The results of physico-chemical characteristics of water quality parameters are summarized for high and low tide in Table 2 and the description for different parameters is elaborated.
Hydrobiology of coastal water during tidal influences
Surface water temperature in all the stations fluctuated between 32.1°C - 34.1°C during both of the tides. The temperature difference between HT and LT was marginal and ranged from 32.7°C (S6) to 34.1°C (S3) during HT, while it was between 32.1°C (S1) to 34.1°C (S5) during LT. Only the S1 site showed the fluctuations in the surface water temperature to the tune of 1.06°C between both the tides attributed to site geographic location viz., in the mouth of the Gulf (Shetye, 1999; Kumar et al., 2015). The absence of fluctuation in the tidal temperature is because of sampling close to the open sea coastline at Arabian sea.
The pH of the water governs the chemical state of the water. In the present study, the pH of the coastal water was recorded in a range of 8.0 to 8.7 during the HT while, 7.9 to 8.4 during the LT. The observed pH results were similar to the previous studies in the GoK (Devi et al., 2014; EIA & EMP, 2015). The electric conductivity (EC) of water is dependent on the presence of total dissolved solids. The present study, EC values ranged from 6.3 mS/cm (S2) to 9.5 mS/cm (S7) during the HT, whereas 6.5 mS/cm (S4) to 9.9 mS/cm (S7) during the LT. The influence of tides on conductivity was highly observed at S6 and S7, during both tides the conductivity was recorded maximum (>9 mS/cm) throughout the study area. These two stations are located at inner gulf where the arid climatic conditions and high temperature regime is prevalent that influence the water quality.
The water salinity during the high and low tide showed negligible variations. The salinity values ranged from 36.0 ppt (S1) to 52.0 ppt (S4) during HT and 37.0 ppt (S1) to 51.0 ppt (S4) during LT. The high evapo-transpiration rate along with an absence of inflow of freshwater into the marine system has accounted for higher salinity in the Gulf. Other authors have reported similar results from the GoK (Saravanakumar et al., 2008; Devi et al, 2014; EIA & EMP, 2015). The presence of high salinity at S4 indicates the release of brine from the salt pans located in the coastal region of the sampling site.
There was no significant difference in the DO values between the HT and LT in major stations except S1 and S6. These both stations S1 and S6 have highest oxygen fluctuations ~2.03 mgO2/L due to high tidal flux (Kunte et al, 2005). The study area S1 lies in the open sea and is under the higher influence of high tide which brings in oxygenated rich water from the sea. On the other hand, S6 lies at inner gulf where the biggest industrial cluster is established and various towns developed for the workers. The major source in the S6 was these industrial effluents and domestic input near shoreline has resulted in low DO during the LT. Similar results also investigated by various researchers and conclude that the lower DO conditions in the coastal areas were mainly due to the anthropogenic influence or industrial activities (Vijayakumar et al., 2000; Bhadja and Kundu, 2012; Devi et al, 2014). The BOD values were 1.63 mgO2/L (S2) to 3.28 mgO2/L (S7) during the high tide whereas, 1.65 mgO2/L (S3) to 4.49 mgO2/L (S5) during low tide indicated the highly dilution effect in the Gulf.
The TSS results revealed that all the stations had higher suspended solids during HT (2.24-6.98 g/L) as compared to LT (1.88-3.53 g/L). The sampling was conducted during the onset of monsoon due to the highly active currents. Among that, there is heavy churning of the sea bottom bringing the sediments into the water suspension. The long-shore currents are active bringing in Indus sediments into the GoK and finally exiting from the mouth of the Gulf at Okha (Nair et al., 1982). The observation revealed that the presence of high TSS at S6 and S7 (6.89-6.98 g/L) as compared to other stations (2.42-2.68 g/L) supports the earlier fact that sediment travels from east to west in the southern coastline of the GoK (Shetye, 1999; Vethamony and Babu, 2010).
Turbidity in study sites was recorded from 3 NTU (S3) to 21 NTU (S2) during HT and from 4 NTU (S5&S6) to 32 NTU (S2) during LT. Similar results were previously reported by various researchers (Shetye, 1999; Rasheed and Balchand, 2001; Sinha et al., 2010; Masood et al, 2015). Spatial distribution showed that S2 and S3 have higher turbidity during LT attributed to geographic location and bay-like conditions. The presence of low turbidity at S5 located in the central part of the Gulf that incidentally represents Marine National Park (MNP) justifies the occurrence of corals and various marine biota at the site (Panseriya et al. 2021).
The nutrient level in terms of total phosphates was comparatively fluctuating throughout the studied area. The total phosphates levels were higher during the HT (0.10 mg/L to 1.54 mg/L) as compared to the LT (0.17 mg/L to 0.65 mg/L). Earlier Saravanakumar et al., 2008 and Srilatha et al., 2013 obtained similar results in the GoK. Total nitrogen in the present study ranged from 0.38 mg/L to 0.63 mg/L during both tides. A similar results were also reported previously at many locations from GoK (ICZM, 2013), indicative of no major input in nitrogen load. Sulphate values varied from 23.66 mg/L to 37.35 mg/L during both tides. A comparison of sulphate values among different stations showed higher concentration at S6. The presence of heavy transportation of coal and cement industry nearby accounted for the presence of higher sulphate at S6. Therefore total sulfate and chlorine ratio of the present study was higher than the standard seawater sulphate : chlorine ratio of 1.18. The sulphate : chloride of HT and LT were 1.34 and 1.36, respectively. Moreover, the results indicate higher sulphate concentration could be attributed due to land-based activities especially anthropogenic sources at the coastline during summer and pre-monsoon seasons (Stromberg and Cumpston, 2014).
Distribution of physico-chemical water quality during tidal variation
The box plot or box and whisker plot is summarized and interpreted in tabular data and provides a visual impact of the location and shape of a primary distribution. The circles and asterisks are also called the outliers and far outliers which indicates a change in a variable between the stations. The box plot results of physico-chemical parameter illustrated in Fig. 2, which explain long whiskers at the top of the box (e.g. pH and EC box plot) and indicate primary distribution has been scattered towards high concentration. The pH, TSS and TS in the box plot showed a visual difference between the low tide and high tide values (Fig. 2). The circles and asterisk were prominent for turbidity and sulphates indicative of a difference in the values between the stations. Turbidity represents a quantum of suspended solids present in the water as explained in the previous section, whereas the TS differed between two tides attributed to current movements. A similar observation was confirmed from the box plot method. There are coal transportation jetties close to S1 and S7, which accounted for high sulphate levels in the water column. The box plot justifies this observation and relates the influence of anthropogenic activity on the water quality during the low and high tides.
Pearson’s correlation analysis
The correlation analysis indicates nutrients were significantly correlated during LT as compared to HT (Table 3). This could be due to the discharges of anthropogenic activities from industries or domestic waste bringing in high nutrients.
The Pearson’s correlation analysis of HT (Table 3) highlighted that surface water temperature correlates with TS (p<0.05). As the temperature raises the dissolution of minerals ions in the water column increases, therefore a positive correlation was observed. DO and BOD showed a positive correlation (p<0.05), the source of DO is because of churning and oxygen mixing during the upwelling process of high tide. However, the presence of inorganic waste from the industrial outfall points has contributed to high BOD in the GoK samples. The results from Table 3 indicated high physico-chemical parameter load during LT by industrial or anthropogenic pressure. This indicates that there was low flushing of waste out of the Gulf and the presence of waste by industrial clusters will result in high nutrient concentration in the Gulf in coming years disrupting the coastal and marine ecosystem process. In the longer run, the mangroves and corals are likely to be affected by the release of industrial and domestic sewage or anthropogenic impact.
Spatio-temporal variability in coastal water quality: Hierarchical cluster analysis (HCA)
The result obtained by HCA based on physico-chemical parameters classified similar groups of dendrogram into two statistically significant clusters. The HCA of the current study express tidal characteristics of coastal water quality viz., nutrients, pH, temperature, conductivity, turbidity, TSS, DO and BOD. The clusters obtained for both the tides were highly distinct from each other, largely due to differences in the contaminant load prevalent during the respective tide (Fig. 3).
During HT, HCA results indicate the distribution of all the stations in two clusters, clusterI and cluster II. ClusterI have similar water quality and contains S4, S6 and S2 study zone. However, the sources of pollution differ in all these stations. The S4 receives concentrated brine discharged from salt pans, while industrial clusters and effluent discharge points represent S6. The S2 is a geographical bay, where the tidal current played a significant role. The clusterI results indicated salinity sulphate, TSS, and turbidity were key parameters that influenced the water quality. The ClusterII results were includes the water quality of S3, S5, S7 and S1. The S3 receives water from the open sea and enters into estuarine of the study area which increase salinity, and DO. The S5 zone recognized as MNP which sustains corals, marine creatures and mangroves forest. The reported higher concentration of DO is important for the aquatic life such as bacteria, fish, invertebrates, and plants. The BOD possibly received from the effluent discharge points located close by to the S5. The S7 has well-developed industrialization, fishing, transportation of coal and cement, etc. along with outlets of industrial effluents which influenced the water quality. The S1 is located at the mouth of the gulf which opens in the Arabian Sea, therefore dilution effect is very high as described in previous studies by Shetye, 1999; Bhadja and Kundu, 2012.
During LT, the HCA result generated two clusters, i.e. clusterI and clusterII. ClusterI includes S2 and S4 with similar water qualities. In which S2 is a bay condition wherein the water column contained a mixture of nutrients and litter detritus that increased the phosphate, nitrogen, and turbidity of coastal water. The field observation suggests S4 had salt pans which influenced the DO, salinity, and sulphate. ClusterII includes stations S5, S6, S7, S1 and S3. The water quality parameters such as salinity and EC observed variation in the stations S5, S6 and S7. Stations S5 and S6 were located in the inner gulf in the study area. The results indicated that domestic waste influenced S5 coastal water quality, whereas S6 is an industrial zone but most of the industrial effluents were disposed into the deep-sea through pipelines while discharges of domestic waste were near the coastline. In contrast, S7 has both contaminants from the domestic waste of Jamnagar city and industrial waste from the surrounding land area. In other stations, S1 was located at the mouth of the open sea and S3 received waste from domestic discharges, fishing activity, transport and boat construction activities. The key impacted parameters were EC, salinity and DO during the low tide.
The overall analysis indicates industrial pressure was on the coastal area of the Gulf of Kachchh. The present study points out that during the high tide, the effluents that are discharged into the sea return back with the water column towards the coastline. Further, land-based domestic and industrial activity contributes to deteriorating water quality during low tide.
Principle indicators evaluation during different tide by factor analysis
The results of factor analysis were separately applied to the water quality data set of high and low tide. Factor analysis includes eigenvalues, factor loading, variable loading, total and cumulative variance and represented in Table 4. The results of the eigenvalue and scree plot were used for significant factors, whereas the factor loadings were sorted as strong, moderate and weak according to the absolute loading values of >0.75, 0.75-0.5 and 0.5-0.3 respectively (Liu et al., 2003; Panseriya et al., 2021). Chemometric or factor loadings were illustrated in Fig. 4.
A chemometric analysis produced four significant factors with >1 eigenvalue that explained the total variance (93.75%) of the high tide dataset. The factor 1 (PC1) indicates 29.10% of total variance which includes strong positive loading of total nitrogen, total phosphate and turbidity, Hence, PC1 comprises of a common source of nutrients due to natural tidal activity and also included with outfall of industrial waste from the deep sea. The second (PC2) explains 24.79% of total variance which includes strong positive loading of salinity, pH, and sulphate. PC2 suggests the combined effect of natural and anthropogenic activities. The PC3 explains 21.60% of total variance with strong positive loading of DO and BOD and strong negative loading of total solids. The PC3 indicates the natural scenario of the Gulf of Kachchh and is indicative of the biological activities during the summer season. During summer high evaporation rate and low inflow of water from the existing streams have ultimately affected TS and TSS. The last PC4 explains 18.26% of total variance with strong positive loading of EC and TSS.
Similarly, Factor analysis of the low tide data set produced three significant factors that explained 80.22% of the total variance of the data set. In FA, PC1 explains 28.87% of total variance which includes strong positive loading of salinity, temperature, sulphate, and TS with moderate negative loading of pH. The PC1 indicates the loadings were due to the receding water effect on the substratum, which re-suspends the bottom sediments and degrades mangrove stands. The high level of suspended solids in low tide is purely due to re-suspension of bottom and coastal eroded sediment by tidal effect. Moreover, positive loading of salinity is mainly due to brine water from salt industries near Okha, Pindara, Dhani, Sikka and Rozy (Fig. 2). The PC2 explains that 27.44% of total variance with strong positive loading of phosphate, nitrogen and turbidity, as well moderate negative loading of DO. The positive loading of nutrients and negative loading of DO clearly shows the main source of contaminant load from land-based activities, sewage disposal or other anthropogenic activities. The third PC3 explains that 23.91% of total variance which include strong positive loading of DO, BOD and EC. The components explained anthropogenic and biological activities. Due to organic matter, microbial activities were increased which increases BOD and all these hydrolysis processes for degradation of acidic material cause a decrease in pH value (Singh et al., 2004).