The rational eutrophication management largely depends on the knowledge of the dynamics in the dissolved inorganic nutrients especially nitrogen forms which trigger exponential primary productivity in eutrophic systems. The present study investigated the phytoplankton interactions with the dissolved N forms, nitrate (NO3) and ammonium (NH4) in a sub-tropical Yangtze River tributary, China vulnerable to multiple anthropogenic stressors following the impoundment of the largest hydraulic structure, the Three Gorges Dam. Results indicated strong NO3 inhibition by the low NH4 pool exerting toxic effects on the major phytoplankton groups, particularly the Bacilliariophyta (relative abundance < 1%) while significant Cyanophyta proliferation prevailed (relative abundance ≥ 90%). Strong N limitation exacerbated by NH4 deficit and P replete condition characterizes the summer bloom in the tributary. The biomass attenuation kinetics revealed significantly fast NH4 metabolism, half-life (t1/2= 1.4 d, K = 0.00750 ± 0.004 d-1) as the first-order rate adequately fitted into the experimental data although, the second-order rate also demonstrated considerable goodness of fit. The growth responses induced by the Si enrichment potentially suggested possible secondary limitation by Si with the likelihood of intensification should the ecosystem phytoplankton community dominance shifts from Cyanophyta to the Bacilliariophyta. The response of P enrichment on growth was attributed to luxury consumption rather than limitation as responses only became significant towards the end of the study. The study, therefore, presents the first report of biomass ageing rate worthy of incorporation into the recent bloom management protocol for the development of predictive ecosystem dynamics.
Keywords: Ammonium; Biomass attenuation; Consumption rate; Kinetics; Nitrate; Phytoplankton.
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