Abstracts
It is well accepted in the literature that lakes are generally net heterotrophic and supersaturated with CO2 because they receive allochthonous carbon inputs. However, autotrophy and CO2undersaturation may happen for at least part of the time, especially in productive lakes. Since diurnal scale is particularly important to tropical lakes dynamics, we evaluated diurnal changes in pCO2and CO2 flux across the air-water interface in a tropical productive lake in southeastern Brazil (Lake Carioca) over two consecutive days. Both pCO2 and CO2 flux were significantly different between day (9:00 to 17:00) and night (21:00 to 5:00) confirming the importance of this scale for CO2 dynamics in tropical lakes. Net heterotrophy and CO2 outgassing from the lake were registered only at night, while significant CO2 emission did not happen during the day. Dissolved oxygen concentration and temperature trends over the diurnal cycle indicated the dependence of CO2 dynamics on lake metabolism (respiration and photosynthesis). This study indicates the importance of considering the diurnal scale when examining CO2emissions from tropical lakes.
pCO2; CO2 flux; diurnal variations; tropical productive lake
É amplamente aceito na literatura que lagos são em geral heterotróficos e supersaturados com CO2 já que recebem carbono alóctone. Porém, autotrofia e insaturação de CO2 podem ocorrer em pelo menos parte do tempo, especialmente em lagos produtivos. Como a escala diurna é particularmente importante para a dinâmica de lagos tropicais, variações diurnas na pCO2 e no fluxo de CO2 através da interface ar-água foram avaliadas num lago tropical produtivo do sudeste do Brasil (Lagoa Carioca) durante dois dias consecutivos. Tanto a pCO2 quanto o fluxo de CO2 foram significativamente diferentes entre o dia (9:00 às 17:00) e a noite (21:00 às 5:00), confirmando a influência desta escala na dinâmica do CO2 na Lagoa Carioca. Foram registradas heterotrofia e emissão de CO2 pela lagoa apenas durante a noite, enquanto durante o dia não houve emissão significativa. Variações na concentração de oxigênio dissolvido e na temperatura ao longo do dia indicaram a dependência da dinâmica do CO2 no metabolismo (respiração e fotossíntese) deste lago. Este estudo indica a importância de se considerar a escala diurna na avaliação da emissão de CO2 por lagos tropicais.
pCO2; fluxo de CO2 ; variações diurnas; lago tropical produtivo
Introduction
It is widely accepted that lakes are typically supersaturated with CO2 relative to the overlying atmosphere (Kling et al., 1991KLING, GW., KIPPHUT, GW. and MILLER, MC., 1991. Arctic lakes and streams as gas conduits to the atmosphere: implications for tundra carbon budgets. Science, vol. 251, no. 4991, p. 298-301. http://dx.doi.org/10.1126/science.251.4991.298. PMid:17733287
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; Cole et al., 1994COLE, JJ., CARACO, NF., KLING, GW. and KRATZ, TK., 1994. Carbon dioxide supersaturation in the surface waters of lakes. Science, vol. 265, no. 5178, p. 1568-1570. http://dx.doi.org/10.1126/science.265.5178.1568. PMid:17801536
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; Duarte and Prairie, 2005DUARTE, CM. and PRAIRIE, YT., 2005. Prevalence of heterotrophy and atmospheric CO2 emissions from aquatic ecosystems. Ecosystems (New York, N.Y.), vol. 8, no. 7, p. 862-870. http://dx.doi.org/10.1007/s10021-005-0177-4.
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). This net heterotrophic condition is believed to predominate as ecosystem respiration frequently exceeds ecosystem primary production in lakes due to the input of allochthonous organic matter from their catchments (Del Giorgio et al., 1999DEL GIORGIO, A., COLE, JJ., CARACO, NF. and PETERS, RH., 1999. Linking planktonic biomass and metabolism to net gas fluxes in northern temperate lakes. Ecology, vol. 80, no. 4, p. 1422-1431. http://dx.doi.org/10.1890/0012-9658(1999)080[1422:LPBAMT]2.0.CO;2.
http://dx.doi.org/10.1890/0012-9658(1999...
; Pace et al., 2004PACE, ML., COLE, JJ., CARPENTER, SR., KITCHELL, JF., HODGSON, JR., VAN DE BOGERT, MC., BADE, DL., KRITZBERG, ES. and BASTVIKEN, D., 2004. Whole-lake carbon-13 additions reveal terrestrial support of aquatic food webs. Nature, vol. 427, no. 6971, p. 240-243. http://dx.doi.org/10.1038/nature02227. PMid:14724637
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). However, recent studies have shown that although heterotrophy is frequent it is not a general rule. CO2undersaturation and/or autotrophy have been recorded in lakes, especially on those with high production rates (e.g. Xing et al., 2005XING, Y., XIE, P., YANG, H., NI, L., WANG, Y. and RONG, K., 2005. Methane and carbon dioxide fluxes from a shallow hypereutrophic subtropical Lake in China. Atmospheric Environment, vol. 39, no. 30, p. 5532-5540. http://dx.doi.org/10.1016/j.atmosenv.2005.06.010.
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; Gu et al., 2011GU, B., SCHELSKE, CL. and COVENEY, MF., 2011. Low carbon dioxide partial pressure in a productive subtropical lake. Aquatic Sciences, vol. 73, no. 3, p. 317-330. http://dx.doi.org/10.1007/s00027-010-0179-y.
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; Laas et al., 2012LAAS, A., NÕGES, P., KÕIV, T. and NÕGES, T., 2012. High-frequency metabolism study in a large and shallow temperate lake reveals seasonal switching between net autotrophy and net heterotrophy. Hydrobiologia, vol. 694, no. 1, p. 57-74. http://dx.doi.org/10.1007/s10750-012-1131-z.
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). Productive lakes support lower respiration rates than the unproductive ones and then tend to be net CO2 sinks (Duarte and Agusti, 1998DUARTE, CM. and AGUSTI, S., 1998. The CO2 balance of unproductive aquatic ecosystems. Science, vol. 281, no. 5374, p. 234-236. http://dx.doi.org/10.1126/science.281.5374.234. PMid:9657712
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). Besides lake productivity, other factors are thought to influence the metabolism and partial pressure of CO2(pCO2) in the surface waters of lakes, such as temperature, dissolved organic carbon concentration, and dissolved inorganic carbon inputs from the watershed (e.g. Hanson et al., 2003HANSON, PC., BADE, DL., CARPENTER, SR. and KRATZ, TK., 2003. Lake metabolism: Relationships with dissolved organic carbon and phosphorus. Limnology and Oceanography, vol. 48, no. 3, p. 1112-1119. http://dx.doi.org/10.4319/lo.2003.48.3.1112.
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; Johnson et al., 2008JOHNSON, MS., LEHMANN, J., RIHA, SJ., KRUSCHE, AV., RICHEY, JE., OMETTO, JPHB. and COUTO, EG., 2008. CO2 efflux from Amazonian headwater streams represents a significant fate for deep soil respiration. Geophysical Research Letters, vol. 35, no. 17, p. L17401. http://dx.doi.org/10.1029/2008GL034619.
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; Kosten et al., 2010KOSTEN, S., ROLAND, F., DA MOTTA MARQUES, DML., VAN NES, EH., MAZZEO, N., STERNBERG, LSL., SCHEFFER, M. and COLE, JJ., 2010. Climate-dependent CO2 emissions from lakes. Global Biogeochemical Cycles, vol. 24, no. 2, p. GB2007. http://dx.doi.org/10.1029/2009GB003618.
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). Lake's pCO2 is then the result of the interaction of many factors and the extent to which each factor controls CO2 likely varies among systems and across time scales.
Many studies in boreal and temperate regions have tested the prevalence of CO2 supersaturation in lakes (e.g. Riera et al., 1999RIERA, JL., SCHINDLER, JE. and KRATZ, TK., 1999. Seasonal dynamics of carbon dioxide and methane in two clear-water lakes and two bog lakes in northern Wisconsin, U.S.A. Canadian Journal of Fisheries and Aquatic Sciences, vol. 56, no. 2, p. 265-274. http://dx.doi.org/10.1139/f98-182.
http://dx.doi.org/10.1139/f98-182...
; Tank et al., 2009TANK, SE., LESACK, LFW. and HESSLEIN, RH., 2009. Northern delta lakes as summertime CO2 absorbers within the arctic landscape. Ecosystems (New York, N.Y.), vol. 12, no. 1, p. 144-157. http://dx.doi.org/10.1007/s10021-008-9213-5.
http://dx.doi.org/10.1007/s10021-008-921...
; Huotari et al., 2009HUOTARI, J., OJALA, A., PELTOMAA, E., PUMPANEN, J., HARI, P. and VESALA, T., 2009. Temporal variations in surface water CO2 concentration in a boreal humic lake based on high-frequency measurements. Boreal Environment Research, vol. 14, suppl. A, p. 48-60.); however, much less is known about CO2 emissions from tropical lakes (e.g. Richey et al., 2002RICHEY, JE., MELACK, JM., AUFDENKAMPE, AK., BALLESTER, VM. and HESS, LL., 2002. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature, vol. 416, no. 6881, p. 617-620. http://dx.doi.org/10.1038/416617a. PMid:11948346
http://dx.doi.org/10.1038/416617a...
; Marotta et al., 2009MAROTTA, H., DUARTE, CM., SOBEK, S. and ENRICH-PRAST, A., 2009. Large CO2 disequilibria in tropical lakes. Global Biogeochemical Cycles, vol. 23, no. 4, p. GB4022. http://dx.doi.org/10.1029/2008GB003434.
http://dx.doi.org/10.1029/2008GB003434...
; Kosten et al., 2010KOSTEN, S., ROLAND, F., DA MOTTA MARQUES, DML., VAN NES, EH., MAZZEO, N., STERNBERG, LSL., SCHEFFER, M. and COLE, JJ., 2010. Climate-dependent CO2 emissions from lakes. Global Biogeochemical Cycles, vol. 24, no. 2, p. GB2007. http://dx.doi.org/10.1029/2009GB003618.
http://dx.doi.org/10.1029/2009GB003618...
; Belger et al., 2011BELGER, L., FORSBERG, BR. and MELACK, JM., 2011. Carbon dioxide and methane emissions from interfluvial wetlands in the upper Negro River basin, Brazil. Biogeochemistry, vol. 105, no. 1-3, p. 171-183. http://dx.doi.org/10.1007/s10533-010-9536-0.
http://dx.doi.org/10.1007/s10533-010-953...
). Knowing precisely how much carbon is delivered to the atmosphere by tropical inland waters and understanding the regulation of this process are crucial steps for better assessment of regional and global carbon budgets.
Diurnal changes in temperature and irradiance may be very large in tropical lakes (e.g. Barbosa and Tundisi, 1989aBARBOSA, FAR. and TUNDISI, JG., 1989a. Diel variations in a shallow tropical Brazilian lake I. The influence of temperature variation on the distribution of dissolved oxygen and nutrients. Archiv fuer Hydrobiologie, vol. 116, no. 3, p. 333-349.). Compared to boreal and temperate regions, seasonal variations on temperature and photoperiod are small in the tropics (e.g. Barbosa, 1997BARBOSA, FAR., 1997. The importance of diurnal cycles for the conservation and management of tropical waters: examples from the Rio Doce Valley lakes system. In TUNDISI, JG. and SAIJO, Y. (Eds.). Limnological studies on the Rio Doce Valley Lakes, Brazil. São Carlos: Brazilian Academy of Sciences. p. 449-456.) while temperature variations within a day can be just as large or even exceed temperature differences between seasons. In tropical lakes, diurnal changes in temperature can be large enough to cause water mixing at night, a pattern described by Lewis (1973)LEWIS, WM., and the LEWIS, 1973. The thermal regime of Lake Lanao (Philippines) and its theoretical implications for tropical lakes. Limnology and Oceanography, vol. 18, no. 2, p. 200-217. http://dx.doi.org/10.4319/lo.1973.18.2.0200.
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in Lake Lanao as atelomixis and more recently re-described in two Brazilian lakes by Barbosa and Padisák (2002)BARBOSA, FAR. and PADISÁK, J., 2002. The forgotten lake stratification pattern: atelomixis, and its ecological importance. Verhandlungen des Internationalen Verein Limnologie, vol. 28, p. 1385-1395.. Metabolic processes may also vary widely over the course of the day, as demonstrated for phytoplankton (Barbosa and Tundisi, 1989b) and bacterioplankton production (Petrucio and Barbosa, 2004PETRUCIO, MM. and BARBOSA, FAR., 2004. Diel variations of phytoplankton and bacterioplankton production rates in four tropical lakes in the middle Rio Doce basin (southeastern Brazil). Hydrobiologia, vol. 513, no. 1-3, p. 71-76. http://dx.doi.org/10.1023/B:hydr.0000018167.43745.33.
http://dx.doi.org/10.1023/B:hydr.0000018...
). Since diurnal changes are large in the tropics and productive lakes are more likely to be autotrophic (Duarte and Agusti, 1998DUARTE, CM. and AGUSTI, S., 1998. The CO2 balance of unproductive aquatic ecosystems. Science, vol. 281, no. 5374, p. 234-236. http://dx.doi.org/10.1126/science.281.5374.234. PMid:9657712
http://dx.doi.org/10.1126/science.281.53...
), it is possible that tropical productive lakes show significant metabolism and CO2 variations over the course of the day, including diurnal changes between heterotrophy and autotrophy.
In this context, the goal of this study was to check for significant variations within a day in surface water CO2 partial pressure (pCO2) and CO2 fluxes between lake and atmosphere in a tropical productive lake (Lake Carioca).
2.Material and Methods
2.1.Site description
Lake Carioca (19°45′26.0″S; 42°37′06.2″W) is located in the Rio Doce State Park (Parque Estadual do Rio Doce, PERD; Figure 1) in an Atlantic Forest remnant in Minas Gerais, Brazil. The park is part of the middle Rio Doce Lacustrine System, which consists of more than two hundred lakes and ponds of varying size, morphometry and trophic state. Roughly 51 lakes are inside PERD, including Lake Carioca, and thus protected from direct human impacts. However, PERD is surrounded by agriculture, pasturelands, and large areas of Eucalyptus spp plantations that have considerable impacts on the lacustrine system and the forest of the park. Carioca is a small (0.14 km2) shallow lake (max. depth = 11.8 m, Bezerra-Neto et al., 2010BEZERRA-NETO, JF., BRIGHENTI, LS. and PINTO-COELHO, RM., 2010. A new morphometric study of Carioca Lake, Parque Estadual do Rio Doce (PERD), Minas Gerais State, Brazil. Acta Scientiarium Biological Sciences, vol. 32, p. 49-54.) and exhibits high production rates (annual average of 497 mgC.m–2.d–1, PELD Technical Report, unpublished data). It mixes vertically once a year during the dry season (between May and August), when primary production is boosted by nutrients from the hypolimnium of the lake.
Rio Doce State Park in southeast Brazil. The circle shows Lake Carioca. Source: Adapted from IBGE/Brasil topographical map by Philippe Maillard – Institute of GeoSciences-IGC, Federal University of Minas Gerais.
Sampling and calculations
Samples were collected at the end of the dry season (August, 2011) just after the water mixing and at the beginning of thermal stratification. Basic limnological data including water temperature, pH, conductivity and dissolved oxygen (D.O.) were taken at the deepest point of the lake with a Hydrolab DS 5 (Hydromet Inc.) probe at depth intervals of 0.5 m. Water samples for analysis of nutrients and chlorophyll-a determination were collected at 4 depths corresponding to 100%, 10% and 1% of surface irradiance, as well as the aphotic zone (defined with a 1400 series International Light Technologies radiometer). These data are not discussed in detail herein and are provided as background information only.
Surface water CO2 partial pressure (pCO2) and CO2 flux across the air-water interface were measured every 4 hours at the deepest point of Lake Carioca over two diurnal cycles. Direct measurements of pCO2 were taken by headspace equilibration according to Cole and Caraco (1998)COLE, JJ. and CARACO, NF., 1998. Atmospheric exchange of carbon dioxide in a low-wind oligotrophic lake measured by the addition of SF6. Limnology and Oceanography, vol. 43, no. 4, p. 647-656. http://dx.doi.org/10.4319/lo.1998.43.4.0647.
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with modification. Three 30 mL glass bottles (triplicates) were filled with 20 mL of lake surface water (0.5 m depth) and immediately capped and sealed with rubber and metal caps. Ten mL of ambient air was introduced to each bottle with a syringe and needle through the rubber cap. The bottles were then shaken vigorously for 60 seconds to allow for equilibration of the air and water phases for CO2. Headspace air was collected with a syringe and injected in an infrared gas analyser (IRGA) (environmental gas monitor EDSEGM4; PP-Systems, Hitchin, Hertfordshire) for pCO2 measurement. pCO2 of ambient air (pCO2air) was also measured by the IRGA. Surface water CO2 concentration (Csur) and the saturation concentration of CO2 (Csat) were calculated from measured pCO2s and Henry's constant at ambient temperature (KH) according to Henry's law (Weiss, 1974WEISS, RF., 1974. Carbon dioxide in water and seawater: the solubility of a non-ideal gas. Marine Chemistry, vol. 2, no. 3, p. 203-215.) (Equation 1 and 2):
Csur = pCO2waterKH(1)
Csat = pCO2airKH(2)
CO2 fluxes across air-water were estimated using the following equation according to Cole and Caraco (1998)COLE, JJ. and CARACO, NF., 1998. Atmospheric exchange of carbon dioxide in a low-wind oligotrophic lake measured by the addition of SF6. Limnology and Oceanography, vol. 43, no. 4, p. 647-656. http://dx.doi.org/10.4319/lo.1998.43.4.0647.
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(Equation 3):
Flux = α k (Csur – Csat)(3)
where α is the factor for chemical enhancement of diffusion (Wanninkhof and Knox, 1996WANNINKHOF, R. and KNOX, M., 1996. Chemical enhancement of CO2 exchange in natural waters. Limnology and Oceanography, vol. 41, no. 4, p. 689-697. http://dx.doi.org/10.4319/lo.1996.41.4.0689.
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) and k is the coefficient of gas exchange for CO2 at a given temperature. k was calculated from k
600 for low wind speeds (Equation 4) (Cole and Caraco, 1998COLE, JJ. and CARACO, NF., 1998. Atmospheric exchange of carbon dioxide in a low-wind oligotrophic lake measured by the addition of SF6. Limnology and Oceanography, vol. 43, no. 4, p. 647-656. http://dx.doi.org/10.4319/lo.1998.43.4.0647.
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) and from Schmidt numbers ratio (Equation 5) (Jähne et al., 1987JÄHNE, B., HEINZ, G. and DIETRICH, W., 1987. Measurements of the diffusion coefficient of sparingly soluble gases in water. Journal of Geophysical Research, vol. 92, no. C10, p. 10767-10776. http://dx.doi.org/10.1029/JC092iC10p10767.
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):
k600 = 2.07 + 0.215 U10 1.7(4)
k = k 600 (Sc / 600)–0.67(5)
U10 is wind speed at 10 m and was estimated with the Smith (1985)SMITH, SV., 1985. Physical, chemical and biological characteristics of CO2 gas flux across the air water interface. Plant, Cell & Environment, vol. 8, no. 6, p. 387-398. http://dx.doi.org/10.1111/j.1365-3040.1985.tb01674.x.
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equation using measured wind speed at 1 m by an anemometer at the centre of the lake. Sc is the in situ Schmidt number for CO2 (Jähne et al., 1987JÄHNE, B., HEINZ, G. and DIETRICH, W., 1987. Measurements of the diffusion coefficient of sparingly soluble gases in water. Journal of Geophysical Research, vol. 92, no. C10, p. 10767-10776. http://dx.doi.org/10.1029/JC092iC10p10767.
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).
Continuous measurements of dissolved oxygen concentration were taken by an automated sensor (D-Opto Logger, Zebra-Tech Ltd.) deployed in the centre of the lake at 0.5 m depth.
2.3.Statistical analysis
T-tests were used to check for differences in surface water pCO2 between day and night and to check for differences between surface water pCO2 and pCO2 in the overlying atmosphere. All tests were performed in Statistica 7.0 software.
3.Results
Limnological conditions of Lake Carioca in August (2011) are summarised in Table 1. Water temperature ranged from 21.4 °C at the bottom to 23.6 °C at the surface, showing the beginning of thermal stratification. The lake exhibited a well-developed oxycline at approximately 4 m and an anoxic hypolimnium. The water was alkaline (pH c. 8) down to 4.5 m but pH decreased to c. 6 at the lower layers. The dissolved oxygen and pH profiles indicated high respiration rates and CO2 concentrations at the bottom of the lake.
Surface water pCO2(pCO2water) ranged considerably (from 389.7 to 643.3 matm) in Lake Carioca during the sampling period and had a mean value of 488.8 ± 78.6 µatm. Mean night pCO2water (565 matm ± 55.3 (± SD) from 21:00 to 5:00) was significantly higher than mean day pCO2water (436.1 matm ± 25.5 (± SD) from 9:00 to 17:00) (t = −8.14; d.f. = 15; p < 0.001, Figure 2), showing significant changes in the CO2 dynamics of this lake within a day. A strong metabolic control of lake's CO2 was evidenced by the similar but opposite trends of dissolved CO2 and O2 over the diurnal cycle (Figure 3A). As expected, during daytime photosynthesis lowered water pCO2 and raised dissolved oxygen (D.O.) concentration. At night, when just respiration occurs, pCO2 increased considerably and the concentration of D.O. in the water reduced (Figure 3A). Diurnal variations in water temperature also showed opposite trend to CO2 with increases during the day and decreases at night (Figure 3B). These mirror-like trends indicate that respiration and photosynthesis are major regulators of CO2dynamics in Lake Carioca. However, small differences between CO2 and O2 variations suggest that other factors may also have some influence on the lake's CO2 concentration.
Mean (±SD, n=16) pCO2(µatm) in Lake Carioca surface water during day and night. p < 0.001 indicating significant difference (t-test).
A. Concentrations of dissolved CO2 and O2 in the surface water. B. Concentration of dissolved CO2 and surface water temperature. All variables were measured at 4-hour intervals over a 48-hour cycle on Lake Carioca in August, 2011. Horizontal bars mean nighttime hours.
Mean pCO2water was significantly higher than mean pCO2 in the overlying atmosphere (pCO2air) during nighttime (from 21:00 to 5:00; t = 4.34, g.l. = 30; p= 0.0001) but were not significantly different during daytime (from 9:00 to 17:00; t = 1.68, g.l. = 29.8; p= 0.10) (Figure 4). This means that Lake Carioca alternated between CO2 supersaturation and atmospheric equilibrium within 24 hours and therefore, the lake was not always a source of CO2 but only during nighttime (Figure 5). Even at night, CO2 emission from Lake Carioca was low, showing a maximum of 2.4 mmolCO2.m–2.d–1 at 1:00 of the first day (Figure 5). Mean CO2 flux throughout the studied period was only 0.9 mmolCO2.m–2.d–1 ± 0.8 (± SD). Low values of CO2 efflux from Lake Carioca are consequences of the relatively low pCO2 in its water and almost null wind speeds registered during the sampling period.
Mean (±SD) pCO2 in the water (pCO2water) and in the overlying atmosphere (pCO2air) measured at 4-hour intervals over a 48-hour cycle on Lake Carioca in August, 2011. Horizontal bars mean nighttime hours.
Mean (±SD) CO2 flux across the air-water interface of Lake Carioca, measured at 4-hour intervals over a 48-hour cycle in August, 2011. Positive values indicate that CO2 flux was towards atmosphere and negative values indicate CO2 flux towards the water. Horizontal bars mean nighttime hours.
Discussion
Our results bring to light significant diurnal changes in pCO2water and CO2 emissions from a productive tropical lake. The use of the diurnal approach demonstrated that despite the belief that lakes are generally net heterotrophic and supersaturated with CO2, Lake Carioca was not a constant source of CO2 and was heterotrophic only during the night.
Diurnal dynamics of CO2 have been recorded in temperate lakes (e.g. Sellers et al., 1995SELLERS, P., HESSLEIN, RH. and KELLY, CA., 1995. Continuous measurement of CO2 for estimation of air-water fluxes in lakes: An in situ technique. Limnology and Oceanography, vol. 40, no. 3, p. 575-581. http://dx.doi.org/10.4319/lo.1995.40.3.0575.
http://dx.doi.org/10.4319/lo.1995.40.3.0...
; Cole and Caraco, 1998COLE, JJ. and CARACO, NF., 1998. Atmospheric exchange of carbon dioxide in a low-wind oligotrophic lake measured by the addition of SF6. Limnology and Oceanography, vol. 43, no. 4, p. 647-656. http://dx.doi.org/10.4319/lo.1998.43.4.0647.
http://dx.doi.org/10.4319/lo.1998.43.4.0...
; Hanson et al., 2006HANSON, PC., CARPENTER, SR., ARMSTRONG, DE., STANLEY, EH. and KRATZ, TK., 2006. Lake dissolved inorganic carbon and dissolved oxygen: changing drivers from days to decades. Ecological Monographs, vol. 76, no. 3, p. 343-363. http://dx.doi.org/10.1890/0012-9615(2006)076[0343:LDICAD]2.0.CO;2.
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) but generally showed small variation. Cole and Caraco (1998)COLE, JJ. and CARACO, NF., 1998. Atmospheric exchange of carbon dioxide in a low-wind oligotrophic lake measured by the addition of SF6. Limnology and Oceanography, vol. 43, no. 4, p. 647-656. http://dx.doi.org/10.4319/lo.1998.43.4.0647.
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for instance, recorded small CO2 changes within a day in midsummer and even smaller ones during the spring in Mirror Lake. Differently, the significant variation between day and night in CO2 dynamics recorded for the tropical and productive Lake Carioca-southeast Brazil can be explained by large metabolic differences between periods of the day, which is not pronounced on higher latitude and/or unproductive lakes. As shown here, in productive and warm lakes, gross primary production can be high enough to equilibrate or eventually exceed respiration rates during daytime. Moreover, although this study was conducted only during the dry season, it is likely that diurnal variation in pCO2 in Lake Carioca is even larger during the rainy season. The higher mean temperature and greater input of nutrients and dissolved organic carbon from the watershed during the rainy season (summer) might favour both autotrophy and heterotrophy in the lake (Brown et al., 2004BROWN, JH., GILLOOLY, JF., ALLEN, AP., SAVAGE, VM. and WEST, GB., 2004. Toward a metabolic theory of ecology. Ecology, vol. 85, no. 7, p. 1771-1789. http://dx.doi.org/10.1890/03-9000.
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; Staehr and Sand-Jensen, 2007STAEHR, PA. and SAND-JENSEN, K., 2007. Temporal dynamics and regulation of lake metabolism. Limnology and Oceanography, vol. 52, no. 1, p. 108-120. http://dx.doi.org/10.4319/lo.2007.52.1.0108.
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; Marotta et al., 2012MAROTTA, H., RICCI, RMP., SAMPAIO, PL., MELO, PP. and ENRICH-PRAST, A., 2012. Variações de curto prazo do metabolismo e da pCO2 na Lagoa Rodrigo de Freitas: elevado dinamismo em um ecossistema tropical urbano. Oecologia Australis, vol. 16, no. 3, p. 391-407. http://dx.doi.org/10.4257/oeco.2012.1603.06.
http://dx.doi.org/10.4257/oeco.2012.1603...
) likely resulting in higher amplitude of diurnal CO2variations (Marotta et al., 2010MAROTTA, H., DUARTE, CM., PINHO, L. and ENRICH-PRAST, A., 2010. Rainfall leads to increased pCO2 in Brazilian coastal lakes. Biogeosciences, vol. 7, no. 5, p. 1607-1614. http://dx.doi.org/10.5194/bg-7-1607-2010.
http://dx.doi.org/10.5194/bg-7-1607-2010...
).
Surface water pCO2 registered in Lake Carioca is very low in comparison to the mean pCO2 for tropical lakes available in the published literature (1804 matm, Marotta et al., 2009MAROTTA, H., DUARTE, CM., SOBEK, S. and ENRICH-PRAST, A., 2009. Large CO2 disequilibria in tropical lakes. Global Biogeochemical Cycles, vol. 23, no. 4, p. GB4022. http://dx.doi.org/10.1029/2008GB003434.
http://dx.doi.org/10.1029/2008GB003434...
). Also differently from what was found by other studies that covered tropical lakes (e.g. Cole et al., 1994COLE, JJ., CARACO, NF., KLING, GW. and KRATZ, TK., 1994. Carbon dioxide supersaturation in the surface waters of lakes. Science, vol. 265, no. 5178, p. 1568-1570. http://dx.doi.org/10.1126/science.265.5178.1568. PMid:17801536
http://dx.doi.org/10.1126/science.265.51...
; Marotta et al., 2009MAROTTA, H., DUARTE, CM., SOBEK, S. and ENRICH-PRAST, A., 2009. Large CO2 disequilibria in tropical lakes. Global Biogeochemical Cycles, vol. 23, no. 4, p. GB4022. http://dx.doi.org/10.1029/2008GB003434.
http://dx.doi.org/10.1029/2008GB003434...
), Lake Carioca is a small source of atmospheric carbon in comparison to higher latitude lakes. While CO2 emissions from Lake Carioca averaged only 0.9 mmolCO2.m–2.d–1, other studies have recorded CO2 effluxes of 20.9 mmolCO2.m–2.d–1 from Arctic lakes (Kling et al., 1991KLING, GW., KIPPHUT, GW. and MILLER, MC., 1991. Arctic lakes and streams as gas conduits to the atmosphere: implications for tundra carbon budgets. Science, vol. 251, no. 4991, p. 298-301. http://dx.doi.org/10.1126/science.251.4991.298. PMid:17733287
http://dx.doi.org/10.1126/science.251.49...
), 55.6 mmolCO2.m–2.d–1 from a boreal humic lake (Huotari et al., 2009HUOTARI, J., OJALA, A., PELTOMAA, E., PUMPANEN, J., HARI, P. and VESALA, T., 2009. Temporal variations in surface water CO2 concentration in a boreal humic lake based on high-frequency measurements. Boreal Environment Research, vol. 14, suppl. A, p. 48-60.), and 1200 and 90 mmolCO2.m–2.d–1 from two clear-water temperate lakes (Riera et al., 1999RIERA, JL., SCHINDLER, JE. and KRATZ, TK., 1999. Seasonal dynamics of carbon dioxide and methane in two clear-water lakes and two bog lakes in northern Wisconsin, U.S.A. Canadian Journal of Fisheries and Aquatic Sciences, vol. 56, no. 2, p. 265-274. http://dx.doi.org/10.1139/f98-182.
http://dx.doi.org/10.1139/f98-182...
). The registered low values of pCO2and CO2 efflux from Lake Carioca are probably a consequence of its high autotrophic activity, particularly during the studied period when vertical mixing induces high production rates (640 mgC.m–2.d–1; PELD Report, 2012) and scarce rainfall limits the input of allochthonous dissolved organic carbon to the lake likely reducing its respiration rates (Cole et al., 2000COLE, JJ., PACE, ML., CARPENTER, SR. and KITCHELL, JF., 2000. Persistence of net heterotrophy in lakes during nutrient addition and food web manipulations. Limnology and Oceanography, vol. 45, no. 8, p. 1718-1730. http://dx.doi.org/10.4319/lo.2000.45.8.1718.
http://dx.doi.org/10.4319/lo.2000.45.8.1...
). Moreover, the lack of rainfall can also lead to lower values of pCO2 in Lake Carioca in the dry season than in the rainy season since rainfall can enhance CO2 inputs from groundwater to lakes (Marotta et al., 2010MAROTTA, H., DUARTE, CM., PINHO, L. and ENRICH-PRAST, A., 2010. Rainfall leads to increased pCO2 in Brazilian coastal lakes. Biogeosciences, vol. 7, no. 5, p. 1607-1614. http://dx.doi.org/10.5194/bg-7-1607-2010.
http://dx.doi.org/10.5194/bg-7-1607-2010...
).
As shown in this study, diurnal changes in CO2 can be significant in tropical waters and ignoring this variation may render misevaluations and misconclusions of the role of tropical aquatic ecosystems in the global carbon cycle. Such conclusions bring new possibilities for further studies concerning the general acceptance of a predominant heterotrophy of lakes, and highlight potential temporal changes between net autotrophy and net heterotrophy in lakes, especially in the tropical and productive ones where temperature and nutrients maintain high autochthonous production rates.
Acknowledgements
The authors thank the financial support of the PELD/UFMG-Brazilian Research Council project (Process No 558175/2009-0) and the support of the Forest Institute of Minas Gerais (IEF-MG).
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Erratum
Due to a formatting error in the article “Diurnal sampling reveals significant variation in CO2 emission from a tropical productive lake” published in volume 74, issue 3 (suppl.), p. 113-119, in the page 116, first column, lines 8, 10 and 12 and in the page 117, second column, line 24 where you read “matm”, you should read “μatm”.
Publication Dates
-
Publication in this collection
Aug 2014
History
-
Received
11 Mar 2013 -
Accepted
18 June 2013 -
Reviewed
30 Nov 2014