| Summary: | One debate about lake restoration is whether there is overemphasis of a "Phosphorus (P)-only" paradigm, while overlooking the limiting effect of nitrogen (N). In particular, the roles of internal nutrient cycling which could act as drivers of algal blooms are not yet well assessed. However, it is hardly possible to identify the complex mechanisms of nutrient limitation patterns in lakes only by in-situ experiments or monitored data. Numerical modeling can serve as a complementary approach by providing quantitative internal variations with high spatial and temporal resolutions. A three-dimensional model-based N:P stoichiometric approach was developed to explore the storage pools and fluxes that affect the limiting nutrient in hyper-eutrophic Lake Dianchi, the most eutrophic large lake in China with seasonal N limitation. The results highlighted the role of benthic P fluxes, which fluctuated considerably within a year and could supply enough P for algae, contributing over 50% of P input and leading to relative N deficiency during algal bloom. Further insights into N cycling indicated that N deficiency could be attributed to low N fixation and extremely high N losses from denitrification (~ 50% of total loss). Considering the continuous sediment P release and N losses that promote N deficiency, controlling both N and P loadings will benefit lake restoration.
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