The river–estuarine continuum of nutrients and phytoplankton communities in an estuary physically divided by a sea dike

Spatial and temporal variation in nutrients, physical variables, primary productivity and the size and taxonomic composition of phytoplankton was investigated over an annual cycle in a macrotidal estuary transected by a sea dike in a temperate region. Our aim was to evaluate whether the river continuum approximation was valid in the highly altered estuary. Ambient nutrient concentrations were generally much higher in the freshwater than in the seawater zone, but decreased downstream. The chlorophyll a concentrations were also much higher in freshwater and decreased downstream along the river–estuary continuum. Primary productivity displayed a similar pattern, except in February and August, when it increased rapidly in seawater following freshwater discharge. This suggests that nutrient availability could have been important in determining the spatial variation in phytoplankton biomass and production. Winter and summer blooms of nano-sized phytoplankton developed in freshwater dominated by Stephanodiscus sp. and Eudorina elegans, which favour low and high temperatures, respectively. The nutrient increase following eutrophic freshwater discharge may have supported phytoplankton blooms dominated by Thalassiosira rotula (micro-sized) and Heterocapsa sp. (nano-sized) in the late winter and monsoon season, respectively, in the upper regions of the seawater zone. However, blooms and primary productivity decreased downstream and the taxonomic composition also varied, corresponding to significant spatial changes of nutrients, salinity and water transparency that were validated by statistical analyses. This suggests that the river continuum was sustained between the fresh and seawater zones, as well as within individual zones, although they were physically transected by the sea dike. The river continuum in highly altered estuaries that can extend seaward during monsoons may be important to the primary production and food web of the Yellow Sea.