Influence of macrophytes on 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.

Legacy Phosphorus and Ecosystem Memory Control Future Water Quality in a Eutrophic Lake

“…The lake physical model solves the energy budget and mixing dynamics on an hourly time step and produces 1 m vertical resolution temperature output. The lake physical model follows an integral energy approach for heat transport and solves vertical diffusion using an implicit scheme (see model formulations in Albright et al., 2022; note that we neglected the effects of macrophytes on energy attenuation in the current study). Water clarity, updated daily by the metabolism model, informs attenuation of short‐wave radiation in the physical model.…”

“…Model details are provided in Supporting Information (Tables S1–S3). The process‐based implementation of thermodynamics was adapted from our previous work on pond thermal structure (Albright et al., 2022), lake metabolism modeling (Carey et al., 2018), and lake phosphorus modeling (Hanson et al., 2020). The model is 1‐D in the vertical dimension, with three compartments modeled simultaneously using a box‐modeling approach—water column epilimnion and hypolimnion, and active sediments (Figure 1).…”

Legacy phosphorus and ecosystem memory control future water quality in a eutrophic lake

“…Model details are provided in supplemental information (Tables S1-S3). The process-based implementation of thermodynamics was adapted from our previous work on pond thermal structure (Albright et al, 2022), lake metabolism modeling (Carey et al, 2018), and lake phosphorus modeling (Hanson et al, 2020). The model is 1-D in the vertical dimension, with three compartments modeled simultaneously using a box-modeling approach -water column epilimnion and hypolimnion, and active sediments (Fig.…”

“…The lake physical model solves the energy budget and mixing dynamics on an hourly time step and produces 1 m vertical resolution temperature output. The lake physical model follows an integral energy approach for heat transport and solves vertical diffusion using an implicit scheme (see model formulations in Albright et al, 2022; note that we neglected the effects of macrophytes on energy attenuation in the current study). Water clarity, updated daily by the metabolism model, informs attenuation of short-wave radiation in the physical model.…”