From human-driven eutrophication to effective management: Controlling brown tides in the coastal waters of Qinhuangdao, China

Abstract

The coastal waters of Qinhuangdao, Bohai Sea, China, experienced recurrent brown tides caused by Aureococcus anophagefferens from 2009 to 2015, severely impacting scallop mariculture and coastal tourism. While eutrophication is a global phenomenon, A. anophagefferens blooms have been reported only in a small number of isolated ocean regions first in the USA and South Africa, and more recently in the coastal waters of Qinhuangdao, Bohai Sea, China. Mechanisms underlying such few isolated bloom patterns remain to be addressed. This study integrates historical water quality data, socioeconomic statistics, and field observations to investigate how rapid urbanization, industrialization, and agricultural development in the Qinhuangdao region, combined with intensive shellfish mariculture in the adjacent coastal waters, contributed to bloom formation. Increased nutrient loading—particularly dissolved organic nitrogen from land-based runoff and mariculture—together with phosphorus limitation (indicated by high ratios of inorganic nitrogen to phosphorus) and restricted water exchange, created conditions favorable for A. anophagefferens, consistent with global understanding of its bloom ecology. Genetic evidence suggests either ballast water introduction or indigenous activation from dormant sediment populations. Since 2012, comprehensive environmental policies, including wastewater treatment and aquaculture restructuring, have significantly reduced nutrient loads and improved water quality, leading to the cessation of brown tides after 2015 and a ∼95 % reduction in other harmful algal blooms (HABs) by 2018–2020. The Qinhuangdao case underscores the strong linkage between anthropogenic activities and HABs and provides a transferable management framework for other vulnerable coastal systems.