Variabilities in autumn cyanobacterial responses to ecosystem external enrichments based on nutrient addition bioassay in Pengxi River, Three Gorges Reservoir, China

Abstract

Nutrient availability, is a crucial anthropogenic stressor promoting freshwater eutrophication and rapid expansion of harmful algal blooms (HABs), deteriorating water quality and threatening public health worldwide.The estimation of the HABs community responses to diel changes in the nutrients while characterizing the ecosystem growth limiting factors, is key to prudent watershed management. The present study investigated the short-term variabilities in autumn cyanobacterial responses to the external nutrient inputs into the Pengxi River using the nutrient addition bioassay approach. Results reveal phytoplankton community structure dominated by the cyanobacteria: Anabaena and Aphanizomenon spp. (relative abundance = 46.20% equilibrium abundance), followed by the diatoms, out of which Lindayia bodaniica, are preponderant. Nutrient enrichment triggered strong variabilities in dominance and successions among the cyanobacterial group, with maximum dominance (76.34%) exhibited by the Aphanizomenon sp. upon NH4 addition. Fe enrichment led to the succession of cyanobacteria, Leptolyngbya tenuis, which was below the detectable limit in the control, indicating the role of Fe in its proliferation. Studies on nutrient limitation demonstrated P/NH4 co-limited ecosystem, with P as the primary and NH4, a secondary limiting factor. The nitrate preference index (NO3−RPI = 0.991) shows a high preference for NH4 while NO3 constitutes the bulk of the ecosystem TN. Considering the elevated NO3 concentration, we posit that a shift in the phytoplankton community structure from cyanobacteria to diatoms dominated ecosystem, is expected following Fe depletion and a further stretch on the current ecosystem NH4 limitation. The study provides useful and first-ever insights for nutrient reduction in the middle Three Gorges Reservoir (TGR) before the onset of the heavy HABs during spring in the Pengxi River.