Estimating the Importance of Hydrologic Conditions on Nutrient Retention and Plant Richness in a Wetlaculture Mesocosm Experiment in a Former Lake Erie Basin Swamp

Abstract

The western basin of Lake Erie, the shallowest of the Laurentian Great Lakes in North America, is now plagued by harmful algal blooms annually due to nutrient discharges primarily from its basin. Water quality was impacted so significantly by toxic cyanobacteria in 2014 that the city of Toledo’s water supply was shut off, affecting hundreds of thousands of residents. A new agricultural land management approach, ‘wetlaculture (=wetland + agriculture)’, has a goal of reducing the need for fertilizer applications while preventing fluxes of nutrients to downstream aquatic ecosystems. A wetlaculture mesocosm experiment was set up on agricultural land near Defiance, Ohio, on the northwestern edge of the former ‘Great Black Swamp’. The mesocosms were randomly assigned to four hydrologic treatments involving two water depths (no standing water and ~10-cm of standing water) and two hydraulic loading rates (10 and 30 cm week−1). Nearby agricultural ditch water was pumped to provide weekly hydraulic loading rates to the mesocosms. During the two-year period, the net mass retention of phosphorus from the water was estimated to have averaged 1.0 g P m−2 in the wetland mesocosms with a higher hydraulic loading rate, while the highest estimated net nitrogen mass retention (average 22 g N m−2) was shown in the wetland mesocosms with 10 cm of standing water and higher hydraulic loading rate. Our finding suggests that hydrologic conditions, especially water level, contribute directly and indirectly to nutrient retention, partially through the quick response of the wetland vegetation community. This study provides valuable information for scaling up to restore significant areas of wetlaculture/wetlands in the former Great Black Swamp, strategically focused on reducing the nutrient loading to western Lake Erie from the Maumee River Basin.