Are Changes in Biogeochemical or Hydrologic Processes Responsible for Increasing DOC Concentrations in Headwater Streams of Northeastern North America?

The recent recognition of widespread and significant upward trends in dissolved organic carbon (DOC) concentrations in surface waters of northeastern North America and Europe has stimulated research to understand the cause of these trends. Several factors have been offered to explain these DOC trends including climate warming, chronic atmospheric nitrogen deposition, decreasing atmospheric sulfur deposition, and increasing surface water pH. Changes in these factors have acted to either increase the solubility of DOC or increase the rates of biogeochemical processes that generate labile carbon in the soil. Additionally, it is well known that rain events and snowmelt increase DOC concentrations in many surface waters through flushing along shallow flow paths where most labile carbon is stored. Changes in hydrologic flushing rates have generally not been explored as a possible explanation of these widely reported upward trends in DOC concentrations. Biscuit Brook, a 9.9 km2 catchment in the Catskill Mountains of New York has shown a significant increasing trend in DOC concentrations since 1992, consistent with other streams in this region. Stream chemistry has been monitored at Biscuit Brook on a weekly basis supplemented with event samples since 1983, providing a detailed data set with which to examine the causes of changes in DOC concentrations. Here, we examine the relative roles of climate warming, decreasing sulfate (SO42-) and nitrate (NO3-) concentrations, and changes in the frequency and size of hydrologic events on the long-term temporal pattern (1992 to 2004) of DOC concentrations in Biscuit Brook. DOC concentrations increased significantly in weekly samples collected primarily during low flow conditions. No similar trend was apparent in the high flow samples. Mean annual SO42- plus NO3- concentrations showed a strong inverse relation (r2 = 0.91, p < 0.01) to DOC concentrations, but these concentrations were not related to stream pH nor to air temperature. These results suggest that the DOC trends largely result from biogeochemical processes associated with the decreasing SO42- and NO3- concentrations such as an increase in the solubility of DOC caused by the decreasing ionic strength of the stream water. The frequency of hydrologic events as determined by hydrograph separation has not changed during 1992 to 2004, so it seems unlikely that changes in hydrologic flushing can explain the upward DOC trends in this stream. Events of similar size in the latter part of the record (2000 to 2004) produced higher DOC concentrations than were evident in the early part of the record (1992 to 1996) suggesting that changes in biogeochemistry and not hydrology are largely responsible for the trends observed at this site, which is representative of upland forested catchments of northeastern North America.