## 4 Discussion  
### Impact of Nutrients on Brown Tide  
During the Thirteenth Five-Year-Plan period, the Science and Technology Ministry of China conducted special studies on typical harmful algal blooms in offshore areas. In 2017, a project examining the formation mechanism, monitoring, and forecasting and estimate technology of red tide (include brown tide) started. Aiming at algal bloom in offshore areas, many scholars have conducted studies into monitoring and water environmental treatment. Using modified clays is an important technique for the mitigation of harmful algal blooms, especially the red tide [49], and has been successfully applied in offshore red tide management in China, greatly improving coastal water quality. Hebei province has promulgated the plan for comprehensive control in the Bohai Sea and advocated improving  
Figure 11: (**a**) The distribution of TN concentration in August 2014; (**b**) the transformation results from TN concentration to _A. anophagefferens_ cell density, and amplification of the bloom area (**c**). \begin{table}
\begin{tabular}{c c c c} _A. anophagefferens_ Density (Cells/L) & **Grade** & **Represent Color** & **Measures** \\ \hline \(<5\times 10^{5}\) & Safe & Green & Conduct routine monitoring \\ \(5\times 10^{5}\)\(\sim\)\(5\times 10^{6}\) & Third-degree & Yellow & Conduct Third-degree forecast \\ \(5\times 10^{6}\)\(\sim\)\(5\times 10^{7}\) & Second-degree & Orange & Warning for shellfish aquaculture \\ \(>5\times 10^{7}\) & First-degree & Red & Brown tides—Conduct emergency \\  & & & handling for shellfish aquaculture \\ \hline \end{tabular}
\end{table} 
Table 5: The brown tide warning and fast forecast monitoring indicator of _A. anophagefferens_ density. the marine environmental risk monitoring and forecasting and emergency management capability. Policy demands that in the ecologically fragile region and the brown tide-prone area, the feeding aquaculture model is forbidden. Meanwhile, carrying out the risk assessment of emergencies in coastal cities to improve the ability of the government to prevent environment risk events is also a vital measure. All of these measures have great effects on reducing harmful algal bloom. Figure 12 shows the occurrence number of red tide (include brown tide) and the coverage of _A. anophageferens_ from 1990 to 2020 (data collected from the Bulletin of China Marine Disaster, [[http://www.mnr.gov.cn/si/sjfw/hy/gbgg/zghyzhgb/](http://www.mnr.gov.cn/si/sjfw/hy/gbgg/zghyzhgb/)]([http://www.mnr.gov.cn/si/sjfw/hy/gbgg/zghyzhgb/](http://www.mnr.gov.cn/si/sjfw/hy/gbgg/zghyzhgb/)), accessed from 26 November 1990 to 26 April 2021. [48]). It is obvious that, after undertaking the relevant measures, the algal bloom decreased sharply, and the ecological environment was improved. Sufficient nutrients and low grazing pressure by zooplankton and scallops can accelerate the growth of small-celled phytoplankton [50], including _A. anophageferens_. Previous studies have revealed the nutrient effects on brown tide blooms. As one of the dominant species in brown tide, _A. anophageferens_ has a considerable contribution to phytoplankton abundance [51; 15] because it can outcompete other phytoplankton in the presence of elevated organic matter concentration [52]. The decreasing concentration of DIN, increasing human aquaculture activities, and suitability of summer temperatures for the growth of _A. anophageferens_ may be the primary causes of brown tide outbreaks [53]. [PERSON] et al. [54; 55] analyzed the in situ data of the stations in Qinhuangdao and found that the DOP (dissolved organic phosphorus) can restrain _A. anophageferens_ growth, while DON showed great importance in causing brown tide. With the continuous increase in the amount of national attention paid to the marine ecological environment, the concentration of nutrients, including nitrogen, in the seagoing rivers has significantly reduced. Considering the appropriate temperature, we can see that brown tide always blooms in summer, so the predicted time period is set from April to September.