WATER COLUMN AMMONIUM REGENERATION SUPPORTS PRODUCTIVITY IN TWO LARGE, EUTROPHIC LAKES

Harmful cyanobacterial blooms often rely on recycling of ammonium to produce biomass and nitrogen-rich toxins, despite low or unmeasurable concentrations in water. Thus, measuring ammonium turnover rates (uptake and regeneration) is necessary to determine its actual availability. Here, water column ammonium dynamics, in-lake water quality parameters driving these dynamics, and the importance of internal nitrogen loading in supporting community ammonium demand were explored in two large, shallow, eutrophic lakes (Võrtsjärv and Peipsi, Estonia). Stable isotope incubations were conducted almost monthly (including during winter) in Võrtsjärv and several times per year in Peipsi (from March 2019 to March 2022). Ammonium turnover rates in Võrtsjärv and Peipsi were similar to those reported for other large eutrophic lakes, despite being located at much higher latitude. Ammonium dynamics were strongly related to seasonally changing water quality variables, such as temperature, nutrient concentrations, and phytoplankton biomass, which, combined, explained 68–71% of variation in measured rates. Water column ammonium regeneration supported, on average, 65% (Võrtsjärv) and 76% (Peipsi) of community ammonium demand during the warm season (May – October). Internal nitrogen loading from ammonium regeneration in the water column vastly exceeded external loading into Võrtsjärv. These results emphasize the importance of internal nitrogen loading in driving primary productivity in eutrophic lakes and the necessity to reduce external nitrogen loading, in addition to phosphorus, into lakes.

Presentation Preference: Oral

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

Authors:

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

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

Silvia Newell, University of Michigan  (senewell@umich.edu)

Justin Myers, Wright State University  (justin.myers@wright.edu)

Triin Hunt, Estonian University of Life Sciences  (triin.hunt@emu.ee)

Kadi Palmik-Das, Estonian University of Life Sciences  (kadi.palmik-das@emu.ee)

Lea Tuvikene, Estonian University of Life Sciences  (lea.tuvikene@emu.ee)

Peeter Nõges, Estonian University of Life Sciences  (peeter.noges@emu.ee)

Tiina Nõges, Estonian University of Life Sciences  (tiina.noges@emu.ee)

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