Hydrology, water quality and trophic state of Pergau Reservoir, Kelantan, Malaysia

ABSTRACT To acquire baseline data for a remote Pergau Hydroelectric Power Plant reservoir, a hydrological and water quality examination of feeder rivers and reservoir water was conducted in 2013. The water balance of the reservoir was determined, and gauging and sampling were carried out in dry and wet periods. Dilution gauging was used to estimate the feeder river discharge. The total water flow into the reservoir increases nearly fivefold between the dry and wet seasons, while river discharge increases two to fivefold. All of the river intakes had Class 1 NWQS water quality. The lake’s water quality was Class 1 up to the top 3 meters, but below that, at some places, the water quality deteriorated to Class II. In the dry season, the trophic status of Pergau Reservoir is eutrophic as measured by TP (59.21) and chlorophyll-a (52.36) and the TSI (SD) was 53.93 Eutrophication occurrences will cause serious limitations in water use applicability. This research contributes to the biogeographical and limnological understanding of the Pergau catchment, as well as laying the groundwork for more sophisticated hydro-ecological investigations. Anthropogenic activities, together with run-off from agricultural operations and the presence of algae, are some of the sources of contamination noted in the study. Stricter legislation, stricter enforcement of current standards, matching of non-technical and techno-social remedial actions, and education are among the recommendations made for the Pergau Reservoir’s protection.


Introduction
The reservoirs for power generating plants are typically created in the hinterland and distant upland locations.In many parts of the world, the construction of hydroelectric power plant reservoirs has become the predominant lake type (Lewis, 2000).The upland lake, or headwater lake, collects water from multiple tiny tributary streams, direct surface rainfall, and groundwater intrusion and is sometimes referred to as a reservoir in upland areas.Almost all of these lakes have only one river outflow.Lakes further downstream in river basins have a single major intake and outflow, with the water balance fluctuating depending on other water sources.Despite containing 50.01 percent of the water on the planet's surface, lakes possess 49.8% of the world's liquid surface freshwater (Bhateria & Jain, 2016).
The act of damming and impounding a river causes a significant physical shift in the river's flow (Winton, Calamita, & Wehrli, 2019).Lakes, in general, have inflowing and out-flowing water, making them complex and dependent on three key characteristics: extended water retention time; complex response dynamics; and the integrating nature of the water body (Nakamura, 2007).
The water quality of lakes and reservoirs has attracted the attention of many researchers (Alemayehu & Hackett, 2016;Bertoni, 2011;Bucci, Delgado, & De Oliveira, 2015;Lobato et al., 2015;Nandini, Ramirez Garcia, & Sarma, 2016;Rangel-Peraza, De Anda, González-Farías, & Rode, 2016).Excessive nutrient inputs, eutrophication, acidification, heavy metal contamination, organic pollution, and unpleasant fishing techniques are the most common causes of water quality degradation in reservoirs.The effects of these "imports" into the reservoir have a severe impact not only on the reservoir's socioeconomic functions but also on its structural biodiversity (Mustapha, 2008).
Pergau Reservoir is one of 90 lakes and reservoirs in Malaysia that the Ministry of Natural Resources and Environment (NRE) Malaysia has not inventoried (ASM NAHRIM 2009).Every lake manager is now expected to create their own lake management plan.The goal of this study was to gather and prepare background information on the Pergau lake and reservoir and its catchment area, as well as its lake and reservoir status and environmental impact.As part of the Energy Board, Tenaga Nasional Berhad (TNB)'s lake management plan is required by Malaysia's Ministry of Natural Environment.This paper report is part of a larger study of the Pergau reservoir baseline study and lake brief that includes socio-economic studies, biodiversity and forest ecology of the area.

Study area and sampling stations
The Pergau Hydroelectric Plant Reservoir is located in the State of Kelantan, Malaysia, at latitude 5° 35' to 5° 38'N and longitude 101°38′-101°41′E (Figure 1).The plant was constructed in January 1991 on the Pergau river and completed in 1996.The development involves the construction of a dam at Kuala Yong together with an aquaduct system to divert four side streams (Sg.Suda, Sg.Terang, Sg.Long, and Sg.Renyok) (Figures 2 and 3).
Its drainage area is near the Malaysia-Thailand border, while the western part is near the Perak state border.This watershed is comprised of an upper catchment area of 224 km 2 .
The reservoir area belongs to the tropical monsoon climatic region with an annual mean temperature of 21-30 °C at Batu Melintang.Both the temperature (maximum 35°C) and the humidity are high, about 85%.Annual precipitation is subject to monsoonal variability.It amounts to 3000-3500 mm, with half of it falling between October and January, i.e. during the North East monsoon season (Ismail & Haghroosta, 2018).In the period between 1998 and 2013, the mean monthly maximum precipitation was 658 mm in December and the mean minimum was 142 mm in July (Figure 4).The catchment's topography is quite rough, and it is completely covered in deep rain forest.Its elevation ranges from 1200 to 1500 meters in the south to around 900 meters in the north at the dam site.The catchment's average elevation above the dam site is around 960 meters.The gradient of Sg.Pergau above the dam site decreases from 0.21 for the first three kilometers to 0.029 for the following 11 kilometers, then 0.004 for the final 8.5 kilometers.The Sg. Pergau dips swiftly below the dam site, reaching a gradient of 0.005 at Batu Melintang, which is only approximately 90 meters above sea level (Figure 2).
The typical level for Pergau Reservoir is at elevation (EL) 633-624 m, whereas the minimum operating level is at EL 615 m to cater for the monsoon season.The water level is frequently reduced to EL 624 m.At the Full Supply Level (FSL) of EL 636 m, the volume of Pergau Lake is 54.4 million m 3 .The lake's deepest point is 49 meters.FSL has a lake surface area of 4.63 km 2 .The length of the lake is 4.25 kilometers, the width of the lake is 3.36 kilometers, and the lake shoreline length is 69.58 kilometers with a mean depth of 35.97 meters.Figure 5 depicts the bathymetry of Pergau Lake in 2013 (Energy Board, TNB, pers comm).
Water from the upper Pergau river's tributaries was collected in the Pergau Reservoir before being used to power the underground turbine, 600 meters below the dam (Figure 2).Pergau Reservoir receives water input from eight water sources (Figure 3) that supply water to run the hydroelectric turbine at the main station when needed.The Sg Pergau Inlet is the main river that flows directly into the lake (IP).Five sampling sites were selected within the reservoir (Figure 6).Other sites are river intakes namely Sg.Long, Sg.Renyok, Sg.Suda, and Sg.Terang (Table 1; Figure 2 and 3).

Water balance calculation
The water balance approach is applied to large areas such as watersheds.The difference between inflow and outflow over a relatively short period of time, such as a season, is a measure of evapotranspiration.A general surface water balance model (Bras, 1990) is: where, ΔS lake = Change in lake storage, P = Precipitation (rainfall), Q = Discharge through regulating outlet, E = evaporation, and WS = water supply from seven intakes plus dead storage.

Hydrology and in situ measurement
Water sampling and hydrological monitoring has been carried out twice during rainy and dry seasons in 2013.The main hydrological procedure involved the measurement of river flow and water discharge i.e. the entire volume of water flowing out of a river catchment.For a straight river stretch, water discharge was determined using the cross-sectional area method (Gordon, 1992) and using the tracer dilution approach when river flows were turbulent, especially in upstream rivers (Moore, 2005).

Water quality sampling and laboratory analysis
Water quality sampling involved collecting water samples in triplicates from each station close to the right and left banks and in the middle of the river.Water samples for the reservoir lake were taken using a van Dorn sampler at three depth intervals, i.e. surface (10 cm below the surface), mid-depth, and bottom (0.5 m from bed).Samples were again collected in triplicates.
Water samples were collected in specific bottles according to American Public Health Association, APHA (2005).Samples were stored in sterile glass flasks (bacteriology) and acid-washed plastic bottles (chemistry), cooled, transported to a licensed laboratory, and processed within 6 h of collection.

Water quality classification
Dissolved oxygen (DO), Biological Oxygen Demand (BOD), Chemical Oxygen demand (COD), suspended solids (SS), ammoniacal nitrogen (AN), and pH are the six characteristics used to classify water quality.The calculations are done on the sub-indices of the parameters rather than the parameters themselves.SIDO, SIBOD, SICOD, SIAN, SISS, and SIPH are subindices that are fit to a common scale and combined into a single number according to a chosen method or model of computation (Table 3;DOE (2007)).
The prime objective of the WQI system is to use it as a means of assessing a body of water's compliance with the established criteria for six different types of beneficial uses.The following is the best fit equation for estimating the six sub-index values: Sub-index for DO (in % saturation); SIDO SIDO = 0 for x ≤ 8% = 100 for x ≤ 92 % = −0.395+ 0.030 ×2-0.000020x 3 for 8 % < x < 92% Sub index for BOD; SIBOD SIBOD = 100.4-4.23× for x ≤ 5 = 108 e-0.055x − 0.1× for x > 5 Sub index for COD; SICOD SICOD = −1.33×+ 99.1 for x ≤ 20 = 103 e-0.0157x − 0.04× for x > 20 Sub index for AN; SIAN SIAN = 100.5 -105× for x ≤ 0.3 = 94 e-0.573x − 5 |x-2| for 0.3 < x < 4 = 0for x ≥ 4 Sub index for SS; SISS SISS = 97.5 e-0.00676x +0.05× for x ≤ 100 Q = 71 e-0.0016x − 0.015× for 100 < x < 1000 = 0for x ≥ 1000  Once the respective sub-indices have been calculated, the water Quality Index (WQI) were then calculated according to Department of Environment (DOE, 2007); where Based on many water quality measures, the water quality index delivers a single value that indicates total water quality at a specific location and time.A water quality index based on some key parameters can be used to generate a basic indication of water quality.Water quality indices, in general, combine data from several water quality criteria into a mathematical equation that assigns a numerical value to the health of a body of water (Yogendra & Puttaiah, 2008).The sum of all the weightage for all of the sub-indices must be equal to one.

Hydrology
Pergau Reservoir's river inputs and outputs were measured as river discharge, or Q, which is the volume of water measured at a river cross-section.The stream discharge statistics for the dry (June) and wet (December) seasons were shown in Table 5.In June 2013, the total water input into the reservoir was projected to be 0.496 million m3/day.The river flow varies from as low as 0.22 m 3 /s at Sg.Long 2 to as high as 1.1 m 3 /s at Sg. Terang.The Sg Pergau upstream entering the lake has a dry season flow of 2.02 m 3 /s and a wet season discharge of 4.34 m 3 /s.The tributary inputs range from as lowest as 0.45 m 3 /s at Sg.Long 1 to the highest value of 9.9 m 3 /s at Sg Terang.However, the outlet in Sg.Pergau at Batu Melintang recorded almost the same data as the dry season because the Station Hydroelectric Pergau controls the water release to prevent flooding in the downstream area.
The percentage increase in the water discharge range from almost 100% to 372% in Sg.Renyok; Sg.Long 1 experience a decrease about 4% but Sg.Long 2 experience an increase in discharge about 130%; Sg.Terang and Sg.Suda had the highest increase of discharge by about 800%, and in the main Pergau river input was about 115%, and Sg.Pergau Batu Melintang by 0.6% decrease.

Water balance
Table 6 tabulates the results of the water balance calculation for Pergau Reservoir.The changes in reservoir storage were approximately 13,959.5 mm/year.The reservoir received water inputs in various forms from drainage basins (as a river flows) and precipitation from the atmosphere.The filling time for Pergau Reservoir is 36.36days during normal storage at water level EL 636 m.Pergau Reservoir has a retention time of 9.42 days.The dry period sampling yielded 495 590  m 3 of water input per day, while the wet period sampling yielded 2.26 million m 3 of water input per day.

Water quality
The primary purpose of this lake is to serve as a hydroelectric dam.However, the Pergau Reservoir catchment area within the Gunung Basor Forest Reserve provides good water quality to the reservoir.Tables 7 and 8 summarize the physical-chemical parameter data of Pergau Reservoir's water quality based on input and output (dry and wet seasons).In their natural state, streams, and rivers are often diverse and biologically active habitats.

Temperature
One of the elements in stream ecology that impacts the overall health of aquatic habitats is water temperature (Caissie, 2006).The temperature in upstream rivers varies from 21.1 to 23.0 °C during the dry season and from 20.9 to 24.6 °C during the wet season.In the dry season, the Pergau downstream at Batu Melintang is 26.0 °C, and in the wet season, it is 24.6 °C.In the lake, there was a little temperature stratification.In the dry month, the lake temperature declined from 27.2 to 27.9 °C on the surface to 23.8 to 24.5 °C at the lake's bottom (Table 7).The temperature is lower during the rainy season, with surface temperatures ranging from 22.7 to 24.9 °C and falling to between 21.3 and 21.8 °C at the bottom (Table 8).
The amount of dissolved oxygen in the water can also be affected by temperature.Water at a lower temperature can hold more dissolved oxygen than water at a higher temperature.Because of the major consequences of water temperature for biotic responses, it became obvious that river thermal regimes play a significant role in stream productivity and are thus worth studying and comprehending (Caissie, 2006).
pH.The pH of water determines the solubility (amount that can be dissolved in water) and biological availability (amount that can be used by aquatic life) of chemical constituents.The amount of dissolved carbon dioxide (CO 2 ), which forms carbonic acid in water, determines the pH (Abdullah & Musta, 1999;Hem, 1985).Organic acids from decaying vegetation or the dissolution of sulfide minerals can lower the pH of ground water (Todd, 1980).The pH of most natural waters ranges between 6.0 and 8.5, with lower values occurring in dilute waters with high organic content and higher values always occurring in eutrophic conditions (Chapman, 1992).The pH values from 7.2 to 8.7 are suitable for aquatic organisms Klein (1973) cited by Toufeek and Korium (2009).
In the Pergau Reservoir catchment area, the mean pH value in the river intakes is 7.22, ranging from 6.98 to 7.39 in the dry season.During the wet season, the mean pH was 6.70 and the ranges are from 6.45 to 7.2.The results are within the standard range and are classified under class I based on NWQS for Malaysian rivers (Table 2).The higher pH during dry season because photosynthesis consumes hydrogen molecules, causing the concentration of hydrogen ions to decrease and, as a result, the pH to rise.As a result, pH may be higher during daylight hours and the growing season when photosynthesis is at its peak (Middelboe & Hansen, 2007).pH was reduced as a result of respiration and decomposition processes.pH in a lake, like dissolved oxygen concentrations, can change with depth due to changes in photosynthesis and other chemical reactions.Algal photosynthesis can convert carbon dioxide into organic matter and oxygen, whereas aquatic respiration uses organic matter and produces carbon dioxide (Zhang, Huang, Yan, & Zhang, 2009).
Fortunately, reservoir water is complex; it contains chemical "shock absorbers" that prevent significant pH changes.Because small or localized pH changes are quickly modified by various chemical reactions, little or no change can be measured.The ability to resist changes in pH is referred to as buffering capacity.The buffering capacity not only controls potential localized pH changes, but it also controls the overall range of pH change under natural conditions.Natural water has a pH of between 6.5 and 8.5 (Wilson, 2010).When pollution causes higher productivity (e.g.increased temperature or excess nutrients), pH levels rise, as allowed by the lake's buffering capacity.
The pH of water in an aquatic system is one of the most important water quality parameters because it dramatically influences the bioavailability of some nutrients, metals, and pesticides to plants and animals.The pH of most natural waters falls into the 6 to 9 range due to bicarbonate buffering (Wilson, 2010).Although these minor pH changes are unlikely to have a direct impact on aquatic life, they have a significant impact on the availability and solubility of all chemical forms in the lake and may exacerbate nutrient problems.A change in pH, for example, may increase phosphorus solubility, making it more available for plant growth and resulting in a higher long-term demand for dissolved oxygen.According to the Environmental Protection Agency, a pH of 5-6 or lower is directly toxic to fish (EPA).During the summer months, the pH of a productive or eutrophic lake will typically range between 7.5 and 8.5.pH is lower at the lake's bottom or in less productive lakes, possibly 6.5 to 7.5.This is a broad statement intended to illustrate the types of differences that could be measured.

Dissolved oxygen
The amount of dissolved oxygen (DO) in rivers is one of the most important indicators of their biological health, with significant variations across a wide range of spatial and temporal dimensions.DO solubility decreases with increasing water temperature (Loperfido, Just, & Schnoor, 2009), and thus the changing hydrometeorological conditions (Rajwa-Kuligiewicz, Bialik, & Rowiński, 2015) have the greatest influence on DO solubility.When dissolved oxygen levels in water fall below 5.0 mg/l, aquatic life suffers.
If oxygen levels fall below 1-2 mg/l for several hours, the fish will perish.
Dissolved oxygen concentrations in all rivers tested in this study are fairly constant, ranging from 7.02 to 9.09 mg/l during the dry season and 8.01 to 9.20 mg/l during the wet season.The DO ranges in rivers are tiny in comparison to the ranges in lakes, which range from 1.1 to 10.26 mg/l in the dry season and 6.5 to 9.7 mg/l in the wet season.In the dry season, the disparity between the top and bottom strata is larger than in the rainy season, indicating that the dry season has more stratification.This is to be expected, given that all rivers are shallow and fastflowing, with rapid re-oxygenation to saturation (Nash, White, & Henry, 2003).In addition, these results are within the standard acceptable levels of NWQS for Malaysian river, which is more than 7 mg/ L as well as categorized under class I.In the lake a case of low DO concentration of 1.1 mg/l was observed at the lake bottom render it as class IV.This could be an exceptional case and in wet season generally wet season is class 1.
The concentration of dissolved oxygen decreases from the surface to the bottom for all lake stations, although dissolved oxygen data is normal in all river stations.Temperature and density are two basic qualities of water that have a significant impact on dissolved oxygen levels in a lake.As the temperature rises, the amount of dissolved oxygen drops (Kalff, 2004).

Total dissolved solids (TDS)
The dissolved portion of a water sample is known as total dissolved solids (TDS).The level of TDS in the water provides a general indication of its usefulness as a drinking source as well as for some agricultural and industrial applications.Agricultural land, residential runoff, leaching of soil contamination, and point source pollution discharge from industrial or sewage treatment plants are the main causes of TDS in receiving waters.
Weathering and dissolving of rocks and soils produce some naturally occurring total dissolved solids.Because of the breakdown and weathering of rock and soil, all natural streams include some dissolved materials.TDS is also used to measure the level of pollution in aquatic systems (Jonnalagadda & Mhere, 2001).TDS was measured using a YSI professional handheld meter and is expressed in mg/l (Rahaman, Che Rus, Omar, & Ismail, 2016).
The mean TDS in Pergau Reservoir was 0.016 ± 0.002 mg/l (ranging from 0.014 mg/l to 0.02 mg/l) in the dry season (Table 6), and 0.014 ± 0.001 mg/l (ranging from 0.012 mg/l to 0.17 mg/l) in the rainy season (Table 7).The maximum TDS was recorded at L3 Bottom because TDS increases in values from the surface to the bottom area in the lake (Toufeek & Korium, 2009).The TDS in the wet season was higher than in the dry season, with a value of 0.025 mg/l at Sg. Pergau Batu Melintang, due to the flow of water from the hydroelectric station.The increase in TDS in river water causes an increase in BOD while decreasing DO.Besides, TDS results are within the standard allowable levels of Malaysian rivers (500 mg/l), and are classified as class I determined NWQS (DOE, 2005).

Electrical conductivity (EC)
The geology of the area through which the water travels has the greatest impact on the electrical conductivity (EC) in streams and rivers (Gupta, 2010).According to Chapman (1992), the EC of water is normally in the range of 0.010 mS/cm to 1000 mS/ cm, although sometime EC could surpassed the range if drainage systems is contaminated by surface runoff.
The EC in pristine water ranged from 0.028 to 0.259 mS/cm at different places, with an average value of 0.067 mS/cm (Mokhtar et al., 2009).EC values are a good indication of the relative difference in water ion components between different places, and a higher EC value usually indicates the existence of a higher quantity of dissolved salts in the river water (Abdullah, Ying, Aris, & Park, 2007).
During the dry season, the highest EC was observed at Sg. Pergau upstream and L3 bottom with a value of 0.03 mS/cm, while the lowest data was obtained at Sg. Renyok 2 with a value of 0.016 mS/cm (Table 6).During the rainy season, the highest data was obtained at Sg. Pergau Batu Melintang (0.039 mS/cm) and the lowest at Sg.Long 2 (0.012 mS/cm) (Table 7).This shows that the EC of the intakes and lake is still within Chapman's (1992) range, which, if exceeded, would be considered polluted.In addition, the results are within the standard acceptable levels of National Water Quality Standards, Malaysia (NWQS).The NWQS classification based on EC for the river intakes and lake are of class 1.
Furthermore, because of the substantial amount of dissolved salt, there is a high level of EC and because the high yearly rainfall in this area of 3000 mm/year, the EC during the rainy season is significantly higher.In the dry season, however, high EC indicates water with a high electrolyte concentration due to evaporation.

Total suspended solids (TSS)
The mass (mg) or concentration (mg/l) of inorganic and organic particles held in the water column of a stream, river, lake, or reservoir by turbulence is referred to as suspended solids (SS).Fine particulate particles having a diameter of less than 62 mm are typical of SS (Bilotta & Brazier, 2008;Waters, 1995).
TSS is a critical measure since it influences other variables such as turbidity, TDS, DO, pH, nutrients, and temperature.TSS from surface runoff in the catchment really moves to a river or lake.The flow rate will quickly increase on rainy days.The suspended solid will be carried in the river water, and if the TSS is quite high, it means that soil erosion in the catchments is a severe problem (Shamshad et al., 2006;Thomas, 1985).
Tables 6 and 7 revealed TSS data for the Pergau Reservoir catchment region during our sampling period (dry and wet seasons).During the dry season, the highest reading was 5.20 ± 0.14 mg/l at L1 surface and the lowest reading was 0.8 ± 0.28 mg/l at Sg Terang station.During the wet season, the greatest reading was recorded at L2 bottom with a value of 7.4 ± 4.81 mg/l and the lowest was recorded at Sg. Renyok 3 (1.0 ± 0.47 mg/l).In the Pergau Reservoir catchment region, overall mean concentrations for river intake are 1.70 ± 0.86 mg/l (dry season) and 3.47 ± 1.63 mg/l (wet season), respectively.The average value for the reservoir station is 3.30 ± 0.97 mg/l during the dry season and 3.5 ± 1.75 mg/l during the wet season.Based on the NWQS, the maximum threshold limit of TSS for Malaysian rivers which support aquatic life is 150 mg/L (DOE, 2006;Rosli, Gandaseca, Ismail, & Jailan, 2010).Therefore, the TSS values in this study were within this limit and were categorized as class I and according to WQI (DOE, 2007) the water is considered clean (see also Tables 8 and 9).

Turbidity
Turbidity is a significant water quality factor influencing freshwater fish communities (Judy et al., 1984).High levels of turbidity can be indicative of poor water quality, and can also make it difficult for aquatic organisms to obtain the light and nutrients they need to survive.Turbidity is caused by organic and inorganic particles scattering light in water.However, high turbidity is usually caused by suspended inorganic particles, particularly sediment (Bright, Mager, & Horton, 2018).
Turbidity of 10 NTU or less indicate very clear waters; turbidity of 50 NTU indicate cloudy waters; and turbidity of 100-500 or higher indicate very cloudy to muddy waters.Long-term exposures of 25 NTU or higher may cause stress in some fish species.Barnes, Meyer, and Freeman (1996) recommended that random monthly values in Georgia Piedmont rivers and streams never exceed 100 NTU; that no more than 5% of the samples exceed 50 NTU; and that no more than 20% of the samples exceed 25 NTU in order to maintain native fish populations.The amount of fine particles suspended in water is indicated by turbidity.
The most acute and visible ecological and physical effects of sedimentation occur near river channels as a result of human activities.Storm runoff naturally adds a tremendous amount of fine suspended silt and turbidity to major drainage systems.Human activities that impact turbidity in otherwise clear water habitats include placer mining, timber harvesting, logging, and road construction (Wood & Armitage, 1997).Placer gold mining on New Zealand's West Coast, for example, resulted in a worsening of the water's optical characteristics and the deposition of fine sediment onto and within the riverbed (Davies-Colley, Hickey, Quinn, & Ryan, 1992).The high amounts of suspended particles and related turbidity were blamed for the low densities of benthic flora and macroinvertebrates that resulted (Caruso, 2001;Quinn, Davies-Colley, Hickey, Vickers, & Ryan, 1992).
In this study, during the dry season, turbidity at river intake locations ranged from 1 to 4.5 NTU, whereas it was 1-3 NTU during the wet season, according to this study (Table 6).Turbidity is lower in the wet season than in the dry season.Higher turbidity of 5.5 NTU (wet season) and 8.5 NTU (dry season) was measured downstream of Sg Pergau at Batu Melintang.In the dry season, Pergau Reservoir water has the lowest turbidity (less than 10 NTU) and clearer water in the wet season (less than 7 NTU).In addition, these concentrations were within standard permissible limits of NWQS for Malaysian rivers and categorized as class I for turbidity of 5 NTU.

Biochemical oxygen demand (BOD) and chemical oxygen demand (COD)
The indirect indices of organic matter, biochemical oxygen demand (BOD) and chemical oxygen demand (COD), are indicative criteria for sewage water quality.The amount of oxygen absorbed by microorganisms during the breakdown of organic materials is measured by BOD.The most widely used criterion for calculating the oxygen demand of a municipal or industrial discharge's receiving water is BOD.These organic substances are markers of organic pollution, with a BOD value of less than 5 mg/l in unpolluted natural water (Mustapha, Aris, Juahir, Ramli, & Kura, 2013).Chemical oxygen demand (COD) levels are always higher than BOD levels, although COD measurements can be done in a matter of hours compared to five days for BOD measurements.Tables 6 and 7 illustrate the BOD and COD for the dry and wet seasons, respectively.
In the dry season, BOD levels in the rivers range are not detectable (ND) except at Sg Pergau Batu Melintang with a value of 1 ± 1.41 mg/l, whereas in the rainy season, they are not detectable (ND) in rivers.In the dry season, the BOD content in the lake ranges from 0.5 ± 0.71 mg/l (Class 2) to 3 ± 4.24 mg/l, whereas in the wet season, it ranges from 2 ± 1.41 mg/l to 3 ± 2.12 mg/l (Table 7).As a result of the lower BOD concentration, the river water were within the recommended permissible limit by NWQS and were categorized as class 2 to class 3.
COD concentrations in the rivers range from 7 ± 9.89 mg/l at Sg. Pergau Batu Melintang to 60.5 ± 33.23 mg/l at Sg. Terang during the dry season.During the dry season, COD levels in the lake range from 2.5 ± 3.53 to 21 ± 9.89 mg/l.In the wet season (Table 7), COD levels in rivers range from 4.0 ± 1.41 mg/l to 12.3 ± 0.35 mg/l, while COD levels in lakes range from 1.0 ± 0.71 to 16.5 ± 4.95 mg/l.Additionally, the COD values of surface water were within the recommended permissible limit by NWQS and were categorized as class I but some time becoming and class II in wet season.

Total coliform (TC)
The total coliform (TC) bacteria test is the most basic test for bacterial contamination of a water supply.TC counts provide a general indication of a water supply's sanitary quality (Wright, Gundry, & Conroy, 2004).Bacteria found in the soil, water impacted by surface water, and human or animal waste are all included in the total coliform.It is not pathogenic in individual samples, but it is a good indicator of pathogenic microorganisms.According to Khan et al. (2013), typhoid fever, hepatitis, gastroenteritis, dysentery, and ear infections can all result from drinking water with high levels of total coliform.The unit for the TC counts is known as colonies forming units per 100 ml of sample (CFU/100 ml).
The TC counts in this study was higher in the dry season than in the rainy season, and the TC in lake water samples was lower than that discovered in rivers.The average TC levels in rivers vary from 241.5 ± 200 CFU/100 ml in the wet season to 575 ± 210 CFU/100 ml during the dry season.This is higher than the NWQS limit for class 1 of 100 CFU/100 ml (DOE, 2005) but not high enough to be in Class II.The average TC levels in reservoirs are lower below 171.5 CFU/100 ml (Figure 7).The average TC counts in the reservoir are much lower at 60 ± 20 CFU/100 ml in the dry season, and 20 ± 30 CFU/100 ml in the wet season, indicating that TC is of Class 1 in reservoir lakes.
The presence of wastewater and septic systems, as well as animal waste, run-off, high temperatures, and nutrient-rich water, can all affect the concentration of these bacteria.Animal feces, drainage, and nutrientrich water in the forest could all contribute to the high numbers seen in river waters in this study.Mcdonald, Kay, and Jenkins (1982) suggest that bacteria are stored in the catchment as a land store and a channel or near-channel store.Movement from the land to the channel must be linked to hillside hydrological activities, although movement within the channel fluvial system could be linked to sedimentary processes.

Water quality classification
Water quality in forest areas is better than in other types of land use.The water in the river upstream of the dam site is generally cleaner and of greater quality than the water downstream (including diverted flows from tributary streams).In both the dry (Table 8) and rainy seasons, the water quality index of current water bodies is derived using a range of physicochemical parameters (Table 9).This water quality rating study clearly demonstrates that the body's status in lakes and rivers is of class I (Clean), with just three stations in class II (Slightly Polluted) (L2 Bottom, L3 Middle, and L3 Bottom), and that it is safe for human consumption.All stations in the Pergau Reservoir region were recorded in class I (clean) during the wet season.

Trophic status
Trophic state is an important aquatic ecosystem property because it reflects the anthropogenic influence on water quality and the ecological functioning of rivers, lakes, and reservoirs.Carlson's Trophic State Index (TSI), also known as the Carlson Index (Carlson, 1977), was created to compare Secchi disc (SD) depth, chlorophyll-a concentrations, and TP concentrations, as well as estimate algal biomass separately.Carlson (1977) chose SD depth as the key indicator, despite the fact that chlorophyll-a is the most direct indication of algae biomass.Trophic state indexes reveal how nutrients, light availability, and other factors stimulate algal biomass development (typically measured as chlorophyll a, Chl a) and contribute to the enrichment of aquatic systems (Duka & Cullaj, 2009).
The overall trophic state index (TSI) of a lake is the average of the TSI for phosphorus, the TSI for chlorophyll-a and the TSI for secchi depth; therefore, it can be thought of as the lake's condition taking into account phosphorus, chlorophyll-a and secchi depth.(https://www.rmbel.info/primer/lake-trophic-states/).
The average SD depth transparency throughout the entire study period was 1.53 m, and the TSI (SD) was 53.93, indicating that the reservoir was eutrophic due to sediment and nutrient inputs.Water is the enrichment of nutrients (nitrogen and phosphorus) in an aquatic environment, according to Heisler et al. (2008), and it is one of the most difficult problems in water protection.Organic debris, dirt, suspended particles, and algae obscure the water, and some species may be removed.
Eutrophication of lake water refers to changes in water chemical characteristics caused by the buildup of surplus nutrients like nitrogen and phosphorus.It is the consequence of a sequence of biological, chemical, and physical processes that combine light, heat, and hydrodynamics.Eutrophication of water can result in rapid growth of phytoplankton and other microorganisms, as well as deterioration of water quality, all of which are harmful to aquatic ecology and the proper functioning of water bodies (Vollenweider & Kerekes, 1982).Eutrophication has also been affecting the reservoirs' ecological balance and increasing their environmental vulnerability (Cunha, Calijuri, & Lamparelli, 2013).
The trophic status of Pergau Reservoir was eutrophic with an overall TSI value of 55.17 in the dry period and 51.74 in the wet period (Table 10).Carlson (1977) associates a range of 40-50 with mesotrophic (moderate productivity); index values greater than 50 with eutrophic (high productivity).A TSI of over 50 describes a lake that is eutrophic, with a high density of plants and algae that could be unpleasant for swimming at certain times in the summer.Some lakes may be naturally eutrophic, having a TSI of 50 or greater for the last 100 years.Other lakes have gradually increased in TSI as a result of human activities (https://www.rmbel.info/primer/lake-trophic-states/).Despite the reservoir's potential for self-cleaning, it could be stretched by repeated pollution overloads.Expanding human habitats and thriving socio-economic activity in the catchment areas are credited with this.
Because of a popular belief that nitrogen limitation is more common in tropical systems, the concentrations of various forms of nitrogen and phosphorus in river intakes and lakes were also examined to prove the link between N and P with eutrophication, as shown in Table 11 (Lewis, 2002).Fine sediment transports a portion of the phosphorus (P) that is transported in the freshwater environment (Svendsen, Kronvang, Kristensen, & Graesbøll, 1995).Mobilization and delivery of sediment-associated phosphorus to aquatic environments frequently have negative consequences (Withers & Jarvie (2008).
In many European countries, phosphorus input from point and diffuse sources controls eutrophication of lakes and estuaries (Kristensen et al., 1999).However, several researchers found that nitrogen did not account for most of the variation in chlorophyll in tropical and subtropical water bodies (e.g.Mazumder & Havens, 1998).As a result, there is no clear rule linking nitrogen or phosphorus limitation (or even corestriction) to tropical or subtropical climates.
Our results (Table 12) strongly support the hypothesis that eutrophication is related to the N and P nutrients.TP and TN were both high in the dry and wet months, suggesting and supporting the high eutrophic state of the lake (Table 11).The domestic wastewater discharge surrounding Pergau Lake and its basin is not posing any serious threat to the water quality.Eutrophication in Pergau Lake will be caused solely by decaying biomass in the lake and its catchment area.The concentrations of most N and P nutrients in the lake are higher than the river intakes (Table 12).The mean concentration of phosphate and TP in rivers was 0.011 ± 0.002 mg/l and 5.622 ± 1.03 mg/l, respectively, for the dry period.Taking the natural concentration of background TP of 0.006 mg/l (Smith, Alexander, & Schwarz, 2003), the concentration in the rivers is almost 90 times that in the lake.
In the wet period, the mean TN concentration was 1.26 ± 1.25 mg/l in the lake and 2.33 ± 0.69 mg/l in the rivers.With a TN natural background concentration of 0.02 mg/l (Smith, Alexander, & Schwarz, 2003), the mean concentration in rivers was 10 times greater and about 17 times greater in the lake.
The concentration of N and P nutrient are changing in its domination in the rivers and in the lake storage and changing according to dry and wet to season.This dynamic movement of nutrient with seasonal variation cause the eutrophication process in the system also dynamic.
In general, the environment of Pergau Lake has received little research.Cutting and clearing of tropical forests in watershed areas is a common part of development activities.Pollution mitigation measures that address the need to educate all stakeholders, including perpetrators, those who are harmed, policy and legal authorities, river basin management boards, and other interested parties, and incorporate them in planning and decision-making activities.

Conclusions
The ecosystem of Pergau reservoir and its catchment areas has received little attention.Due to the wide distribution of catchments in the natural tropical forest reserve, Pergau Reservoir has a wealth of water resources.Pergau Reservoir and its inflow are Note: Ammonia 0.3 mg/l; NO 2 0.4 mg/l; NO 3 limit 7 mg/l; TP background of 0.006 mg/l; TN background 0.02 mg/l.
protected by a forest reserve, and the water quality is still in excellent condition.The river flows at river intakes were lower during the dry season compared to the wet season.The percentage increase in water discharge was by as much as 99% at the outlet, and between 200% to 900% increase at the upstream river intakes.The water quality of the river intakes to the Pergau reservoir, the reservoir itself, and, the river outlet at Batu Melintang, varies based on the seasons and location of the sampling stations.
According to the NWQS for Malaysian rivers, all parameters describing the water quality standards namely, temperatures, pH, EC, TDS, TSS, TC, turbidity, and nutrients were all classified as class I, while DO, BOD, and COD were categorized under class II to class III.
The trophic status of Pergau Reservoir was eutrophic with an overall TSI value of 55.17 in the dry period and 51.74 in the wet period.The reservoir was eutrophic was also due to sediment and nutrient inputs.Eutrophication in Pergau Lake is also caused by decaying biomass in the lake and its catchment area.The concentration of N and P nutrients is changing in its domination in the rivers and the lake storage and changing according to dry and wet seasons.This dynamic movement of nutrients with seasonal variation caused the eutrophication process in the system was also dynamic.Interventions in the lake basin should be taken to limit anthropogenic discharges; otherwise, high levels of pollution will have a significant impact on the population.The average TC counts in the reservoir ranged between 50 CFU/100 ml in wet season to about 100 CFU/100 ml in dry season.Thus TC is of Class 1 in the reservoir.These findings should be taken into account when intending to use the lake's water for irrigation, but with some precaution, and it is in need for any form of treatment to be used for domestic purposes.

Figure 2 .
Figure 2. Schematic diagram of the layout of the Pergau Hydroelectric project, showing (a) the longitudinal profiles, (b) the layout of the river intakes and the lake reservoir.Fifty percent of water supply are from the four side streams, while another 50% from Sg. Pergau itself.

Figure 3 .
Figure 3.The river catchments that supply water to the Pergau reservoir.

Figure 6 .
Figure 6.The sampling stations in the Pergau reservoir.

Figure 7 .
Figure 7. Concentration of total coliform counts per 100 ml water sample in the rivers and lake station during dry and wet period.

Table 1 .
Location and elevation of sampling sites for this study.

Table 2 .
Excerpt of the Malaysian National Water Quality Standard (NWQS).

Table 3 .
Department of environment Malaysia water quality classification based on water quality index.

Table 4 .
Department of environment Malaysia water quality classification.

Table 5 .
Water discharge in June 2013 (dry season) and December 2013 (wet period).

Table 6 .
Results of the water balance of Pergau reservoir.

Table 9 .
Water quality index for pergau reservoir and river intakes in dry period.

Table 10 .
Water quality index for Pergau reservoir and river intakes in wet period.

Table 11 .
Trophic status based on indicator for five sites in the pergau reservoir in dry and wet period.

Table 12 .
Descriptive statistics of nitrogen and phosphorus component of river intakes and lakes samples.