Macrophyte-hydrodynamic interactions mediate stratification and dissolved oxygen dynamics in ponds

Abstract

Small waterbodies are sensitive to stressors such as eutrophication and heatwaves; however, interactions between macrophytes and hydrodynamics may mediate the effects of compounding stressors. Leveraging an ecosystem experiment and hydrodynamic model, we evaluated how macrophyte biomass, thermal structure, and dissolved oxygen (DO) responded to the interaction of episodic nutrient loading and periods of high temperatures in two temperate ponds. In one pond we experimentally added pulses of nutrients, simulating storm-driven loading (the other pond served as an unmanipulated reference). Following the first nutrient pulse both ponds experienced a 5-day period of high surface water temperatures. Macrophytes in the nutrient addition pond began to senescence mid-summer due to phytoplankton shading from the nutrient addition and heat stress while macrophytes in the reference pond followed expected seasonal patterns, senescing in early autumn. Field observations and model results indicate that macrophytes structured the thermal environment through vertical attenuation of turbulent kinetic energy and light. Macrophytes reduced the vertical extent of water column warming during the heat event by 0.25-0.5 m and maintained cooler bottom temperatures (up to 2.5 °C cooler) throughout the summer, suggesting that macrophytes may buffer small waterbodies from heatwaves. Seasonal patterns in DO saturation also followed trends in macrophyte biomass; however, during the heat event, DO saturation fell sharply (-22.4 to 50.4 %) in both ponds and remained depressed through the remainder of the summer. This experiment and modeling exercise demonstrated that macrophyte influence on turbulent flows and light are pivotal in mediating how small waterbodies respond to compounding stressors.