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Brazilian Archives of Biology and Technology

On-line version ISSN 1678-4324

Braz. arch. biol. technol. vol.60  Curitiba  2017  Epub May 18, 2017

http://dx.doi.org/10.1590/1678-4324-2017160381 

Environmental Sciences

Floating Aquatic Macrophytes Decrease the Methane Concentration in the Water Column of a Tropical Coastal Lagoon: Implications for Methane Oxidation and Emission

André Luiz dos Santos Fonseca1  * 

Claudio Cardoso Marinho2 

Franscisco de Assis Esteves3 

1Instituto Federal de Educação, Ciência e Tecnologia do Rio de Janeiro, Realengo. , Rio de Janeiro, Brasil;

2Universidade Federal do Rio De Janeiro - Ecologia, Rio de Janeiro, Brazil;

3Universidade Federal do Rio de Janeiro - Núcleo em Ecologia e Desenvolvimento Sócio-Ambiental de Macaé, Macaé, Rio de Janeiro, Brasil.

ABSTRACT

In wetlands, the knowledge accumulated on the role of aquatic plants in the methane cycle focused on emergent macrophytes, to the detriment of other typologies. Herein, we evaluated whether the free-floating macrophyte Salvinia auriculata Aubl. and the floating-leaved macrophyte Eichhornia azurea (Sw.) Kunth. decrease the water column methane concentrations compared to a plant-free surface. We prepared microcosms by inserting an individual of S. auriculata or of E. azurea into chambers filled with lagoon water previously bubbled with CH4. Another set of chambers was incubated only with the prepared water, representing the plant-free surface. Half of the chambers were kept in the dark and half in sunlight to simulate a diel cycle. We observed greater loss of CH4, higher O2 uptake and lower CO2 outflow in the plants treatments. The decrease in methane concentrations in the E. azurea treatments was 93.5% in the light and 77.2% in the dark. In the S. auriculata treatments, the decreases were 74.2% and 67.4% in the light and in the dark, respectively. In plant-free surface the decrease was 58.7% in the light and 36.3% in the dark. These results indicate a role of floating aquatic macrophytes in the methane cycle in the water column. Moreover, our results suggest a diel variation of methane oxidation and methane emission, according to the differences observed in O2 uptake and CO2 outflow between dark and light conditions. Thus, future predictions of global methane budget should include the role played by floating aquatic macrophytes.

Key words: methane; eutrophication; global warming; Eichhornia azurea; Salvinia auriculata.

INTRODUCTION

Methane (CH4) is one of the major greenhouse gases (GHG) present in the atmosphere, contributing to global warming 1. Natural wetlands are the single largest source of atmospheric methane, but their estimated emissions present considerable uncertainties, varying from 177 to 284 Tg CH4 annually, which represents 33.7-34.2% of global methane emission 2. The variation in the amount of methane emitted in wetlands reflects the balance between methanogenesis and methanotrophy 3. These processes involve complex interactions between plants and microorganisms in heterogeneous environments, resulting in variations in the methane emission at the scale of hours and meters 4. In addition to these interactions, we must also consider the importance of the emission through hydrodynamic processes at the air-water interface. Hydrodynamic transport includes both molecular diffusion and advective processes, which are controlled by physical variables such as wind, temperature, thermal convection, current velocity, waves and bubbles 5,6. In this scenario, the vegetation contribution to the global methane budget is one of the least understood 7. Considering the key role of methane in the global climate change and the uncertainties in methane emission determinations in wetlands is essential identify and quantify all sources and driving processes related to methane flux to the atmosphere.

Shallow wetlands, such as coastal lagoons, usually have broad colonization by aquatic macrophytes 8. These plants can influence methane cycle in wetlands through the mechanisms of production, consumption and transport of methane 9. First, aquatic macrophytes release organic matter from root exudates and from the decaying litter, providing substrates for methane production in the anoxic sediments 10,11,12. Second, plants may reduce a large and variable proportion (1-90%) of methane produced in the sediment through the oxidation in the rhizosphere 13,14,15. Third, plant aerenchyma may act as a conduit by which methane is conducted from the sediment to the atmosphere 16. Most of the methane is emitted through the aerenchyma of emerged plants in shallow wetlands 9,17,18.

So far, the scientific knowledge accumulated on the role of aquatic macrophytes in the methane cycle in wetlands focused on emergent macrophytes 9,10,12,19,20. Wetlands vary greatly in aquatic macrophytes species composition, comprising diverse typologies, such as emergent, floating-leaved, submerged rooted, submerged free and free-floating 8. However, in eutrophic environments, apart from colonization by emergent aquatic macrophytes, the floating-leaved and free-floating plants are benefited over submerged forms 21,22. The shift in the plants community structure in eutrophic ecosystems significantly changes the dissolved gases dynamics 23,24. The floating plants release the oxygen produced during photosynthesis directly to the atmosphere 25. At the same time, the floating leaves hinder the light penetration into the water, limiting the phytoplanktonic photosynthesis, as well as the diffusion of O2 from the atmosphere, thereby promoting water anoxia below the leaves 26,27. The floating leaves also represent a barrier against the escape of methane and carbon dioxide (CO2) to the atmosphere, so supersaturation of these gases in the water column is expected 23,28. Despite the concerns about floating-leaved and free-floating aquatic macrophytes in eutrophied environments, the role of these plants in reducing the methane concentration in wetlands, encompassing emission through the leaves and oxidation in the rhizosphere, has not been assessed.

In this study, we aim at evaluating whether the free-floating macrophyte Salvinia auriculata and the floating-leaved macrophyte Eichhornia azurea diminishes the concentration of methane in the water column compared to a plant-free surface through short-term experiment in microcosms. We hypothesize that the aquatic macrophytes have positive effect on methane concentration decrease in water column. We then hypothesized that the influence of macrophytes follows a diel pattern, depending on the light incidence. These issues are relevant at a larger scale due to two main reasons. First, freshwater shore zones have been heavily damaged by human activities historically 29. The major problem in these densely populated areas is the pollution by liquid and solid wastes 30, generating the eutrophication of water bodies 8. Second, the climate change, particularly global warming, is expected to increase the eutrophication process in shallow coastal lagoons 31,32. The interaction between eutrophication and climate change could result in a positive feedback loop, as eutrophication promotes the increase in the emission of greenhouse gases into the atmosphere, contributing to global warming. In turn, the global warming contributes to the magnification of eutrophication, since the unequivocal increase in the sea level may lead to a decrease in the amount of light reaching the bottom of the lagoons. This light attenuation exists as a result of the decrease in water column transparency caused by the suspended solids runoff and the increase in nutrient inputs 31,32.

MATERIALS AND METHODS

Aquatic macrophytes and water samples utilized in the experiment were collected in Cabiúnas Lagoon (22º15' S, 41º40' W), located at Restinga de Jurubatiba National Park, Rio de Janeiro, Brazil (Fig. 1). Cabiúnas is a pristine freshwater coastal lagoon with a surface area of 0.34 km2 and a mean depth of 2.37 m 33. The water is oligotrophic, humic (13 mg C L-1 of dissolved organic carbon, DOC) and slightly acidic (pH 6.3) and has an average annual temperature of 23.6 °C 34. The large perimeter/volume ratio and the dendritic shape enable the dense development of aquatic macrophytes communities in its margins 33, covering 60% of its area 35. Regarding the study of the methane cycle, an earlier study did not observe the presence of methane in Cabiúnas Lagoon 36. On the other hand, the monthly monitoring of this environment between 2001 and 2003 revealed the presence of methane in the entire period of study, with a maximum value reaching approximately 12 µmol L-137.

Figure 1 Schematic map of Cabiúnas lagoon location on the Restinga de Jurubatiba National Park at Macaé, in the Northern region of Rio de Janeiro State (22º24’ S and 41º42’ W). 

We collected samples of the floating-leaved aquatic macrophyte E. azurea and of the free floating aquatic macrophyte S. auriculata and placed them in a plastic container with lagoon water. Samples of E. azurea were detached from the parent stem, keeping the adventitious roots attached to the individual collected. Although this procedure has caused damage to individuals collected, the detachment and subsequent displacement and anchoring of the plants is a common event in flood pulses. The water samples were collected with polyethylene gallons (~ 80 L) in the water column below the leaves of plants.

Approximately two hours after sampling, the water samples were filtered through plankton net with mesh size of 25 µm to remove coarse organic matter, which could alter the results of incubations. After filtration, the water was prepared by bubbling methane with a silicone hose dipped in the water and connected to a methane cylinder for 20 minutes for the dissolution of gas. Next, we established 36 experimental chambers (glass vials, volume of 3 L, 26 cm long x 13 cm internal diameter) containing 2 L of the prepared water, carefully introduced into the chambers. The treatments established were 12 chambers with of 1 individual of E. azurea, 12 chambers with 1 individual of S. auriculata and 12 chambers without plants (plant-free surface). The initial methane concentration after preparation of the chambers was 136 µmol L-1. The methane concentration used in the experiment was about ten times higher than that found in the environment. In the environment there is a constant supply of methane from the sediment, which maintains methane in the water without interruption. In the experiment was necessary dissolving a large amount of methane at once because the tendency is the rapid loss of the gas, since there is no source of renewal. Half of the chambers of each treatment was kept in the dark condition in the laboratory in a pool containing tap water and the other half was kept in the light condition (under the sun) in other pool also containing tap water. The pool with water kept the experiment under controlled temperature (23 ± 2 ºC), according the average temperature found in the lagoon 34. We decided to use the same temperature for dark and light conditions because we aimed to assess the effect of light intensity and not the effect of temperature on methane concentrations.

The chambers were incubated for 12 hours, with sampling to determine the concentrations of methane in the following intervals: 0, 2, 4, 6 and 12 h. In each sampling, the water was collected from each chamber using plastic syringes and needles. The tip of the syringe was dipped into the water close to the roots and the plunger was pulled slowly removing the aliquot of 8 mL of water. After collecting water, the syringe was connected to a needle and the water was injected into an evacuated glass flask of 12 mL closed with rubber stoppers, containing the equivalent of 20% (weight/volume) of NaCl. Under this condition methane is degassed into the head space. The samples were kept under refrigeration in the laboratory and analyzed 48h later. The degassing with NaCl, the cooling and analysis in this short time minimize interference in the sample results. The methane content of the head space was quantified using a gas chromatograph (Star 3400 - Varian Co., EUA) and the operation conditions were FID detector temperature 200 °C, injector temperature 120 °C and a 3 m Poropak-N column (80/100 mesh) at 85 °C and N2 as the carrier gas. Photosynthetically active radiation (PAR) was monitored in light and dark in every time of water sampling using a radiometer (LI-COR - model LI-1000).

The CO2 concentrations were determined from measurements of pH and alkalinity, using ALCAGRAN software 38. pH was measured with a precision of 0.01 pH units using a Analion PM 608 pH meter and total alkalinity by Gran’s titration 39. O2 concentrations were determined with a digital oxymeter (YSI-55). The pH values, CO2 and O2 concentrations ​​in each chamber were determined at the beginning and at the end of the experiment to evaluate the variation in pH values, the CO2 flux and the O2 consumption, respectively.

The methane concentrations (dependent variable) of each region (aquatic macrophytes species and plant-free surface) and condition (light and dark) (categorical variables) per sample time (continuous variable) were compared using an analysis of covariance (ANCOVA; significance level of 0.05), followed by Tukey’s HSD test to determine which groups are particularly different from each other. At each sampling time, 1 sample was collected from each flask (n = 6) to the methane concentration analysis. The dissolved oxygen and carbon dioxide (dependent variable) values were tested using one-way analysis of variance (ANOVA; significance level of 0.05) between sites conditions (categorical variables), followed by Tukey’s HSD test to determine which groups are different. For the analysis of dissolved oxygen and carbon dioxide, 1 sample was collected from each flask (n = 6) at the beginning and at the end of the experiment.

RESULTS

Methane concentration decrease was significantly higher in treatments with plants than in the plant-free surfaces, both in light and in the dark conditions (Fig. 2; Table 1), indicating a positive effect of the aquatic macrophytes studied on the decrease of methane concentration in the water column. There were no significant differences between the plants in light and in the dark (Fig. 2; Table 1). The plant-free surfaces also presented methane concentration decrease with nonsignificant differences between light and dark conditions (Fig. 2; Table 1). The decrease in methane concentrations at the end of the experiment in the treatments with E. azurea was 93.5% in the light and 77.2% in the dark. In the treatments with S. auriculata, the decreases were 74.2% and 67.4% in the light and in the dark, respectively. In plant-free surface treatments the decrease was 58.7% in the light and 36.3% in the dark.

Figure 2 Temporal variation of water CH4 concentrations (µmol L-1) in the plant-free surface and in the treatments with aquatic macrophytes in the light and in the dark conditions. Bars = standard deviation. 

Table 1 Values of significance (p) of ANCOVA test, number of samples (n), degrees of freedom (DF) and values of deviance (F), between regions, conditions and regions-conditions interactions for CH4 concentrations. Values of significance (p) of Tukey’s HSD test between regions in light and dark conditions and between conditions in each region for CH4 concentrations. 

Test Treatment n DF F p
ANCOVA Region 12 2 29.786 < 0.001*
Condition 18 1 0.579 0.448
Region-Condition interaction 12 2 1.700 0.187
Tukey’s HSD
Light p
Plant-free x E.azurea 6 < 0.001*
Plant-free x S. auriculata 6 < 0.05*
E.azurea x S. auriculata 6 0.722
Dark
Plant-free x E.azurea 6 < 0.001*
Plant-free x S. auriculata 6 < 0.001*
E.azurea x S. auriculata 6 0.999
Plant-free
Light x Dark 12 0.652
E.azurea
Light x Dark 12 0.966
S. auriculata
Light x Dark 12 0.914

* indicates statistical difference.

Oxygen consumption was significantly higher in treatments with plants than in the plant-free surface only in the dark (Fig. 3; Table 2), being 4.0 times higher on treatment with E. azurea and 13.6 times higher in the treatment with S. auriculata. In the comparison between plants was observed significantly higher consumption of oxygen (Fig. 3; Table 2) in the treatment with S. auriculata in the dark. The oxygen consumption in the three regions was always higher in the dark, but significant difference (Fig. 3; Table 2) was observed only in treatment with S. auriculata.

Figure 3 O2 consumption (mg L-1) (difference between the values at the beginning and at the end of the experiment) in the plant-free surface and in the treatments with aquatic macrophytes in the light and in the dark conditions. Bars = standard deviation. 

Table 2 Values of significance (p) of ANOVA test, number of samples (n), degrees of freedom (DF) and values of deviance (F), between regions, conditions and regions-conditions interactions for O2 concentrations. Values of significance (p) of Tukey’s HSD test between regions in light and dark conditions and between conditions in each region for O2 concentrations. 

Test Treatment n DF F p
ANOVA Region 12 2 15.736 < 0.001*
Condition 18 1 23.663 < 0.001*
Region-Condition interaction 12 2 13.413 < 0.001*
Tukey’s HSD
Light p
Plant-free x E.azurea 6 0.205
Plant-free x S. auriculata 6 0.999
E.azurea x S. auriculata 6 0.341
Dark
Plant-free x E.azurea 6 < 0.05*
Plant-free x S. auriculata 6 < 0.001*
E.azurea x S. auriculata 6 < 0.05*
Plant-free
Light x Dark 12 0.999
E.azurea
Light x Dark 12 0.820
S. auriculata
Light x Dark 12 < 0.001*

* indicates statistical difference.

Conversely, the loss of CO2 in the treatments with plants was significantly lower than in the plant-free surface in the dark (Fig. 4; Table 3), being 6.8 times lower in the treatment with E. azurea and 1.8 times lower in the treatment with S. auriculata. In the light, the loss of CO2 was significantly lower than plant-free surface only in S. auriculata treatment (Fig. 4; Table 3), being 1.2 times lower. S. auriculata presented significantly lower CO2 loss (Fig. 4; Table 3) than E. azurea only in the dark. In each region, the loss of CO2 was always lower in the dark and, significant differences (Fig. 4; Table 3) were observed in both treatments with plants.

Figure 4 CO2 loss (mg L-1) (difference between the values at the beginning and at the end of the experiment) in the plant-free surface and in the treatments with aquatic macrophytes in the light and in the dark conditions. Bars = standard deviation. 

Table 3 Values of significance (p) of ANOVA test, number of samples (n), degrees of freedom (DF) and values of deviance (F), between regions, conditions and regions-conditions interactions for CO2 concentrations. Values of significance (p) of Tukey’s HSD test between regions in light and dark conditions and between conditions in each region for CO2 concentrations. 

Test Treatment n DF F p
ANOVA Region 12 2 79.344 < 0.001*
Condition 18 1 209.470 < 0.001*
Region-Condition interaction 12 2 37.169 < 0.001*
Tukey’s HSD
Light p
Plant-free x E.azurea 6 0.153
Plant-free x S. auriculata 6 < 0.05*
E.azurea x S. auriculata 6 0.975
Dark
Plant-free x E.azurea 6 < 0.001*
Plant-free x S. auriculata 6 < 0.001*
E.azurea x S. auriculata 6 < 0.001*
Plant-free
Light x Dark 12 0.157
E.azurea
Light x Dark 12 < 0.001*
S. auriculata
Light x Dark 12 < 0.001*

* indicates statistical difference.

The pH values showed a slight decrease, especially in the dark condition, ranging from 7.56 to 7.01 in the light and from 7.56 to 6.65 in the dark, in treatments with E. azurea and, from 7.56 to 7.08 in the light and 7.56 to 6.67 in the dark in treatments with S. auriculata. In the plant-free surface, the pH values increased from 7.56 to 7.88 in the light and decreased from 7.56 to 7.45 in the dark. The decrease in pH values indicates the formation of CO2 by heterotrophy. The light intensity in the light condition ranged from 2 to 1100 µmol photons m-2 s-1 and in the dark condition it was not detected.

DISCUSSION

In this study, the presence of plants significantly promoted a greater loss of CH4 per water volume concomitantly with higher O2 uptake and a lower outflow of CO2 compared to the plant-free surface, supporting our first hypothesis. Moreover, our results indicate a diel variation of methane oxidation and methane emission processes, according to the differences observed between O2 uptake and CO2 outflow, mainly in the dark condition, supporting partially our second hypothesis. In the light condition, differences between plants and plant-free surface were also observed, but were smaller and require confirmation by other studies. We consider that this study identified a role for floating aquatic macrophytes in the methane cycle, which was not considered in studies of greenhouse gases in freshwater ecosystems.

In freshwater ecosystems, methane encounters several zones that oxidize methane before emission. The first methane oxidation zone is the oxygenated rhizosphere of emergent aquatic macrophytes in the sediment, where the methane produced in anoxic sediments diffuses being oxidized by methanotrophic bacteria 40. The second zone of methane oxidation is the thin superficial oxic layer of sediment 41. The third zone of methanotrophy is the oxygenated water column 42,43,44. After that, the remaining CH4 diffuses through the plants and water column to the atmosphere. Our study suggests that the floating aquatic macrophytes are the final zone of methane oxidation before methane emission to the atmosphere, when the plants are present. It is important to note that the aquatic macrophytes are only part of the water-air interface and other processes that influence methane oxidation and emission occur in this boundary layer, such as convective cooling, internal waves and surface organic films. The greater decrease in methane concentration in the treatments with plants resulted probably from methanotrophy in the rhizosphere, according to the associated smallest loss of CO2, which was most likely the result of partial replacement by new CO2 produced during the methanotrophic activity. It is important to note that methanotrophy should be responsible only for part of the CO2 produced, since CO2 is produced by many other heterotrophic processes that can occur simultaneously. The concomitant higher O2 uptake and the decrease in pH in these treatments also indicate the heterotrophic activity with CO2 formation. Previous studies showed the capability of free-floating aquatic macrophytes in transporting oxygen from the foliage to the rhizosphere 45,46. The roots within the water column act as a living substrate for the growth of attached aerobic bacteria, which consume the oxygen to decompose dissolved organic compounds 47. The absence of a substrate for bacteria in plant-free surface probably promotes a less effective heterotrophic activity, with lower O2 uptake and CO2 production. In Cabiúnas Lagoon, these findings are particularly relevant because the aquatic macrophytes cover 60% of its area, including floating aquatic macrophytes 35 and the heterotrophic activity associated with these plants in the water-air interface is being underestimated. Moreover, the sediment of aquatic macrophytes stands showed higher potential CH4 production rate 10 and higher CH4 concentration 48 than the limnetic region free of plants in this lagoon. Another four coastal lagoons located in Restinga de Jurubatiba National Park had higher potential CH4 production in aquatic macrophytes stands than in the limnetic region 10,19, highlighting the importance of aquatic macrophytes in the methane cycle. The natural decrease of methane concentration from the bottom to the surface in the aquatic ecosystems due to the methane consumption processes lead to low methane concentrations near the water-air interface, diminishing the potential of floating aquatic macrophytes in reducing methane concentration. On the other hand, our findings may be particularly important in eutrophic ecosystems. In these ecosystems, the increased amount of organic matter associated with the usual shift to floating aquatic macrophytes covering promotes anoxia below the leaves 26,27 and a barrier against the efflux of gases 23,28, increasing the methane concentration in the water column. Therefore, the thin aerobic boundary layer below the leaves of floating aquatic macrophytes may represent an important zone of methane oxidation in these environments.

We did not observe significant differences between methane concentrations in the light and in the dark in the treatments with aquatic macrophytes, despite the greater loss of CO2 (lower CO2 production) and lower consumption of O2 in the light. One of the factors responsible for this result may be the inhibition of methane oxidation by the incidence of light (2 to 1100 µmol photons m-2 s-1), which could explain not only the greater loss of CO2 (lower CO2 production) as well as the lower O2 consumption. However, the influence of incident light on the inhibition of methanotrophy may have been overestimated in our study, once a single plant was incubated in each glass bottle, allowing an excessive light penetration in the water. In the lagoon, the plants are juxtaposed forming a mat that hampers light penetration. Another factor probably responsible for light attenuation in Cabiúnas Lagoon is the black-colored water 36 originated from humic substances produced mainly by the decomposition of terrestrial plants from the surrounding sandy soils 49. The inhibitory effect of light on the activity of methanotrophs has been reported before with increasing light intensities, ranging from a slightly inhibitory effect with low intensities (4.1 to 21 µmol photons m-2 s-1) to a complete inhibition with higher intensities (57 to 760 µmol photons m-2 s-1) 50,51. The absence of methane oxidation in the surface water of a CH4 supersaturated lake was also observed 52. The proposed mechanism by which light inhibits methane oxidation is the possible photosensitivity of the enzyme methane monooxygenase. Methanotrophs are genetically related to nitrifying bacteria 53, which has the photosensitive enzyme ammonia monooxygenase 54. Ammonia monooxygenase and methane monooxygenase are closely related by their substrate specificities, their active sites structures, and their sensitivities to inhibitors 55. On the other hand, the photosynthetic activity of plants during the light period and, the consequent diffusion of produced O2 into the water could replace the O2 consumed in the other heterotrophic activities, which could explain the lower O2 consumption observed. The processes that occur in the dark may act in an opposite way. Higher methane oxidation is expected in the dark, since the methane oxidation inhibition is not observed in this condition. The O2 consumed cannot be refueled due to the absence of photosynthetic activity in the dark.

Our results also suggest that the emission of methane through the plants is controlled by light, concurrently with the control of the oxidation. In the light, the unoxidized methane and the CO2 produced could be emitted by stomata. On the other hand, methane emission is expected to be lower in the dark, since the stomata are closed in this condition. These results suggest a diel variation of methane oxidation and methane emission, depending on the intensity of light in freshwater ecosystems. Diel variation in methane emission by emergent aquatic macrophytes has been observed 56,57, but the physiological mechanisms that control methane release to the atmosphere from plants are not completely understood. The stomata opening is controlled by factors that present diel variation such as light 58. Therefore, it is expected that the emission of methane present diel variation. However, while some studies have found a correlation between the emission of methane and opening of stomata 59,60, other studies did not observe such correlation 61,62. Stomatal control of methane emission becomes less important as cracks develop in the cuticle of older leaves or damaged leaves that are open to the air 63,64. A study conducted with emergent plants of Cabiúnas Lagoon indicated that very small damages performed by insects in many culms may considerably increase methane fluxes to the atmosphere 20. Presumably, methane could not accumulate in these leaves with less resistance to gas flow 63,64.

Despite no significant differences were observed, O2 consumption in the dark was slightly higher than in the light also in the plant-free surface, concomitant with a smaller loss of CO2, indicating that photosynthetic activity occurred in the light and the heterotrophic activity was higher in the dark, reinforcing the results observed with plants. Therefore, based on the differences observed in light and dark conditions in this study, we suggest that light is an important controlling factor of biological methane oxidation and methane emission primarily in the roots and rhizomes of floating aquatic macrophytes, but also in the plant-free surface.

Several studies have shown that the influence of emergent aquatic macrophytes on methane cycle varies among plant taxa, according to the differences in detritus quality, architecture of lacunar spaces, in oxygen transport mechanisms, and root oxygen demand 10,12,19,65,66. We believe that variation associated to taxa may occur in floating aquatic macrophytes, since we observed differences in O2 consumption and CO2 outflow for treatments with E. azurea and S. auriculata in this study. The extent to which root-associated methane oxidation and emission varies among plant taxa is uncertain. Therefore, a better understanding of such variability is necessary in pristine freshwater ecosystems, but especially in eutrophied environments, since the shifts in aquatic macrophytes communities structures can profoundly affect the relative role of methanogenesis, methanotrophy and methane emission.

CONCLUSION

In summary, two common species of aquatic macrophytes potentialized the decrease of methane concentration in the water column probably by alternating between the predominance of methane oxidation and methane emission in a diel cycle of methane in coastal lagoons. We suggest that an inhibition of methane oxidation occurs during daylight and the emission to the atmosphere predominates. Overnight methanotrophy prevails, since there is no inhibition by light and the stomata are closed, preventing the emission through the plants. The length variation of day and night throughout the year probably provides a seasonal change in the preponderance of oxidation and emission of methane in the water column of coastal lagoons, particularly in regions colonized by floating aquatic macrophytes. Eutrophic environments need special attention because they usually have a wide colonization by floating aquatic macrophytes and high production and concentration of methane. Overall, our findings emphasize the need to include the role played by floating aquatic macrophytes in the biogeochemical cycling of methane as an important component for future predictions of global methane budget.

ACKNOWLEDGEMENTS

The authors are grateful to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e tecnológico (CNPq) for financial support.

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Received: February 03, 2016; Accepted: July 14, 2016

*Author for correspondence: andre.fonseca@ifrj.edu.br

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