Macrozoobenthos diversity and heavy metal contamination in Faunus ater in the Rasian and Asahan rivers in the southern coast of Aceh, Indonesia

Macrozoobenthos are demersal organisms that are sedentary and have slow movements, making them highly susceptible to environmental conditions (Minggawati, 2013). Due to this sensitivity, macrozoobenthos are frequently used as bioindicators for assessing water quality (Dabrowska et al., 2016; Rosdatina et al., 2019; Ramadhaniaty et al., 2021). These species play an important role in the aquatic ecosystems as decomposers (Desmawati et al., 2020; Kumar et al., 2013) and transfer energy to the next trophic levels as prey for other aquatic animals such as fish and water birds (Ridwan et al., 2016; Jędruch et al., 2019).

In Indonesia, several groups of macrozoobenthos, particularly bivalves and gastropods, are used as protein sources by local communities and are traded in local markets. However, these species have been intensively harvested without regard to size, leading to overharvesting and diminishing wild stocks (Almohdar et al., 2017; Aralaha et al., 2015; Sarong et al., 2015). Overharvesting can cause population stress, leading to premature spawning to maintain their population (De Jong et al., 2015). This can affect the stability of the water ecosystem and reduce water productivity.

Studies on the diversity of macrozoobenthos have been carried out in Indonesia waters, for example, Irham et al. (2022) examined the abundance and diversity of the benthic community in Krueng Cut estuary Banda Aceh, and they recorded seven species of benthos with low diversity and moderate dominance indices. Furthermore, Octavina et al. (2018) reported seven species of macrozoobenthos in Gosong Telaga Aceh Singkil with a moderate diversity index. The biodiversity and community composition of freshwater and marine macrozoobenthos in the waters of Gonrontalo were analyzed by Kadim et al. (2022); they concluded that the diversity of macrozoobenthos in these waters was low to moderate. However, in Aceh province, specifically in the southwest coast regions, there is a lack of exploration and reports. Currently, there are only two reports available from this region. The first report was by Munandar et al. (2016) who studied macrozoobenthos diversity in the Kuala Rigaih estuary, Setia Bakti District. Aceh Jaya District recorded 10 species from this estuary. The second report by Rizka et al. (2016) examined macrozoobenthos diversity on the coast of Nagan Raya and concluded that the diversity of these species in the area tends to be moderate, indicating unstable water condition (Rizka et al., 2016). Therefore, the above studies indicate that the diversity of macrozoobenthos in Indonesian waters ranges from low to moderate. This is an indication of a general problem with the quality of water in Indonesia.

The Rasian River and the Asahan River are located in the Pasie Raja and Kluet Selatan subdistricts, Aceh Selatan District in Aceh province, Indonesia. Both traditional and illegal mining activities, such as traditional sand and gold mining, are carried out in this district and the local government has planned to develop the cement industry in this area. Therefore, in this study, we aimed to document the diversity of species and distribution of macrozoobenthos in the Rasian and Asahan rivers and to examine the heavy metal contamination in one of the dominant species of macrozoobenthos, F. ater. These data are crucial for monitoring the impact of development and are important information for developing a management plan for the aquatic resources in this area.

The Rasian River estuary contained 14 species of macrozoobenthos, while the Asahan River estuary contained seven species, resulting in a total of 17 across both rivers (Table 1). These species belonged to two phyla (Mollusk and Arthropoda) and three classes (Bivalvia, Gastropods, and Crustaceans). The external morphology of the macrozoobenthos found during the survey is presented in Figure 3. Analysis of the density of macrozoobenthos in the Rasian River showed that F. ater had the highest average density of 13.33 ind./m², followed by Neritina turrita (7.33 ind./m²), and the species with a low density were Charybdis affinis, Ocypodae kuhlii, and Eriphia smithii. The diversity index values in this estuary were categorized as moderate value (H′ = 2.24), high uniformity (E = 0.84), and no predominant species was present (C = 0.142) (Table 2). Meanwhile, in the Asahan River, the highest density was recorded for Terebralia sulcata (162 ind./m²) followed by F. ater (88.3 ind./m²), while the lowest density was recorded for Metapaneus affinis (0.3 ind./m²). The diversity and uniformity indices in this estuary were also classified as moderate category (H' = 1.05 and E = 0.54, respectively) and no predominant species was present (C = 0.431) (Table 2).

Figure 3.

Heavy metal analysis in the F. ater muscles showed that the species from these two rivers were contaminated with mercury (Hg) ranging from 0.0845% to 0.1115%, cadmium (Cd) 0.0002%, and lead (Pb) 0.0005%. The Hg content exceeded the tolerance threshold for aquatic animals, while the Cd and Pb contents were below the thresholds (Table 3).

The species diversity index of macrozoobenthos in the Rasian and Asahan rivers was in the moderate category, but the value was lower compared to those reported in several related studies in the area, for instance, in the Peat Swamp forest in Nagan Raya and Aceh Barat Daya districts (Riska et al., 2016) and in Kuala Rigah, Setia Bakti Subdistrict, Aceh Jaya District (Munandar et al., 2016). In general, the number of macrozoobenthos species in the Rasian River was higher than in the Asahan River; this may be due to the sampling area in the Rasian River having diverse sediment characteristics and being predominantly muddy and adjacent to mangrove ecosystems dominated by nipah (Nypa fruticans) and Rhyzopora sp., while in the Asahan River, the sediment characteristics were dominated by sandy area with no mangroves. Therefore, the Rasian River estuary was richer in organic matter from mangrove litters. This finding is consistent with the findings of Nagelkerken et al. (2008), which suggested that estuarine areas with mangroves and nipa palms tend to have higher productivity levels than other aquatic ecosystems, leading to a greater variety of organisms (Fithria et al., 2022). F. ater, locally known as chue, is a biota that is present at both locations and is classified as an herbivore that feeds on plants, algae, plankton, and detritus, making its habitat the bottom of water (Agustina et al., 2017). The local population uses it as a source of protein, and it is traded in the local market. This study also examined the potential of heavy metal contamination in this biota. The results confirmed that the chue from these two rivers was contaminated with Hg, Cd, and Pb metals, of which the Hg concentration passed the threshold limit. Detection of Pb and Hg levels contained in foodstuffs such as chue indicates that biomagnification has occurred, where harmful substances accumulate in the food chain (Hilgendag et al., 2022; Murillo-Cisneros et al., 2019). The high Hg content in chue is probably caused by gold mining activities in Mount Manggamat, Central Kluet District, where this is the catchment area for these rivers. Mercury is commonly used by illegal miners to separate pure gold from other particles. Subsequently, mining activities must be licensed to operate prudently based on nondestructive principles for natural resources. However, it has been reported that there are a lot of illegal mining activities that do not pay attention to the impact on the surrounding ecosystem. Mount Manggamat is an upstream part that is connected to the Rasian and Asahan rivers, and therefore, this metal accumulates in the body of F. ater as a bioindicator, which is a filter feeder (Jedruch et al., 2019). Cd and Pb may have come from the chemical fertilizers and pesticides from the plantations and farms close to the rivers (Rumhayati et al., 2018; Atitsogbey et al., 2023). Therefore, it is necessary to carry out further intensive studies related to these initial findings to take early anticipatory action.

A total of 17 species of macrozoobenthos were recorded, of which 14 were found in the Rasian estuary and seven in the Asahan estuary, and therefore, the Rasian River estuary has a high diversity. However, a large number of individuals were encountered in the Asahan estuary. The diversity index of macrozoobenthos in both estuaries was in the moderate category. In addition, F ater was contaminated by mercury, cadmium, and lead, of which the concentration of mercury exceeded the threshold limit.