The trophic status and estimation of fish potential production in Batur Lake, Indonesia: a preliminary study

. Lake Batur is a type of volcanic lake in Bangli Regency, Bali, Indonesia. The main problems in Lake Batur are no outlet, organic matter pollution from accumulated waste from the human population, cages, and agriculture. This study intends to investigate and evaluate the existing trophic status, the condition of the water, and fish production potential in Batur Lake. Observations were conducted at six stations in March 2023 using random purposive sampling. The status of water was calculated using the Trophic State Index (TSI), and fish yield was estimated using the Morphoedaphic Index (MEI). The findings showed that the TSI measurements of Batur Lake ranged from 54.07 to 75.25. In this condition, the lake is categorized as eutrophic. The annual fish production potential ranges from 109.9 to 210.5 kg/ha, with an average of 120.4 kg/ha. With 1600 hectares, the lake can produce 109-130 tons of fish. Based on captured fisheries statistic data in 2022, the potential value of fish production was relatively lower than the fishery production, which amounted to 576 tons. To suggest sustainable fisheries in Batur Lake, we need to maintain the existing fishing regulations and keep stabilizing the water quality.


Introduction
Lake Batur is currently tending to change, especially its quality, due to the impact of community activities around the lake.Eutrophication is a global environmental concern that poses a challenge to ensuring the sustainability of freshwater lakes [1].Lake ecosystems and human health can be impacted by eutrophication [2].The lake has been utilized in various sectors (fisheries, tourism, and agriculture) that have developed previously [3].The aquaculture sector in the Lake Batur area generally uses the floating net cage system, the waste of which directly affects the aquatic ecosystem.In general, the problem in Lake Batur is a closed water system with no outlet, which significantly affects the quality of its waters.In addition, pressure will grow with the increase in community activities around the lake, such as activities in agriculture, tourism, fish farming, and others.
The impact of some of these activities is the accumulation of organic matter, nitrates, and phosphates, which tends to cause high trophic conditions [4,5].Based on the past morpho-edaphic index approach, the fish potential production on Lake Batur was estimated at an average of 159 tons per year [6].There were 12 species of fish and shrimp caught in Lake Batur, which are dominated by tilapia and red devil.Lake Batur has a vital role as the main producer of freshwater fish to fulfill the demands of the Bangli Regency community, especially the people living around the lake [6].To determine the TRIX index productivity classification, it could be calculated by some methods, such as the Secchi depth and the model for the nitrogenphosphorus ratio [7,8,9,10].However, Carlson's trophic state index is the most used approach for estimating it [11].The estimation of the fish potential yield of a water body could predict the morpho-edaphic index, chlorophyll a, and shoreline development [12].As a result, the chlorophyll data in TSI might be used for evaluations of future fish production [13].Environmental and biological data need to be studied as a basis for fisheries development and management.Therefore, research on environmental aspects and estimation of it in Lake Batur needs to be conducted for fisheries sustainability.Environmental aspects can indicate the biological productivity of waters, and data on potential fish production indicate the number of exploitable fish stocks in Lake Batur.

Methods
The research was carried out in 2023 in Lake Batur, Bangli Regency, Bali (Figure 1).The study includes a field survey as well as laboratory analysis.Six research stations were identified and chosen on purpose (Table 1).The water quality measurement methods and equipment tools used in the studies can be seen in Table 2.The TSI of the waters was calculated using the formula [14] with the following sequence: TSI = (TSI-SD+TSI-TP+TSI-Chl)/3 Lake productivity is divided into three groups based on TSI values: oligotrophic (low productivity), mesotrophic (mid-productivity), and eutrophic (high productivity).According to production levels, Carlson's water state situation is divided into four categories: very low, low, medium, and high (Table 3).

Fig. 1. Sampling site in Batur Lake
The fish potential yield with the MEI approach was calculated using this formula [15], i.e.Y = 14.314 * MEI 0.4681 Where: Y = fish production potential value (kg/ha/year) MEI = Morphoedaphic Index = conductivity concentration divided by the mean depth (m).

-80 Heavy eutrophic
There is a significant algal bloom, similar to hypereutrophic environments, aquatic plants layer.

Results and discussion
The trophic status index (TSI) was derived using the amount of chlorophyll-a, total phosphate, and transparency as measured by the Secchi disc.[14] argues that these three factors are highly correlated, with each doubling of algal biomass resulting in a halving of transparency.Chlorophyll-a is the primary indicator of trophic status in tropical lakes.Furthermore, when phosphorus is considered the limiting factor for algae growth, total P maintains a reciprocal relationship with transparency.The measurements of chlorophyll-a, transparency, and total-P in Batur Lake are presented in Table 4. Table 5 shows that stations 2 and 3 have higher chlorophyll-a concentrations than the other sites.As a result, the two sites have the highest primary productivity.Total P concentration follows a similar pattern with larger values seen in sites 2 and 3.Both sides were located in village areas; site 3 had the highest chlorophyll-a and total-P concentrations in agricultural areas.This condition is due to the intensive fish farming and agriculture activities in these areas.[16] stated that the phosphorus concentration in the lake's Batur Lake was caused by fish farming and the use of fertilizers containing phosphates at the lake's borders.Meanwhile, Batur Lake's transparency remains optimal for phytoplankton development because it is below 45 cm [17].Fortunately, the highest indication of phosphorus was only at station 3, indicating that phosphorus levels in the water did not have much effect on transparency and chlorophyll-a.As a result of the relatively high oxygen levels and relatively consistent alkaline pH, the water quality of Lake Batur did not suffer a drastic decline.Nonetheless, the water quality of Lake Batur allows for the proliferation of aquatic creatures.It is imperative to assess the carrying capacity of the floating net cages and the regulation of the boundary lake's use.
The estimations of the potential value of fish production in Lake Batur using MEI (Morphoedhaphic Index) were 77.47-113.09kg/ha/year.The area of Lake Batur is 1605 hectares.So, the potential fish production figures range from up to 77.47 x 1605 = 109,470 kg/ha (109 tons/year) and 113.09 x 1605 = 130,193 kg/ha (130 tons/year) with an average figure of 95.75 x 1605 = 120,880 (120 tons/year) (Table 6).When compared to this value with the MEI approach in Lake Batur before [6], which is 99.15 kg/ha/year, this condition shows that there has been a decrease in potential fish production.2).Fig. 2. Fish production in Batur Lake [6] stated that the average fish production in Lake Batur was 175.1 tons, while the average fish production from 2004-2022 was 486.8 tons [18].It shows that fish catches are increasing.The results showed that estimating potential fish production using the MEI approach was much lower than fish production from fisheries statistics.
Finally, this approach can measure the potential of the lake to produce fish.But, the results show that the fish production is much higher than the production capacity of the water.This is due to the dominant fish species in Batur Lake, such as tilapia, a fast-growing fish that can spawn throughout the year.However, it should be recognized that the management of capture fisheries must still be aware of aquatic habitats, both in terms of fish nurseries and maintaining existing fishing methods.
The average water temperature was 23.68 0 C, with a range of 22.9-24.1 0 C, which was an ideal zone for fish life (Table 7).The life of aquatic species, especially fish, can be assisted by a consistent temperature.[19] Said the existence of fish was not endangered by small variations in water temperature (less than 2.80 °C).Fish can adapt their metabolism and respiration to changes in temperature; as a result, fish need a temperature range of 20°C to 30°C to survive.[20,21].According to this study, Batur Lake's water temperature was still ideal for fisheries and suitable for fish habitat.Batur Lake still had a high oxygen content, with average dissolved oxygen levels of 6.1 mg/L that ranged from 5.4 mg/L to 7.4 mg/L.[22] said that in order to live properly, the dissolved oxygen concentration must be at least 4 mg/L.Therefore, the dissolved oxygen concentration was relatively good and supported the lives of aquatic organisms such as fish.
Total dissolved solids (TDS) range from 940 mg/L to 960 mg/L (Table 7).The transparency value is influenced by weather conditions, measurement time, turbidity, and suspended solids.The relationship between changes in the total suspended solids, conductivity, and transparency concentration is very fulfilling.As a consequence, minerals dissolved in sediment are suspended above the water.The rock erosion is caused by water flow, natural processes, and human activity [23].During flooding, organic and inorganic matter, including suspended and dissolved organic and inorganic matter, as well as the quantity of plankton and other microbes, all have a significant impact on transparency [24].
The water is alkaline, with a pH ranging from 8.83 to 8.99 and an average value of 8.91 (Table 7), indicating the presence of some minerals that can increase the pH.Water with a pH concentration of 5.0-9.0 is needed to support fish life naturally [25].These standards showed that the water quality at Batur Lake was suitable for supporting fish life.When algae are actively growing and removing carbon dioxide, internal phosphorus loading can also increase water pH [26,27].
The total suspended solids (TSS) value is the biophysical status of inland waters, which illustrates the environmental health due to degrading impacts from water bodies, such as sedimentation by rock erosion and landscape changes upstream [28].The value concentration ranged from 5 mg/L to 36 mg/L, showing that the water was good, and for station 3, where the value was higher than the others.Total phosphor material and its free form (orthophosphate) describe the essential nutrients for the growth of living things, especially for aquatic biota.
The average nitrate concentration was 0.034 mg/L, with a range of 0.005-0.008mg/L (Table 7).The waters of Batur Lake were categorized as having moderate fertility levels based on the water fertility criterion, with an average nitrate level of 0.034 mg/L [29,22].
The overall TSI of the six sampling sites ranges from 54.266 to 75.256.It means that the trophic state in Batur Lake is categorized as light eutrophic to heavy eutrophic, and generally, the water was categorized as light eutrophic.The fact that the lake water was highly nutrient-rich also means that the water has become polluted due to rising nitrogen and phosphorus levels.The problem of phosphorus in Batur Lake is important and a concern for relevant stakeholders and the government.Hence, keeping phosphorus input at a safe level is a must to prevent eutrophication in Batur Lake.The trophic level in some volcanic lakes in Indonesia (Kerinci, Limboto, Tondano, and Maninjau lakes) was eutrophic [30,31,32,33].For that, we need to monitor and manage it for the organism's sustainability in the future.
The potential fish production of Batur Lake had been decreasing if compared with the latest.We assume that variations in water quality dynamics cause this, changes in electrical conductivity values and fluctuations in water depth due to sedimentation.In addition, it is influenced by changes in the conductivity value due to the activity of Mount Batur, which is still active.So, it is predicted that it can change depending on the activity of the mountain that produces minerals for the lake.The conductivity value in Lake Batur had significantly decreased when compared with the research before [6].This indicates that there has been a decrease in dissolved materials due to abiotic habitat changes caused by land use changes and minerals produced by active volcanoes.The water quality change will impact the growth and survival of aquatic organisms, which can lead to a decrease in their population [34,35].Finally, fisheries sustainability in Lake Batur is still needed to maintain and make regulations for stabilizing the water quality In Batur Lake.

Conclusion
The status trophic of Batur Lake categories as eutrophic with TSI ranged from 54.07 to 75.25.The average of potential fish production in Lake Batur is 95.17 kg/ha/year, and a lake area of 1600 hectares can produce 120 tons of fish.Lake Batur's potential fish yield has decreased from the previous year, but catch fish production continues to increase.The fish production in Lake Batur continues to increase because of the restocking program and the dominant species of fast-growing fish.

Table 1 .
Name, Coordinate, and description of stations

Table 2 .
Water quality parameters methods

Table 3 .
Categories in Carlson's Trophic State Index

Table 4 .
The variable measurement used for assessing TSI condition In Batur Lake

Table 5 .
TSI value in Batur Lake

Table 6 .
Fish potential yield in Batur Lake

Table 7 .
Water quality in Batur Lake