• Changes in concentration and composition of dissolved and particulate organic matter in the upper San Pedro River, Arizona, in response to changes in flow regime

      Haas, Peter Andrew.; Brooks, Paul D. (The University of Arizona., 2003)
      This study presents the results of 15 months of intensive sampling of the upper San Pedro River at the Boquillas ranch research site. Sampling identified patterns in surface water chemistry associated with seasonal changes in flow regime. The source of organic matter shifted from highly aquatic to highly terrestrial during the monsoon season. Concentrations of Dissolved Organic Carbon (DOC) increased from a nonmonsoon baseflow median of 2.7 ± 0.1 mg/I to a median monsoon baseflow concentration of 3.3 ± 0.2 mg/l. DOC concentrations during flow events were significantly higher (p < 0.05) with a median peak concentration of 7.2 ± 0.4 mg/l. Concentrations of Dissolved Organic Nitrogen (DON) increased from a non-monsoon baseflow median of 0.10 ± 0.01 mg/1 to a median monsoon baseflow concentration of 0.16 ± 0.01 mg/l. DON concentrations during flow events were significantly higher (p < 0.05) with a median peak concentration of 0.48 ± 0.02 mg/l. The 59-day 2002 monsoon contributed 40% of the water volume while transporting nearly 50% of the Dissolved Organic Matter (DOM) and 99% of the Particulate Organic Matter (POM) for the 2002 water year. Annual rates of production were estimated for DOC between 0.008 and 0.0012 g C m^-2yr^-1 and Particulate Organic Carbon (POC) between 0.07 and 0.11 g C m^2 yr^-1 , as well as, DON between 0.00024 and 0.00036 g N m^-2 yr^-1 and Particulate Organic Nitrogen (PON) between 0.004 and 0.006 g N m^-2 yr^-1 . Flux rates were much lower than that of mesic systems and likely reflects the lower terrestrial net primary productivity of the desert environment and below average precipitation during the 2002 water year.