## 4 Discussion
### Factors Affecting Chla Concentration in the Bohai Sea  
The change of Chla concentration in the Bohai sea is not only related to the environmental factors such as seawater temperature, salinity, illumination and nutrient salts, but also related to the vertical mixing of seawater caused by dynamic factors such as wind, flow field and tidal current [51]. The wind field and flow field (such as upwelling), the input of terrestrial materials, the feeding of zooplankton and other factors also have a direct influence on the spatial and temporal distribution of phytoplankton Chla concentration [52].  
Chla distribution is subject to ocean current imaging and moves with ocean currents [57]. Along with the evolution of the coastal topography of the Bohai Sea, the cotidal time line located in the vicinity of the Bohai Sea Strait has basically shifted to the east as a whole, and the cotidal time line in most parts of the Bohai Sea has shifted counterclockwise [58]. The deflection of the line with tidal time leads to the universal advance of the full diurnal equinox tidal time in the Bohai Sea. In most of the sea areas, with no tidal point as the center along the direction of the wave rotation and propagation, the advance of tidal time-time gradually increases [59]. The Bohai sea tidal currents have two types of tidal currents, including regular semi-diurnal tidal currents and irregular semi-diurnal tidal currents, which are regular semi-diurnal tidal current in Liaodong Bay, Bohai Bay and Laizhou Bay, and irregular semi-diurnal tidal current in the central part of Bohai Sea [60]. The tides in the Bohai sea are basically irregular semi-diurnal tides with a tidal cycle of about 6 h [61]. The tidal components M2 and K1 are the most significant tidal components in the Bohai Sea. When the tide is high, the tide is southward; when the tide is low, the tide is northwards, with an average velocity of 0.5-1.0 m/s [62]. When the tidal wave moves, the distribution of Chla concentration in the sea surface layer will change.  
Wind also affects ocean currents, inducing currents such as rip currents [63], thus causing changes in Chla concentrations. The northern Bohai Sea residual current velocity is smaller than other coastal current. The coastal current system not only presents certain circulation law in its own sea area, but also interacts with coastal currents in other sea areas, which has important influence on the material transport among all sea areas. The hydrodynamic analysis of three bays in the north of the Bohai Sea shows that the hydrodynamic force in the north of the Bohai Sea is mainly tidal current and the residual current is weak [57]. Therefore, the transport of Chla in the Bohai Sea mainly depends on the tidal current. In addition, the upwelling and cold vortex of wind often promote the growth of phytoplankton after a significant wind. Cyclonic vorticity caused by wind increases the distribution of surface Chla not only in time but also in space [64]. The short-term Chla change, such as changes in Chla concentration within 10 minutes, is mainly caused by ocean currents and wind. The characteristics of the average state of Chla concentration are that it is distributed in a band from the near shore to the far sea and gradually decreases from the near shore to the offshore deep sea area.