## 3 Results and Discussion
### Changes in Tidal Prism in the Southwestern Part of the Gulf  
Based on the calculation results of the tidal model, the tidal prism calculation formula was used to obtain the tidal prism for a total of 60 tidal cycles in the southwest of Laizhou Bay in 2000 and 2020 (Figure 6). For the Laizhou Bay with irregular semidiurnal tides [9], the tidal range between two adjacent tidal cycles is unequal, resulting in unequal tidal prism between two adjacent tidal cycles. The tidal currents change periodically over time, and the tidal prism in different tidal cycles are also different. Irregular semidiurnal tides have two high tides and two low tides in one lunar day, but the heights of the two adjacent high tides (low tides) are not equal. The higher high tide is called the high tide, and the lower high tide is called the low high tide [24]. Compared with low high tide, the tidal flat area that can be submerged by the tide is wider during high tide, and the reduction in tidal prism caused by the reduction in tidal flat area is greater. Therefore, the change in tidal prism during high tide is greater than that during low high tide. Similarly, during low tide, most of the tidal flats are exposed, and the change in tidal prism caused by the reduction in tidal flat area is smaller than that during high tide. During the 20 years, the tidal prism during high tide in the southwestern part of the bay decreased from 4.59 \(\times\) 10\({}^{9}\) m\({}^{3}\) to 4.37 \(\times\) 10\({}^{9}\) m\({}^{3}\), and the tidal prism during high tide decreased by 4.79%; the tidal prism during low tide decreased from 3.34 \(\times\) 10\({}^{9}\) m\({}^{3}\) to 3.23 \(\times\) 10\({}^{9}\) m\({}^{3}\), and the tidal prism during low tide decreased by 3.29%; the average tidal prism decreased from 4.01 \(\times\) 10\({}^{9}\) m\({}^{3}\) to 3.84 \(\times\) 10\({}^{9}\) m\({}^{3}\), and the average tidal prism decreased by 4.24%. The tidal prism of a bay is positively correlated to the product of the bay area and the bay tidal range, that is, the tidal prism of a bay is not only related to changes in the bay area, but is also affected by changes in the bay tidal range. Although the smaller area of tidal flats has led to an increase in the tidal range in the southwestern part of Laizhou Bay, the reduction in tidal prism caused by the disappearance of tidal flats is greater than the increase in tidal prism caused by the increase in tidal range. Therefore, although the tidal range in the southwestern part of Laizhou Bay continues to increase, the tidal prism is decreasing. Tidal carrying capacity refers to the total amount of tidal water flowing through the bay section during the flood or ebb tide period of the bay. During the ebb tide, the tidal water in the bay flows out through the section, while during the flood tide, the tidal water outside the bay flows into the bay through this section, thereby enabling the exchange of water bodies inside and outside the bay. Consequently, the tidal carrying capacity not only reflects the intensity of tidal water exchange between the bay and the open sea but also, from an environmental perspective, reflects the self-purification capacity of the bay. Under the same water quality standard, a decrease in tidal volume leads to a reduction in the bay's tidal volume, and accordingly, the amount of pollutants that the bay's tidal water can accommodate is also diminished. As a result, pollutants are more prone to accumulate in the bay, leading to a decline in the bay's self-purification capacity.  
Figure 6: Tidal prism change with tidal cycle.