Spatial Biomass Production And Seasonal Nutrient Limitation Transitions In A Tributary of The Three Gorges Reservoir, China

Abstract

The freshwater ecosystem characteristics in terms of nutrient inventory across seasons, spatial variations of chl-a biomass, and the phytoplankton community structure are prudent ecological assessment indices for a bloom management protocol. We evaluated the spatial and seasonal chl-a distribution under different nutrient conditions and phytoplankton community structure in a eutrophic Three Gorges reservoir tributary China. Result showed significant variations in biomass production with the mainstream reaches severely affected. The nutrient addition bioassay demonstrated significant stimulations on growth in both autumn and summer. The nutrient limitation pattern shifted from P in autumn and spring to N limitation during summer. Combined additions of trace metals with N, P, and Si in autumn and Fe alone enrichment in summer and spring showed maximum productivity. The phytoplankton community structure demonstrated strong sensitivities to seasonal variabilities with regime shift from Cyanophyta, dominated by the toxic and hypoxia generating, Microcystis spp in both autumn and summer, the Cryptophyta dominated by the Chroomonas acuta in spring to the Bacilliariophyta dominated by the genera, Cyclotella in winter. This reflected the ability of the Bacilliariophyta to thrive under a low-temperature condition. Combined N&P led to significant growth stimulation in summer while P alone controlled the bulk of the growth in autumn. The study points to the need for extending mitigation steps to the mainstream towards achieving lasting bloom management solution in the impacted tributary.