Sampling was conducted in the eastern water of Java Sea, Gresik, Indonesia. There were five sampling stations that represented different local activities. Station 1 was located in the fish landing area that is busy with fisheries activity, especially in the morning. Station 2 was located in the mangrove area that is frequently inundated during high tide. The mangrove area is vulnerable to the plastic wastes from other places that are carried away by the current and tides. Station 3 was located in the abandoned shrimp pond and was once known for shrimp culture. However, this activity had stopped and many ponds have been abandoned. Some local people discard their waste in these ponds, including plastic waste. Station 4 was located in the river mouth of Bengawan Solo River that connected to the open sea. Bengawan Solo River is the longest river in the Java Island, passing through many cities in the Central and East Java Provinces and along the way could bring domestic wastes to its end point in the Java Sea. Station 5 was located in the open sea, about 1 km from the river mouth ( Figure 1).

Sampling was conducted in March 2018 and samples were collected in the surface sediment using an Ekman Grab soft sediment sampler. In total, three replicates were obtained and about 500 g of sediment samples from each replicate of the sampling station were stored in sealed plastic bags. Sediment samples were taken to the laboratory for further analysis. For more information about the study areas regarding the present of the plastic wastes, we also held informal discussions with the local fishermen, especially those who have concern for the mangrove ecosystem. Since this was considered low-risk personal communication, ethical approval and consent were not sought.

Microplastics analysis was conducted by modifying the section 3.5 of the NOAA method ( Masura et al., 2015). In the laboratory, 150 g of sediment samples were oven-dried for 24 hours at 90°C. Density separation was performed by adding 20 ml 0.05 M Fe(II) and 20 ml 30% H ₂O ₂. The samples were then homogenized on a hotplate with stirrer for 30 minutes at 60°C. To remove organic materials, an additional 20 ml of 30% H ₂O ₂ and NaCl were added and left overnight. After one night, the floating microplastics were collected by filtration using an 0.3-mm mesh filter. Visual identification of microplastics was conducted under a microscope with three distinct rules to separate the types of microplastics. A fragment is a particle that cannot be torn apart with tweezers, with sharp and broken edges of irregular shape and size of degradational plastic; a fiber is a particle that equally thick throughout the entire length and is not tapered at the end; and film that is very thin, part of the sheets of plastic bags and similar ( Dai et al., 2018; Hidalgo-Ruz et al., 2012; Nor & Obbard, 2014; Zobkov & Esiukova, 2017). The results of this study were compared with the results from other studies to understand more of the sources of plastic pollution.

Normality test was performed to determine the data distribution and to decide whether to use parametric or nonparametric tests for the statistical analysis. The abundance of microplastics proven to be distributed normally, thus, one-way ANOVA was used to compare the abundance of microplastics among the sampling stations (p < 0.01) and the post hoc Tukey`s test was run to confirm the differences of microplastics between sampling stations. Kruskal-Wallis H test was conducted to test the difference in type of microplastics found in the study areas in which the distributions were found to be nonparametric. All statistical tests were carried out using SPSS 16.0 for Windows.

Microplastics were detected in the surface sediments of the sampling stations in the eastern waters of Java Sea. Anthropogenic activities mostly contributed to the present of microplastics in the study areas. The areas have been known to be highly populated. This is also home to many industrial activities which discharge wastewater to the eastern water of the Java Sea. Microplastics appeared in the range of 206.04−896.96 particle/kg ( Figure 2). The highest abundance of microplastics was found at Station 2 in the mangrove area (896.96±160.28 particles/kg), followed by Station 1 (772±336.75 particles/kg) and Station 5 (639.51±121.58 particles/kg). Stations 3 and 4 were observed to contain rather similar amounts of microplastics, which were three times lower than the abundance in the mangrove area (206.04±84.49 particles/kg and 215.54±64.58 particles/kg, respectively). According to one-way ANOVA, there was a statistically significant different in the abundance of microplastics among sampling stations (p < 0.01). Tukey’s post-hoc test revealed that the abundances of microplastics at Stations 1 and 2 were significantly different to the abundances at Stations 3 and 4 (p = 0.05). There was no statistically significant difference between the abundance of microplastics at Station 5 and the other stations. Raw data on microplastic abundance, along with all other raw data, are available on OSF ( Yona, 2018).

Microplastics in the mangrove ecosystems have been studied in the Singapore`s coastal areas ( Nor & Obbard, 2014), where there were much lower amounts compared to those found in this study. Even though the abundances were very much different between these studies, but the reasons of the occurrence of the microplastics in the mangrove ecosystems were rather similar. High occurrence of microplastics at Station 2 in the mangrove area was observed, which could be the result of root system of mangroves, which can trap many different type of rubbish including plastics. Moreover, tides that frequently inundated mangrove beds could bring more plastic wastes from the surrounding waters.

Due to their small size and ability to float in the water column, microplastics can be transported for long distance by ocean currents ( Iwasaki et al., 2017). Eastern water of Java Sea is the end point of a very big river, Bengawan Solo, which passes many cities in Java Island and carried plastic wastes along its way to the ocean. Therefore, microplastics found in this study might not just from the local sources, but also from faraway places. According to information from local fishermen, during west monsoon season (November-February), mangroves in the study area are filled with plastic waste from the Bengawan Solo River. Similar results were also found in the study in the Saigon River canal system crossing a megacity, Ho Chi Minh City, in which the source of the plastic pollution was from the land-based due to local habits and waste management ( Lahens et al., 2018).

On the other hand, limited interaction of Station 3, which located in the shrimp pond, with the surrounding waters resulted in a low abundance of microplastics. Even so, the abundance of microplastics in the pond was not that low (206.04 particles/kg). This may be because the pond has been abandoned for quite some times and some people from local village may have discarded their rubbish inside the pond.

There were three type of microplastics found in the study areas: plastic fragments, plastic fibers and plastic films ( Figure 3). Fragments and fibers dominated most of the sampling stations, while films occurred in very low number compared to the other two types of the microplastics. In total, half of the microplastics found were plastic fragments (54.34±6.39%), followed by plastic fibers (41.45±4.59%) and plastic films (4.21±3.90%). Kruskal-Wallis H test showed that there was a statistically difference in the type of microplastics (fragment, fiber and film) among the study sites (p < 0.01).

Fragments contributed the most to the composition of microplastics in the study areas. This type of microplastic is the result of fragmentation of large plastic pieces into smaller particles ( Alomar et al., 2016), and mostly the contribution comes from the domestic waste. Fisheries activity in the study areas might contribute to the present of fiber, as mostly local fishermen use plastic fishing nets to catch their fish. A high abundance of fibers was also observed in the North Yellow Sea as the result of constant use of plastic fishing and nets and ropes as the main fishing tools ( Zhu et al., 2018).

The highest abundance of plastic fragments appeared at Station 2 in the mangrove area (537.25 ± 160.28 particle/kg). The highest abundance of plastic fibers was also found at Station 2 compared to the other stations. On the other hand, plastic films were detected the highest at Station 1 in the fish landing area.

Compared to the results from the other studies ( Table 1), the levels of microplastics found in this study were similar to values obtained in the Western Mediterranean Sea ( Alomar et al., 2016) and much higher than those in the North Yellow Sea ( Zhu et al., 2018). The dominant type of microplastics found among the studies was also different. The study in the North Yellow Sea found that plastic films predominated; in the Bohai Sea the most common type was plastic fibers ( Dai et al., 2018), while this study obtained the highest percentage of plastic fragments. The results revealed that microplastics found in this study are mostly from the degradation of the plastic wastes from human activities as stated by Barnes et al. (2009) that fragments are the result of the breakdown of a wide range of everyday plastic products. The lack of awareness from the citizen on how dangerous plastic materials are to the environment is the main reason for plastic pollution ( Derraik, 2002). Efforts from the government and the community are therefore needed to combat plastic use and production.