Water column ammonium regeneration supports productivity in two large, eutrophic lakes

Phytoplankton often rely on the regeneration of ammonium () to produce biomass and, for some cyanobacteria, nitrogen‐rich toxins, despite low or unmeasurable concentrations in water. Thus, measuring turnover rates (i.e., uptake and regeneration) is necessary to determine its actual availability. The objectives of this study were to quantify water column turnover rates in two large, shallow, eutrophic, cyanobacteria‐dominated lakes in Estonia (Lakes Võrtsjärv and Peipsi), explore which in‐lake variables drive these processes, and evaluate the importance of internal nitrogen loading in supporting community uptake. Stable isotope () incubations were conducted almost monthly in Võrtsjärv and several times per year in Peipsi (from March 2019 to March 2022). Despite being located at a higher latitude, turnover rates in Võrtsjärv and Peipsi were similar to those reported for other large, eutrophic lakes. turnover rates were strongly related to seasonally changing water quality variables, including temperature, nutrient concentrations, and chlorophyll a concentrations, which, combined, explained 68–71% of the variation in measured rates. regeneration supported, on average, 65.2% ± 9.6% (in Võrtsjärv) and 75.6% ± 16.8% (in Peipsi) of community uptake during the warm season (May to October). For 2020, estimated internal nitrogen loading from regeneration in the water column exceeded, by ~ 18 times, estimated external nitrogen loading to Võrtsjärv from the three main inflows. These results emphasize the importance of internal nitrogen loading in driving primary productivity in lakes and highlight the necessity to reduce external nitrogen loading, in addition to phosphorus, into eutrophic lakes.