###### Abstract  
Qinhuangdao, a famous coastal resort city in northeastern China, has been affected by the rapid growth of harmful algae. The brown tide induced by _Aureococcus anophageferrens_ (_A. anophageferrens_) first occurred along Qinhuangdao's coastline in 2009, and it then developed into a harmful alga for the Bohai Sea. Based on MIKE 21, we construct a 2D coupled hydrodynamic and transport model to study the variation and distribution of total nitrogen (TN), and establish the relationship between _A. anophageferrens_, dissolved organic nitrogen (DON), and TN to provide a fast forewarning system for brown tide in Qinhuangdao. This model considers the decay, diffusion, and settling of TN, and the model results are in good agreement with the measured tidal level, current, and TN, indicating that the model is capable of capturing the observed TN distribution during the brown tide period. The transfer function relating TN to _A. anophageferrens_ leads to a reliable fast forewarning and monitoring system. Moreover, the transferred _A. anophageferrens_ cell density can be used to forecast the regional risk level of brown tide with a specific color indicator. The implementation of national policy relating to marine ecosystems decreases the nearshore concentration of N, P, and other nutrients, and therefore decreases harmful algal blooms. The fast routine assessment of brown tide by the present warning system provides robust guidance for the government to take action. brown tide; Bohai Sea; hydrodynamic and transport model; _Aureococcus anophageffrens_; forewarning system; harmful algal bloom +
Footnote †: journal: Applied sciences  
0000-0002-2000-2000-2000-20000-2000-2000-2000-20000-2000-2000-20000-2000-20000-20000-20000-20000-200  
Aureococcus anophagefferens (A. anophagefferens)-induced algal bloom events have received considerable attention for decades since their first occurrence in the 1980s along the eastern coast of the USA [6; 7; 8], and they were also found in the coastal bays of South Africa [9; 10]. As a type of pico-sized red tide, this algal bloom has been named \"brown tide\" to distinguish it from traditional red tide [11], as it can discolor waters into a dark brown color. Brown tide is a discoloration of marine water due to the explosive reproduction or aggregation of planktonic algae, protozoa, or bacteria in the ocean driven by anomalous hydro-meteorological, hydrodynamic, and biological processes [12; 13]. The brown tide induced by the _A. anophagefferens_ in Qinhuangdao was first observed in the coastal waters near a scallop cultivation farms in 2009 [14; 15; 16; 17], which motivated extensive investigations on the geographic and historical presence of this species in the seas of China. [PERSON] et al. [18] investigated 3000 km of coastline and the 1500-year presence of _A. anophagefferens_ and revealed that it is an indigenous species in Qinhuangdao. It took several years to confirm the causative species by microscopy, pigment chemotaxonomy, and genetic analyses [19]. [PERSON] et al. [12] found that all eight pelagophyte clones were similar to _A. anophagefferens_, which caused the brown tides along the coasts of the eastern Unites States and South Africa. Dissolved organic nitrogen (DON) and shellfish farms are mainly responsible for brown tide [20; 21]. Brown tide outbreaks have become a major ecological hazard in the Bohai Sea, with significant impacts on the marculture industry (scallops), coastal ecosystems [14; 20], and zooplankton [22; 23; 24; 25; 26; 27]. The main tributaries to Qinhuangdao Bay have been polluted to various degrees by industrial effluent, aquaculture wastewater, and domestic sewage. During brown tide events, peak _A. anophagefferens_ concentrations were always observed along the Funing and Changli coast, where scallop cultivation farms are located in Qinhuangdao Bay. Human activity-induced eutrophication offshore is well-recognized as the main cause of harmful algal bloom [28; 29; 30].