Risk of eutrophication in the seawater of the coastal Red River aquaculture zone (Thai Binh province, Vietnam)

Coastal aquaculture represents an important economic activity in many countries. Nutrient (N, P and Si) concentrations and phytoplankton diversity and abundance in the coastal aquaculture zone, Tien Hai district, Thai Binh province (North Vietnam) were investigated during 2019–2020. Nutrient concentrations in five types of wastewater were also determined. The mean Eutrophication Index value of surface seawater at the 9 studied sites varied from 0.27 to 0.52, ranking as oligotrophic to mesotrophic. Five phytoplankton groups were observed (Bacillariophyceae (62% relative abundance), Dinophyceae (16%), Chlorophyceae (11%), Cyanobacteria (7%), and Euglenophyceae (4%)). The temporal and spatial distribution of phytoplankton diversity and abundance was governed by water temperature, pH, TSS, and TP and reflected the strong interactions between seawater and riverine water in the area. The presence of certain potentially toxic algal species combined with increasing inorganic nitrogen concentrations and N:P ratios further underlined the risk for harmful algal blooms. The investigation of different nutrient sources in this area revealed that while nitrate and total phosphorus concentrations are related to diffuse sources, point sources may be important in providing ammonium and dissolved phosphate concentrations to coastal seawater in this aquaculture zone. These results provide a scientific basis upon which better management strategies can be constructed in order to assure the sustainable development of aquaculture in the Red River coastal area, in particular, and in the coast of Vietnam, in general.