Study of water degradation due to community activities in The Lake Batur ecosystem, Bangli District, Bali

. Lake Batur is a aquatic ecosystem that has a role for domestic and economic needs of the people in Kintamani District, Bangli Regency even widely used within the Bali Province. This lake is one of the national priority that must be immediately saved due to degradation and sedimentation based on the stipulation in the Indonesian Presidential Regulation. This research aims to determine the level of water quality of Lake Batur, identify community activities and its correlation with water degradation, and formulate strategies for controlling water ecosystem. Primary data for this research are 11 different water sample points determined based on land use around the lake. The water sample datas then are tested in the laboratory to observe physical, chemical and biological parameters, then determine water quality index using NSF-WQI method and pollution index. Secondary data is interviews with communities around. Respondents were selected using purposive sampling classified based on the livelihoods of local residents. Based on calculations using the NSF-WQI method, Lake Batur water has a score of 68.97 or classified as moderate status. The water quality of Lake Batur calculated using polution index method has a score of 8.56, which is classified in the moderately polluted. Community activities are very dependent on the existence of lakes and are very close to water bodies. Waste generated from various activities has a risk of water degradation and sedimentation. Waste and sedimentation can occur due to the use of chemicals. Communities should switch for using organic materials such as compost for farmers and natural fish food for fish cultivators, as well as being orderly in implementing existing policies. The government needs to provide physical and non-physical facilities to support for reducing the risk of. Investors are expected to follow policies and apply ecotourism principles.


Introduction
Lake is one of the ecosystems that has a topography in the form of a basin filled with water and surrounded by land.The lake is part of an ecosystem that has economic, ecological, historical, cultural, and close relationships with the lives of Indonesian people.In general, lakes have three kinds of functions, namely ecological, social, and economic functions.It is said to be an ecological function because lakes are habitats for organisms, have the ability to control groundwater balance, and control microclimates.The social function of the lake is as a provider of community needs for daily needs, while the economic function of the lake is as a source of water that supports the work of the surrounding community such as agricultural irrigation, aquaculture and fishing, and local and international tourism activities [1].
The existence of Lake Batur has a crucial role as a provider of the main water source for the surrounding community.The lake with a rainwater catchment area of 105.35 km 2 has an important role in the fisheries, agriculture, plantations, tourism, and religious activities sectors [2].As a fairly extensive aquatic ecosystem, this lake has a very rapid development for various community activities.This lake water is also used for various activities such as raw water for several regions in Bali, lake boundaries for agriculture, capture fisheries and aquaculture fisheries, tourism, germplasm sources, where the life cycle of important types of flora and fauna takes place, natural reservoirs, storage of excess water derived from rainwater, surface flow, maintaining microclimates where the existence of lake ecosystems can affect humidity and rainfall levels, and educational facilities [3].Moreover, this lake is a tourism area and one of the sites that has been designated as world heritage by UNESCO so that Batur Global Geopark was formed.
Although it has a crucial role, Lake Batur has significant environmental problems.This lake is known to have been degraded due to water pollution.The increase in pollution is thought to be caused by the habit of people who throw domestic waste into the waters.The waste is in the form of liquid waste and solid waste which then puts pressure on the lake aquatic ecosystem [4].The condition of Lake Batur has decreased in quality accompanied by other problems due to community activities [3].Moreover, according to data from the Ministry of Environment and Forestry of the Republic of Indonesia, sedimentation occurred as much as 124.71 tons m3 based on research for 37 years (1957 -2012), so Lake Batur is estimated to experience silting as high as 7.80 meters with a sedimentation rate of 0.21 meters per year.
The First Lake Indonesia National Conference on 13 -15 August 2009 resulted in the 2009 Bali Agreement on sustainable lake management in anticipation of global climate change.The Lake National Conference produced several ideas, including the forerunner of the discovery of national priority lakes that need to be saved from the threat of environmental degradation.This idea then resulted in Presidential Regulation of the Republic of Indonesia Number 60 of 2021 concerning National Priority Lake Rescue.There are 15 lakes that receive priority, namely: Lake Toba, Lake Maninjau, Lake Singkarak, Lake Kerinci, Lake Tondano, Lake Limboto, Lake Rawa Pening, Lake Tempe, Lake Matano, Lake Mahakam, Lake Sentarum, Lake Sentani, Rawa Dano, and Lake Batur [5].Based on the Presidential Regulation of the Republic of Indonesia Number 60 of 2021 concerning National Priority Lake Rescue, the determination of national priority lakes is carried out based on several criteria such as: experiencing pressure and degradation in the form of damage to lake catchment areas, damage to lake boundaries, damage to lake water bodies, reduction in lake reservoir volume, reduction in lake area, increase in sedimentation, decrease in water quality, and decrease in biodiversity resulting in ecological, economic, and socio-cultural issues for society.
Monitoring water quality in aquatic ecosystems such as lakes is important because it is an effort to identify lake conditions [6].Monitoring the condition of the lake or the status of water quality in the lake water body is determined by physical, chemical, and biological parameters that interact with each other, then the monitoring results can be used as a reference to determine the most suitable use for the lake specifically [6].There are several previous studies that have monitored water quality in Lake Batur.Research conducted by Handayani et al. (2011) on the water quality of Lake Batur, the results identified several points that have conditions that are not in accordance with quality standards with the level of water pollution, which is lightly polluted [7].According to research by Laili et al. (2020), the fertility rate of Lake Batur is at a low or oligotrophic level [2].Changes in water conditions are very likely to occur due to various factors as mentioned above, so water quality monitoring must be carried out continuously.This study was conducted to assess the level of damage to the waters of Lake Batur in terms of physical, chemical, and biological parameters; identify community activities that have the potential to damage the environment to the aquatic ecosystem of Lake Batur; and formulate strategies to control environmental degradation in the aquatic ecosystem of Lake Batur.

Material and method
The monthly water samples collection was conducted at eleven stations in Lake Batur.The location or research area includes a volcanic lake area located at an altitude of 1,080 m with an area of 1,607 ha [8].The lake in question is Lake Batur located in Bangli Regency, Bali Province.The sample station is determined based on the character of the location or land use around the sampling point.The eleven stations represent the character of land use, namely tourist boat docks, non-cultivated vegetation, jungles, floating net cages, shrubs, settlements, farmland, hot springs, farmland, floating net cages, and the middle of the lake (Figure 1).

Fig. 1. Map of sampling station in Lake Batur
The level of pollution to aquatic ecosystems was analyzed using the National Sunitation Foundation's Water Quality Index (NSF-WQI) and Pollution Index methods to determine the water quality index.Both methods of calculating the water quality index are able to answer questions about the quality of aquatic ecosystems and can simplify complex water quality data consisting of various parameters into a single number so that it is easy to understand [9].The use of Water Quality Index in general can be done in three steps, namely: 1) determination of parameters; 2) determination of water quality functions for each parameter, and 3) aggregation using mathematical equations [10].
The equation for determining the National Sanitation Fund Water Quality Index (NSF-WQI) is stated: Where:  is the weight of the parameter and  is the sub-index value of each parameter, while n is the sum of the water quality parameters.Sub-indices are used to transform various water quality values into the same score value with a range of 0 -100 [9].The sub-index value () of each parameter is obtained by accessing the http://www.waterresearch.net/watrqualindex/index.htm site [11].The water quality index is determined based on class divisions that have been adjusted to the calculated score with mathematical equations on the parameters used in the National Sanitation Fund as in Table 1 [12].The equation for calculating water quality index with the pollutant index method is as follows [13]: with information:  is a pollution index,  is the concentration of parameter ,  is the quality standard of parameter  for the designation .
the use of minimum/maximum  on the divisor is based on the value of , if  is smaller than  a then the minimum  is chosen as the divisor, but if  is greater than  a then the maximum  is chosen.
Unlike NSF-WQI, the results of the pollution index are calculated based on numbers 0-10 with the division of four categories, namely good or meeting quality standards, lightly polluted, moderately polluted, and heavily polluted.The division of water quality classes based on the results of the calculation of the pollution index is listed in Table 2 [15].Secondary data collection was carried out in Kintamani Village, one of the villages directly adjacent to Lake Batur.The determination of location is based on the condition of the community in the area which has the most complete job classification compared to other areas.According to data from the Kitamani District Government, the population (N) of Kedisan sub-district is 1,384 people.The number of respondents to be interviewed is determined using the Slovin equation to determine the number of samples in a population.The error tolerance limit (d) used is 20% so that the interview results have a confidence level of 80%.The equation for determining the number of respondents is as follows: = 24,94 25 respondents (6) Mixed analysis methods are used to determine the relationship between damage to aquatic ecosystems and the influence of community activities.The method used to analyze the relationship between water quality and community activities is explanatory sequential mixed methods which is a method used to integrate quantitative and qualitative data [16].This method is used by collecting quantitative data then qualitative data is collected to strengthen the results of quantitative data analysis.This method tends to be used in research that has a stronger quantitative data orientation [16].Then, environmental control strategy is identified using a collaborative PAOC and SWOT method.

Result and discussion
Water quality index (WQI) in this study was calculated using the National Sunitation Foundation's Water Quality Index (NSF-WQI) method and Pollution Index.The results of these measurements can be used to provide a specific picture of water quality conditions in aquatic ecosystems.Measurement with the NSF-WQI method is more comprehensive because this method is elaborated with the Delphy method in determining parameters, scale development, and weight assessment [9].The water quality of Lake Batur based on the calculation results of the NSF-WQI method obtained results such as Table 3.Based on the calculation results in Table 3, it is known that Lake Batur has a water quality with a value of 68.97 on a scale of 0 -100.The value was obtained after calculating nine parameters according to the National Sanitation Foundation Water Quality Index method.The calculation of Lake Batur's water quality value, which is 68.97, is in the range of 50-69 or class C, so that Lake Batur has moderate water quality.This class can be interpreted as the water quality of Lake Batur is not in a pollution-free condition so it is not good for consumption, but also not bad or dangerous in general.Water quality that is in the medium category range is more common in algae growth and has little diversity of biota or aquatic life [9].
The calculation of water quality by the pollution index method was carried out against eleven parameters.Calculations through this method allow researchers to compile parameters according to research needs.The measurement of Lake Batur water quality index using the polluteion index is adjusted to class I water quality standards in Government Regulation No. 22 of 2021 concerning the Implementation of Environmental Protection and Management.These calculations are listed in Table 4.The results of observations and laboratory tests on quality parameters are compared with lake water quality standards according to Indonesian government regulations.Lake water quality standards are classified into four classes according to their designation, namely for raw water, recreational facilities, fish farming, and irrigation water or irrigation of rice fields.Testing the water quality index of Lake Batur with the pollution index method is calculated by taking into account class I water quality standards.Class I water quality standards have standards that tend to be stricter than other classes because they are intended for human consumption.The variety of parameters set includes physical, chemical, and biological.The choice of class I is because Lake Batur is an ecosystem used as a source of drinking water [17] so that in this discussion the classification used is the class I lake water quality standard.Based on the data in Table 4, the calculation of the Lake Batur water quality index using the pollution index method produces a value of 8.56.This figure is classified as moderately polluted because it is in the range of 5 ≤ Pij ≤ 10.The calculation of Lake Batur water quality index using NSF-WQI and IP methods results in different statuses.Testing with the NSF-WQI method states that the water quality of Lake Batur is in moderate status, while calculations with the pollution index method state that Lake Batur water is moderately polluted.This difference can occur due to differences in parameters that are used as a basis for calculation.The NSF-WQI calculation method uses limited parameters because it has been set [12], this can result in measurements being less flexible to research objectives.The calculation of nine parameters in the NSF-WQI method is used to express that a body of water is more affected by organic matter.Differences in parameters also have an impact on the value or number that determines the calculation result [18].For example, in this study there are two additional parameters in the IP method, namely COD and salinity which are not contained in the NSF-WQI calculation, where the COD value based on laboratory test results on average passes the established class I quality standards.This can push the IP water quality index value higher, which means the worse the status classification.NSF-WQI has shortcomings because the parameters used are not comprehensive and sub-index curves can be subjective [15].
Each calculation method has its own advantages and disadvantages.The NSF-WQI method has the advantage of having equations that tend to be easy and do not require too many parameters, while this method has the disadvantage that it can only describe one moment of water quality and tends to be subjective [12].The polluter index or IP method has the advantage of being able to assess off-stream pollution, while the disadvantage of this method is that measurements are not carried out sustainably [15].Based on the calculation results, it is also known that the IP method has advantages in terms of flexibility in parameter selection so that it can be adjusted to the research objectives.However, it is uncertain which measurement method is most valid or best, but different water quality indices provide options for determining overall lake water quality and are suitable for research purposes [6].
Community activities in the Batur area produce discharges that affect the water quality of Lake Batur.Various waste products or waste enter water bodies and can result in changes in the lake ecosystem.Various human activities contribute waste to reservoir or lake waters, either directly or indirectly [18].Some materials with chemical content are used to support the economic activities of the community.All respondents who work as farmers use fertilizers to fertilize plants and use pesticides to overcome plant pests.Fish farmers feed fish using pellets every day twice.Only one respondent used vitamins for fish endurance.The boat as a means of crossing to go to Trunyan Village uses a mixture of fuel oil and oil as fuel.While hotel and restaurant managers use water and fish to support their business.The materials used to support community business activities are shown in Table 5.The data in Table 5 contains the use of materials used by the people of Batur Area to increase business productivity.Respondents who work as farmers amounted to 9 people with the use of fertilizer, which is 137.8 per day.Thus, the average use of fertilizer for each farmer can be obtained which is 15.31 kg every day.Fish farmers in Lake Batur feed daily using fish pellets.From nine respondents, the number of fish pellets needed to feed is 208 kg, so that the average value of fish pellet use by each farmer can be obtained which is 23.1 kg every day.Boat entrepreneurs need fuel in the form of a mixture of fuel oil and oil in order to run the crossing from the pier located in Kedisan Village to tourist attractions in Trunyan Village.Of the five respondents as boat captains, a total of 70 liters of fuel consumption is required, so it can be assumed that each boat captain needs 14 liters of fuel per trip.Activities that do not pay attention to environmental conservation such as agricultural businesses and fisheries that are not environmentally friendly are very prone to cause sedimentation or siltation [19].
Data on every parameters then compared with class I quality standards in accordance with the government regulation of the Republic of Indonesia number 22 of 2021 concerning the Implementation of Environmental Protection and Management.Class I was chosen because of the use of lake water as raw water for the community.Data on parameters that are not in accordance with quality standards and are suspected to be affected by community activities are shown in Table 6.Based on the data above, it is known that agricultural land has an adverse impact on COD, pH, phosphate, and water surface temperature parameters.High concentrations of phosphate in waters can be caused by runoff of agricultural materials such as chemical fertilizers.The main content of chemical fertilizers is nitrate and phosphate [4].Phosphate residues on farmland by the lake affect lake water when it overflows or when it rains.The phosphate content in chemical fertilizers also triggers an increase in pH value.According to research conducted by Zhao et al. in 2022, higher phosphate concentrations in water will be followed by higher pH values [20].High COD values at sample points adjacent to agricultural land are due to high organic matter dissolved in water.Organic matter in the form of humus from humification results is collected which then results in more oxygen needed to decompose the organic matter [21].
Floating net cages are thought to cause high COD, pH, and phosphate values and may have an effect on lake sedimentation.The large number of floating net cages results in the large number of fish feeds that are poured every day.Fish feed in the form of pellets used by farmers has the main content in the form of nitrogen and phosphorus [22].Fish feed that is not absorbed by fish becomes a burden on aquatic waste which results in high phosphate concentrations.In addition, unabsorbed fish feed waste causes lake sedimentation as shown in Figure 4.27.Each feeding, the average amount absorbed by fish is 30% while the rest becomes a polluting burden https://doi.org/10.1051/e3sconf/202346803008ICST UGM 2023 [23].The high pH value is triggered by the high concentration of phosphate [20].Fish pellets containing phosphates and nitrates also have an impact on the high value of COD.There is a positive relationship between nitrate and phosphate concentrations and COD values [24].
Sampling at locations around settlements provides BOD, COD, pH, phosphate, and water surface temperature values that are not in accordance with class I quality standards.Domestic waste as a result of human presence will affect the level of acidity or pH, both towards being alkaline and acidic [25].The high value of phosphate is caused by domestic waste generated by household activities such as washing clothes.This is because detergents contain phosphate ions [26].Phosphate contamination in waters is often caused by detergent compounds [27].In addition, domestic waste in the form of human waste can increase the phosphate content in water [28].There are about 30 mg of phosphate in every 1 liter of human feces [29].High COD values that exceed class I quality standards can be caused by detergent contaminants from residential areas.Detergent liquid waste contains 25 complex compounds that adversely affect the environment, one of which is COD in waters [30].
In the dock area, several parameters like BOD, COD, phosphate, and surface temperature values exceeding class I quality standards.The higher the tourist boat traffic, the greater the risk of oil or oil spills in the waters.Oil and oil contain chemicals that are difficult to decompose and can kill biota.The more pollutants in the waters result in more oxygen needed to oxidize the organic matter.This triggered an increase in the value of BOD and COD.The phosphate value of the dock area exceeds the quality standard due to runoff originating from agricultural land around the site.Runoff carrying phosphate nutrient elements occurs due to the influence of direction, speed and surface current patterns that affect the distribution of phosphate [31].The high surface temperature of the water is caused by the measurement of the sample during the day.The surface temperature of the water somewhere has a strong correlation with solar activity [32].
The existence of hot spring attractions has an influence on the temperature of the water surface which increases sharply and the value of phosphate and COD that is not in accordance with quality standards.The higher surface temperature of the lake around the hot spring location is the impact of the proximity of the object to the lake so that the flow of water from the attraction enters the lake body.The high concentration of phosphate is influenced by human waste that can come from these attractions.According to research conducted by Aprianto et al. in 2022, human feces can increase phosphate levels in water [28].Human waste is one of the sources of phosphate content in waters [26].The high value of COD is influenced by the large distribution of organic matter in waters, the more organic matter, the more oxygen is needed to decompose the organic matter chemically [33].Large concentrations of organic matter can come from tourist attractions, because domestic and industrial waste contains high organic matter [21].
Other land cover such as forest forests, shrubs, and non-cultivated vegetation with varying slopes have an effect on pH, COD, phosphate, and surface temperature values.Surface temperature values in some areas show low values, this can be caused by sampling on the edge of the cliff, so that water bodies are blocked by cliffs and not exposed to sunlight.High phosphate values can be affected by sampling sites adjacent to diverse vegetation, making them prone to contamination of rotting litter.The litter rots there are bacteria that produce phosphate [34].
In addition to the consequences caused by human activities, the pH value can depend on natural factors such as the condition of CO2 gas in water, the concentration of carbonate and bicarbonate salts and the process of decomposition of organic matter on the bottom of the water [35].The high value of COD is caused by soil runoff that is on the shore of the lake.The soil contains 3-5% organic matter [36].These organic materials cause an increase in COD values because more oxygen is needed to oxidize the particles.Phosphate, COD and pH values that pass quality standards also occur in the measurement results of water samples from the middle of the lake due to the influence of wind and strong currents at the time of sampling [31].
Controlling strategies can be formulated with the POAC method (planning, organizing, actuating, controlling).The POAC method represents management in an effort to achieve the main objectives.In addition, the SWOT (Strength, Weakness, Opportunity, Threat) method can be used to map the strengths and weaknesses of ecosystem conditions as well as external opportunities and threats [37].The combination of POAC and SWOT methods allows for comprehensive strategy development from pre-implementation to postimplementation.
Planning or planning is the determination of goals and setting ways to achieve these goals [38].The goal setting process is carried out by policy makers so that it is expected to produce public policies that are beneficial to all parties [39].The determination of goals and plans is carried out SMART by considering various factors such as Specific (clear), Measurable (can be measured success rate), Achievable (can be achieved), Realistic (in accordance with existing capabilities and resources), Time (there is a clear time limit).Based on observations and interviews, Lake Batur control can be aimed at maintaining and improving water quality, overcoming lake sedimentation problems, and spatial planning and regulations in the lake area.
Organizing is the adjustment or grouping of available resources according to predetermined criteria [40].Organizing aims to be able to implement plans and support the achievement of goals.Policymakers can map the availability of human resources and consider other stakeholders to collaborate.Stakeholders can be divided into the government, private sector, and community.The cooperation between the three pillars is commonly referred to as collaborative governance [41].
The process in achieving goals can be done with the distribution of tasks and responsibilities so as to create collaboration between stakeholders is expected to encourage a more precise and directed process.
Actuating is the implementation of plans to achieve goals or implementation carried out to achieve goals [38].The preparation of plans to achieve predetermined goals can be combined with the SWOT method through consideration of several factors such as Strength, Weakness, Opportunity, Threat.Lake Batur currently has the following conditions::

Strengths:
• Lake Batur is one of the national priority lakes • The Batur area is designated as a cultural heritage site (UNESCO Batur Global Geopark) • Some of the surrounding people have the knowledge and desire to protect the lake Weakness: • Water quality is in poor status • Sedimentation and reduced lake capacity • There is no entry and exit for water, so the lake water will overflow inland during the rainy season • There are many buildings in the lake border area • Waste from community activities that may become pollutants Opportunity: • Status as a national priority lake, Lake Batur receives direct attention and support from the central government • Lake Batur has no input source (inlet) so that polluting sources in the form of runoff of organic matter or waste from other areas can be suppressed • The Batur area has abundant resource potential and is one of the economic centers for the surrounding population • Strategic area that becomes one of the magnets for tourists • Improving community welfare • The potential of people's cooperatives has not operated well Threat : • Tourists who disobey the rules and litter • Tourists' habit who are less concerned about the environment • Various materials or materials left over from community activities enter the lake so that it can affect the characteristics of the lake • Community employment land can be threatened if there is a policy regarding the revitalization of regional functions • On some shores of the lake surrounded by cliffs prone to landslides Based on various factors of conditions in the Lake Batur aquatic ecosystem area, by considering strengths, weaknesses, challenges, and threats, the proposed environmental control strategy of the Lake Batur aquatic ecosystem that can be done is as listed in Table 7.After planning, organizing, and actuating, the next thing that needs to be done after strategy implementation is controlling or supervision.Controlling is the supervision of the process that is being carried out so that things that must be achieved can be determined.In addition, controlling is defined as the process of determining the suitability of the implementation process being carried out so that an evaluation can be carried out if needed [38].In addition to post-program observations, assistance also needs to be carried out if the program involved community elements.Assistance is carried out to assist stakeholders in achieving sustainable goals.

Conclusion
According to the water quality index calculation using the NSF-WQI method, Lake Batur has a score of 68.97 or is classified as moderate.Lake Batur's water quality index was measured using pollution index with moderately polluted status with a score of 8.56.The temperature, pH, COD and BOD parameters show values higher than the quality standards at several observation points, while the phosphate concentration at all sampling points exceeds the quality standards.It might be caused by people that use lake water for various purposes, both domestic and economic including agricultural, fishing and tourism activities.These activities use chemicals such as fertilizer, fish pellets and fuel, so that the waste material from these various activities directly comes into contact with the water of Lake Batur and has an impact on environmental changes in the form of decreasing water quality and sedimentation.Lake Batur needs to be managed by involving synergy between stakeholders such as the community, government and investors.Substitution of chemicals is the use of organic materials such as compost and natural feed.The government needs to provide facilities so that the public has a clear reference in implementing policies.Meanwhile, investors are expected to comply with development regulations in lake border area.This research was supported by RTA Program Universitas Gadjah Mada with the Grant Number 5075/UN1.P.II/Dit-Lit/PT.01.01/2023.The authors would like to express their gratitude to the reviewers for their thorough reading of the manuscript and constructive suggestion

Table 2 .
Water Quality Index with Pollution Index Classification Pij ≥ 10 Heavily polluted Source: Hoya et al., 2022

Table 3 .
Lake Batur Result of Water Quality Index using NSF-WQI

Table 4 .
Lake Batur Water Quality Index using Pollution Index

Table 5 .
Material Used by Communities

Table 6 .
Land Use and Parameters That Exceed Quality Standards

Table 7
SWOT Analysis Redesignation of lake boundary line 3. Ecoenzymes to neutralize water quality 4. Dredging sediment or picking up garbage on the bottom of the lake 5. Preparation of the concept of green economy for the people of the Batur area so that activities can run well and sustainably 1. Provision of fish feed filter nets in floating net cages or the use of smart floating net cages 2. Use of organic fertilizers 3. Substitution or mixing fish feed with organic matter 4. Spatial arrangement in Lake Batur area 5.The use of soil nailing on slopes by the lake Source: Data analysis (2023) E3S Web of Conferences 468, 03008 (2023) https://doi.org/10.1051/e3sconf/202346803008ICST UGM 2023