Regional and seasonal variability in planktonic photosynthesis and planktonic community respiration in Amazon floodplain lakes

Results from two regional surveys and multi-lake seasonal studies were used to investigate the variability of phytoplankton photosynthesis and planktonic community respiration in central Amazon floodplain lakes. Hypothesized effects of optical and chemical variables on planktonic photosynthesis and respiration were examined statistically. Changes in dissolved oxygen in light and dark bottles distributed along light-gradients in a shipboard incubator or in situ were used to calculate volumetric community respiration (Rc), volumetric gross photosynthesis (P), daily integral gross photosynthesis (Π), and daily integral community respiration rates (Λ). Π varied significantly among all lakes with source-water river stage and source-water river type. Λ also varied significantly with source-water river stage. Variation in maximum depth linked to source-water river stage was a key factor controlling seasonal variations in Π through its influence on total suspended solids and total phosphorus concentrations which affected light extinction and light-saturated photosynthesis, respectively. The predominance of sub-saturated dissolved O2 in the pelagic surface waters of Amazon floodplain lakes was attributed to high integral Rc:P ratios, indicating the existence of large sustained inputs of non-phytoplankton organic carbon to these environments.