Nutrient enrichment and N:P ratio decline in a coastal bay–river system in southeast China: The need for a dual nutrient (N and P) management strategy

Eutrophication and harmful algal blooms (HAB) have become the primary water quality concerns in China. A comprehensive analysis was conducted using a dataset obtained from long-term monitoring of a coastal bay–river system in southeast China (Xiamen Bay–Jiulong River Basin), to examine the nutrient [nitrogen (N) and phosphorus (P)] dynamics and the management options. The analysis showed that increasing human stresses and external nutrient loads over the past 30 years were the main causes of water degradation and eutrophication. A significant decline of N:P ratio was observed in both river and estuarine waters since the 1990s due to relatively high P loadings as a consequence of waste discharges from the proliferating husbandry of livestocks and the application of excessive phosphate fertilizers to cash crops. Continued nutrient enrichment and the decline of N:P ratio have changed the nutrient stoichiometry and supply ratio in waters, which have increased the risk of nutrient-enhanced algal bloom. According to the findings and current knowledge concerning eutrophication and HAB processes, a dual nutrient (N and P) management strategy is necessary for management of water quality in the Xiamen Bay–Jiulong River Basin with a focus on animal wastes in the north Jiulong tributary and over-fertilization in the west Jiulong tributary.