Lake depth modulates phytoplankton resource use efficiency during long-term restoration: a seasonal study of 20...

Phytoplankton resource use efficiency (RUE), reflecting how efficiently nutrients are translated into phytoplankton biomass, may be a useful indicator of phytoplankton responses to a nutrient decline during lake restoration. However, the key drivers and seasonal dynamics of RUE under such conditions remain uncertain. We analyzed temporal patterns (1989-2008) and controlling factors of RUE in 20 Danish lakes in recovery from eutrophication, focusing on dissolved nutrient availability, lake depth, plankton community dynamics, and trophic regulation. We found that the response of RUE_Chl (resource use efficiency based on Chl a) varied among lakes and was largely determined by early-period (first three monitoring years) mean efficiency during lake restoration. In lakes with low RUE_Chl, subsequent nutrient decline appeared to promote compensatory increases in efficiency, thereby buffering phytoplankton responses. XGBoost models identified dissolved nutrients (phosphate and dissolved inorganic nitrogen) and lake depth as the key predictors of RUE_Chl. SHAP analyses revealed nonlinear nutrient associations, with model-derived transition ranges of 0.014‒0.021 mg P L^-1 for phosphate and 0.45‒1.14 mg L^-1 for dissolved inorganic nitrogen. Lake depth modulated seasonal RUE_Chl dynamics mainly by shaping nutrient availability and mixing regimes. These patterns were accompanied by seasonal shifts in the relative importance of individual predictors. Several, but not all, phytoplankton groups contributed positively to RUE_Chl at high relative biomass, and documented community shifts and biomanipulation coincided with abrupt RUE_Chl changes in specific lakes. By elucidating the seasonal and mechanistic drivers of RUE_Chl dynamics, this study advances our understanding of nutrient-phytoplankton interactions, with implications for lake-specific restoration strategies.