The effect of seasonal drying on sulphate dynamics in streams across southeastern Canada and the northeastern USA
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Within the southeast Canada and northeast USA region, a peak in sulphate (SO42-) concentration has been reported for some streams following periods of substantial catchment drying during the summer months (ON, Canada; VT, NH and NY, USA). However, it is currently unclear if a SO42- response to seasonal drying is widespread across the broader region, or to what extent the level of response varies among catchments. In our study, SO42- response to seasonal drying was compared in 20 catchments from 11 locations across southeastern Canada (ON, QC and NS) and northeastern USA (NH, NY, VT, WV and ME). Using long-term monitoring data of stream discharge and chemistry, the number of days for each month of the dry season (# d) when discharge (Q) was below a threshold level (25th percentile; Q25) was calculated for each catchment to give a measure of 'seasonal dryness' (# d Q\Q25). A SO42- response score (rs) was then calculated for each catchment based on linear regression analysis of # d Q\Q25 versus either the annual SO42- concentration, or the residual of annual SO42- concentration as a function of time (year). The final rs values for each catchment provided an estimate of the proportion of variation in annual SO42- concentration which could be explained by seasonal drying (possible rs range = 0–1). Of the 20 catchments, 13 exhibited some level of a SO42- response to seasonal drying (rs = 0.04–0.72) with an additional two catchments exhibiting a SO42- response for one or more seasons. SO42- response scores were positively related to percent wetland area(w) (rs = 1.000 - 0.978e-0.054*w, r2 = 0.44) and percent saturated area (sat) (rs = 0.481 – 0.488e-0.101*sat, r2 = 0.54) indicating that wetlands/ saturated areas were an important driver of regional variation in the SO42- response to seasonal drying. Our results suggest that any shift towards drier summers as a result of climate change could impact SO42- dynamics in a large number of catchments throughout the region.
Kerr, J.G.; Eimers, M.C.; Creed, I.F.; Adams, M.B.; Beall, F.; Burns, D.; Campbell, J.L.; Christopher, S.F.; Clair, T.A.; Courchesne, F.; Duchesne, L.; Fernandez, I.; Houle, D.; Jeffries, D.S.; Likens, G.E.; Mitchell, M.J.; Shanley, J.; Yao, H. 2012.The effect of seasonal drying on sulphate dynamics in streams across southeastern Canada and the northeastern USA. Biogeochemistry. 111(1-3): 393-409. https://doi.org/10.1007/s10533-011-9664-1.