire breaks.
On the seasonally flooded floodplains there is a positive correlation between mean frequency of flooding and the frequency of fires up to a level of seven flooding years and three fires (Ramberg et al. 2009) after which the fire frequency drops. These trends are readily explained by the increase of macrophyte primary production in the floodplains caused by higher flooding frequency and the resulting higher fuel load as a determinant for fire frequency (Heinl et al. 2006); fire is less likely where flood frequency is greater than 7 years in fifteen because the increased wetness reduces the possibility of burning.
In the Okavango Delta area there are no long lasting differences in plant biodiversity between areas that have been burnt compared to those with a long period without fires (Heinl 2005) but records of burning only go back 15 years. Fires in Africa and many other tropical parts of the world have been prevalent for several 100,000 years and the biota that now exists in areas with high fire frequencies is adapted to this. Fires will however start new plant successions beginning with annual and pioneer species that after a few years are replaced by short lived perennials and after about 10 years woody plants are taking over. The highest species diversity on a larger scale is often found where there are a number of fire patches of different age.
Conclusions: tropical wetlands and climate change
If climate change affects the dynamics of the inter-tropical convergence zone (ITCZ), will hydrology and biochemistry of tropical wetlands be affected and to what extent?
What are the planetary roles of tropical wetlands in global carbon cycling, particularly in carbon sequestration and methane emissions?
If hydrologic conditions in the tropics shift, what are the global implications of less (or more) methane emissions, fires and other carbon oxidations or reductions in tropical wetlands?
Any changes in the ITCZ could have dramatic effects on the carbon dynamics and productivity of tropical wetlands. Wetlands found in monsoonal climates seasonally affected by the ITCZ may emit more methane than do permanently wet humid tropical wetlands; fires and subsequent release of CO2 are more frequent there as well.
This research was partially supported by the U.S. Department of Energy Grant DE-FG02-04ER63834 (EARTH University/OSU Program on Collaborative Environmental Research in the Humid Tropics; D Hansen, PI); the U.S. Environmental Protection Agency grant EM-83329801-0 (Olentangy River Wetland Research Park: Teaching, research and outreach; W Mitsch, PI); a 2007 Fulbright Senior Specialist grant (Project 2426 to WJ Mitsch) for collaboration with the Harry Oppenheimer Okavango Research Centre, University of Botswana; and by support from the Olentangy River Wetland Research Park, The Ohio State University. We were assisted in so many ways by Bert Kohlmann, Carlos Hernandez, and Jane Yeomans of EARTH University, Costa Rica; by John Holm, University of Botswana; and by Dave Klarer, Old Woman Creek National Estuarine Research Reserve, Huron, Ohio, USA. This paper is based on an invited presentation at the Society of Wetland Scientists (SWS) 2007 conference in Sacramento, CA. Anne Mischo kindly prepared some of the illustrations. Olentangy River Wetland Research Park Publication 2010–001.