Persistent chemostatic behaviour of stream solutes in a northern hardwood forest under climatic and atmospheric deposition changes

Concentration - discharge (C - Q) relationships in streamflow can serve a variety of purposes, including elucidating catchment water sources, quantifying chemical weathering, and estimating solute fluxes. Four main C - Q relationships have been identified: dilution (decreasing C with increasing Q), flushing (increasing C with increasing Q), chemostatic (little or no change in C with increasing Q) and chemostochastic (great variation in C relative to that of Q) behaviour. We used a 40-year data record for 10 headwater forested catchments in central Ontario, Canada, to examine C - Q relationships for a range of geogenic, exogenic and bioactive solutes. The thin overburden cover of this landscape and general absence of deep groundwater contributions to streamflow suggest that streamflow solute concentrations and their relationship with discharge would be sensitive to pronounced changes in climate and atmospheric deposition chemistry over the monitoring period. We explored the temporal and spatial stability of C - Q relationships in response to these changes at annual and seasonal timescales for relatively wet and dry conditions and for periods with relatively high and low atmospheric deposition. Chemostasis dominated C - Q relationships at the annual and seasonal scales for geogenic (e.g., SiO2) and exogenic (e.g., Cl-) solutes, although bioactive solutes (e.g., NO3-, NH4+) exhibited non-chemostatic behaviour for some catchments, time periods and wetness conditions. C - Q relationships derived at the annual timescale often masked pronounced seasonal shifts in bioactive solute concentration response to changes in streamflow and provide only partial insight into the production and delivery of such solutes in this forest landscape. Relatively subtle changes in overburden thickness across this landscape appeared to explain some of the deviations from widespread chemostasis. Nevertheless, this general chemostatic behaviour was largely temporally stable across catchments and solutes, highlighting its resilience in the face of such non-stationary influences as climate conditions and atmospheric deposition.