Inferring scale-dependent processes influencing stream water biogeochemistry from headwater to sea

Understanding how scale-dependent processes regulate patterns of water chemistry remains a challenge in aquatic biogeochemistry. This study evaluated how chemical properties of streams and rivers vary with drainage size and explored mechanisms that may underlie nonlinear changes with increasing scale. To do this, we contrasted concentrations of total organic carbon (TOC) with pH and cations (Ca and Mg) from 69 catchments in northern Sweden, spanning a size gradient from headwaters (<0.01 km(2)) to major rivers and estuaries (>100,000 km(2)). Across this gradient, we evaluated (1) changes in average concentrations and temporal variation, (2) scale breaks in catchment area-concentration relationships, and (3) the potential importance of groundwater inputs and instream processes as drivers of change. Results indicated that spatial and temporal signals converge at approximate to 2-10 km(2) as streams draining distinct headwater catchments coalesce and mix. Beyond 10 km(2), streams tended to lose headwater signatures, reflecting a transition from shallow to deep groundwater influence. This was accompanied by a second break at approximate to 70-500 km(2) corresponding to reduced spatial variability and a convergence of the response to snowmelt, as the dominance of deep groundwater influence increased with catchment scale. Larger catchments showed greater effect of instream processing on TOC, as concentrations predicted from the conservative mixing of upstream signals and dilution with deep groundwater were lower than measured. This study improves the understanding of scaling biogeochemical patterns and processes in stream networks, highlighting thresholds that imply shifts in the factors that shape variation in chemistry from headwaters to the sea.