Diffusive gas fluxes in neotropical rainforest streams
Björn Skoglund
Student
Thesis in Earth Science 15hp Bachelor level
Report passed: 2015-06-04 Supervisor: Jan Karlsson
Abstract
Rainforests are of great importance to global carbon cycling, but the importance of
deforestation and change in land use is poorly understood due to a lack of studies quantifying the difference in carbon fluxes between original rainforest and agricultural land.
Furthermore, the aquatic outgassing of neotropical systems have been proven to have greater impact on global carbon cycling than previously anticipated (Richey et al 2002).
In this study we investigated the aquatic concentration and daily diffusive gas flux of CO2 and CH4 from 4 pristine sites and 4 impacted sites, respectively, in 4 streams running along a gradient of anthropological impaction in the Atlantic Rainforest, Brazil. Statistically
significant differences between pristine and impacted sites were found in all streams for both CO2 and CH4. On average, the impacted sites were found to be emitting almost three times as much C into the atmosphere as the pristine sites, mainly owing to CO2 emissions
(14172±5226 mg C m-2 d-1). Exploring an area of the neotropical carbon cycle that is not yet fully understood, the study draws attention to the significant difference in aquatic outgassing from rivers observed at different impaction levels and highlights the need for further field studies.
Table of Contents
1 Introduction ... 1
1.1 Background ... 1
1.2 Aim ... 1
2 Material and Methods ... 1
2.1 Site Description ... 1
2.2 Flux Measurement ... 2
2.3 Gas Measurement ... 2
2.4 Temperature and Atmosphere data ... 3
2.5 Flux calculation ... 3
2.6 Statistics ... 4
3 Results ... 4
4 Discussion ... 7
4.1 Carbon dioxide trends ... 7
4.2 Methane trends ... 8
4.3 Conclusions ... 9
5 References ... 10
1
1. Introduction
1.1 Background
A comprehensive understanding of global carbon cycling is an important part of accurately predicting future impact of global climate change and its potential effects on different ecosystems (Intergovernmental Panel on Climate Change, 2007), and rainforests are known to play a major role in the global carbon cycle (Schimel et al 2001). However, neotropical rainforests have been subject to extensive anthropological disruption in the form of
increasing agricultural activity and expanding urbanization. An area of particular interest for the restoration of previously disrupted natural carbon cycling is the Atlantic Rainforest, which currently spans 7,5% of its original area (Mics et al 2013) and has been massively impacted by logging and anthropological conversion ever since colonization began.
While the negative impact on carbon cycling caused by rainforest logging is well-documented regarding the neotropics’ ability to act as a carbon sink (Schimel et al 2001, Gloor et al), the effects on the global carbon cycle caused by aquatic outgassing from riverine sources in tropical areas are not as well explored. Outside of the tropics, however, studies of riverine outgassing’s impact on carbon cycling from streams as well as lakes have been conducted in boreal and sub-arctic areas (Lundin 2013) using methods that could be applied in the tropics, and to assess the carbon dynamics of an aquatic system several studies have successfully used the primary method employed in this paper, measuring the water-air flux interaction in order to extract information concerning respiration in aquatic systems (Jonsson et al 2008, Alin et al 2011, Jähne et al 1987). Studies concerning similar outgassing in reservoirs located in temperate areas show that water contained in reservoirs generally act as a large carbon sink with comparatively low carbon fluxes (Knoll et al 2013). Studies conducted in subtropical China found that outgassing from reservoirs is largely dependant on seasonal variations in precipitation (Wang et al 2015). These findings imply that both turbulence and water cycling are important factors in anticipating flux conditions, especially for tropical conditions where precipitation can vary greatly depending on season, which is interesting for the riverine outgassing assessment conducted in this study. Although studies of aquatic carbon outgassing in the neotropics are rare, rainforest methane fluxes, especially from aquatic sources, are even less documented. However, some soil chemistry studies in rainforest areas are attempting to discover more about methane’s role in carbon fluxes and the effect different types of land usage has on it (Lammel et al 2015).
1.2 Aim
This study aims to provide results showing that there is an observable and significant difference in CH4 and CO2 concentrations as well as diffusive gas fluxes between areas covered in pristine rainforest and areas impacted by anthropological activities such as deforestation and urbanization, and that deforestation in the neotropics has a direct impact on aquatic carbon outgassing.
2. Material and methods
2.1 Site Description
Four streams located in REGUA (Reserva Ecologica de Guapiacu), all situated within the watersheds of Serra do Mar in the upper Guapiacu river basin, were chosen based on their proximity to pristine and impacted rainforest. Guapiacu is located in the Atlantic Rainforest, 120 kilometers north of Rio de Janeiro, Brazil. The selected streams are situated in
2
catchment areas where anthropological impaction is abundant, caused mainly by
deforestation and subsequent conversion of the landscape into agricultural land or grazing pastures. The distance and elevation difference between streams and their sites were minimized as far as local topography and available streams allowed. The average stream distance between sites was 4 kilometers. The average elevation for pristine sites was 173,2 m above sea level, while the impacted sites had an average elevation of 53,1 m a.s.l.
With this stream selection in mind, two sampling sites were chosen along each of the four main streams. The primary sampling site for each stream served as a representative for pristine rainforest, while the secondary site was located in an area where the stream was surrounded by agricultural land or heavily impacted rainforest. As GIS data was not available for the area, publically available satellite imagery was used to create a rudimentary map over the sample streams and immediate surroundings (Google Earth, Figure 1). At each sampling site, three subsites within 10 meters of each other were used to obtain triplicate samples, in order to represent the different turbulence conditions within the locale. Water sampling was conducted two days a week during four weeks, two in October and two in November. Due to equipment malfunction, only three weeks of flux chamber data was usable. As REGUA is located in a neotropical zone (Aw), the sampling period coincided with the start of the rain period.
Figure 1 Satellite imagery of sample streams and surrounding area. Note the agricultural or urbanized areas proximal to all impacted sites.
2.2 Flux measurement
At the subsites, the CO2 emission flux was measured using a polymethylmethacrylate flux chamber equipped with a SensAir sensor suspended above the stream with a tripod. A custom built control unit collected emission data for three cycles of 30 seconds each, with an 18 second window of gas analysis between the cycles. The chamber was then flushed with the same procedure before another collection cycle resumed. This procedure was performed
3
three times for each sample point, yielding three CO2 flux slopes per sampling occasion and subsite. The program DAS100 Logging Software was used to process the data received from the sensor.
2.3 Gas measurement
Gas concentrations of stream water was sampled by injecting 4ml of water in a 22 ml glass vial, prepared by introducing 100 µl of 1,2 molar HCl into the vial and subsequently flushing it with with N2O for 1,5 minutes. The samples were later processed in a gas cromatograph (Clarus 500, PerkinElmer Inc., U.S.) equipped with a FID (Flame Ionization Detector) at 250°C. The samples were introduced to the GC by manual injection, where 0,1 ml gas was injected to determine the concentration of CH4 and CO2 in the water. Three standards with triplicates for each standard was used for calibrating the instrument. The samples were ran three times, with calibration being performed before each run. Water CO2 measurements were also conducted in realtime using a Vaisala handheld CO2 meter.
2.4 Temperature and atmosphere data
Data for air temperature was collected with the SensAir sensor at the time of flux chamber placement. Water temperature was collected with a thermometer at the time of flux chamber placement. Atmospheric CO2 was recorded before flux chamber placement using the SensAir sensor, while an atmospheric CH4 concentration of 1,8 μatm was inferred from the global atmosphere average (Dlugokencky et al 2009).
2.5 Flux calculation
Using the measurements of CO2 concentration along with the flux data collected, the gas transfer coefficient was calculated in order to estimate the diffusive flux of CO2 from the different streams.
𝑘 =(
𝐹𝐶𝑂2 𝐶𝑂2𝑤𝑎𝑡𝑒𝑟−𝐶𝑂2𝑎𝑖𝑟)
𝑆𝑑 (1) Where k is the gas transfer coefficient, FCO2 is the CO2 flux as estimated by the logged CO2
slopes (ppm/s-1) along with flux chamber height (cm), Henry’s constant for CO2 (mol/atm) (Wanninkhof 1992) and atmospheric pressure (bar), CO2water is the CO2 concentration in water (µmol/m-3), CO2air is the CO2 concentration of the water-air equilibrium (µmol/m-3) and Sd is stream depth (cm). After Alin (2011), k values were then normalized to a Schmidt number (viscous diffusion divided by molecular mass diffusion rate) of 600 (20°C) to compare gas transfer velocity among sites using the following formula:
𝑘600= 𝑘(600
𝑆𝑐𝑡)𝑛 (2)
Where k is the measured k value of a specific site, ScT is the Schmidt number for the
temperature T, and 600 is the Schmidt number for CO2 at 20°C. As all stream surfaces were determined to be turbulent, n was set to -0,5 (Jonsson et al 2008). ScT, the Schmidt number for CO2, is calculated as a function of temperature (Wanninkhof 1992):
4
𝑆𝑐𝑇 = 1911,1 + 118,11𝑇 + 3,4527𝑇2− 0,04132𝑇3 (3)
To determine the specific gas transfer coefficient for temperatures across the different sites, the following equation (similar to equation 2) was used after Jähne et al (1987).
𝑘𝑡 = 𝑘600 (𝑆𝑐𝑡/ 𝑆𝑐600)n (4)
Finally, using the temperature-specific kt, the diffusive fluxes for CO2 were estimated for the flux chamber runs using the following equation after Jonsson et al (2008):
𝐹𝑙𝑢𝑥 = 𝑘𝑡(𝐶𝑂2𝑤𝑎𝑡𝑒𝑟− 𝐶𝑂2𝑎𝑖𝑟) (5)
The Sensair sensor mounted in the employed flux chamber was not capable of measuring CH4 emissions to produce CH4-specific slopes. Thus, k600 (equation 4) was used both for calculating diffusive CO2 and CH4 fluxes as done in Lundin et al (2013). However, Sct is in the case of CH4 replaced with ScTCH4, the temperature specific Schmidt number for CH4
(Wanninkhof 1992):
𝑆𝑐𝑇𝐶𝐻4= 1897,8 + 114,28𝑇 + 3,2902𝑇2− 0,039061𝑇3 (6)
2.6 Statistics
All data was logarithmically transformed to reduce skew, whereafter an Anderson-Darling test was performed in Minitab to determine normality within the dataset. As all data was determined to be normally distributed, two-sided t-tests with confidence intervals of 95%
were performed in Minitab. The t-tests compared gas concentrations and fluxes, respectively, between pristine and impacted streams.
3. Results
As shown in table 1, mean CO2 concentration in pristine sites was found to be 27±6 mg/l, compared to mean CO2 concentration in impacted sites where measurements showed an average concentration of 39±12 mg/l. All streams were supersaturated with CO2 when compared with atmospheric CO2 on every sampling occasion; mean stream CO2
concentration observed across all sites was 2165±742 ppm, while atmospheric CO2 was recorded at a mean of 415±30 ppm. In contrast to the CO2 results, average CH4 concentration for pristine sites was determined to be 205±35 µg/l, while impacted sites sported a slightly lower mean CH4 concentration at 179±24 µg/l.
For diffusive fluxes, the values follow the same general trends as the gas concentrations, albeit with considerable standard deviations; mean CO2 flux in pristine sampling sites across
5
all weeks was found to be 5265±4208 mg CO2 m-2 d-1, while impacted sampling sites were determined to be emitting an average CO2 flux of 14172±5226 mg per square meter and day (Table 2). Thus, a trend of lower diffusive flux in pristine sites compared to impacted sites can be observed across all weeks of sampling (Figure 3). The same trend holds true for
aquatic CO2 concentrations, while the CH4 relationships appear more complicated (Figures 2, 4).
A two-sided t-test revealed that all subsets of data had statistically significant differences between pristine and impacted sites, as indicated in table 3.
Table 1 Mean aquatic gas concentrations from impacted and pristine sites across all sampling occasions.
CO2 Pristine
(mg/l) CO2 Imp. (mg/l) CH4 Pristine
(µg/l) CH4 Imp. (µg/l)
Mean 27 39 205 179
Stdev 6 12 35 24
Table 2 Calculated mean diffusive flux emissions per square meter and day in impacted and pristine sites across all sampling occasions.
CO2 Pristine flux (mg C m-2 d-1) CO2 Impacted flux (mg C m-2 d-1)
Mean 5265 14172
Stdev 4208 5226
Table 3 Variables compared in a two-sided t-test and their statistical significance as determined with an α-value of 0,05.
Pristine/ Impacted
CO2 concentrations Pristine/ Impacted
CH4 concentrations Pristine/ Impacted CO2
Flux emissions
P-value 0,002 0,017 0,002
6
Figure 2 Mean aquatic CO2 concentrations from pristine and impacted sites across 4 weeks of sampling
Figure 3 Diffusive CO2 flux emissions in impacted and pristine sites across 3 weeks of sampling.
0 10 20 30 40 50 60
Week 1 Week 2 Week 3 Week 4
CO2 content (mg/l)
Mean CO
2content of rainforest streams
Pristine Impacted
0 5000 10000 15000 20000 25000
Week 1 Week 2 Week 3
CO2 flux [mg C m-2d-1]
Mean CO
2flux of rainforest streams
Pristine Impacted
7
Figure 4 Mean aquatic CH4 concentrations from pristine and impacted sites across 4 weeks of sampling.
4. Discussion
While carbon fluxes from neotropical streams have been previously investigated (Mcdowell &
Asbury 1994), few studies have focused on quantifying the difference in diffusive gas fluxes between pristine and impacted areas although riverine flux emissions are known to represent an important factor in neotropical areas (Richey et al 2002). As this study was carried out at a remote area with very limited laboratory resources, and with the additional limitation of a single person’s carrying capacity, certain sampling which would have been useful for this study had to be omitted. Regardless of this unfortunate scarcity of data, the trends shown in this study clearly indicate that deforestation may have larger impacts on carbon cycling in the neotropics than previously anticipated, due to carbon mobility from tertiary sources such as diffusive flux emissions from streams increasing significantly with anthropological
impaction.
4.1 Carbon dioxide trends
As shown in table 1 and 2, an immediate increase in aquatic CO2 concentration and
subsequent diffusive flux emission from streams can be observed as a neotropical landscape covered in rainforest transitions into farmland or settlement. These trends are consistent with similar studies conducted on non-aquatic flux emissions from pristine rainforest versus agricultural land (Fowler 2011). The source of the increased carbon dioxide content in impacted streams is somewhat uncertain as carbon speciation could not be conducted due to
0 50 100 150 200 250
Week 1 Week 2 Week 3 Week 4
CH4content [µg/l]
Mean CH
4content of rainforest streams
Pristine Impacted
8
limited resources - however, as Mayorga et al (2005) has shown, high carbon dioxide content in rivers stem largely from respiration of a local organic carbon source. As deforestation and establishment of agricultural or pasture areas profoundly changes the source of organic carbon being introduced into the stream, affected strongly by a change in vegetation and thus making the area a much less efficient carbon sink (Gloor et al 2012), the mechanism
proposed by Mayorga could explain the trend observed in this study. This cannot be concluded by simply looking at unspeciated gas concentrations and fluxes however, but would need to be confirmed with further studies utilizing isotopic signatures to determine the source of the elevated CO2 concentration in impacted areas.
4.2 Methane trends
The CH4 trends in this study are less conclusive than their CO2 counterparts, and the
response of methane fluxes to land use change aren’t as well understood (Lammel et al 2015).
Due to aquatic CH4 content not following as strong of a trend as was the case with the CO2
content, it is unlikely that CH4 differs between impacted and pristine rainforest segments.
Flux calculations were carried out, but as the water-air CH4 flux was determined to be positive although atmospheric concentration were higher than water concentrations, it is fairly likely that, given the method used in this paper to calculate CH4, the strong CO2 slopes may have affected the CH4 flux results. Thus, the CH4 flux results were considered to be unreliable and were omitted. To further investigate this, a larger-scale experiment with methane-specific sampling during a longer time period would likely have to be done.
According to the data presently collected from these streams however, CH4 is responsible for only 0,008% of total C emissions.
4.3 Conclusions
As large areas of neotropical rainforest are deforested in favor of agriculture, the results shown in this study describe an often overlooked effect of deforestation in the neotropics.
In conclusion, the findings in this study indicate that outgassing from aquatic systems in the neotropics are directly affected by changes in land use, and that it may have a significant impact on the carbon cycle of the area. Specifically, the results show that streams located in anthropologically impacted areas are likely to release more greenhouse gases into the
atmosphere than streams located in pristine rainforest. Further studies of this subject should include carbon isotope speciation, DIC/DOC/POC sampling as well as expanded water and soil chemistry work to expose the underlying mechanisms behind the trends shown in this study. Interestingly, the chosen study area is currently in the process of being reforested - the non-profit organization behind REGUA, concerned with rainforest replantation, has during the last decade been purchasing impacted land in the Guapiacu basin with the purpose of reforestation. Given the results in this study, this opens the possibility of a secondary study to quantify the positive impact of reforestation on stream flux emissions and thus
reforestation’s impact on neotropical carbon balance.
9
5. References
Alin, S. R., Rasera, M., Salimon, Cleber., Richey, J., Holtgrieve, G. W., Krusche, A. V. and Snidvongs, A. 2011. Physical controls on carbon dioxide transfer velocity and flux in low- gradient river systems and implications for regional carbon budgets. J. Geophys. Res.
116:G01009
Dlugokencky, E. J., Bruhwiler, L., White, J., Emmons, L., Novelli, P., Montzka, S., Masarie, K., Lang, P., Crotwell, A., Miller, J., Gatti, L. 2009. Observational constraints on recent increases in the atmospheric CH4 burden. Geophys. Res. Lett. 36:LI8803
Fowler D., Nemitz E., Misztal P., Di Marco C., Skiba U., Ryder J., Helfter C., Cape J. N., Owen S., Dorsey J., Gallagher M. W., Coyle M., Phillips G., Davison B., Langford B., MacKenzie R., Muller J., Siong J., Dari-Salisburgo C., Di Carlo P., Aruffo E., Giammaria F., Pyle J. A. and Hewitt C. N. Effects of land use on surface–atmosphere exchanges of trace gases and energy in Borneo: comparing fluxes over oil palm plantations and a rainforest. 2011. Philosophical Transactions of the Royal Society 366:1582
Gloor, M., Gatti, L., Brienen, R., Feldpausch, TR., Phillips, OL., Miller, J., ; Ometto, JP., Rocha, H., Baker, T. and de Jong, B. 2012. The carbon balance of South America: a review of the status, decadal trends and main determinants. Biogeosciences 12:5407-5430
Google Earth 7.1.2.2041. 2015. Guapiacu Basin 22°26'60"S,
42°46'0.001"W. <http://www.google.com/earth/index.html> [Viewed 4 January 2015].
10
Intergovernmental Panel on Climate Change. 2007. Climate change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change.
Jonsson, A., Åberg J., Lindroth A. and Jansson M. 2008. Gas transfer rate and CO2 exchange between an unproductive lake and the atmosphere in northern Sweden. J.
Geophys. Res 113:G04006
Jähne, B., Münnich, K. O., Bösinger, R., Dutzi, A., Huber, W. and Libner, P. 1987. On the parameters influencing air-water gas exchange. J. Geophys. Res. 92:1937-1949
Knoll, L., Vanni, M., Renwick, W., Dittman, E., Gephart, J. 2013. Temperate reservoirs are large carbon sinks and small CO2 sources: results from high-resolution carbon budgets.
Global Biogeochemical Cycles 27:52-64.
Lammel, D. R., Nüsslein, K., Tsai, S. M. and Cerri, C. C. 2015. Land use, soil and litter chemistry drive bacterial community structures in samples of the rainforest and Cerrado (Brazilian Savannah) biomes in Southern Amazonia. European Journal of Soil Biology 66:32-39
Lundin, E., Giesler, R., Persson, A., Thompson, M. and Karlsson, J. 2013. Integrating carbon emissions from lakes and streams in a subarctic catchment. J. Geophys. Res. 118:1200-1207.
Mayorga, E., Aufdenkampe, A., Masiello, C., Krusche, A., Hedges, J., Quay, P., Richey, J. and Brown, T. 2005. Young organic matter as a source of carbon dioxide outgassing from
Amazonian rivers. Nature 436:538-541.
McDowell, W. H, Asbury, C. E. 1994. Export of Carbon, Nitrogen, and Major Ions from Three Tropical Montane Watersheds. Limnology and Oceanography 39:111-125.
Mics, F., Rozak, AH., Kocsis, M., Homoródi, R. and Hufnagel, L. 2013. Rainforests at the beginning of the 21st century. Applied ecology and environmental research 11:1-20 Richey, J., Melack, J., Aufdenkampe, A., Ballester, V. and Hess, L. 2002. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature 416:617-620.
Schimel, D.S., House, J. I., Hibbard,K. A., Bousquet, P., CiaisP., Peylin, P., Braswell, B.H., AppsM.J., Baker,D., Bondeau, A., Canadell, J., ChurkinaG. and Cramer,W. 2011. Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems. Nature 414:169-172.
Wang, F., Cao, M., Wang, B., Fu, J., Lyo, W., Ma, J. 2015. Seasonal variation of CO2 diffusion flux from a large subtropical reservoir in East China. Atmospheric Environment 103:129-137.
Wanninkhof, R. 1992. Relationship between wind speed and gas exchange over the ocean. J.
Geophys. Res. 97:7373-7382
