The Importance of pH, Particulate Carbon, and Photosynthesis in Controlling Water-Column Respiration Rates in the Central and Southwestern Amazon Basin
Ellis, University of Washington, firstname.lastname@example.org
Richey, University of Washington, email@example.com
Aufdenkampe, Stroud Water Research Center, firstname.lastname@example.org
Krusche, University of Sao Paulo, email@example.com
Although water-column respiration has been hypothesized to be the main source of outgassing CO2 from Amazonian rivers, we lack an understanding of the variability of the observed rates. This study examines the variation of water-column respiration rates as a function of aquatic chemical parameters, bacterial abundance, photosynthetic production, and organic carbon (OC) size fractions (coarse OC, fine OC, dissolved OC and size fractions within the dissolved pool: < 5 kDa, 5-100 kDa, > 100 kDa). Throughout Amazonas and Acre, respiration rates ranged from 0.034 to 1.77 µM/hr of O2 consumed. Respiration was positively correlated with pH (r2=0.58) and with bacterial abundance (r2=0.78). Our results from a partial correlation analysis suggest that pH may indirectly control respiration by limiting bacterial abundance in acidic sites. Further, sites with a pH < 7 (low pH sites) had a higher ratio of respiration to photosynthesis (R:P) (2.3 ± 0.3) than sites with a pH > 7 (high pH sites) (1.1 ± 0.2). This is consistent with the isotopic signature of respired CO2, which suggests that algae is consumed at high pH sites, whereas C3 and C4 vegetation fuel respiration at low pH sites. Out of all size fractions of OC studied, FPOC and the percentage of DOC < 5 kDa were the most positively correlated with respiration rates (r2=0.70 and r2=0.29, respectively). Kendall’s Coefficient of Concordance indicated that rivers with high respiration rates also have high pH values, high fine particulate OC concentrations and low R:P ratios. Thus the results of this study suggest that pH may control respiration rates by limiting bacterial abundance in acidic conditions, that fine particulate OC is a labile size fraction of OC, and that autochthonous production may play a larger role in riverine carbon cycling in the Amazon than previously thought.