Weather and land use mediate the C:N:P stoichiometry of watershed exports

Background/Question/Methods

Many areas, including the Midwestern US region, are experiencing an increased frequency of large storm events. Our goal was to determine how increased pulse flow affects the carbon : nitrogen : phosphorus (C:N:P) stoichiometry of watershed exports. Variation in C:N:P exports is important because it can influence downstream aquatic nutrient limitation, nutrient and carbon sequestration, and biodiversity. We quantified exports using a high-frequency, flow-dependent sampling regime for two water years in two Ohio watersheds of contrasting land use (agricultural, three streams; forested, one stream) to get daily exports of C, N, and P in dissolved organic and inorganic (DOC, DON, DOP, ammonium, nitrate, SRP) and particulate (PC, PN, PP) fractions. We then used hydrograph separation techniques to obtain daily base flow and pulse flow exports for each C:N:P fraction. We calculated a pulse- to base-flow ratio (P:B) during high flow events for C:N:P export ratios (DIN:SRP, DOC:DON:DOP, PC:PN:PP) to see how increased pulse flow affects these ratios in watersheds of contrasting land use. We also regressed P:B’s for individual discrete events against average discharge during each event.

Results/Conclusions

P:B ratios for DIN:SRP were < 1 in both watersheds suggesting that pulse flow elevates SRP over DIN. In general, P:B ratios for DOC:DON and DOC:DOP were > 1 for both watersheds and DON:DOP were < 1 for both watersheds. Increased DOC relative to DON and DOP during pulse flow could negatively impact microbial communities in downstream waters. P:B ratios for PC:PN:PP in the forested watershed were near or < 1 while they were > 1 in the agricultural watershed likely a consequence of differential exports of mineral sediments. Overall, regressions for P:B against average storm discharge explained more variation when considering events within a season (i.e., fall, winter, summer, and spring) than when combining all seasons together. We also typically found that within specific C:N:P stoichiometric ratios, discharge correlated with P:B’s in the same direction for both watersheds; exceptions were found with PC:PP, PN:PP, and TC:TN. Taken together, our results suggest that increased pulse flow due to more frequent storm events will have complex effects on C:N:P stoichiometry of watershed exports. Further, results suggest a flow-land use interaction for many ratios, i.e., the extent to which C:N:P ratios increase or decrease with increased pulse flow depends on land use.