Why do algal blooms intensify under reduced nitrogen and fluctuating phosphorus conditions: The underappreciated role of non-algal light attenuation

Light availability for phytoplankton in shallow lakes is closely related to non-algal light attenuation (Kd_NALA, the fractional light absorption by non-algal substances, such as suspended sediments); thus, significant changes in global wind speed in recent decades may have a profound effect on light availability and algal blooms in shallow lakes. Herein, the eutrophic shallow Lake Taihu was selected to investigate the long-term dynamics of light availability and its effect on algal blooms. The results showed that Kd_NALA decreased from approximately 3.5 to 2.5 m⁻¹ with decreasing wind speed from 2005 to 2021, indicating the significantly increased light availability for phytoplankton despite surface photosynthetically active radiation exhibiting limited variability during the study period. In addition, both experimental (i.e., in situ nutrient enrichment experiments) and statistical approaches (i.e., deviations of Trophic State Index subindices) indicated that phytoplankton growth was primarily light-limited during the study period in Lake Taihu. Consequently, considerable increases in fraction of observed and maximal chlorophyll a yield at given nitrogen or phosphorus concentration were observed and were mostly related to decreasing Kd_NALA, which implied that decreasing Kd_NALA allowed phytoplankton utilize “unused nutrient-capacity” until the additional algal-turbidity induce further light limitation or nutrient limitation. As the effect of changes in global wind speed on Kd_NALA and algal growth received limited attention in the existing research, we revealed an underappreciated mechanism by which global changes in wind speed significantly affects algal biomass by influencing light availability, which may have profound effects on future algal bloom mitigation efforts in shallow lakes.