We investigated plankton metabolism and its influence on carbon dioxide (CO) dynamics in a central Amazon floodplain lake (Janauacá, 3°23' S, 60°18' W) from September 2015 to May 2016, including a period with exceptional drought. We made diel measurements of CO emissions to the atmosphere with floating chambers and depth profiles of temperature and CO partial pressure (pCO) at two sites with differing wind exposure and proximity to vegetated habitats. Dissolved oxygen (DO) concentrations were monitored continuously during day and night in clear and dark chambers with autonomous optical sensors to evaluate plankton metabolism. Overnight community respiration (CR), and gross primary production (GPP) rates were higher in clear chambers and positively correlated with chlorophyll-a (Chl-a). CO air-water fluxes varied over 24-h periods with changes in thermal structure and metabolism. Most net daily CO fluxes during low water and mid-rising water at the wind exposed site were into the lake as a result of high rates of photosynthesis. All other measurements indicated net daily release to the atmosphere. Average GPP rates (6.8gCmd) were high compared with other studies in Amazon floodplain lakes. The growth of herbaceous plants on exposed sediment during an exceptional drought led to large carbon inputs when these areas were flooded, enhancing CR, pCO, and CO fluxes. During the period when the submerged herbaceous vegetation decayed phytoplankton abundance increased and photosynthetic uptake of CO occurred. While planktonic metabolism was often autotrophic (GPP:CR>1), CO out-gassing occurred during most periods investigated indicating other inputs of carbon such as sediments or soils and wetland plants.

The river downstream from a dam can be more contaminated by mercury than the reservoir itself. However, it is not clear how far the contamination occurs downstream. We investigated the seasonal variation of methylmercury levels in the Balbina reservoir and how they correlated with the levels encountered downstream from the dam. Water, plankton, and fishes were collected upstream and at sites between 0.5 and 250 km downstream from the dam during four expeditions in 2011 and 2012. Variations in thermal stratification of the reservoir influenced the methylmercury levels in the reservoir and in the river downstream. Uniform depth distributions of methylmercury and oxygen encountered in the poorly stratified reservoir during the rainy season collections coincided with uniformly low methylmercury levels along the river downstream from the dam. During dry season collections, the reservoir was strongly stratified, and anoxic hypolimnion water with high methylmercury levels was exported downstream. Methylmercury levels declined gradually to 200 km downstream. In general, the methylmercury levels in plankton and fishes downstream from the dam were higher than those upstream. Higher methylmercury levels observed 200-250 km downstream from the dam during flooding season campaigns may reflect the greater inflow from tributaries and flooding of natural wetlands that occurred at this time.

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The first extensive set of measurements of methane concentrations and fluxes for the Negro River and its major tributaries combined with complementary data for the Solimões and Madeira rivers and several tributaries are presented and their temporal and spatial variations examined. Fluxes were measured using floating chambers, and dissolved CH4 concentrations were measured by the headspace technique. In the Solimões basin, tributaries had higher fluxes when water levels were low; no statistical difference among periods for lakes and the main stem river was observed. In the Negro basin, rivers had higher fluxes with greater variations among rivers during high water than during low water based on fluxes calculated from the concentration gradient and modelled gas transfer coefficients. We estimate a regional methane emission of 0.31 Tg C yr−1 for large river channels in the lowland Amazon basin.