De Mello NAST, Brighenti LS, Barbosa FAR, Staehr PA, Bezarra Neto JF. 2017. Spatial variability of methane (CH₄) ebullition in a tropical hypereutrophic reservoir: silted areas as a bubble hot spot. Lake Reserv Manage. 35:105—114. The concentration of methane (CH₄) has doubled in the atmosphere over the last 200 yr, raising the need to understand emissions of this potent greenhouse gas from inland waters. CH₄ ebullition is the dominant pathway in shallow aquatic environments and is difficult to quantify due to its episodic nature and heterogeneous spatial distribution. We investigated the temporal and spatial variability of CH₄ ebullition during 2013 in a shallow hypereutrophic urban reservoir, in Belo Horizonte City, Brazil. The average emission measured during summer was 780 mg CH₄/m²/d, ranging from 1 to 3070 (n = 75). During winter, the average emission was 316 mg CH₄/m²/d, ranging from 4 to 1253 (n = 75). A strong spatial variation (P < 0.001) was observed across the reservoir in both seasons. Several folds higher (39—58% of the total) emissions were recorded at the mouth of the main tributaries, which therefore was considered to be a hot spot ebullition zone. This was expected due to its shallow area (mean depth 1.30 m) with low hydrostatic pressure and 2 to 6 C (winter and summer, respectively) higher sediment temperatures, which is aggravated by the intense siltation process resulting from insufficient management of the sewage water entering the reservoir. In this article we demonstrate the consequence of siltation as an enhancing factor for CH₄ emission from the hot spots ebullition zones.

AIM: This study aims to demonstrate the potential of hydroacoustics in the study of behavior of the invertebrate Chaoborus and fish in neotropical water environments; METHODS: Synoptic campaigns were conducted in May and June-2008 in Dom Helvécio and Carioca lakes, at the Parque Estadual do Rio Doce (MG), and Nado reservoir, Belo Horizonte (MG). The acoustic scattering of targets was studied using a downward-oriented split-beam 200 kHz echosounder; RESULTS: We detected clear echo signals from fish and Chaoborus larvae, which can be viewed in high density in all environments studied. The normal migratory behavior of the larvae of Chaoborus could be easily monitored in the Dom Helvécio Lake and the Nado reservoir. However, this behavior was not seen in Carioca Lake; CONCLUSIONS: This study revealed the potential application of acoustic approaches to study the behavior of fish and zooplankton organisms in freshwater aquatic systems.

We tested how depth-linked hydrostatic pressure, sediment temperature, and sediment texture, represented here by grain-size distribution, partition methane (CH₄) between dissolved storage in sediment porewater and ebullitive release in a tropical monomictic lake. Along a shoreline-to-center transect spanning 1.5–7 m depth, we measured 24-h ebullition with replicated inverted funnels over three consecutive days and quantified porewater methane from short cores while recording depth, sediment temperature, grain size, wind, and barometric pressure. Ebullition was greatest at shallow, warmer, sandier sites, whereas porewater methane accumulated at deeper, cooler sites, yielding an inverse spatial pattern of the two phases. Site-level regressions identified sand content as the strongest spatial correlate of bubbling, while porewater methane increased with depth and declined with sediment temperature. Under weak winds and near-constant barometric pressure, ebullition showed no detectable day-to-day variation over the sampled 72-h interval, indicating that between-site differences exceeded short-term temporal variability. Together, these results describe a spatially resolved storage–release pattern consistent with depth-, temperature-, and texture-related controls on methane retention and bubbling in Lake Carioca.