Ammonium dynamics and internal nitrogen loading from the water column of eutrophic lakes: implications for managing watershed nutrient loads

Synthetic and organic fertilizer use has increased nitrogen (N) levels and combined with climate change, drives eutrophication and cyanobacteria blooms in aquatic systems. Internal nutrient loading from microbial activity in sediments and the water column can intensify these blooms, and ammonium (NH4) is key for internal N cycling and algal growth. NH4 regeneration and potential uptake rates were measured in two medium-sized, shallow, eutrophic lakes in Estonia (Kaiavere and Veisjärv). Water samples from two sites in each lake were amended with 15N-labelled NH4 and incubated in light and dark bottles for 24 hours at near-in situ temperature and light. Results from autumn 2023 and spring 2024 (similar water temperatures, 13 – 14 ˚C) showed 2 – 6 times higher potential NH4 uptake rates in autumn in both lakes, with larger differences in dark bottles. Potential uptake rates in Kaiavere were higher than in Veisjärv, but NH4 regeneration could support ~60% of potential uptake in Veisjärv in spring. In autumn in both lakes and Kaiavere in spring, NH4 regeneration could support 22 – 35% of potential uptake. Combined with high NH4 releases from sediments, internal N loading, ultimately driven by external N loading, could support large proportions of community NH4 demand in the water column (dominated by non-N-fixing cyanobacteria), even when ambient NH4 concentrations were low or undetectable. Thus, watershed management and lake restoration efforts focusing solely on reducing external or internal phosphorus loads are insufficient to mitigate eutrophication in these and other lakes.

Presentation Preference: Poster

Primary Presenter: Gizem Oguz, Estonian University of Life Sciences (gizemoguz9806@gmail.com)

Authors:

Gizem Oğuz, Estonian University of Life Sciences, Institute of Agricultural and Environmental Sciences, Chair of Hydrobiology and Fisheries  (gizemoguz9806@gmail.com)

Esther Nwume, Estonian University of Life Sciences, Institute of Agricultural and Environmental Sciences, Chair of Hydrobiology and Fisheries  (esther_amalachukwu.nwume@emu.ee)

Kadi Palmik-Das, Estonian University of Life Sciences, Institute of Agricultural and Environmental Sciences, Chair of Hydrobiology and Fisheries  (Kadi.Palmik-Das@emu.ee)

Triin Hunt, Estonian University of Life Sciences, Institute of Agricultural and Environmental Sciences, Chair of Hydrobiology and Fisheries  (triin.hunt@emu.ee)

Margot Sepp, Estonian University of Life Sciences, Institute of Agricultural and Environmental Sciences, Chair of Hydrobiology and Fisheries  (margot.sepp@emu.ee)

Mark McCarthy, Estonian University of Life Sciences, Institute of Agricultural and Environmental Sciences, Chair of Hydrobiology and Fisheries  (mark.mccarthy@emu.ee)