## 4 Results and Discussion
### Correlation Matrix Evaluation and Seasonal Variation
Magnesium is an essential nutrient required for numerous biochemical and physiological functions [59]. The TDS of water generally increases with the level of dissolved pollutants (such as nitrate, ammonium, and phosphate). Conductivity of ions in water depends on water temperature, and ions move faster when water is warm. Hence, conductivity apparently increases when water has a higher temperature [60]. In addition, [PERSON] et al. [61] pointed out a strong relationship between these variables or ions, such as nitrate, ammonium, and phosphate, and stated that high concentrations of EC indicate high concentrations of soluble salts. There are strong correlations between EC/TDS, as evidenced by an increase in conductivity as the concentration of all dissolved constituents increases [62] Table 6.  
The BOD (COD) of rivers was 19.7 (96.5) mg/L in dry seasons and 6.9 (89.4) mg/L in the wet season, and the BOD and COD of lakes was 28.1 (133.3) mg/L in dry season and was 19.1 (112.9) mg/L in wet season. The BOD and COD concentrations for point sources were 116.2 (398.6) mg/L in dry season and 111.6 (353.7) mg/L in wet season (Table 6). The DO of rivers was 3.5 mg/L in dry season and 6 mg/L in wet season, and the DO of lakes was 4.2 mg/L in dry season and 4.4 mg/L in wet season. The DO of point sources was 2 mg/L in dry season and 2.3 mg/L in the wet season (Table 6). The DO of the rivers in the dry seasons and Lake Hawassa were well below the standard value. This indicates that the discharge of industrial and domestic effluents has resulted in serious organic pollution of these rivers, as the decrease of DO was mainly caused by the decomposition of organic compounds. Moreover, an extremely low DO content usually indicates the degradation of an aquatic system [63]. The DO showed a negative correlation with most parameters in both dry and rainy seasons, revealing the value of DO decreases with the increase in other water quality parameters. This could explain the temporal variations, as more oxygen was available for reaction with the pollutants, especially metals and organic pollutants, during dry seasons. Additionally, the characteristics of temporal variation in water quality of LHW were affected by DO. DO was strongly correlated with organic matters, nutrients, and metals, and thus seasonal variation should be considered when DO is used as an indicator to evaluate surface water quality. Low dissolved oxygen (DO) is primarily the result of excessive algal growth caused by nutrients. As the algae die and decompose, this process consumes dissolved oxygen. This may result in insufficient dissolved oxygen for fish and other aquatic life. Temperature was significantly correlated with water quality parameters such as EC, TDS, TP, PO\({}_{4}-\)P, and DO in both seasons. Temperature had significant negative correlation with DO in the dry and wet seasons, indicating that when water temperature increases, the metabolic rate of microorganisms also increases, and the amount of DO in the water decreases. This might be because faster biodegradation of organic matter during dry seasons can effectively improve water quality. The solubility of oxygen was inversely related to temperature, as the water becomes warmer and more easily saturated with oxygen, hence holds less DO during the dry season. [PERSON] et al. [32] observed the inverse relationship between temperature and DO in natural processes, as water can hold less DO with increasing temperature.