###### Abstract
[PERSON] and [PERSON] [31] investigated algal blooms in Hong Kong and found that red tide can be formed under the right temperature, nutrient, and hydrodynamic conditions and transported to other locations. [PERSON] et al. [32] studied the relationship between _A. anophagefferens_ distribution and environmental factors and found that certain temperature and salinity levels are required for intensive brown tide to occur in Qinhuangdao coastal water with high DON and low dissolved inorganic nutrients (DIN). [PERSON] et al. [28] and [PERSON] and [PERSON] [33] found that eutrophication is induced by nutrient pollution close to the harmful algal area offshore. Eutrophication can impact the distribution and diversity of species and then change the preponderant algae. [PERSON] et al. [34] proposed that algal blooms are strongly controlled by the interaction of physical and biological process. In a changing climate, an increased carbon dioxide concentration in the atmosphere is expected to intensify global warming and ocean acidification, therefore altering the distribution, frequency, and occurrence season of the red tide [35]. Brown tide events will become more frequent due to urbanization along the coast. Previous studies into harmful algae mainly focused on species invasion or ecology deterioration. There is a lack of understanding of the formation and outbreak mechanisms of brown tide, which would be critical for establishing a reliable monitoring, forewarning, and treatment system for hot spots to prevent and reduce brown tide events. [PERSON] [36] proposed a three-level hierarchy of effects of brown tide based on algal cell components. He used 35,000 and 200,000 cells/mL as concentration criteria to identify the risk level. [PERSON] et al. [37] used traditional modified clays to remove _A. anophagefferens_ and found the removal efficiency to be low. [PERSON] [38] used H\({}_{2}\)O\({}_{2}\) to control brown tide in the laboratory and found that it can remove high-density _A. anophagefferens_ efficiently, but the cost is high. In recent years, advanced technology has been developed to monitor harmful algae and to create a warning system. Remote sensing and satellite observations have been applied to track the spatial distribution and growth of algae. Numerical models have been applied to provide harmful algal warnings [39; 40]. Using physical and chemical methods to treat brown tide and other harmful algal bloom events, however, is still in its infancy. In this study, a coupled hydrodynamic and particulate transport model based on MIKE21 is employed to predict the temporal and spatial distribution of the total nitrogen (TN). This model considers the decay, diffusion, and settling of TN, and the model results are in a good agreement with the measured tidal level, current, and TN, which indicates that the model is capable of capturing the observed TN distribution during the brown tide period. A relationship between _A. anophageferens_, dissolved organic nitrogen (DON), and TN is obtained from field observations and combined with numerical simulation results of TN to establish a fast forewarning system for brown tide in Qinhuangdao. The objective is to make full use of the observation data to establish a reliable relationship between _A. anophageferens_ and TN to represent the complex process of algal growth in order to provide fast alerts for the stakeholder. The main simulated area in this study is shown in Figure 1.