Water Quality Assessment of Lake Maninjau After the Mass Fish Kill Event

A mass fish kill is often occurring in Lake Maninjau. A lack of oxygen has reportedly resulted in numerous fish kills, including the recent lake Maninjau condition. Deoxygenation of lake water is a natural phenomenon that often occurs after heavy rain events and low sunlight intensity. Additionally, during the strong wind for a long period, the decrease in oxygen could be chemically triggered by the diffusion of sulfide from the bottom layer of the water column. Furthermore, this study examines water quality parameters after the mass fish mortality occurred across locations in Lake Maninjau. Field surveys were conducted by measuring physiochemical parameters and total sulfide at the Fifteen sampling points (five points in the middle and ten points around the lake) from November 26 to December 2, 2022. They were measured directly using the Horriba©U 52 water quality checker (WQC), DO meter YSI ProDO© International and the HACH spectrophotometer DR3900 method. The results show that the average DO levels at each observation point were close to 0 mg/L while the average sulfide level on the surface was 9.5 micro g/L. The average Oxidation Reduction Potential (ORP) of -79.26 mV indicates that the lake is in a reduced state and signifies that much of the dead and decaying material in the water would be slowly broken down and decomposed. Low ORP values indicate that the compound in the water is available in reduced form, which is more toxic. Our diel DO measurement show that sunlight intensity was effective in increasing oxygen concentration in the lake.


Introduction
Lake Maninjau is one of 15 lakes that are prioritized for management (Perpres No. 60).Recently, the decrease in water quality due to eutrophication appears to be serious.Indications of the eutrophication are a decrease in the water clarity (Secchi Depth) and frequent occurrence of a mass fish kill.This could be due to a much lower oxygen level in the epilimnion zone now (Fukushima et al., 2017) than that of the condition of Lake Maninjau in 1929 (Rutner, 1931).According to previous studies, the input of organic matter overloading from the intensive activity of floating net cages (FNC) has caused the deterioration of the water quality (Henny, 2009;Henny & Nomosatryo, 2012;Syandri et al., 2014;Syandri, 2016;Anggraini, 2019).The enhancement of organic matter in the water column could accelerate the rate of aerobic decomposition of organic matter by consuming a lot of oxygen in the oxic layer of the water column, which subsequently impacts the oxygen deficit in lake surface water (Komala et al., 2019;Santoso & Triwisesa, 2020).Additionally, organic matter triggers the generation of sulfides via sulfate reduction by anaerobic sulfate-reducing bacteria (Holmer & Storkholm, 2001;Sakai et al., 2013;Luek et al., 2017).
Changes in Dissolved Oxygen (DO) concentration in Lake Maninjau occur on a daily and seasonal scale.DO concentrations drop dramatically at night.In contrast, the DO concentration increases during the day when the sun is shining, moving in tandem with the increasing photosynthesis process.(Romaire & Boyd, 1979;Lukman et al., 2014;Ajie & Setiadewi, 2020).The seasonal factors that influence DO fluctuations in the lake included the rainy, cold season, and strong winds event (Araoye, 2009;Mihu-Pintilie et al., 2014;Siriwardana et al., 2019).Increased precipitation and minor sunlight prevent photosynthesis activity by phytoplankton and lower the temperature gradient in the water column (Liu et al., 2020).The water column's ability to stratify is weakened by a decreasing temperature gradient, which triggers the movement of contaminants from the anoxic layer that act as oxygen-reducing agents (Santoso et al., 2018).Furthermore, a period of strong winds can cause mixing to transport various reducing agents including sulfide, up to the oxic layer through mixing.Sulfide that diffuses to the surface will reduce oxygen to form sulfate (Henny, 2009;Couture et al., 2016).
The elevate of sulfide to the surface is a phenomenon that frequently occurs in Lake Maninjau.The local people called it tubo belerang.Tubo is a dialect for noxious while belerang is sulfur.Tubo belerang often occurs during extreme weather, such as prolonged periods of cloudy days and heavy rain accompanied by strong winds ( Henny, 2009;Lukman et al., 2013;Anggraini, 2019).Typically, tubo belerang takes place between the end and the beginning of the year.The tubo belerang occurrence causes a significant increase in fish mortality and deteriorating water quality.
Even though the mass fish kill in this lake has been the subject of numerous studies, none of them specifically described the lake water quality conditions following the incident.Knowing the lake water quality condition after the incident is crucial to understand the root cause of the mass fish kill.Therefore, this study will reveal the water quality conditions specifically after the mass fish kill that occurred on November 2, 2022.The goal of this study is to provide the fundamental knowledge needed for effective lake management.

Study site
Lake Maninjau is located in West Sumatra Province, Indonesia (0°19′S, 100°12′E).This tropical basin was originally formed from a volcanic eruption around 51000 ± 6000 years ago by ejecting volcanic material as much as 220-250 km 3 and spreading as far as 75 km ( Alloway et al., 2004;Pribadi et al., 2007).Lake Maninjau has a surface area of 99.5 km 2 , a volume of 10.1 km 3 , an average depth of 105 m, and a maximum depth of 165 m (Santoso et al., 2018).The height of the Maninjau cliff wall is about 459 m above lake level, with the south side being the steepest part.This lake stretches from north to south with a length of 17 km and a width of about 8 km (Pribadi et al., 2007;Nontji, 2016).Lake Maninjau has been exploited by floating net cages since 1992.These fishery activities had deteriorated the water quality of Lake Maninjau, which is indicated by the frequency of mass fish kills every year and caused enormous economic losses.Until 2017, 11 times fish were killed happened, which increased in 2016 (Makmur et al., 2020).

Water Quality Measurement and Sampling
The survey was conducted in November 2022 at 15 sampling points.The temperature, DO, and conductivity parameters were measured with the CTD Rinko Profiler and DO meter ProDO YSI© International, while pH and Oxidation Reduction Potential (ORP) were measured by the Horriba©U 52 water quality checker.Each parameter is measured in strata to see the variation in the water column.
Sulfide was analyzed by taking a water sample using a syringe to prevent it from oxidizing by oxygen in the air.The water sample was quickly transferred into the screw tube, which had previously been added with Sulfide 1 Reagent.Sulfide 2 Reagent then was added quickly to the tube and measured with a DR3900 spectrophotometer (DR 2800Spectrophotometer, 2007).
The diel pattern of DO was estimated by measuring the water surface every hour at Tan Dirih station using DO meter ProDO YSI© International.To identify the weather condition around Lake Maninjau, we analyzed the meteorological data from Padang Panjang Geophysics station, which can be freely accessed from the Meteorological, Climatological, and Geophysical Agency (BMKG) website (https://dataonline.bmkg.go.id/data_iklim).

The water quality condition
Lake Maninjau's condition after the mass fish kill event is shown in Figure 2. The lake water is whitish green with a foul odor.The stench comes from fish carcasses inside the cage of several FNC or thrown outside the FNC.The DO concentration at the lake's periphery on the first day of the survey was 0.52 mgL -1 , and the temperature was 27.2 o C. The critical DO level indicated that the water quality of Lake Maninjau after the mass kill was still very low.
Figure 3 depicts the profile of the temperature and thermocline depth in the middle of the lake in November 2022 measurements.The vertical structure of the temperature at each station looks the same.This condition is caused because the amount of heat energy absorbed from sunlight by each station is minimal.Based on the BMKG station measurements, the sun duration (SD) when measuring the temperature of the water column is short (SD = 3 hours).In addition, because the lake is narrow and there is no other heat source outside sunlight, the variation in sampling points does not produce a substantial difference in temperature.
The average surface temperature of the lake is 28.07 o C, with the temperature difference between the bottom and surface of 1.16 o C. The temperature difference in this period is smaller than the measurement in February 2017, which is 3 o C (Santoso et al., 2018).Furthermore, the water column did not have a thermocline layer, except at station DM5 with a thermocline layer depth of 0.46 m.The disappearing thermocline layer accompanied by a homogeneous water column indicates that the convective process occurred in Lake Maninjau that mixes each layer of the water column.The mixing process can be identified not only by the vertical temperature of the water column, but also by the vertical structure of homogenous conductivity and DO (Fukushima et al., 2021).Mixing in the water column was indicated by the DO profile in the water column (Figure 4).When the water column is not stratified, the thermocline layer as a barrier to the mixing process between the epilimnion and hypolimnion layers disappears, causing pollutants and reducing oxygen to easily rise from the anoxic hypolimnion to the oxic epilimnion.This condition causes the oxygen level in the epilimnion to decrease.If this occurs continuously, the oxygen level in the epilimnion layer will balance with the hypolimnion layer to form a homogeneous layer.
A straight-line profile with DO level close to zero from the surface to the bottom is also due to the increase in rainfall.Local people have observed constant rain since the beginning of November, which inhibited the photosynthesis process.On the other hand, respiration by autotrophs and heterotrophs (the decomposition of organic and inorganic matter) continued to increase, causing DO concentrations to decrease drastically (Santoso & Triwisesa, 2020).
The effect of photosynthesis and respiration on DO dynamics in Lake Maninjau is demonstrated by surface DO measurements for 24 hours.Figure 4. B shows that at 8:00, the DO content began to increase with the appearance of sunlight.DO surface reached its peak at 18.00.Along with the loss of sunlight, the process of photosynthesis stopped, as indicated by the decreased DO content, and reached a critical condition at 05:00.Based on Figure 4. B, it can be seen that although that sunlight intensity was effective in increasing oxygen concentration in the lake, the DO concentration shown was still below the standard quality set for fisheries, (4 mg L -1 ) (Andria & Rahmaningsih, 2018;Paputungan et al., 2022).This condition occurs because the lake needs time to purify itself (Komala et al., 2019).Because of the low DO levels, the fish carcasses monitored were dispersed around Lake Maninjau.Biogeochemical processes in waters often involve oxidation-reduction processes (Wetzel, 2001).The state of the ongoing reduction or oxidation process in a body of water can be tested by measuring the ORP.By measuring the ORP, it can be seen the ability of water to recover itself from the pollutants that enter it.A high ORP value shows much available oxygen, so bacteria effectively break down organic

B)
matter.Conversely, if the ORP value is low, it indicates low oxygen levels, and the water is in the reduction process.
Figure 5 shows that the water column at almost all sampling points had a negative ORP, except at surface and 2 m depth in DM3.A negative ORP indicates that a reduction process of various ions and molecules has occurred in the water column (Sulastri et al., 2016). Wetzel. (2001) stated that an ORP of less than 100 mV, the sulfate-to-sulfide reduction would occur in eutrophic lakes.As a result, a negative ORP (Figure 5. A) indicated that Lake Maninjau was in a reduced state, including sulfate reduction to sulfide.
ORP value of the water column can be used as a proxy in identifying the response of the water column to seasonal conditions.The ORP of the water column measured during the dry season will differ from the rainy season.Figure 5 B shows the ORP measured during the transition from rainy to dry seasons.The ORP remains positive to a depth of 10 m at each sampling point, indicating that oxygen is still available at that depth (Sulastri et al., 2016).This condition can be caused by the intensity of the sun starting to increase, thereby increasing the photosynthetic process.In addition, the water column stratification has formed forms of thermocline layers that act as a barrier to the mixing process.

Spatial distribution of sulfide in lake surface November 2022
The weakening of thermal stratification and the oxic layer of the water column triggers the transport process from the hypolimnion to the epilimnion.Sulfides that accumulates in the hypolimnion layer (Henny & Nomosatryo, 2016) could quickly diffuse to the surface and become toxic to fish in the FNC.We found that dissolved sulfides were detected over the entire surface of the lake at an average of 9.5 µgL -1 (Figure 6).The sulfide concentration in the lake exceeded the class III water quality standard set at 2.0 µgL -1 (Piranti et al., 2018).Based on Figure 5, the lake's periphery (points 1, 7, and 8) had a higher sulfide concentration than in the center.With low sunlight intensity and high organic matter of dead fishes, we estimate the recovery of the lake will take a long time.

The weather patterns in 2022 related to changes in the water quality of Lake Maninjau
Biogeochemical processes in the waters are strongly influenced by physical factors, such as the stratification of the water column and the mixing process between the epilimnion and hypolimnion (Katsev et al., 2010).An unstratified water column will trigger the exchange of several molecules, ions, or compounds between the epilimnion and hypolimnion (Henny, 2009).The strength of the stratification and mixing of the lake water column is greatly influenced by the weather conditions above (Yang et al., 2018;Kunz & Wildman, 2019).Therefore, the DO decrease and the rise of sulfide to the surface during this observation period are strongly influenced by weather variabilities, such as rainfall and air temperature.
Figure 7 show the daily rainfall and maximum air temperature during 2022.The rainfall has increased since June and reached its maximum in November (average rainfall is 21.34 mm/day) (Figure 7. A).This follows the report of Fakhrudin et al. (2002), which states that rain in Lake Maninjau reaches its maximum in November.The rainfall increase since May corresponds to the air temperature decrease from May until the end of the year (Figure 7. B).Rain is a weather parameter that significantly influences heat distribution in the lake water column, such as lowering the lake's surface temperature (Liu et al., 2020).Therefore, decreasing the lake's surface temperature will reduce the temperature gradient between the surface and the bottom, resulting in the loss of lake stratification.In addition, a decrease in air temperature on the surface layer will trigger a flux of heat energy from the surface to the air, resulting in the mixing and deepening of the surface layers (Bouffard & Wüest, 2019).Fukushima et al. (2021) reported that large convection in Lake Maninjau corresponds to a decreased air temperature.Therefore, rising sulfides to the surface and decreasing DO in Lake Maninjau are closely related to significant weather changes.
Changes in lake ecosystem parameters due to changes in meteorological parameters depend on the magnitude of each parameter.Due to the short research time and limited data on physicochemical parameters, in this study, we did not quantify in detail how long it took for ecosystem parameters to change as a result of their response to climatological parameters.However, for a rough estimate, November 2022's heaviest rainfall occurred on the 14th (75 mm), and local media reported that on the 17th, there was a mass fish kill (Nugroho, 2022).The mass fish kill in that period was probably preceded by a decrease in oxygen levels due to inhibition of the photosynthetic process or sulfides rising from below reducing oxygen on the surface.Therefore, we hypothesize that in these conditions with extreme weather, ecosystem parameters can respond immediately to climatological parameters.

Conclusions
A few days after the mass fish kill, the water column temperature appears to be homogeneous with a very low temperature gradient of 1.16 ℃.In addition, the water quality of Lake Maninjau was still poor, as indicated by deficient DO levels on the surface.Although the DO increased when sunlight reached the lake's surface, DO levels were still too low to provide the aquatic biota demand.High levels of detectable sulfide (an average 9.5 µgL -1 ) on the surface also indicated poor water quality.The decrease in DO and the presence of sulfide on the lake surface water is an implication of increased rainfall and decreased air temperature, which are directly related to primary production and the regulation of water column stratification patterns.

Figure 2 .
Figure2.The water quality condition of Lake Maninjau after the mass fish kill event.Some fish carcasses are still inside and outside FNC.The water in the screw tube was taken from the surface.The green color indicates water containing sulfide

Figure 4 .
Figure 4. A) DO vertical profile of water column and B) Diel of pattern DO at the surface water

Figure 6 .
Figure 6.Distribution of sulfides on the surface November 2022.