Ellen Amara Albright , Robert Ladwig, Grace Marie Wilkinson 

1. Small waterbodies are sensitive to stressors such as nutrient enrichment and heatwaves. However, when present, macrophytes may mediate these compounding stressors through their influence on water column thermal structure. Canopy-forming macrophyte beds can induce thermal stratification, which may limit the depth and degree of water column warming during heatwaves.
2. We leveraged an ecosystem experiment and hydrodynamic model to evaluate how macrophyte biomass, thermal structure, and dissolved oxygen (DO) responded to the interaction of episodic nutrient loading and periods of high temperatures in two shallow, temperate ponds (mean depth 0.8 m, maximum depth 2 m). We added nutrients to one pond, simulating storm-driven loading, while the other pond served as an unmanipulated reference. Following the first nutrient addition both ponds experienced a 5-day period of high surface water temperatures.
3. Submersed macrophytes in the nutrient addition pond began to senesce mid-summer, likely a result of phytoplankton shading from the nutrient addition and heat stress, while macrophytes in the reference pond followed expected seasonal patterns, senescing in early autumn.
4. We found that macrophytes structured the thermal environment in the ponds through vertical attenuation of turbulent kinetic energy and light. Macrophytes reduced the vertical extent of water column warming during the sustained 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 (declined by 22.4 to 50.4%) in both ponds and remained depressed through the remainder of the summer.
5. Synthesis: Our findings reveal that canopy-forming aquatic plant beds can buffer ponds from brief aquatic heat events but also that the plants themselves are sensitive to nutrient loading and temperature extremes. These results contribute to our mechanistic understanding of the effects of compound, extreme events in small waterbodies and the role aquatic plants can play in mediating these stressors. This understanding is necessary for adaptive management of small waterbodies such that these systems will continue to support freshwater biodiversity.