No abstract

Inland waters have a significant influence on atmospheric methane (CH 4 ) levels. However, processes determining the strength of CH 4 emissions from these systems are not well defined. Aerobic oxidation is a major sink of CH 4 in freshwater environments and thus an important determinant of aquatic CH 4 emissions, yet strikingly little is known about its drivers. Here we assessed the extent of water column CH 4 oxidation at the whole ecosystem scale using stable carbon isotopic (δ 13 C) mass balance of CH 4 in 14 northern lakes spanning wide range of dissolved organic carbon (DOC) concentrations. We show that the extent of oxidation can vary from near zero to near complete, and for concentrations of 1.9-11 mg/L, DOC is a key modulator of CH 4 oxidation during the summer stratification period. Increasing DOC concentrations enhances oxidation in the upper layers by reducing light inhibition on methanotrophic activity, while reducing oxygen available for oxidation in the deeper layers. The effect of this light inhibition was also observable over the diurnal cycle. We developed simple predictive empirical models (r 2 > 0.82) to estimate the extent of oxidation in the different layers of lakes for the summer period. Applying our surface layer model to a larger data set suggests that about 30% of CH 4 transported to or generated within the epilimnion of Québec lakes is oxidized during summer. Our results imply that DOC concentration, through its effect on the light regime of lakes, has the potential to affect strongly the magnitude and patterns of summer CH 4 emissions.

Lake methane (CH 4 ) emissions are largely controlled by aerobic methane-oxidizing bacteria (MOB) which mostly belong to the classes Alpha-and Gammaproteobacteria (Alpha-and Gamma-MOB). Despite the known metabolic and ecological differences between the two MOB groups, their main environmental drivers and their relative contribution to CH 4 oxidation rates across lakes remain unknown. Here, we quantified the two MOB groups through CARD-FISH along the water column of six temperate lakes and during incubations in which we measured ambient CH 4 oxidation rates. We found a clear niche separation of Alpha-and Gamma-MOB across lake water columns, which is mostly driven by oxygen concentration. Gamma-MOB appears to dominate methanotrophy throughout the water column, but Alpha-MOB may also be an important player particularly in well-oxygenated bottom waters. The inclusion of Gamma-MOB cell abundance improved environmental models of CH 4 oxidation rate, explaining part of the variation that could not be explained by environmental factors alone. Altogether, our results show that MOB composition is linked to CH 4 oxidation rates in lakes and that information on the MOB community can help predict CH 4 oxidation rates and thus emissions from lakes. Received

Methanogenesis is traditionally considered as a strictly anaerobic process. Recent evidence suggests instead that the ubiquitous methane (CH 4 ) oversaturation found in freshwater lakes is sustained, at least partially, by methanogenesis in oxic conditions. Although this paradigm shift is rapidly gaining acceptance, the magnitude and regulation of oxic CH 4 production (OMP) have remained ambiguous. Based on the summer CH 4 mass balance in the surface mixed layer (SML) of five small temperate lakes (surface area, SA, of 0.008−0.44 km 2 ), we show that OMP (range of 0.01 ± 0.01 to 0.52 ± 0.04 μmol L −1 day −1 ) is linked to the concentrations of chlorophyll-a, total phosphorus, and dissolved organic carbon. The stable carbon isotopic mass balance of CH 4 (δ 13 C-CH 4 ) indicates direct photoautotrophic release as the most likely source of oxic CH 4 . Furthermore, we show that the oxic CH 4 contribution to the SML CH 4 saturation and emission is an inverse function of the ratio of the sediment area to the SML volume in lakes as small as 0.06 km 2 . Given that global lake CH 4 emissions are dominated by small lakes (SA of <1 km 2 ), the large contribution of oxic CH 4 production (up to 76%) observed in this study suggests that OMP can contribute significantly to global CH 4 emissions.

It is well accepted in the literature that lakes are generally net heterotrophic and supersaturated with CO 2 because they receive allochthonous carbon inputs. However, autotrophy and CO 2 undersaturation 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 pCO 2 and CO 2 flux across the air-water interface in a tropical productive lake in southeastern Brazil (Lake Carioca) over two consecutive days. Both pCO 2 and CO 2 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 CO 2 dynamics in tropical lakes. Net heterotrophy and CO 2 outgassing from the lake were registered only at night, while significant CO 2 emission did not happen during the day. Dissolved oxygen concentration and temperature trends over the diurnal cycle indicated the dependence of CO 2 dynamics on lake metabolism (respiration and photosynthesis). This study indicates the importance of considering the diurnal scale when examining CO 2 emissions from tropical lakes.Keywords: pCO 2 , CO 2 flux, diurnal variations, tropical productive lake.Amostragem diurna demonstra variação significativa na emissão de CO 2 em um lago tropical produtivo Resumo É amplamente aceito na literatura que lagos são em geral heterotróficos e supersaturados com CO 2 já que recebem carbono alóctone. Porém, autotrofia e insaturação de CO 2 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 pCO 2 e no fluxo de CO 2 através da interface ar-água foram avaliadas num lago tropical produtivo do sudeste do Brasil (Lagoa Carioca) durante dois dias consecutivos. Tanto a pCO 2 quanto o fluxo de CO 2 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 CO 2 na Lagoa Carioca. Foram registradas heterotrofia e emissão de CO 2 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 CO 2 no metabolismo (respiração e fotossíntese) deste lago. Este estudo indica a importância de se considerar a escala diurna na avaliação da emissão de CO 2 por lagos tropicais.Palavras-chave: pCO 2 , fluxo de CO 2 , variações diurnas, lago tropical produtivo.

Cecropia genus comprises pioneer trees which are characteristic elements of forest borders and gaps in the Neotropics (Vasconcelos & Casimiro, 1997; Sposito & Santos, 2001). Most species are inhabited by mutualistic ants that nest inside hollow internodes where they store eggs, larvae and pupae (Harada & Benson, 1988), and feed on glycogenrich Müllerian bodies called trichilium located at the base of leaf petioles (Yu & Davidson, 1997). Although Cecropia can host a variety of resident ant genera, Azteca spp are the most common ant inhabitants (Longino, 1991a). Usually only one mature Azteca colony inhabits a Cecropia tree, however up to five Azteca species may be found in a Cecropia population (Longino, 1991a). In addition to a resident Azteca colony, other ant species can be found living or foraging on the same Cecropia tree (Vieira et al., 2010). However, the diversity of non-Azteca ants living on Cecropia trees or using them as foraging substrates is still poorly known, as well as the factors that might affect ant richness and abundance on these trees.

No abstract

Trophic connections among habitats may be central to food-web dynamics in lakes. Lacustrine zooplankton can rely on basal carbon (C) sources from different origins and plays an important link between these and organisms in higher trophic levels. We investigated the basal C sources supporting the planktonic food web and the trophic relationships among zooplankton size fractions in a tropical lake (Carioca) in Brazil. To do so, we measured the C and nitrogen (N) stable-isotope ratios in basal C sources originated in terrestrial, littoral, and pelagic habitats and in zooplankton size fractions, and data were analysed through Bayesian mixing models. Mesozooplankton showed seasonal variation in resource use, specifically a smaller dependence on algae in the wet than in the dry season. In the wet season, mesozooplankton relied more on the detritivore food chain eating mostly microzooplankton (mode: 95.1%), which in turn consumed mostly terrestrial C in this season (mode: 74.7%). Zooplankton size fractions also occupied different relative trophic positions between seasons. These variations seem to follow the seasonal dynamics of in-lake primary production and of terrestrial C inputs. Also, all size fractions of zooplankton, and particularly Chaoboridae larvae, showed low C staple-isotope values, suggesting the consumption of a missing C source.