Examining the impact of stream pH changes on transient P storage in streambed sediments across multiple temporal scales

Canadian Laurentian Great Lakes Basin. Study examines the potential for stream pH changes to modulate the storage of phosphorus (P) in streambed sediments across multiple temporal scales by (i) synthesizing experimental data that quantify the relationships between pH and sediment P exchange, and (ii) comparing these relationships to measured long-term, seasonal, diel and event-based pH changes observed in streams. Synthesis of experimental data indicate that P exchange is highly sensitive to pH changes over the pH range typically observed in Ontario streams (pH >7). Of 157 monitored streams, 84 % experienced significant pH increases over the last 35 years, averaging + 0.24 pH change, and favouring long-term P release from streambed sediments or limiting P retention. Seasonally, most streams exhibited higher pH during summer, suggesting P release from sediments at times of high-productivity. Over shorter timescales, a subset of streams demonstrated diel pH fluctuations of > 1 pH unit in summer, and rapid pH changes (up to 1.7 pH units) during precipitation and snowmelt events. pH changes of this magnitude may drive rapid and extensive P exchange from streambed sediment. Overall, our results indicate pH changes may be an under-appreciated control on the transient P storage within streams, with relevance at multiple temporal scales. This finding has implications for P load predictions by mechanistic watershed nutrient models. • pH is an under-appreciated driver of transient P storage in streams. • P exchange highly sensitive to pH over pH range typically observed in streams. • Long-term pH increase in Ontario streams favours P release from sediments. • Seasonal, diel and event-based pH changes expected to drive P exchange.