Nutrient Budgets Calculated in Floodwaters Using a Whole-Ecosystem Reservoir Creation and Flooding Experiment

Abstract

Flooding in forested areas can release nutrients via organic matter flushing and organic matter decomposition and contribute to nutrient loading in aquatic ecosystems. Organic matter content may set an upper limit for nutrient release during flooding, but variation in timing, depth, duration of inundation, and site organic matter substrate inhibit comparison across sites and events. We used data collected during a whole-ecosystem flooding experiment conducted at the IISD-Experimental Lakes Area (ELA) in northwestern Ontario to examine the relationship between site organic matter content and nutrient release on both a short-term (first 3 weeks) and annual scale (open-water season). Three upland forest sites with differing amounts of organic matter stored in soil and vegetation were flooded to create experimental reservoirs. We hypothesized that the magnitude of nutrient release would be related to the organic matter content in each site. We found that nutrient concentrations increased relative to low-nutrient water pumped into each reservoir at both scales, but there was no relationship between site organic matter content and short-term total nitrogen (TN) and total phosphorus (TP) concentrations or annual TN and TP fluxes. Additionally, N and P release rates differed, decreasing TN:TP ratios in reservoirs and outflows relative to inflows. Nutrient concentrations increased immediately after inundation each year and continued for 5 years of repeated flooding. Predicted increases in flooding due to changing precipitation patterns and reservoir creation may cause forested areas to be a long-term nutrient source to aquatic ecosystems. Over time, nutrient accumulation may affect water quality, shift biological communities, and influence ecosystem functioning.