**3. Results**

### *3.1. 24 Hourly Variations in Temperature and Salinity*

The trend of the surface temperature in the three regions was offshore > near-shore > front, and the bottom temperature showed a trend of near-shore > offshore > front (Figure 3a–c). The trend of salinity in the surface and bottom layer showed a distribution trend of offshore > front > near-shore (Figure 3d–f). The difference between the surface and bottom temperature in the front region was the largest, followed by the offshore region, and the temperature difference in the near-shore region was the smallest. The temperature and salinity changes in the near-shore region fluctuated with a semidiurnal frequency. The temperature and salinity at 06:00 and 18:00 both had extreme values (Figure 3a,d). The relative standard deviation of surface salinity changes was as high as 25.82% in 24 hours. In the front region, the relative standard deviation of the temperature variation at 10 m reached 8.90%, and the salinity variation at 5 m was as high as 21.01%. In the offshore region, the temperature and salinity changes were small in 24 hours: the relative standard deviation of the temperature at 10 m was 5.15%, and the relative standard deviation of the changes in salinity at the surface in 24 hours was 1.01%; the others were less than 1%.

**Figure 3.** Twenty-four-hour variations in temperature in the near-shore (**a**), front (**b**), and offshore (**c**) regions and salinity in the near-shore (**d**), front (**e**), and offshore (**f**) regions in summer.

### *3.2. Variation in pH, TA, DIC, and Sea Surface pCO2 within 24 Hours*

In the near-shore region, the surface daily averages (standard deviations in brackets) of pH increased from 7.92 (±0.02) to 7.95 (±0.02) at the bottom (Figure 4a), TA increased from 1936.06 (±40.21) to 1993.43 (±15.01) μmol kg−<sup>1</sup> at the bottom (Figure 4d), DIC increased from 1889.75 (±28.04) to 1919.82 (±9.35) μmol kg−<sup>1</sup> at the bottom (Figure 4g), and *p*CO2 decreased from 996 (±71) to 868 (±53) μatm at the bottom (Figure 4j).

In the front region, the surface daily averages (standard deviations in brackets) of pH decreased from 8.33 (±0.11) to 7.94 (±0.03) at the bottom (Figure 4b). TA increased from 2157.80 (±34.17) to 2244.12 (±0.68) μmol kg−<sup>1</sup> at 30 m and then decreased to 2244.06 (±0.50) μmol kg−<sup>1</sup> at the bottom (Figure 4e). DIC increased from 1833.97 (±68.63) to 2102.68 (±12.88) μmol kg−<sup>1</sup> at the bottom (Figure 4h), and *p*CO2 increased from 283 (±87) to 735 (±59) μatm at the bottom (Figure 4k).

In the offshore region, the surface daily averages (standard deviations in brackets) of pH decreased from 8.38 (±0.03) to 8.05 (±0.02) at the bottom (Figure 4c), TA increased from 2229.48 (±4.72) to 2240.15 (±0.53) μmol kg−<sup>1</sup> at the bottom (Figure 4f), and DIC increased from 1791.73 (±24.87) to 2018.65 (±13.21) μmol kg−<sup>1</sup> at the bottom (Figure 4i). Daily average *p*CO2 was 227 (±23) μatm at the surface, and it decreased to 226 (±32) μatm at 5 m and then increased to 566 (±38) μatm at the bottom (Figure 4l).

Overall, from the vertical distribution of the water column, pH was generally highest at the surface and lowest at the bottom. On the contrary, TA, DIC, and *p*CO2 were generally lowest at the surface and highest at the bottom. Spatially, pH and TA generally increased from the near-shore to the offshore region. On the contrary, DIC and *p*CO2 generally decreased from the near-shore to the offshore region.

**Figure 4.** Twenty-four hour variations in pH in the near-shore (**a**), front (**b**), and offshore (**c**) regions; TA in the near-shore (**d**), front (**e**), and offshore (**f**) regions; DIC in the near-shore (**g**), front (**h**), and offshore (**i**) regions; and sea surface *p*CO2 in the near-shore (**j**), front (**k**), and offshore (**l**) regions in summer.

### *3.3. Variation in NEP within 24 Hours*

In the near-shore region, there were negative NEP values, and the NEP at the bottom was slightly larger than that at the surface (Table 3). The minimum NEP value was −0.36 mmol C m<sup>−</sup><sup>3</sup> day−<sup>1</sup> at 12:00 at the surface, and the maximum value was 0.13 mmol C m<sup>−</sup><sup>3</sup> day−<sup>1</sup> at 15:00 at the bottom (Figure 5a).

**Table 3.** Minimum, maximum, mean, and standard deviation of NEP in the three regions in summer (mmol C m<sup>−</sup><sup>3</sup> day−1).


**Figure 5.** Twenty-four-hour variation in NEP in the near-shore (**a**), front (**b**), and offshore (**c**) regions in summer.

In the front region, the maximum NEP value (1.89 mmol C m<sup>−</sup><sup>3</sup> day−1) was observed at 03:00 at the surface, and the minimum value (−0.32 mmol C m<sup>−</sup><sup>3</sup> day−1) was observed at 21:00 at 10 m (Figure 5b). In the vertical direction, the daily variation in the surface NEP was slightly larger than that at the bottom. In the front region, the daily mean NEP from the surface to the bottom generally decreased, and the 24-h variation in NEP in the mixed layer (Table 3) was larger than that under the mixed layer (Table 4).

**Table 4.** Mixed layer depth (m) at each measurement time within 24-h in three regions.


In the offshore region, the maximum NEP (0.52 mmol C m<sup>−</sup><sup>3</sup> day−1) was observed at 09:00 at 5 m, while the minimum NRP (−0.54 mmol C m<sup>−</sup><sup>3</sup> day−1) was observed at 10 m (Figure 5c). The largest variation in NEP within 24-h was at 10 m, and the smallest variation was at the bottom (Table 3).
