Spatiotemporal variation in water quality
The temperature of water increases the sensitivity of several organisms towards toxic waste. Industrial process waste discharge and air temperature are two major factors that influence water temperature. Higher temperatures might raise the rate of metabolism in living organisms within their thermal tolerance range; beyond this range cause a decline of metabolic processes and decrease performance ( Kroeker et al. 2013). Moreover, the rate of photosynthesis is also controlled by temperature (Brown et al. 2004) and physiological processes like the release of stimulus for breeding in fish species, both under artificial and natural processes(Muralidharan et al. 2014). One of the extreme anthropogenic stresses is the release of hot water from power stations and industries that cause an abrupt change in water temperature ( Riđanović et al. 2010).
Measurement of pH is very significant as an indicator of water quality. Aquatic life is sensitive to ecological pH variations, and the suitability of drinking water is also pH-dependent (Al Shujairi et al. 2013). In the present study conducted in Sialkot district regions, it was found the water pH was slightly alkaline to neutral during different seasons; thus, during the current study, slightly higher pH values were observed in freshwater streams. Rainwater during the wet season dissolve many gases present in the air, which later became a part of the freshwater streams through the atmospheric wet deposition. Water pH reduced as these gases were acidic. It is also affected by bicarbonate, carbonate, and carbon dioxide equilibrium. Fluctuations in the pH value of the water might be attributed to the anthropogenic stress in catchments like domestic sewage disposal, improper irrigation practices, weathering of the parent rock, and industrial discharge (Kumar et al. 2015). Decomposition process going on in aquatic ecosystem which on biological oxidation releases carbon dioxide is another significant factor (Verma et al. 2012). Moreover, decomposition activities, temperature exposure, and high respiratory rate might also be attributed to high pH during th e dry season. Uchchariya and Saksena 2012 also obtained similar observations (Uchchariya et al. 2012). Variations in pH might influence pollution and nutrient solubility. In Pakistan, similar values of pH were also observed and reported by Qadir et al. 2008; Mahmood et al. 2014; Eqani et al. 2015. Another important ecological health indicator in the aquatic ecosystem is that the aquatic organisms utilize dissolved oxygen in respiration and biochemical reactions(Mustapha et al. 2008). The previous study shows that the DO data obtained was tolerable to support life in the aquatic environment during the whole year. ( qadir et al. 2008; Kumar et al. 2012). Moreover, DO was exhibited above 4 mg/L in all samples of water in previous studies which is the minimum tolerable limit as per Chinese standard (3mg/L). According to findings reported from the Kra River Tehran (Iran), Indus River, Chocancharava River in Argentina, and Ebro River (Spain) variations in the level of DO was found to be comparable (Ali et al. 2004; Gholikandi et al. 2012; Bouza-Deaño et al. 2008; Gatica et al. 2012). Similarly, the low level of DO represents sewage addition to the water body as microorganisms decompose organic waste as food and, in return, utilizes oxygen present in water for their metabolic activities; hence they deplete DO level in the water (Staley et al. 1985; Sundaray et al. 2006). In the case of biological oxygen demand (BOD) indicates low temporal variations but considerable spatial variations. Biological oxygen demand (BOD) is the quantity of dissolved oxygen desired by aerobic organisms in water bodies to break down organic material found in a given sample of water at a specific temperature over a definite time and is considered as a significant indicator of water quality. Biological oxygen demand can be preferred over COD as it represents the biodegradable pollutants in the aquatic environment better (Ali et al. 2013). A high level of biological oxygen demand (BOD) produces an intolerable smell ( Muralidharan et al. 2014). According to the standard set by China (1997), 6mg/L is the maximum permissible limit for aquatic organisms. It was found BOD in water samples exceeds the Chinese limit (6mg/L) for the propagation of aquatic organisms, emphasizing the fitness status of Streams water for aquatic life. So during the current study concentration of BOD were also comparable to those reported Gomati River, India (0.8–35.8 mg/L) and Pearl River in China (1.5-15.58 mg/L) (Zhang et al. 2009; Singh et al. 2005) and higher than those Kara River, Iran (5.6-11mg/L), Surma River, Bangladesh (3.5-7.6mg/L) and Mahanadi River, India (7.7–7.73) (Sundaray et al. 2006; Gholikandi et al. 2012; Alam et al. 2007).
In the freshwater stream system, natural flow regimes are a significant element for monitoring fish fauna assemblage (Church et al. 2002). Flow regimes are changed due to formations of water regulating structures like barrages and dams that critically impact biological activities in the aquatic ecosystem (Yoon et al. 2016). Furthermore, overfishing by using pesticides and illegal mesh size nets can significantly reduce fish fauna from some parts of the study area. However, increasing fishing pressure, pollution, habitat destruction, introduction of substituting exotic species, and other stress factors continue to exert strong pressure on fish populations around the world ( Malakoff et al. 1997). Our study area is facing fishing pressure, pollution, destruction of habitat and many other factors that are causing decline in fish population.
Different environmental factors are responsible for the distribution of different fish species in different habitat (Fig. 4). The group of fishes shown in lower left corner has association with high pH, total dissolve substances (TDS), electrical conductivity (EC), and alkalinity. The fishes on the lower right corner are distributed on the basis of high temperature, alkalinity, bicarbonate and low oxygen content (BOD). However the fish species on the upper left and right corner are mostly non-specific.