A LONG-TERM PERSPECTIVE OF DISSOLVED ORGANIC-CARBON TRANSPORT IN SYCAMORE-CREEK, ARIZONA, USA

Dissolved organic carbon (DOG) dynamics were examined over five years (1989-1993) in Sycamore Creek, a Sonoran Desert stream, specifically f ocusing on DOC concentration in surface and hyporheic waters, and rate s of export. In 1989 and 1990, the years of lowest stream discharge (0 .08 and 0.04 m(3) s(-1) annual mean of daily discharge, respectively), DOC was high, averaging 7.37 and 6.22 mgC l(-1) (weighted annual mean s). In contrast, from 1991 through 1993, a period of increased flow (1 .1, 1.2 and 4.3 m(3) s(-1)), concentration was significantly lower (P < 0.001) with annual mean concentrations of 3.54, 3.49 and 3.39 mgC l( -1). Concentration exhibited little spatial variation between two samp ling stations located 6 km apart along the mainstem or between surface and hyporheic waters. Annual export of DOC from Sycamore Creek varied 100-fold over the five-year period from a mean rate of only 24 kgC d( -1) in 1990 to 2100 kgC d(-1) in 1993. Ninety percent of DOC was expor ted by flows greater than 2.8 m(3) s(-1), and 50% during flows greater than 24 m(3) s(-1); flows of 2.8 and 24 m(3) s(-1) occurred only 9 an d 1% of the time. The export of organic matter in Sycamore Creek appea rs to be coupled to El Nino-Southern Oscillation phenomena. The years of highest export, 1991-1993, had Fl Nino conditions while 1989 and 19 90 had medial conditions.