Physico-chemical parameters
The physico-chemical parameters of Lake Dapao from the four designated sampling stations were tested for pH level, dissolved oxygen, turbidity, total dissolved solids (TDS) and temperature.
Temperature
The water temperature measurements ranged from 25.57±0.07 to 26.43±0.19⁰C. There was no significant difference in water temperature between the four sampling stations of the lake, as depicted in Figure 2.
This lack of variation in temperature could be attributed to the presence of a relatively humid environment and a greater amount of shade due to the surrounding forest and mountains. Other factors that may have contributed to temperature differences among the sampling stations include direct exposure to sunlight, water depth, air temperature, and thermal disturbance from human activities (Surubaru et al., 2012).
The recorded water temperatures from the three sampling stations, ranging from 23 to 24⁰C, were within the permissible limit set by the Department of Environment and Natural Resources (DENR) since there was no recorded increase of more than 3⁰C compared to the ambient temperature.
pH
In terms of pH, the water exhibited a range of 8.66±0.08 to 8.85±0.09, indicating a highly alkaline nature (Fig. 3). There were no significant differences in pH between the four sampling stations of the lake. However, the pH range was slightly higher than the lowest basic range specified in the DAO 2016-08, which is 6.5-8.5.
The elevated pH values may be attributed to various factors, including waste discharge, microbial decomposition of organic matter in the water, and sewage discharge from the surrounding human population (Raja, 2008; Patil et al., 2011; Patil et al., 2012; Vyas et al., 2015). These higher pH values suggest that changes in physico-chemical conditions have a significant impact on the carbon dioxide and carbonate-bicarbonate equilibrium (Karanth, 1987; Sharma et al., 2015).
The pH level is a crucial factor in determining the productivity of an ecosystem, as it indirectly affects most of the biochemical parameters of aquatic bodies. Sawant et al. (2013) reported that pH can have a significant impact on the health of fish, with a pH range of 6.5 to 9.0 being suitable for most freshwater species. However, marine animals cannot tolerate such a wide pH range, and the optimum pH for them is usually between pH 7.5 and 8.5, as noted by Boyd and Tucker (2012, 2014).
Dissolved oxygen
In terms of dissolved oxygen (DO) levels, the results from the four stations in Lake Dapao indicate that the DO concentration is higher than the prescribed level set by DAO 2016-08, which is 5 mg/L. The level of DO ranges from 7.10±0.07 to 7.25±0.03, and there is no significant difference among the four stations. This finding suggests that the lake water in Lake Dapao can sustain the oxygen demands of the aquatic organisms within it, as per the standard set by DENR.
The dissolved oxygen concentration is an important indicator of water purity and the distribution of algal groups (Shyamala et al., 2008; Patil et al., 2011; Patil et al., 2012; Niba Rawlings and Nchang, 2013; Vyas et al., 2015).
Turbidity
Turbidity, on the other hand, is caused by suspended matter such as clay, silt, organic matter, phytoplankton, and other microscopic organisms. Algae are often the dominant influence on the transparency of lake water (Fuller and Minnerick, 2007). The turbidity levels from the four sampling stations in Lake Dapao ranged from 4.97±0.03 to 6.43±0.03 meters, with Station 1 being the most turbid and Station 4 being the least. The high turbidity level in Station 1 could be attributed to human activities such as washing and bathing, while Station 4, located at the center of the lake, had the least disturbance on the water surface. No visible algal growth was observed during the sampling, and the high turbidity level could be due to the presence of floodwater, surface runoffs, and suspended materials after rainfall (Ibrahim et al., 2009).
The presence of high turbidity in lakes and streams can have detrimental effects on their aesthetic quality, ultimately impacting recreation and tourism. Additionally, it can increase the cost of water treatment for drinking and food processing purposes. Furthermore, high turbidity can harm fish and other aquatic life by reducing their food supplies, degrading spawning beds, and affecting gill function (Minnesota Pollution Control Agency, 2008).
Total Dissolved Solids (TDS)
The total dissolved solids (TDS) measured at the four sampling stations ranged from 31.67±0.33 to 35.33±0.33 (Fig. 6), which is relatively lower than the level specified in the DAO 2016-08, set at 50. Streams can contain suspended, volatile, and dissolved solids. Suspended solids include silt, stirred-up bottom sediment, decaying plant materials, and sewage treatment effluent. Dissolved solids, on the other hand, can pass through a filter. Various factors influence the concentration of TDS in a body of water (APHA, 2005; Rice et al., 2012).
It is important to note that higher levels of TDS in drinking water can lead to the development of diseases that are not water-borne but are caused by excessive salt intake (Vyas et al., 2015). The station with the highest TDS concentration is typically associated with increased human activity. As we move towards Station 4, located in the middle of the lake where human activity is minimal or nonexistent, the TDS level decreases.
Table 1 presents the mean values of each physico-chemical parameter mentioned above for each station, comparing them to the standards set by the DENR in their DAO 2016-08 for potable water (Subspecific Class A).
Microbiological Characteristics
The researcher identified four sampling stations and collected three samples from each station to examine the microbiological characteristics of the water. The results showed a common variable among the samples from each station, as presented in Table 2. However, the level value obtained from the test at all stations was found to be greater than 8 (> 8.0), which is significantly higher than the level recommended by the PNSDW from the Department of Health Administrative Order No. 2017-0010. This may be attributed to the close proximity of human activity around the lake and the flow of water to nearby irrigations, particularly in Brgy. Linuk (Station 2), where the undercurrent of the lake carries microbes to such openings. These findings indicate that the water samples are not suitable for drinking purposes according to DOH recommendations.
To measure the degree of pollution and sanitary quality of healthy water, total coliforms and E. coli are commonly used as indicators, as testing for all known pathogens is a complex and expensive process. The primary source of pathogens in drinking water is recent contamination from human or animal waste, which can occur through improperly treated septic and sewage discharges, leaching of animal manure, stormwater runoff, or domestic animals or wildlife. During and after precipitation, bacteria and other harmful microorganisms from these sources may be washed into rivers, lakes, or groundwater. Poor construction or maintenance of water systems can increase the risk of groundwater contamination.