Though the number of sluices and dams in coastal areas has increased rapidly in recent years, the influence of their construction on phytoplankton in estuary areas is hardly known. This paper aims to provide a reference for quantitative research on the ecological influence of sluice construction and give ecological justifications for the setting of environmental standards in the estuary areas. The survey data gained at the lower reach of the Yongjiang River and its estuarine areas in June 2015 were used in MIKE21 software (Danish Hydraulic Institute (DHI), Denmark)) for establishing a two-dimensional numerical model to simulate the salinity field distribution after sluice construction. Based on the simulation results, the salinity gradient changes caused by the construction were analyzed. The one-dimensional Gaussian model was applied to calculated the phytoplankton’s ecological threshold interval over the salinity changes, which helped predict the influence of salinity changes on phytoplankton cell density. The study shows that salinity in the Yongjiang estuary increases obviously, beyond the phytoplankton ecological threshold, after sluice construction without water discharge. Salinity will become a restriction factor to phytoplankton growth after sluice construction in the study area, which may cause a sharp decrease of certain phytoplankton species.

The estuary—located in the joint zone between river and ocean—has created unique ecosystems and rich biodiversity due to its complex habitat conditions. As the critical period for the growth and reproduction of estuarine organisms, the flood season is also crucial for estuary sluice to play a role in flood control and drainage. This study aims at proposing an ecological scheduling scheme that comprehensively considers the adverse factors brought by draining of the estuarine sluice to maintain different ecological functions of the Jiaojiang River estuary. A two-dimensional mathematical model in MIKE 21 was used to ascertain the change of salinity and nutrients in the study area, and results were analyzed to understand the impact of draining on estuarine organisms. Based on an analysis of the change and redistribution of salinity and nutrients, the sea area could be divided into three parts during the operation of the estuarine sluice: nearshore area with low-salt and high eutrophication, medium mixed salinity under nitrogen limitation area and high-salt under phosphorus limitation area. The aquaculture area was alternately situated in the mesohaline (>5.0‰–18.0‰) and polyhaline (>18.0‰) zones. The after-effects of salinity change—including the lowest value and the sharp change of salinity, especially the sharp change of salinity (11.8‰–12‰) caused by large-volume discharge—would affect the growth of organisms to a certain extent, but would not be lethal. Moreover, a high-incidence red tide area was determined to be formed within 8–14 km away from the estuarine sluice after the sluice opening, and the area of this zone was firmly related to the opening schemes. The study suggested that the proposed sluice should choose the mode of draining frequent with the small flow at neap tide in order to reduce the stress of sluice construction and operation on mariculture organisms and phytoplankton in Jiaojiang River estuary on the strength of the ecological restoration measures for estuary and riparian zones.

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