Matching scale to processes in stream DOM biogeochemistry

Abstract:

We conducted temporal and synoptic sampling of a seasonally irrigated agricultural watershed in the Sacramento River valley, California, in order to investigate 1) temporal trends in dissolved organic carbon (DOC) compositions and concentrations related to seasonal climate and land-use activities, and 2) the extent to which DOC concentrations and compositions from throughout the catchment are represented at the mouth. DOC concentrations ranged from 1.8 to 13.9 mg L^-1, with the lowest values in headwater 1^st and 2^nd order streams, and the highest values associated with flood irrigation and storm events. DOC concentrations and compositions at or near the mouth (3^rd and 4^th order streams) appear to be primarily influenced by land use (agriculture) in the lower reaches, thereby masking much of the headwater chemistry (1^st and 2^nd order streams). Temporally, DOC composition and chemistry is distinctly linked to three hydrologic regimes: 1) winter baseflow in which water and organic matter (OM) primarily derives from subsurface flow through aged soil organic matter, 2) spring storms in which fresher OM is released from surface flow/soils, and 3) summer irrigation in which field runoff can leach OM directly from plant tissues and surface soils. Sampling of the latter revealed large pulses of compositionally distinct DOC that drives stream biogeochemistry in the summer. Optical proxies exhibited varying degrees of correlation with chemical measurements, with strongest relationships to DOC and dissolved lignin (r² = 0.95 and 0.73, respectively) and weaker relationships to carbon-normalized lignin yields and C:V (r² from 0.31 to 0.42). Demonstrating the importance of matching (time) scale to processes, we found no relationship between dissolved lignin concentrations and total suspended sediments (TSS) synoptically, in contrast to the strong relationship observed temporally at the mouth. In contrast, a strong relationship (r² = 0.56) between carbon-normalized lignin yields and ratios of cinnamyl to vanillyl phenols evident synoptically was not apparent at the mouth. In both cases, the common element appears to be processing time in the stream, with time to partition generating the lignin:TSS relationship, while time for degradation and source inputs masks the latter relationship.