Catchment-scale biogeochemical filtering: A preliminary evaluation of chemical export in four Experimental Watersheds. (Invited)

Catchments may be conceptualized as hydrological, biological and geochemical filters, which, forced by precipitation, generate patterns of flow and chemical export. We examine these patterns to obtain insight into the operation of the filters, and consequently into key process interactions within catchments. This study examined patterns of precipitation, flow, nutrient and weathering export from four Experimental Watersheds (Hubbard Brook, Loch Vale, HJ Andrews and Fernow). The following questions guided the study: 1) How was precipitation linked to patterns in stream flow? 2) What patterns emerged with respect to hydrology and chemistry, and how did they vary across sites? 3) Which time scales appeared to be important? Hydrologically, large differences in precipitation-flow patterns emerged at annual and within-year timescales, which appeared to be linked to variability in snowpack density and timing of snowmelt. Across the majority of sites and for a majority of chemical species examined, apparent chemostatic behavior was observed at seasonal timescales, such that the variability in flow-weighted concentration was small in comparison to the variability in hydrological drivers. The variations in the magnitude of this concentration between sub-catchments and between research sites were influenced by geology and atmospheric deposition. Exceptions to the apparent chemostatic response were also observed, notably in bioactive compounds. Potential process controls and decoupling between hydrologic and biogeochemical processes that might lead to such chemostatic behavior are discussed.