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.
Phosphorus (P) flux across the sediment-water interface in lakes and reservoirs responds to external perturbations within the context of sediment characteristics.Lentic ecosystems experience profound spatiotemporal heterogeneity in the mechanisms that control sediment P fluxes, likely producing hot spots and hot moments of internal loading. However, spatiotemporal variation in P fluxes remains poorly quantified, particularly in the context of sediment chemistry as a controlling variable. We measured P flux rates and mobile sediment P forms along the longitudinal gradient of a temperate reservoir every 2 months from February to October 2020. Both aerobic and anaerobic processes mobilized sediment P throughout the year. High flux rates at littoral sampling sites (8.4 and 9.7 mg P m À2 day À1 ) occurred in late summer under oxic conditions in the overlying water and mobilized labile organic P. High fluxes at the profundal site coincided with hypolimnetic anoxia under ice cover and in mid-summer (11.2 and 17.2 mg P m À2 day À1 , respectively) and released redox-sensitive P. Several high fluxes substantially skewed the flux rate distribution, providing evidence of hot spots and hot moments of internal loading. We further scaled the measured sediment P flux rates to representative areas of the lakebed to estimate internal P loads at an ecosystem scale. We found that P release from littoral sites under oxic conditions in the overlying water had an outsized impact on total loads. Our findings demonstrate the importance of considering spatial and seasonal variation in sediment P pools and fluxes in order to more accurately estimate internal loads and identify the dominant biogeochemical mechanisms involved.
Phosphorus (P) release from lakebed sediments may fuel algal blooms, especially in shallow systems. A primary mechanism that controls internal P loading is the size and chemical composition of the sediment P pool. However, variation in sediment P speciation within and among shallow lakes remains poorly quantified, limiting efforts to scale and model sediment P pools. We measured the degree of spatial heterogeneity in the size and composition of the sediment P pool, both within and among lakes, for seven shallow glacial lakes by measuring sediment P fractions from 10 cores in each lake. There was a 1.6x difference in total sediment P among the study lakes, with sediment P composition varying based on the dominant watershed soil series. We also found that higher mobile P (as a fraction of total P) in the profundal sediments was significantly correlated with higher average chlorophyll-a concentrations (p=0.04), indicating the influence of sediment P composition on algal biomass in shallow lakes. Additionally, we measured substantial within-lake heterogeneity in total and loosely-bound sediment P among the 10 sites sampled in each lake. Concentrations were positively correlated with the depth of the water column above the sediments such that extrapolating measurements from the deep site alone could overestimate whole-lake mean P concentrations. Our results provide insight into the magnitude and pattern of inter- and intra-lake variation in sediment P pools that should be accounted for when sampling, scaling measurements, and modeling sediment P dynamics.
Among freshwater systems, coldwater habitats are among the most threatened by climate change. Examining the impacts of increasing water temperature requires the use of both traditional biomonitoring efforts and measures of ecosystem function and structure. We examined fish and macroinvertebrate communities, leaf decomposition rates, periphyton production, and ecosystem metabolism to compare two branches of a trout stream in Minnesota with differing thermal regimes. The cooler South Branch had more coldwater fish, a higher index of biological integrity for fish but a lower index for macroinvertebrates. There were no differences in leaf decomposition rates between branches, although nonnative buckthorn leaves decomposed faster than native black cherry leaves. Periphyton production was higher in the North Branch than the South Branch. Both branches had high nitrogen but low phosphorus levels. Nutrient enrichment with phosphorus enhanced periphyton production in both branches. Measures of stream metabolism, based on diurnal variation in oxygen levels, showed that both branches were heterotrophic. Despite higher periphyton production in the North Branch, gross primary production was higher in the South Branch. The bioassessment measures used in our study yielded inconsistent results, pointing to the need for multiple methods to examine and better describe potential responses to warming from climate change.
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