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

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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Publication Dates

• Publication in this collection
  2017

History

• Received
  03 Feb 2016
• Accepted
  14 July 2016