Savchuk O.P., Isaev A.V., Filatov N.N.
A three-dimensional coupled hydrodynamical biogeochemical model of the nitrogen and phosphorus cycles has been used for a long-term reanalysis of the Lake Onego ecosystem. The comparison between simulation and sparse irregular observations, presented in the first part of this paper, demonstrated plausibility of the reconstructed temporal and spatial features of biogeochemical dynamics at a long-term scale, while seasonal dynamics of variables and fluxes are presented here. As new regional phenological knowledge, the reanalysis quantifies that the spring phytoplankton bloom, previously overlooked, reaches a maximum of 500 ± 128 mg C m–2 d–1 in May, contributes to approximately half of the lake’s annual primary production of 17.0 – 20.6 g C m–2 yr–1 , and is triggered by increasing light availability rather than by an insignificant rise in water temperature. Coherent nutrient budgets provide reliable estimates of phosphorus and nitrogen residence times of 47 and 17 years, respectively. The shorter nitrogen residence time is explained by sediment denitrification, which in Lake Onego removes over 90 % of the bioavailable nitrogen input, but is often ignored in studies of other large lakes. An overall assessment of the model performance allows us considering the model a necessary and reliable tool for scenario simulations of possible changes in the Lake Onego ecosystem at the requested spatial and temporal scales.