Implications of future changes in climate extreme events for aquatic biogeochemical cycles and associated ecosystem services of the St. Croix National Scenic Riverway

Abstract:

Both Minnesota and Wisconsin have declared Lake St. Croix, the lowermost 40 km of the St. Croix National Scenic Riverway, as impaired due to excessive phosphorus and chlorophyll levels and have agreed to reduce phosphorus loads to the river by 27% over the next several decades. On the other hand, the projected increase of extreme climate events (ECEs), such as more frequent and severe rainfall, may increase the risks of nutrients delivery from terrestrial to aquatic ecosystems. Based on the Soil and Water Assessment Tool (SWAT), we developed the St. Croix watershed model to explicitly simulate rainfall-runoff, erosion, and nutrient-transport processes along the terrestrial-aquatic continuum. Notably, the SWAT model used here incorporates the CENTURY carbon, nitrogen and phosphorus algorithms and phosphorus supply from soil weathering to better describe nutrients cycling. Available digital data sets of topography and hydrography were used to characterize flow paths, numerous wetlands and ponds, and 39 largest lakes intersecting the stem channel. Surveyed land use and management practices were used to account for crop rotations, tillage practices, livestock grazing, and applications of manure and inorganic fertilizer. The model was calibrated and validated using observed data for 1990-2007 with satisfactory model fits for suspended sediment, total phosphorus, and total nitrogen. By analyzing climate projections from the CMIP5 scenarios, we find that both annual mean and maximum daily precipitation will increase substantially by the end of 2100. Driven by spatially downscaled and bias-corrected CMIP5 climate scenarios, projected flood intensity could increase as much as 35% and cause significant increase in the delivery of sediments and nutrients from terrestrial to aquatic ecosystems and deteriorate water quality. Next, we evaluate the effectiveness of alternative cropland best management practices under future uncertain changes in ECEs, and derive probabilistic outcomes that are valuable for making science-based decisions in how best to restore and protect the St. Croix National Scenic Riverway.