Global regulation of methane emission from natural lakes.

Methane (CH4) emissions from lakes are the largest of the emissions from freshwater ecosystems. We compile open water CH4 emission estimates from individual lakes from all over the world and consider the three main emission pathways: diffusive; ebullitive; and storage. The relationships between emissions, environmental variables, lake characteristics and methodological approaches are investigated for the measurements from 297 lakes. We show that environmental factors, such as temperature and precipitation, act as important driving factors for CH4 emissions, with higher emissions occurring where air temperature and precipitation are high. The diffusive flux of CH4 was found to be positively related to dissolved organic carbon concentration. Diffusive flux is the most frequently estimated component of the total flux, while the other emission pathways are often neglected. Based on the cases where all three components of the total flux were measured (30 lakes), we estimate that measuring the diffusive emission only, and then assuming that the value obtained is a good surrogate for the total emission, would have led to a 277% underestimation of the real total flux. In addition we show that the estimation of fluxes is method-dependent with substantial differences revealed between the flux estimates obtained from different measurement techniques. Some of this uncertainty is due to technical constraints which should not be neglected, and lake CH4 flux measurement techniques require thorough re-evaluation.

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.

Mercury methylation in sediments of a Brazilian mangrove under different vegetation covers and salinities.

The presence and formation of methylmercury (MMHg), a highly toxic form of Hg, in mangrove ecosystems is poorly studied. Therefore the aim of this study was to evaluate mercury methylation potentials in sediment, litter and root samples (Avicennia shaueriana and Spartina alterniflora) from different regions of a mangrove ecosystem, as well as the influence of salinity on methylation. Sediment was sampled under different depths and in mangrove regions with different plant covers and salinities. All samples were incubated with (203)Hg and MM(203)Hg was extracted and measured by liquid scintillation. MMHg was formed in all samples and sites tested including plant roots and litter. Higher Hg methylation was found in the superficial fraction of sediments (0.47-7.82%). Infralittoral sandy sediment had low MMHg formation (0.44-1.61%). Sediment under Rhizophora mangle had lower MMHg formation (0.018-2.23%) than under A. shaueriana (0.2-4.63%) and Laguncularia racemosa (0.08-7.82). MMHg formation in sediment tended to increase with salinity but the differences were not significant. Therefore, MMHg formation occurs in different sites of mangrove ecosystems and may be an important threat that requires further study.

Methane concentration in water column and in pore water of a coastal lagoon (Cabiúnas lagoon, Macaé, RJ, Brazil)

O objetivo dessa pesquisa foi determinar a concentração de metano na coluna d'água e na água intersticial do sedimento nas regiões limnética e litorânea de uma lagoa costeira (Lagoa Cabiúnas, Macaé, RJ). Na região litorânea as amostras foram coletadas em três estandes de macrófitas (Typha domingensis, Eleocharis interstincta e Potamogeton stenostachys). A concentração de metano na água intersticial na região litorânea foi maior do que aquela encontrada na região limnética em cada fração do sedimento. A maior concentração de metano na fração superficial do sedimento (0-2 cm) foi observada no estande de P. stenostachys (3.7 mM). Este resultado foi o único significativamente diferente (p<0.05) da região limnética. A concentração de metano na água intersticial aumentou com a profundidade nos estandes de macrófitas. A concentração de metano na coluna d'água não variou significativamente entre os pontos de coleta (p>0.05). Os resultados sugerem que há uma considerável influência das macrófitas aquáticas estudadas na concentração de metano na água intersticial do sedimento.