A Comparative Assessment of Microplastic Abundance in Conservation vs. Residential Coastal Sediments in Bitung, North Sulawesi Indonesia

Abstract Microplastic pollution represents a pervasive threat to marine ecosystems, yet the extent of its intrusion into protected conservation areas compared to industrial zones remains critical to understand, particularly within the Coral Triangle region. This study investigates and contrasts the abundance and morphological characteristics of microplastics in coastal sediments of a Nature Reserve (Tangkoko) versus a high-density industrial port area (Maesa) in Bitung City, North Sulawesi. Sediment samples were collected from the upper intertidal zone using purposive random sampling and analyzed via density separation (NaCl) and oxidative digestion (). The results revealed significant spatial heterogeneity () in microplastic distribution driven by anthropogenic proximity. The industrial zone exhibited a mean density of 158.7 particles dry sediment, predominantly composed of fragments (69%), indicative of direct fragmentation from domestic and industrial macro-debris. Conversely, the conservation area, despite its protected status and distance from residential centers, remained contaminated with a density of 67.24 particles . Notably, fibers were the dominant morphotype (64%) in the conservation zone, suggesting pollution intrusion via long-range hydrodynamic transport or fishing activities. These findings challenge the assumption that conservation boundaries effectively shield coastal habitats from marine debris. Consequently, this study underscores the urgency for integrated waste management strategies that transcend administrative zonations to mitigate the “fiber paradox” threatening critical nesting habitats in protected areas. Keywords: Marine debris, sediment pollution, protected areas, microplastic fibers, Bitung City. 1) Faculty of Fisheries and Marine Science Unsrat; 2) Correspondence Author 1. Introduction Global marine ecosystems are currently facing an unprecedented crisis driven by plastic pollution, now recognized as a distinct geological marker of the Anthropocene. Among these pollutants, microplastics (particles mm) have emerged as a pervasive threat due to their persistence, bioavailability, and potential to adsorb toxic organic compounds (Thompson & Napper, 2019). While floating debris often garners public attention, marine sediments act as the ultimate sink for plastic loads, accumulating particles at rates significantly higher than the water column (Woodall et al., 2014). The fragmentation of macro-debris through photo-oxidative degradation and mechanical abrasion further exacerbates this accumulation, particularly in coastal zones exposed to high anthropogenic pressure (Barnes et al., 2009). Indonesia, positioned at the heart of the Coral Triangle, is frequently cited as a major contributor to marine plastic debris (Jambeck et al., 2015). However, existing research in the archipelago has predominantly focused on heavily urbanized estuaries and tourist hotspots (Ayuningtyas, 2019; Cordova et al., 2019). A critical knowledge gap remains regarding the intrusion of microplastics into Marine Protected Areas (MPAs) and nature reserves. While conservation boundaries are effective against overfishing and habitat destruction, they offer little physical defense against waterborne pollutants. Understanding the extent to which “protected” coastal sediments are compromised by microplastic infiltration—specifically via long-range transport or adjacent industrial activities—is vital for evaluating the efficacy of current conservation zoning. Bitung City, located in North Sulawesi, presents a unique “natural laboratory” to investigate this conservation-pollution paradox. As a bustling port city and industrial hub, Bitung hosts significant maritime traffic and manufacturing activities. Yet, it lies in immediate proximity to the Tangkoko Nature Reserve, a critical habitat for endemic biodiversity and a nesting ground for sea turtles. This stark juxtaposition offers a rare opportunity to perform a head-to-head comparison of microplastic footprints between a high-intensity anthropogenic zone and a theoretically pristine conservation area. This study aims to assess and compare the abundance, distribution, and morphological characteristics of microplastics in the coastal sediments of the Tangkoko Nature Reserve (conservation site) and Maesa District (industrial/residential site). By analyzing the distinct morphological signatures (e.g., fragments vs. fibers) at these contrasting locations, this research seeks to trace potential sources and determine whether administrative protection status correlates with lower plastic contamination levels. The findings are expected to provide a scientific baseline for refining waste management strategies in port cities that border sensitive ecological zones. 2. Materials and Methods 2.1. Study Area The study was conducted in the coastal waters of Bitung City, North Sulawesi, Indonesia. Sampling was performed at two stations selected via purposive sampling to represent contrasting environmental conditions: Station 1 (Tangkoko): Located at the Tangkoko Nature Reserve (1o30'0"N, 125o10'48"). This station represents a non-residential, conservation area characterized by white sandy beaches and serves as a nesting site for sea turtles. Access is limited, minimizing direct human habitation. Station 2 (Maesa): Located in Maesa District (1o36'0"N, 125o10'48"E). This station represents a high-density anthropogenic zone, situated near residential settlements, industrial factories, and a port. The sediment is characterized by black sand and rocky substrates, receiving direct runoff from domestic and industrial drainage. 2.2. Sample Collection Sediment samples were collected from the upper intertidal zone, specifically near the backshore, where marine debris accumulation is most prominent (Hidalgo-Ruz et al., 2012). At each station, three sampling points were determined using a random sampling method to serve as replicates. Samples were extracted using a PVC core sampler (pipe) to a depth of approximately 10 cm. The collected bulk sediment was transferred into labeled ziplock bags and transported to the Coastal and Small Islands Laboratory, Faculty of Fisheries and Marine Science, Sam Ratulangi University, for further analysis. 2.3. Sample Preparation and Extraction In the laboratory, wet sediment samples were weighed and subsequently air-dried in aluminum foil trays covered with gauze to prevent airborne contamination. Drying was conducted under sunlight for 3–5 days until a constant weight was achieved. The dried samples were re-weighed to determine the dry weight (dw) and sieved through a 0.5 mm mesh screen to separate large debris. Microplastics were extracted using a modified density separation and chemical digestion protocol (Masura et al., 2015): 1. Density Separation: A saturated sodium chloride (NaCl) solution was prepared by dissolving 6 g of NaCl per 20 ml of distilled water. Approximately 300 ml of this solution was added to the sediment sample, stirred, and allowed to settle for 24 hours. The supernatant was filtered through a 0.5 mm mesh. 2. Organic Digestion: To eliminate organic matter, the filtered solids were treated with 100 ml of Hydrogen Peroxide (). The mixture was covered and left to react for 24 hours. Post-digestion, the samples were rinsed with distilled water and filtered again for final sorting. 2.4. Microplastic Identification Recovered particles were examined under a stereo-microscope and classified into four morphological types: fragments, films, fibers, and granules. The abundance was expressed as particle density per unit of dry sediment weight (particles ), calculated as: Where is density, is the number of particles, and is the dry weight (kg). 2.5. Data Analysis Statistical analysis was performed using a paired t-test () to compare microplastic density and morphological distribution between the conservation and industrial zones, was performed using Microsoft Excel to compare microplastic density and morphological distribution between stations. Data visualization and graphical plotting were conducted using Python (v3.10) with the Seaborn and Matplotlib libraries. 3. Results 3.1. Microplastic Abundance and Distribution A total of 660 microplastic particles were recovered from the sediment samples. The abundance varied considerably between the two sites. The highest accumulation was observed at the industrial/residential site (Station 2, Maesa), where 465 particles were retrieved, compared to 195 particles at the conservation site (Station 1, Tangkoko). Station 2 exhibited a mean microplastic abundance of 158.7 particles dry sediment, which was more than double the density recorded at Station 1 (67.24 particles ). Statistical analysis using a paired t-test confirmed that this spatial difference was significant (). 3.2. Morphological Composition Four morphotypes were identified: fragments, films, fibers, and granules. No foam particles were detected. The composition differed notably between the stations (Table 1). Table 1. Average density of microplastics by morphotype (particles dry sediment). Morphotype Station 1 (Conservation) Station 2 (Industrial) -value Significance Fragment 18.28 109.22 0.003 Significant Film 5.86 31.40 0.029 Significant Fiber 43.10 16.04 0.094 Not Significant Granule 0.00 2.05 - - Fragments dominated the industrial site (Station 2) with a density of 109.22 particles , significantly higher than Station 1 (). Conversely, fibers were the dominant morphotype at the conservation site (Station 1) with 43.10 particles , compared to 16.04 particles at Station 2, although the difference was not statistically significant (). Films were significantly more abundant at Station 2 (), while granules were exclusively found at Station 2. 4. Discussion 4.1. Spatial Heterogeneity and Anthropog