Community structure is a concept that examines the arrangement or composition of various species and their abundance in a community (Dibyanti, 2023); (Hanny, 2023); (Anggie, 2022); (Daulima et al., 2021); (Montesinos-Navaro et al., 2018); (Hidayani, 2015). Three approaches are generally used to describe community structure, namely 1) species diversity, 2) species interactions, and 3) functional interactions (Jennifer, 2023); (Hikmatul, 2023); (Ashari, 2023); Schowalter 1996). Community structure can be studied through the composition, size, and diversity of species which are closely related to habitat conditions. Changes in habitat will affect community structure because changes in habitat will affect the species level as the smallest component that makes up the population in the community. One of the organisms that make up the basic aquatic ecosystem is mollusks, such as gastropods and other benthic biota (Amin et al., 2023); (Aryanti et al., 2023); (Hasanah et al., 2023); (Mustofa et al., 2023); (Yulianto et al., 2023); (Lestari et al., 2021); (Kinasih et al., 2018); (Sarong et al., 2017); (Mushthofa et al., 2014); (Nugroho et al., 2012).

Gastropod community in the mangrove ecosystem of Dawi-Dawi Village, Southeast Sulawesi is strongly influenced by the input of overburden waste or reddish-orange sediment originating from nickel mining activities, both on a temporal and spatial scale. Overburden waste is carried by river flows that are directly connected to the upstream area where nickel mining is located and trapped in the mangrove ecosystem of Dawi- Dawi, Pomalaa, Indonesia (Hamzah et al., 2015); (Hamzah, 2009); (Zubayr, 2009).

Gastropods are benthic organisms that walk with their legs on the ground floor of the mangrove ecosystem so that almost all their biological activities are carried out on the bottom substrate (Harminto, 2003); (Chukaeva & Petrov, 2023); (Menon et al., 2022-12-24); (Campbell et al., 2008)). The “substrate” parameter is one of the key variables for the survival of the gastropod community in the mangrove ecosystem (Litaay et al., 2023); (Mustofa et al., 2023); (Yuanike, 2023). Disturbances that occur in the substrate directly have implications for the destruction of their natural habitat and can slowly cause the death of individual gastropods (Magfirah & YML, 2014). This condition has occurred in the Dawi-Dawi mangrove ecosystem since 5 decades ago (1968) or half of century ago until now (2023). Overburden waste is the waste rock in the form of mud/acid rock drainage which contains low mineral content (Hamzah et al., 2015); (Hamzah, 2009); (Zubayr, 2009).

Long and continuous exposure times cause the thickness of the overburden sediment to increase range from 10-30 cm, so that overburden exposure in the Dawi-Dawi mangrove ecosystem systemically degrades the living habitat and causes a drastic reduction in the abundance and taxa of the gastropod community. Empirical research on the structure of gastropod communities in overburden exposure areas in Southeast Sulawesi is not yet available, even in Indonesia generally. Therefore, research regarding the existing conditions of gastropod communities in mangrove ecosystems affected by overburden is very important, considering that scientific information regarding gastropod taxa that can survive in environments exposed to overburden waste is not yet available. Apart from that, this research has never been carried out on the coast of Dawi-Dawi Village, Southeast Sulawesi. This research aimed to determine the existing condition of mangrove gastropods affected by overburden.

Ecologically, the mangrove ecosystem is valued as a sediment trap. This ecological function is beneficial from an environmental perspective in a broad sense. However, it may harm aquatic organisms that inhabit the bottom of the waters if the trapped waste sediment from industrial activities for example contaminated with nickel. River flows carrying sediment with waste materials from nickel mining activities in upstream areas can directly cover the mangrove ecosystem’s ground floor. On the other hand, the mangrove substrate becomes a living space and feeding ground area for benthic organisms, one of which is the gastropod community. These conditions impact changes in the ecological niche and structure of the gastropod community. The impact is proven in this study by the research results in the Dawi-Dawi mangrove ecosystem in the Pomalaa district. The results showed a decrease in the diversity (medium) and species richness (low) of gastropods in areas affected by overburden.

The number of gastropod species found inhabiting the Dawi-Dawi mangrove were only seven species (Telescopium telescopium, Terebralia sulcata, Terebralia palustris, Faunus ater, Herba nigra, Littoraria scabra, and Littoraria melanostoma), Meanwhile, in areas not affected by overburden (reference site), the number of species was much higher, namely 34 species.

In contrast, in the reference site area or areas with minimal disturbance, it was found to be ± 30 species. The low richness of gastropod species in the Dawi-Dawi mangrove ecosystem has implications for high species uniformity (E = 1.12). If this condition continues, a tendency for dominance by certain species may exist, although this has not happened (C = 0.19). The results of the gastropod ecological index analysis in the Dawi- Dawi mangrove ecosystem, Pomalaa, are presented in Table 1.

The gastropod ecological index analysis reveals that ecological pressure or an environmental imbalance has disrupted the homeostasis system in the Dawi-Dawi mangrove ecosystem (Table 1). The community’s expression of its environment’s quality is evident in the gastropod community structure, where diversity metrics, specifically species richness, indicate low indices. The Dawi- Dawi mangrove ecosystem exhibits a low number of gastropod taxa (species), directly influencing the ecological index. The diversity of gastropods in this ecosystem fell into the medium category with a tendency for low index values (H’ = 1.81), while the richness was in the low category (R = 1.42).

The impact of low species richness in the Dawi- Dawi mangrove ecosystem is further manifested in the high gastropod uniformity index (E = 1.12). This condition raises concerns about the potential domination by certain species in the future, even though it is not observed currently; the trend is moving in that direction (C = 0.19) (Table 1).

These ecological components serve as benchmarks for evaluating environmental quality or the health condition of an aquatic ecosystem. Overburden waste pollution in the Dawi-Dawi mangrove ecosystem not only alters the color of the substrate and water to reddish-brown in the mangrove area but also induces systemic changes in the life of aquatic organisms. This impact is visibly reflected in the structural composition of the gastropod community in the Dawi-Dawi mangrove landscape. This observed condition aligns with the findings of (Sinapoy et al., 2020) regarding the diversity index (H’) of gastropods in a mangrove ecosystem in the former nickel mining area in Tokowuta Village, Lasolo. The diversity index was found to be in the low category (H’ = 1.36). The evenness index (E) indicated evenness with an unstable community at 0.70, while the dominance index, categorized as moderate in 0.35. The gastropod species diversity index in this area was classified as low, with moderately polluted water quality. This is attributed to the accumulation of chemicals from nickel mining, transported by water and settling in sediment (overburden).

A low diversity index value indicates low species richness, and there tends to be only one or a few species that have an abundant number of individuals. This condition causes this area to have low species diversity, the distribution of the number of individuals of each species is low, and the stability of the community is low due to the waters being polluted by nickel metal levels in the waters and sediments. According to (Rachmawaty, 2011), species diversity in an area is influenced by several factors, namely polluted substrate, availability of food sources, inter- and intra-species competition, disturbances, and conditions of the surrounding environment so that species that have high tolerance will increase while those that having low tolerance will further decrease. The types of gastropods affected by overburden in the Dawi-Dawi, Pomalaa mangrove area are also relatively the same as the gastropods found during research in the mangrove forest area of the former nickel mining area in Tokowuta Village, Lasolo, consisting of 7 species, namely Terebralia sulcata, Cerithidea cingulata, Cerithidea quadrata, Nerita lineata, Littoraria scabra, Littoraria melanostoma and Telescopium telescopium (Sinapoy et al., 2020).

The dominance index value obtained in this area was 0.35. This value is similar to the gastropod dominance index in the Dawi-Dawi mangrove ecosystem (C = 0.19), both in the low category. This value showed that in general, there are no dominant gastropods. The absence of dominant gastropod species in this area is thought to be due to the relatively small number obtained due to the decreasing quality of the aquatic environment, which is no longer suitable for gastropod species to live, except for species that have a high tolerance range for changes in environmental conditions. (Kharisma et al., 2012) stated that the dominance index determines the type of gastropod that dominates a community and the influence of environmental quality on an individual’s community. The results of another study which also had the same conditions, namely by (Supratman et al., 2018), stated that the results of the analysis of the gastropod diversity index in all research locations were categorized as small diversity because the diversity index value was < 2 (Odum, 1993).

The low diversity index at this location was caused by the extreme environmental conditions, such that certain species dominate the gastropods. The high dominance of a species will affect the low diversity of gastropods at the research location. The high dominance of a species can be caused by several factors, such as polluted habitat conditions so that only species that are tolerant of pollution can live in that habitat or the availability of abundant food sources for certain species so that other species cannot compete. Likewise, according to (Arbi et al., 2019) and (Cappenberg, 2006), high and low species diversity index values can be caused by several factors, namely the number of species obtained, the presence of species that dominate other species, substrate conditions, and ecosystem conditions as habitat for gastropods. Ecosystem or environmental conditions are good if they show low Diversity Index (H’) and Dominance Index (C) values. The abundance of gastropod species in the Dawi-Dawi, Pomalaa mangrove ecosystem is presented in Figure 3.

The gastropod types F. ater (6-14 ind./m2) and T. sulcata (2-9 ind./m2) have an average abundance far exceeding other species in the Dawi-Dawi mangrove ecosystem. The average abundance with the lowest value was found in the climber snail, L. melanostoma (1-2 ind./m2). The image below shows the actual condition of the area affected by overburdened waste in the Dawi-Dawi mangrove ecosystem and the gastropod community living in it (Figure 4 & Figure 5).

Dawi-Dawi waters, Pomalaa receives overburden waste input from nickel mining activities in upstream areas with connectivity (flow) with estuary or mangrove areas. This condition has been going on for the last ± 2 decades, resulting in many changes to the life of aquatic organisms in the Dawi- Dawi mangrove ecosystem (Hamzah et al., 2015). Among the many macrozoobenthos organisms affected by overburden waste, the gastropod community is a typical aquatic biota of the bottom of the waters (benthos), which has experienced a decline in population density (Figure 3) and its taxa. This decline happens because the ground floor of the mangrove ecosystem, which is the living and feeding ground for various types of gastropods, has been covered by overburdened sediment waste. In other words, the ecological niche which is rich in organic material and is needed by the gastropod community for its survival is directly degraded and lost due to input of overburden waste which continues to occur until now. Only seven types of gastropods were found in the Dawi-Dawi mangrove ecosystem, which has a very low average density (2-5 ind./m²).

Several types found in a large abundance, namely F. ater (6-14 ind./m2) and T. sulcata (2-9 ind./m2), are typical types of mangroves that have high resistance to changes in the quality of the aquatic environment. in mangrove areas, as well as other species such as H. nigra, L. scabra, L. melanostoma, T. Telescopium and T. palustris (Kurniawati et al., 2022); (Arbi et al., 2019); (Sarong et al., 2017); (Sarong et al., 2015); (Arbi, 2014); (Ayunda, 2011); (Wells, 2003). Furthermore, (Rachmawaty, 2011) stated that species diversity in an area is influenced by several factors, namely polluted substrate, availability of food sources, inter- and intra-species competition, disturbances, and conditions of the surrounding environment so that species with high tolerance will increase. In comparison, those with low tolerance will decrease further. (Rachmawaty, 2011) explanation above is very appropriate to the condition of the Dawi- Dawi mangrove ecosystem, which is polluted by

overburdened sediment waste, and the impact is to eliminate feeding ground areas and other ecological spaces for the biological needs of the gastropod community. (Arbi et al., 2019) emphasized that changes in the function of mangrove areas are the main factor that can threaten the stability of their flora and fauna, including the gastropod communities associated with mangrove areas.

Figure 4 and Figure 5 above show the massive impact of overburden waste in the Dawi-Dawi mangrove area, where not only is the entire substrate area covered by overburden waste, but the surface of the leaves and trunks of mangrove trees is also covered by reddish-orange dust. The gastropod community that lives inside is affected, where the surface of the shell and operculum is covered by overburdened sediment waste, making it even more difficult for gastropod movement when moving (Figure 5). The impact on the gastropod community is because, during high tide, the leaves and lower trunk/twigs of mangrove trees are submerged by water, which carries sediment particles (overburden). The types of gastropods found in the Dawi-Dawi mangrove ecosystem are shown in Figure 6.

The existing condition of gastropod taxa (species) in the Dawi-Dawi mangrove ecosystem that is affected by overburden is directly related to the unique characteristics of these types of snails as native inhabitants of mangrove habitats and are not facultative or migratory species. This correlation can be seen from the adaptability of these 7 species to changes in water quality or disturbances that occur in their ecological environment. (Rangan, 2010) stated that the adaptability of a gastropod community to exponential changes in the habitat environment is an indicator or sign that the species can be categorized as native, facultative, and/or visitor species. The group of mangrove gastropods is known as a native species if the individual/ population of gastropods spends its entire life cycle in the mangrove ecosystem. Meanwhile, the facultative group can be found in mangrove areas and beaches, where this type of gastropod species tends to have euryhaline properties. In contrast, the visitor gastropod group is the type of gastropods in marine areas carried to the mangrove forest by current activity and dynamic waves in coastal areas.

(Odum, 1993) also explained that a species with the highest density indicates that the organism can occupy a larger space and thus develop more. The condition of the gastropod community in the Dawi-Dawi mangrove ecosystem is in line with the results of research by (Maturbongs & Elviana, 2016), where the Littorinidae and Potamididae

families are the majority groups that dominate mangrove forest areas. The input of overburden waste reduces the number of sensitive mangrove gastropod taxa and slowly and massively reduces the number of individuals of each species in the mangrove ecosystem due to the nature of gastropod communities, which tend to be sessile or have limited movement. (Maturbongs & Elviana, 2016) stated that the activities around the mangrove ecosystem greatly influence the density of the gastropod community, which affects the survival of gastropods because living gastropods tend to stay settled with limited movement. Gastropods are very sensitive to changes in the water quality in which they live. Changes in the aquatic environment, for example, in river basins, affect the composition and diversity of the population of this class (Odum, 1993). According to (Fachrul, 2007), pristine waters usually have high species diversity and vice versa for poor or polluted waters.

The abundance and number of taxa (species) of the gastropod community exposed to overburden waste in the Dawi-Dawi mangrove ecosystem experienced a drastic decrease from their natural conditions (without exposure to overburden). Overburden waste has a negative impact on the structure of the mangrove gastropod community. The diversity index is in the medium category (H’

= 1.81), with low species richness (R = 1.42), high evenness (E = 1.12), and no dominance of certain species (C = 0.19).