C/N Ratio of Resuspended Sediment Regulates Phytoplankton Growth in Shallow Eutrophic Lakes

Internal nutrient loading is a key challenge during shallow and eutrophic lake restorations, especially after effective external loading controls. Sediment organic nitrogen (SON) released through resuspension is a critical internal nitrogen source for algal blooms. However, the role of SON released from sediment resuspension in stimulating algal blooms has not been fully elucidated. In this study, sediment exposure bioassays were conducted to explore the response of algal biomass to SON and further evaluate the relative importance of SON content, composition (e.g., carbon/nitrogen (C/N) ratios), and molecular weight in driving algal growth. Results showed that Chl-a concentrations in the sediment exposure bioassays increased by 24%. The generalized additive model showed that the SON content, sediment organic carbon content, and C/N ratio contributed 16.1% to the increase in Chl-a concentration. The partial least-squares path model further revealed that sediment C/N ratios played stronger positive effects in regulating algal biomass than SON contents and molecular weights, mainly through indirect effects on water column dissolved organic nitrogen (DON) concentrations, DON lability, and ammonification rates. Our results thus provide new insights into factors, especially sediment C/N ratios, that support algal growth and persistence in eutrophic shallow lakes.