Influence of plankton metabolism and mixing depth on CO₂ dynamics in an Amazon floodplain lake

We investigated plankton metabolism and its influence on carbon dioxide (CO₂) 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 CO₂ emissions to the atmosphere with floating chambers and depth profiles of temperature and CO₂ partial pressure (pCO₂) 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). CO₂ air-water fluxes varied over 24-h periods with changes in thermal structure and metabolism. Most net daily CO₂ 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 (6.8gCm^-2d^-1) 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, pCO₂, and CO₂ fluxes. During the period when the submerged herbaceous vegetation decayed phytoplankton abundance increased and photosynthetic uptake of CO₂ occurred. While planktonic metabolism was often autotrophic (GPP:CR>1), CO₂ out-gassing occurred during most periods investigated indicating other inputs of carbon such as sediments or soils and wetland plants.