Influence of hydropattern and vegetation on phosphorus reduction in a constructed wetland under high and low mass loading rates

Abstract

Large constructed wetlands are extensively used in south Florida for surface water quality improvement as well as for flood control. Due to the periodic wet–dry climate of this region, it is likely that soils in these large constructed wetlands may become dry or partially dry (experiencing drawdown), which represent a challenge to managing and optimizing phosphorus (P) reductions in these systems. Therefore, we designed an experiment to study the interactive effect of hydropattern (batch vs. continuous flow) and the presence or absence of emergent vegetation on P exchange between surface water and organic soil in two sets of 12 mesocosms. These 24 mesocosms were filled with 30-cm deep peat soil, and each set were subjected to either high (12.5gm−2year−1) or low (3.4gm−2year−1) P loading from surface water. All treatments performed similarly prior to surface water drawdown with P reductions being relatively high for high P loading rates and being low for low P loading rates. Mesocosms subject to wet–dry–wet cycles exhibited a three- to four-fold increase in surface water effluent P concentrations immediately following soil re-flooding, which lasted 1–3 and 3–10 weeks for low and high P loading rates, respectively. The magnitude of P flux from sediment to surface water and the time period over which P release took place were P loading-dependent (higher loading led to higher P flux compared to lower P loading rates), and season-dependent (a longer duration of higher P flux experienced during dry- compared to wet-season drawdowns). Results indicated that hydropattern was the dominant factor affecting P flux to overlying surface water for the high P loading rate, while the presence or absence of emergent vegetation was the dominant factor influencing P release for the low P loading rate. Treatments lacking emergent vegetation generated the most particulate P (PP) for both high and low P loading rates. All P fractions were correlated to either hydropattern or inflow concentrations, for both low and high P loading rates, and, with the exception of PP, correlated to vegetation at low-P loading rate. Our results indicated that the presence or absence of emergent vegetation is a critical factor in the management of large constructed wetlands receiving low P loadings while hydropattern should be the focus in managing treatment systems receiving high loads of P. Regardless of the P loading rates, maintaining moist soils in large constructed wetlands is a good management strategy, particularly during dry climatic periods, to minimize soil P oxidation and P flux to surface water after soil re-flooding.