Spatial and temporal distribution of dissolved oxygen in the Ciliwung River, DKI Jakarta Province

The Ciliwung River is one of the main streams that pass Jakarta. This river flows through the city’s center and passes many villages, highly inhabited homes, and slums, necessitating special attention to the Ciliwung River’s water quality. Because dissolved oxygen is essential for aquatic life, it is the most critical measure of freshwater quality. This study aimed to analyze the spatial and temporal distribution of dissolved oxygen in the Ciliwung River in DKI Jakarta Province. Water quality was monitored during the rainy (February and June 2021) and the dry seasons (August and September 2021). The study area comprised 18 observation sites along the Ciliwung River in DKI Jakarta. Spatially, in Central and North Jakarta, the dissolved oxygen content of the Ciliwung River is relatively low. Temporarily, the dissolved oxygen concentration was somewhat more significant during the rainy season, but statistical testing revealed no statistically significant difference between the rainy and dry seasons.


Introduction
A total of 13 river systems flow through the DKI Jakarta area, most of which originate in West Java and flow into Jakarta Bay.Consequently, the river in DKI Jakarta is the final discharge point for these wastes.Water quality in the DKI Jakarta region is already a major problem.This is based on the Regional Environmental Control Service (BPLHD) of DKI Jakarta, which tests 13 rivers in the Jakarta region and demonstrates that both river water and groundwater contain significant amounts of inorganic and organic contaminants.Thus, the river water no longer fulfils the quality criteria for its designation, namely for raw materials for drinking water, fisheries, agriculture, and other urban enterprises (quality standard class I).The Ciliwung River constitutes one of the busiest rivers that run through Jakarta.It passes through the city's core and through several towns and densely populated residential areas, so its water quality needs special attention.This river is also considered the most severely polluted by domestic waste compared to other rivers in Jakarta [1].
The Ciliwung watershed occupies 347 km 2 and stretches from Mount Gede Pangrango (West Java) to Jakarta Bay (DKI Jakarta) [2].Bogor Regency, Bogor City, Depok City, and Jakarta City are the six governmental divisions of the Ciliwung watershed [3].Increased human activity degrades water quality by introducing a variety of pollutants, including residential, industrial, agricultural, and livestock waste [4].Because the Ciliwung River is a free ecosystem, it receives pollution from various sources, including 1260 (2023) 012017 IOP Publishing doi:10.1088/1755-1315/1260/1/012017 2 domestic, agricultural, livestock, and industrial waste [5].The river pollutant load is determined by the spatial pattern or land use along the river [6].The Ciliwung River is also in bad shape because of rapid expansion, which creates garbage, especially in DKI Jakarta [7].Alteration in land utilization from agricultural to urban areas also impacts water quality.These factors contribute to considerable impairment of the Ciliwung River's water quality [8,9].
Several researchers have already conducted operations to monitor the quality and contamination of the Ciliwung River.According to Rahardjo and Widayat [10], the Ciliwung River is weakly to significantly contaminated, with various water quality metrics surpassing quality standards, including detergent, phosphate, and organic matter.Furthermore, Aprilia et al. [11] discovered that the Ciliwung River (DKI Jakarta segment) is in moderate to highly polluted, with parameters total dissolved solids (TDS), total suspended solids (TSS), dissolved oxygen (DO), biochemical oxygen demand (BOD5), chemical oxygen demand (COD), ammonia (NH3), nitrite (NO2), total nitrogen (TN), total coliform (TC), and fecal coliform (FC) not complying with the class II river quality standard.According to Sujati et al. [12], the pollution level in the Ciliwung River upstream (Bogor Regency) was categorized as moderate.Pollution levels alter due to changing water conditions at any time or season.
The type and status of water may be examined using a range of parameters (physical, chemical, and biological) [13].To maintain water supply, river water quality is essential to environmental management and sustainable development.River water quality can be managed by maintaining water functions to achieve the quality requirements assigned to them [14].Dissolved oxygen (DO) is critical in measuring freshwater water quality because it is essential for aquatic life [15].Changes in dissolved oxygen concentration in freshwater can impact biological and chemical activities in the body of water [16].This study aimed to analyze dissolved oxygen's spatial and temporal distribution in the Ciliwung River, DKI Jakarta Province.River dissolved oxygen monitoring is becoming mandatory to maintain freshwater resources.

Time, place, and water sampling analysis
Water samples were obtained at each observation point during the rainy (February and June 2021) and dry seasons (August and September 2021).The research site was in the Ciliwung River, DKI Jakarta.Water samples were collected from 18 observation points.The study area and observation stations are depicted in Figure 1, and the coordinates of the observation stations are presented in Table 1.The site was chosen following the regulations of SNI 6989.57:2008concerning Surface Water Sampling Methods, which refers to broad criteria for site selection, such as natural water sources, contaminated water sources, utilized water sources, or the place of water entrance into reservoirs/lakes/seas.DO concentrations were measured immediately in situ using a DO meter by SNI 6989.57:2008.

Spatial and temporal distribution of dissolved oxygen
The Inverse Distance Weighting (IDW) method was applied using ArcMap 10.8 software to investigate dissolved oxygen's spatial distribution mapping in the Ciliwung River.This deterministic interpolation method seeks to anticipate or estimate an unsampled location value based on data from nearby points [17].The dissolved oxygen's temporal distribution was compared to the class II river water quality standards in Attachment VI to Government Regulation of the Republic of Indonesia Number 22 of 2021 (water designated for water recreation infrastructure/facilities, freshwater fish farming, animal husbandry, irrigation, or other water uses requiring the same water quality).This is due to Government Regulation of the Republic of Indonesia Number 22 of 2021 Article 527 letter f, which states that if the Government or Regional Government has not established a standard for the most recent quality in surface water bodies, then class II water quality standards must be used.
Seasonal effects were investigated using one-way ANOVA (Analysis of Variance) with Minitab 20 software.A one-way ANOVA was employed for one dependent variable to compare the mean between two or more groups.The one-way ANOVA test requires at least one independent variable (season in this case) and one dependent variable (dissolved oxygen).It is presumed to be regularly distributed if at least 30 data points exist.As a result, the study met the requirements because there are 36 data points from both seasons.The minimum of 30 data points is intended to decrease the likelihood of a mistake in failing to reject the null hypothesis (failure to show a statistically significant difference) [18].Station plots are given per river subnetwork to identify dissolved oxygen's broad temporal distribution pattern.The dissolved oxygen concentration fluctuates over both seasons, and most do not meet the quality level (Figure 4).Based on one-way ANOVA to examine the effect of season, the dissolved oxygen concentration was not substantially different in both seasons (Table 2).The average dissolved oxygen concentration is 3.53 mg/L during the rainy season and 3.27 mg/L during the dry season.

Discussion
Dissolved oxygen (DO) is essential for the survival of various biological life forms in water.The system's oxygen balance largely determines the impact of waste disposal in an aquatic environment.Salinity, temperature, air pressure, and water turbulence all influence the quantity of oxygen in natural water bodies.The colder the water is, the more oxygen it can transport.At high altitudes, DO is more significant than at low levels.The more organic contaminants contaminate the water, the more oxygen is consumed to break down these molecules, reducing the water's dissolved oxygen level [19].Variations in dissolved oxygen spatial distribution are assumed to be directly connected to anthropogenic activities along the river.According to Hasibuan et al. [20], human actions near rivers can impact water quality.Agriculture, plantations, domestic trash, traditional markets, rubbish leaching, and industry along the Ciliwung River are all sources of anthropogenic activity.The degradation of the Ciliwung River's water quality was caused mainly by the introduction of biodegradable organic sewage into the waterways [20].Organic matter may be found in nature and industrial and home activities.Household wastewater is divided into two categories: washing water (including detergent, soap, oil, and grease) and latrine water (such as urine and feces).Inorganic elements, floating and dissolved organic components, and microbes are all in wastewater [21].The pollutant load is introduced through groundwater runoff (non-point source pollution) and sewage lines in residential and commercial properties (point source pollution).

Rainy Season Dry Season
This idea was embraced by Yudo [22], which discovered various small and home-scale enterprises that contaminate the Ciliwung River, including tofu and tempeh manufacture, batik dyeing, printing, and automotive and motorcycle repair operations.The kind of pollution source is strongly connected to the temporal distribution of dissolved oxygen in DKI Jakarta's Ciliwung River.Dust, oil, agricultural fertilizers, animal feces, silt, and other items can pollute rainwater that falls to the ground.Rainfall as run-off is the primary non-point source pollution that affects river water quality in metropolitan areas.
When the intensity of rainfall is excellent, materials delivered to river water bodies become more plentiful, increasing the oxygen required to break down these materials [6].
The presence of organic materials can be assessed using the BOD5 and COD measurements.The entire quantity of oxygen required to degrade biodegradable organic matter is called the Biochemical Oxygen Demand (BOD5).In contrast, the total amount of oxygen needed by microorganisms (decomposing bacteria) in water to degrade biodegradable and non-biodegradable organic materials is called Chemical Oxygen Demand (COD) [13].Figures 5 and 6 show the BOD5 and COD concentrations during the study.The high levels of BOD5 and COD in the Ciliwung River suggest that organic waste is the primary contaminant.Organic waste in rivers is caused by anthropogenic activity in the river's vicinity.Since decomposing bacteria utilize oxygen to digest organic waste, high BOD5 and COD concentrations result in low dissolved oxygen [23].
The dissolved oxygen concentration decreases downstream of the river, with the lowest values at Stations 4 and 5 in Central and North Jakarta, respectively.This is assumed to be due to household and industrial garbage being discharged into the rivers.These findings are consistent with the previous study [24], which found that DO concentrations decreased water quality from headwaters to downwards.Field observations show several densely inhabited communities on the river's banks, particularly at Station 1, followed by massive structures and industrial operations farther downstream, particularly at Stations 4 and 5. Domestic waste is waste generated by urban populations' activities.Domestic waste generally consists of liquid waste from home operations (such as washing) and solid waste in faeces [21].Moreover, small-scale tofu, tempeh production, batik dyeing, and other textile enterprises along the Ciliwung River contribute to industrial waste [25].The surrounding land use has a strong influence on water quality.Thus, it is impossible to conclude that the water quality downstream could be better than upstream [26].The average dissolved oxygen concentration was slightly more significant during the rainy than during the dry season, but statistical testing revealed no statistically significant difference.According to Zhang et al. [27], the dissolved oxygen content decreases during the dry season as the temperature rises.In the rainy season, the river receives more water flow, bringing more dissolved oxygen into the river.Due to the higher water flow, there is more aeration or agitation of the water, so oxygen from the atmosphere can mix with the water more efficiently.Thus, the river's DO will usually be higher in the rainy season.However, in the dry season, the river receives less water flow.Due to low water flow, lack of aeration and water agitation, and the process of photosynthesis by aquatic plants, which increases oxygen levels during the day, DO in rivers can be lower during the dry season.This can cause water quality to decline and harm the aquatic organisms that live in it.In rivers, dissolved oxygen regulates various biogeochemical processes and biological populations.Aquatic organisms are sensitive to changes in dissolved oxygen levels in water bodies, mainly when dissolved oxygen levels fall.A considerable drop in concentration can result in fish mortality and changes in population composition and trophic status [28].The river's total dissolved oxygen content is balanced by surface water reaeration, primary production (from photosynthesis), and oxygen consumption for organic matter breakdown [29].
Changes in dissolved oxygen content are also affected by the nitrification process.Nitrification is the aerobic breakdown of ammonia (which requires oxygen).This ammonia transformation, known as nitrification, is a crucial stage in the nitrogen cycle.The Nitrosomonas group converts ammonia to nitrite in the first step.The Nitrobacter group, which converts nitrite to nitrate, has a function in the second step of nitrite oxidation [30].If the nitrifying bacteria grow correctly, the nitrification process will proceed smoothly.The ideal temperature range for nitrifying bacteria development is 25-35 o C, pH 7.5-8.6,and dissolved oxygen content >1 mg/L [31].
According to field studies, the suckermouth catfish (Hypostomus plecostomus), an invasive fish from South America and a demersal fish [32], regularly rises to the surface to acquire oxygen.This indicates that oxygen at the bottom of the water is insufficient to meet the fish's respiration demands.Station 4-3 had the lowest average DO content of 1.55 mg/L in this investigation (Penjaringan, North Jakarta).Yet, in February (rainy season), the DO concentration at Station 4-4 (Penjaringan, North Jakarta) reached 0.95 mg/L.Boomfish can survive in low oxygen environments (even as low as 1 mg/L), implying that the DO concentration in the Ciliwung River has exceeded this limit at multiple measurement stations.Low oxygen levels in water can induce stress in fish due to a lack of oxygen delivery to their brain and mortality from oxygen deprivation (hypoxia) [33].

Rainy Season Dry Season
These fish are known as suckermouth catfish because they eat the leftovers of feed, algae, moss, and the remains of dead biota found at the bottom of bodies of water, including sewage.Suckermouth catfish is a source of sustenance for most people in the Ciliwung Watershed [34].In general, suckermouth catfish can live in poor-quality water and become the dominating fish in various environments.Two respiratory organs corroborate this, the fish's gills and labyrinth.The gills' primary organs are inhaled in clear water, whereas the labyrinth is used by organisms living in mud or dirty water [32].The findings of studies [35] conducted around the Cirata Reservoir's floating net cages revealed similar effects, notably low DO at the bottom owing to the breakdown of organic matter, mainly feed leftovers, which use oxygen in the waters.
Most of the station's features are occupied by temporary and semi-persistent residents.Residences across the river may harm the physical state of the terrain, river water quality, and riparian scenic features.Riparian zones are the ecological links between the aquatic and terrestrial components of the environment.Riparian landscapes that function effectively may filter subsurface and surface water flow, prevent riverbank deterioration, filter lousy air quality, and provide refuge and active paths for animals to navigate increasingly diverse urban environments [36].According to Yohannes et al. [37], uneven settlements frequently require appropriate sanitation.Thus, they are identified by their irregular structure, meaning they are not connected to Wastewater Treatment Plant (WWTP) services.As a result, management practices are required to restrict the entry of nutrients and organic matter into the Ciliwung River, DKI Jakarta.

Conclusion
Spatially, the dissolved oxygen content of the Ciliwung River is relatively low in Central Jakarta and North Jakarta.Temporally, the dissolved oxygen concentration was slightly higher during the rainy season.However, based on statistical tests, there was no significant difference between the dissolved oxygen concentration in the rainy and dry seasons.
3 show the dissolved oxygen distributions from 18 observation stations during the rainy and dry seasons.The dissolved oxygen concentration values presented for each sampling station were the mean values.The maximum dissolved oxygen concentrations were reported at Station 1-1 (Cimanggis, Depok) in the rainy season and at Station 1-2 (Jagakarsa, South Jakarta) in the dry season (Figure3).Stations 4 and 5 in Central and North Jakarta had the lowest concentrations across the two seasons.Dissolved oxygen concentrations ranged from 1.25-6.72mg/L (rainy season) and 1.55-5.39mg/L (dry season).

Figure 2 .
Figure 2. Spatial distribution of DO during the rainy season.

Figure 3 .
Figure 3. Spatial distribution of DO during the dry season.

Figure 4 .
Figure 4. Temporal distribution of DO.

Table 1 .
Description of the coordinates, subnetwork, and address of each station.

Table 2 .
Statistical test of the effect of season on dissolved oxygen.