Factors controlling water-column respiration in rivers of the central and southwestern Amazon Basin
Limnol. Oceanogr., 57(2), 2012, 527-540 | DOI: 10.4319/lo.2012.57.2.0527
ABSTRACT: We examined the factors controlling the variability in water-column respiration rates in Amazonian rivers. Our objectives were to determine the relationship between respiration rates and the in situ concentrations of the size classes of organic carbon (OC), and the biological source (C3 and C4 plants and phytoplankton) of organic matter (OM) supporting respiration. Respiration was measured along with OC size fractions and dissolved oxygen isotopes (δ18O-O2) in rivers of the central and southwestern Amazon Basin. Rates ranged from 0.034 µmol O2 L-1 h-1 to 1.78 µmol O2 L-1 h-1, and were four-fold higher in rivers with evidence of photosynthetic production (demonstrated by δ18O-O2 < 24.2‰) as compared to rivers lacking such evidence (δ18O-O2 > 24.2‰; 1.35 ± 0.22 vs. 0.30 ± 0.29 µmol L-1 h-1). Rates were likely elevated in the former rivers, which were all sampled during low water, due to the stimulation of heterotrophic respiration via the supply of a labile, algal-derived substrate and/or the occurrence of autotrophic respiration. The organic composition of fine particulate OM (FPOM) of these rivers is consistent with a phytoplankton origin. Multiple linear regression analysis indicates that [FPOC], C : NFPOC ratios, and [O2] account for a high amount of the variability in respiration rates (r2 = 0.80). Accordingly, FPOC derived from algal sources is associated with elevated respiration rates. The δ13C of respiration-derived CO2 indicates that the role of phytoplankton, C3 plants, and C4 grasses in supporting respiration is temporally and spatially variable. Future scaling work is needed to evaluate the significance of phytoplankton production to basin-wide carbon cycling.