Distribution of microplastics in Lusi Island, Sidoarjo, Indonesia

Lusi Island, situated downstream of the Brantas River, is anticipated to harbour significant microplastics from Brantas River runoff. Consequently, a thorough analysis of the microplastic presence on Lusi Island is crucial for effective pollution mitigation. Despite the critical need for such investigations, more studies must report the abundance of microplastics in this locale. This study aims to fill this gap by examining the abundance of microplastics in the downstream area of the Brantas River, specifically Lusi Island. The research not only delves into the prevalence of microplastics but also investigates various water quality parameters, including dissolved oxygen, pH, temperature, turbidity, and total dissolved oxygen. The study reveals that the abundance of microplastics in Lusi Island ranges from 597 to 787 particles per litre in surface water samples. Three distinct types of microplastics—fragments, fibres, and film—were identified on the island. Notably, microplastic fragments dominate in number compared to other forms. Despite the favourable water quality parameters suggesting a healthy aquatic environment, the study underscores the necessity of implementing mitigation measures to address microplastic pollution effectively.


Introduction
Plastic is one of the materials that humans widely use in daily activities, both personal and commercial.Plastic production experienced a rapid increase in 1950 and the amount of plastic used increased from 279 Mton (2011) to 390 Mton (2021) [1,2].Humans have an important role in increasing the use of microplastics, which, without realising it, has many negative effects in the long term [3,4,5].Plastic waste produced by various human activities is generally thrown into the aquatic environment [6,7,8].The more human activities that use plastic, the higher the amount of plastic waste thrown into the aquatic environment.
Plastic waste that is thrown into the aquatic environment is found in fresh waters (rivers) and brackish waters (estuaries) and ends up in the sea [9].Plastic is the primary type of waste found in Indonesian waters [10,11].Around 95% of the total waste accumulated along the coastline, surface, and seabed is plastic waste.Plastic waste is found in almost all aquatic environments in various parts of the world [12,13].Plastic waste is found in various sizes (microscopic and macroscopic) and is spread in various benthic and pelagic habitats.This plastic waste is even found in remote locations such as the Arctic, southern, and deep seas [14,15,16].
The plastic decomposition process takes a very long time.It takes hundreds of years for plastic to degrade into microplastics and nanoplastics through various physical, chemical, and biological processes.Microplastics are particles smaller than 5mm in size [17,18].Because the size of microplastics is tiny, it is easy to find microplastics in the aquatic environment.Microplastics are found in various colours, compositions, shapes, and types [12,13].
The size of plastic waste is one of the things that affects the life of biota in the marine environment [19].Large plastic waste, such as nets, often entangles marine life.Small plastic waste, such as bottle caps and lighters, can be ingested by organisms, causing intestinal blockages and potential chemical poisoning.On the other hand, microplastics can be ingested by microorganisms in the marine environment and cause various serious problems.
Poor awareness of the impacts caused by microplastic contamination in water areas only appears when it is already at a chronic level [20].Various forms of research are carried out to determine the spread of microplastics in the aquatic environment.One of them is knowing the distribution of microplastics in freshwater (rivers), brackish waters (estuaries), and the sea.
Sidoarjo Mud Island (Lusi) is an example of brackish waters where many microplastics are found [21,22,23].The island was formed due to hot mud deposits in Porong, Sidoarjo, thrown into the river, and formed in the estuary.In previous research, microplastics have been studied in the upstream part of the Brantas River, which represents freshwater areas [20].Therefore, it is crucial to conduct research observing microplastics' presence in the downstream part of the Brantas River, which represents brackish waters.This research aims to determine the distribution of microplastics on Lusi Island.

Sampling area
This research was conducted at 3 sampling points on Lusi Island.All the sampling locations are downstream of the Brantas River.Water samples were taken in June 2023 using a purposive sampling method, which assumes that the sampling points taken will represent the overall conditions of Lusi Island.

Characteristic of Microplastics
Microplastic characteristics were analysed using a modification of the Masura method [24].A 0.3 L of water was taken from each sampling point and filtered using a plankton net.All filtered materials, including microplastics, are isolated for the subsequent analysis stage.Then filtered again using a multilevel sieve with mesh sizes of 50, 18, and 4.Then, the results were washed using distilled water.A total of 20 mL of 30% hydrogen peroxide and 20 ml of Fe 2+ 0.05M were added to the beaker glass.This addition aimed to digest any materials present in the filtered water, excluding microplastics.The Fe solution was meticulously prepared by diluting 7.5 g of FeSO4•7H2O to 500 mL of water and 3 mL of concentrated sulfuric acid.This addition aims to process filtered materials but does not include microplastics.The solution is heated using a 90°C hotplate; after cooling, it is filtered using a vacuum pump with Whatman paper no.42.Then, the retained microplastics are oven-dried at 50°C for 1 hour.The characteristics of microplastics were observed using a microscope (Olympus BX-50, Japan).
3 Microplastic abundance was calculated by calculating the ratio between the microplastics obtained and the volume of water samples collected, expressed in particles per litre (particles/L).

Analysis Water Quality Parameters
The water quality parameters analysed in this research are temperature, pH, DO, conductivity, turbidity, TSS, and BOD.Water temperature and dissolved oxygen were measured using a digital DO metre integrated with a thermometer ((Do-Meter 5509, Lutron, USA).pH was measured using a digital pH metre (pH Meter 201, Lutron, USA).Turbidity was measured using a pH metre (WQC-224, TOA DKK, Japan).

Water Quality Parameters
The water quality parameters at Lusi Island were analysed in this study (Table 1).The parameters measured are the main environmental factors affecting the presence of microplastics in Lusi Island (i.e., dissolved oxygen, pH, temperature, turbidity, and total dissolved oxygen).The results of this study indicate that the temperature of the Lusi Island analysed varies from 29.2 to 29.4 °C.This value is approximately ±3°C from the surrounding air temperature (meets water quality standard).Temperature is an environmental factor that influences plastic fragmentation and increases the number of microplastics in aquatic environments [25,26,27].The pH value in Lusi Island is 7.3; this affects the presence of microbes in the aquatic environment, including the life of bacteria attached to microplastics.In previous studies, it was found that microbes could degrade plastic.Therefore, the pH affects the existence of microplastics on Lusi Island [28,29]].DO measurements showed that DO values in the Lusi Island ranged from 8.6 mg/L to 8.7 mg/L.The lowest and highest DO values were found at Site 3 and Site 1, respectively.This favourable dissolved oxygen concentration indicates a healthy aquatic ecosystem capable of sustaining the life of aquatic organisms [30].
For turbidity values, the water in the Lusi Island is 20.The water in Lusi Island is downstream of the Brantas River, so it is suspected to be contaminated.The range of TSS values obtained is 9.0 to 9.10 mg/L.The TSS indicates the presence of microplastics in aquatic ecosystems [31,32,17].BOD values in sampling sites 1, 2, and 3 were 4.7, 4.6, and 4.65 mg/L, respectively.BOD indicates the amount of oxygen microorganisms used [20].In the context of the downstream Brantas River, these parameters have an essential role as the primary indicators used to monitor the presence of microplastics.Considering that Lusi Island is downstream of the Brantas River, which is close to various human activities, regular environmental quality monitoring at these islands is very important and requires regular evaluation.

Microplastics Analysis
Microplastics were found in all the sites on Lusi Island in this study.The forms of microplastic found are fibres, fragments, and films (Figure 2).Microplastics were observed in varying forms (fibre, fragments, and film).Fragments were the dominant shape in this study and the most common shape observed was related to the disintegration of larger-sized plastic floated in water due to their low density and exposure to ultraviolet light [33,12].The fibre is characterised by a long and thin shape similar to synthetic fibres [20].Fibre has a close relationship with the clothing washing procedures activity, the textile industry, lines in agriculture, and fishing nets for aquaculture [33,12].Microplastics on Lusi Island come from various human activities along the Brantas River.The input from the Brantas River has been thought to constitute a primary channel for the transport and accumulation of microplastics in the estuary and marine environments.Although most microplastics tend to be buoyant and transported through surface water from rivers to the ocean, the presence of microplastics at the water's surface cannot be ignored.Most microplastics were discovered downstream of large urban areas, including Sidoarjo [34,35].
The quantities of each type of microplastic found at Site 1 are 70 particles/L, 140 particles/L for film, and 353 particles/L for fragments (Figure 3).Similarly, at Site 2, the respective amounts for fibres, film, and fragments are 60 particles/L, 123 particles/L, and 293 particles/L.Microplastics at site 3 were found in 80 particles/L for fibres, 163 particles/L for film, and 380 particles/L for fragments.Moreover, the abundances of microplastics for each type (fibre, fragment, and film) are shown in Figure 4.The total amount of microplastic found in Sites 1, 2, and 3 is 710 particles/L, 597 particles/L, and 787 particles/L, respectively.The presence of microplastics in Lusi Island is not surprising, given their widespread presence.The microplastic abundance found in Lusi Island is relatively higher than those found in other surface water studies conducted in the Cisadane downstream [12].The abundance of microplastics in Lusi Island is relatively lower than in other studies from China and Japan [12] but higher than in North America and Europe [12].However, the microplastic abundance in Lusi Island is higher than the number of microplastics detected downstream in various Indonesian rivers [36,37,38].The source of microplastic pollution is point and nonpoint sources [33].Anthropogenic activities become one of the significant sources of microplastic pollution.The number of anthropogenic activities is increasing downstream of the Brantas River.The Brantas River passes through 15 districts and cities in East Java with multiple activities (domestic, fisheries, and agriculture).Microplastic pollution is also closely related to the higher population numbers along the Brantas River.The results of this study suggest that mitigation techniques in high population areas are important to develop because most of the plastic pollution in the ocean comes from the river [39,40].The number of microplastics in Lusi Island would provide essential data on microplastic pollution downstream.More studies are needed to explore the number of microplastics in ecosystem components abundant in aquatic ecosystems, like microbial biofilm [41,42] or gastropods [43].Moreover, the study in other locations, such as the beach and ocean, connected to the Brantas River and Lusi Island to provide the distribution data of microplastics from upstream, downstream, and estuaries to marine areas.

Conclusion
The study showed that microplastics were found in Lusi Island.Three distinct types of microplasticsfragments, fibres, and film-were identified on the island.Notably, microplastic fragments dominate in number compared to other forms.Despite the favourable water quality parameters suggesting a healthy aquatic environment, the study underscores the necessity of implementing mitigation measures to address microplastic pollution effectively.

Figure 1 .
Figure 1.Location of the Lusi Island where the samples were taken

Table 1 .
Measurement results of water quality parameters This research received funding from the Directorate of Research, Technology, and Community Service.Ministry of Education, Culture, Research and Technology, Republic of Indonesia, Master Contract No: 015/E5/PG.02.00.PL/2023.