Spatial-temporal hydrodynamic and algal bloom modelling analysis of a reservoir tributary embayment

The tributary embayments of Three Gorges Reservoir have been frequented by algal blooms over the past decade. We develop a three-dimensional hydrodynamic and water quality model to study the spatial and temporal variation of hydrodynamics and algal bloom dynamics in a eutrophic tributary embayment, Xiangxi Bay. The study area exhibits distinct longitudinal gradients in hydrodynamics, and could be generally characterized into four longitudinal zones: riverine, intermediate, lacustrine, and mainstream influenced. Compared with the typical longitudinal zonation for a pure reservoir, there is an additional mainstream influenced zone (about 5-10 km long) near the mouth due to direct effects of its adjacent reservoir mainstream. The transport and transformation of key water quality constituents associated with algal blooms, such as chlorophyll, nitrogen, phosphorus, and silicon are simulated satisfactorily. We further investigate the algal bloom dynamics by combining the model results and in situ water quality observations. There is a strong spatial dependence of algal bloom events (occurrence and distribution) in Xiangxi Bay: (i) during 2003-2012, some 73.9% of algal bloom events were first observed in a 10 km long reach within the intermediate and lacustrine zones, and (ii) the spatial pattern of chlorophyll concentrations is closely related to the longitudinal variance of hydrodynamics. A conclusion from this paper is that the relatively calm environment of the intermediate and lacustrine zones provides favourable conditions for algal growth, suggesting the possibility of using longitudinal zonation for risk assessment of algal bloom initiation and levels in reservoir tributary embayments.