**3. Results and discussion**

Descriptive statistics of nine water quality parameters is shown in **Table 2**. The NO<sup>2</sup> -N level at most of sampling stations is below the detected limit (data not shown). pH ranged from 8.00 to 8.06, with the narrow range. Chl-a has a relatively low value in the range from 0.02 to 0.06 μg L−1. COD varied from 0.16 to 0.31 mg L−1, with the mean value of 0.24 mg L−1. Nutrients display a big change with higher standard deviation than pH and COD. According to the seawater quality standard of China (GB 3097–1997), all these water quality (pH, DO, COD, and nutrients) ranges from the first class quality standard scale. Thus, Xisha waters belong to the first class water quality, which is namely, the pristine water.


**Table 2.** Descriptive statistics of nine water quality parameters.

In this study, different water quality parameters display different spatial characteristics (**Figure 2**). From the graph, it is observed that pH and Chl-a are higher in YX than the rest of the study stations. Salinity, COD, NH<sup>4</sup> -N, and TP are maximum in SD. The maximum of NO<sup>3</sup> -N is observed in ZS, with the minimum of DO also in ZS. However, results of one-way analysis of variance show the insignificant difference among the four sampling areas, suggesting the water quality mainly affected by the natural progress, with the least disturbed anthropogenic activities.

pH is significantly and positively correlated with DO (*r* = 0.71, *p*-value = 0.00). It is possible that in an environment with low levels of organic matter and low levels of respiration, any hydrogen ions released are being absorbed by the alkalinity of the surrounding seawater [10]. COD is lower than the threshold limit of the first class water quality standard and is insignificantly related with other water quality parameters, implying the low organic matter in this environment. On the other hand, the chemoautotrophic process of nitrification is also associated with DO and pH. The process uses up oxygen and also releases hydrogen ions which can cause a fall in pH. However, NO<sup>3</sup> -N is insignificantly related with NH<sup>4</sup> -N and DO. It implies that nitrification is not a main controlling factor on dissolved nitrogen for transformation. That is to say, other physical-biochemical processes play an important role in dissolved nitrogen cycle. NH<sup>4</sup> -N is significantly negatively related with Chl-a (*r* = −0.50, *p* –value = 0.02), while NO<sup>3</sup> -N is insignificantly related with Chl-a. NH<sup>4</sup> -N may be the preferred N source for phytoplankton growth in the study area. NH<sup>4</sup> -N is assimilated primarily by phytoplankton in Pearl River Estuary, nSCS [11].

Robust principal component analysis rendered the first three significant factors (eigenvalue >1.0) that explained 67.02% of the total variance of data set. The results of the RPCA were visualized in the form of ordination diagrams based on PC1 and PC2. The parameter lines were obtained from the factor loadings of the original variable (**Figure 3a**). The closer the two-parameter lines lie together, the stronger the mutual positive correlation is [12]. pH and DO have the highest positive correlation coefficients, so the two lines are very close with acute angle between them. Similarly, an obtuse angle between two lines represents a negative correlation, etc., NH<sup>4</sup> -N and Chl-a. The lengths of the parameters represent the relative explanatory power of sampling stations within the ordination in PC1 and PC2 (**Figure 3b**). In the fourth quadrant, sampling stations in YX are characterized by high DO, salinity and Chl-a, and low nutrients, as these parameter lines are located in this quadrant. It suggests that this area may have a high photosynthesis rate and high primary production. According to Redfield ratio, the average ocean photosynthesis and aerobic respiration can be expressed as follows:

**3. Results and discussion**

(BD) and Zhaoshu (ZS), respectively.

162 Water Quality

Descriptive statistics of nine water quality parameters is shown in **Table 2**. The NO<sup>2</sup>

Mean 8.04 33.35 6.61 0.24 0.07 0.68 0.97 0.03 0.20 Min 8.00 32.99 6.02 0.16 0.03 0.36 0.43 0.02 0.13 Max 8.06 33.50 7.81 0.31 0.29 2.25 1.79 0.06 0.45 Std 0.02 0.12 0.45 0.04 0.06 0.47 0.39 0.01 0.08

first class water quality, which is namely, the pristine water.

**pH salinity DO COD PO4**

**Table 2.** Descriptive statistics of nine water quality parameters.

most of sampling stations is below the detected limit (data not shown). pH ranged from 8.00 to 8.06, with the narrow range. Chl-a has a relatively low value in the range from 0.02 to 0.06 μg L−1. COD varied from 0.16 to 0.31 mg L−1, with the mean value of 0.24 mg L−1. Nutrients display a big change with higher standard deviation than pH and COD. According to the seawater quality standard of China (GB 3097–1997), all these water quality (pH, DO, COD, and nutrients) ranges from the first class quality standard scale. Thus, Xisha waters belong to the

**Figure 1.** Monitoring stations in the studying area. Four sampling areas are Yongxing south (YX), Shidao (SD), Beidao

**-P NO3**

**-N NH4**

**(mg L−1) (μmol L−1) (μg L−1) (μmol L−1)**

**-N Chl-a TP**


$$106\,\mathrm{CO}\_{2} + 16\,\mathrm{NH}\_{3} + \mathrm{H}\_{3}\mathrm{PO}\_{4} + 122\,\mathrm{H}\_{2}\mathrm{O} \rightarrow \left(\mathrm{CH}\_{2}\mathrm{O}\right)\_{106}\,\mathrm{(NH)}\_{16}\mathrm{(H}\_{3}\mathrm{PO}\_{4}\right) + 106\,\mathrm{O}\_{2}\tag{1}$$

Due to photosynthetic activities, nutrients (NH<sup>4</sup> -N and PO<sup>4</sup> -P) are quickly taken up by the algae, which result in a shift of the equilibrium [Eq. (1)]. That is to say, phytoplankton photosynthesis is stronger in YX island waters than rest of the areas, suggesting phytoplankton consumes more nutrients from the water, and then produces more oxygen. Photosynthetic organisms release O<sup>2</sup> and assimilate nutrients, thereby community respiration attenuating the pH and DO decline. It suggests that photosynthesis keeps balance with respiration, therefore leaving the system without extra organic matter in these areas. The loading of COD is negative in PC1, indicating less organic matter in these areas than rest of the areas (**Figure 3**).

**Figure 2.** Spatial distributions of water quality, (a) pH, (b) salinity, (c) DO, (d) COD, (e) phosphate, (f) nitrate, (g) ammonia, (h) chlorophyll and (i) total phosphate. ZS, BD, SD, and YX denote Zhaoshu, Beidao, Shidao, and Yongxing, respectively.

**Figure 3.** Results of robust principal component analysis: (a) loadings of water quality parameters and (b) scores of monitoring stations. The first two letters denote the monitoring area. "S" and "B" in the third letter denote the surface and bottom layers, respectively. The number denotes the monitoring station.

On the other hand, sampling stations in ZS1, SD1 and most of sampling stations in BD are characterized by high NH<sup>4</sup> -N and COD, and low pH (**Figure 3**). From this phenomenon, it implies that these areas may have higher organic matter decomposition than rest of the areas. Algal debris and other organic matters can be converted into carbon dioxide and NH<sup>4</sup> -N [Eqs. (2) and (3)] under biochemical oxidation, resulting in the increase of NH<sup>4</sup> -N and a decrease in pH.

$$\text{(CH}\_2\text{O)}\_{106}\text{(NH)}\_{16}\text{(H}\_3\text{PO}\_4\text{)} + 106\text{O}\_2 \rightarrow 106\text{CO}\_2 + 16\text{NH}\_3 + \text{H}\_3\text{PO}\_4 + 122\text{H}\_2\text{O}\tag{2}$$

$$\mathrm{NH}\_3 \mathrm{+H}^\* \to \mathrm{NH}\_4^\* \tag{3}$$

NH<sup>4</sup> -N in these stations is insignificantly related with NO<sup>3</sup> -N (*r* = −0.31, *p*-value > 0.10), while ammonia concentration is higher than NO<sup>3</sup> -N except in ZSB1, ZSS2, and SDS1. NH<sup>4</sup> -N production has excess ammonia reduction during nitrification process, resulting in that nitrification may be secondary biochemical progress. This is in disagreement with the study [13]. In this study, the presence of nitrate is mainly due to processes such as nitrification. Water quality may be associated with coral community in Xisha waters. Different dominant coral species reside in the different areas around Yongxing Island, with different species index and cover [5].

As the abovementioned, Xisha waters have spatial variations due to different ecological characteristics. Nutrient levels are below the threshold of the first class water quality of China, Xisha waters are considered in pristine environment so far. However, Sansha city on central Yongxing Island of the Xisha islands was built up by China in 2012. With the development of Sansha, more and more infrastructure has been built in Xisha islands. Special marine features have a great deal of potential for attracting various types of tourists. The elevated nutrient level may promote negative responses such as an increase in bleaching susceptibil-

**Figure 2.** Spatial distributions of water quality, (a) pH, (b) salinity, (c) DO, (d) COD, (e) phosphate, (f) nitrate, (g) ammonia, (h) chlorophyll and (i) total phosphate. ZS, BD, SD, and YX denote Zhaoshu, Beidao, Shidao, and Yongxing, respectively.

164 Water Quality

ity of coral community [14]. Consequently, Xisha islands is intensively facing the stress of anthropogenic activities.
