The Lake Mesocosm Warming Experiment (LMWE) in Silkeborg, Denmark, has been running since 2003, with the goal of studying climate warming and nutrient pollution effects on lake ecosystems. Water levels in the 24 mesocosms (1.9 m diameter, 1 m water depth, 0.2 m sediment depth) are maintained with groundwater, continuously mixed using mechanical paddles, and heated to treatment temperatures with submerged heating elements. Mesocosm temperatures are either ambient, ambient plus ~2 deg C, or ambient plus ~4 deg C, and mesocosms within each temperature group are further treated with low (groundwater only) or high nutrients (added NO3-N and PO4-P). During summer 2021, mixing paddles were turned off/on for 2-week periods. We collected water samples above and below thermoclines during no mixing, and depth-integrated water samples during mixing, for initial and post-24 h incubation dissolved O2 and N2 concentrations. In most cases, we observed PO4, NH4, and N2 accumulation in bottom waters during stratification, along with O2 consumption. N2 accumulation, presumably due to denitrification, and water column O2 respiration rates varied and were not predictable based on temperature or nutrient treatment. Sediment O2 consumption and bottom-water NH4 and PO4 accumulation were generally higher in elevated nutrient and temperature mesocosms, but variability was high. In general, results followed expected patterns commonly observed in stratified lakes and reinforce previous work showing that internal nutrient loading from lake sediments includes both N and P, both of which represent nutrient legacies that can delay water quality improvements after watershed nutrient loading reductions.

Primary Presenter: Mark McCarthy, Estonian University of Life Sciences (mark.mccarthy@emu.ee)

Authors:

Mark McCarthy, Estonian University of Life Sciences  (mark.mccarthy@emu.ee)

Kayla Gonzalez-Boy, Kennesaw State University  (kgonza36@students.kennesaw.edu)

Margot Sepp, Estonian University of Life Sciences  (margot.sepp@emu.ee)

Marju Tamm, Estonian University of Life Sciences  (marju.tamm@emu.ee)

Thomas Davidson, Aarhus University  (thd@ecos.au.dk)

Erik Jeppesen, Aarhus University  (ej@ecos.au.dk)