Internal loop sustains cyanobacterial blooms in eutrophic lakes: Evidence from organic nitrogen and ammonium...

Algal bloom species can live upon internal regenerated ammonium (NH₄⁺) for growth during the nitrogen-limited period. However, the linkages between NH₄⁺ regeneration and phytoplankton biomass and community composition dynamics remain largely unknown. To unravel the interactions between NH₄⁺ regeneration and phytoplankton community, we measured water column NH₄⁺ regeneration rates (REGs) during a continuous phytoplankton growing period and a contrast summer/winter turnover in eutrophic Lake Taihu. Measured REGs were higher in summer than in winter and significantly correlated to total phytoplankton biomass, Cyanophyta biomass and its biomass proportions, and the concentrations of particulate nitrogen and dissolved organic carbon as well as the relative abundance of labile components (proteins and lipids). Random forest regression analyses displayed that variation of REGs were mainly controlled by water temperature and algal-related parameters (including chlorophyll a, total phytoplankton biomass, and Cyanophyta biomass). Partial least squares path model further revealed that algal-related parameters were the direct and significant factors regulating REGs, and contributed to the largest effect of the variance in REGs. Of the algal community, Cyanophyta was the dominant phylum to accelerate REGs. Correspondingly, rapid internal NH₄⁺ turnover may strongly support the persistence of cyanobacterial blooms, thus forming a positive feedback between cyanobacterial blooms and REGs during the nitrogen-limited summer months. We therefore deduced that the internal loop between cyanobacterial blooms and REGs during summer may be a key self-maintenance mechanism of continuous cyanobacterial blooms.