Microplastic occurrence and short-term variability across seaweed-farming and non-farming coastal sites in Takalar Waters, Indonesia.

Microplastic (MP) pollution is an emerging concern along tropical seaweed-producing coasts, where aquaculture occurs within broader coastal systems influenced by settlements, fisheries, rivers, and shoreline use. This study assessed the occurrence, spatial patterns, short-term variability, and particle characteristics of MP pollution in surface seawater and surface sediment along the Takalar coast, South Sulawesi, Indonesia. Water and sediment samples were collected from 12 stations, comprising nine seaweed-farming sites and three non-farming sites, during two sampling repetitions in January and March 2022. After laboratory extraction, MPs were visually identified and characterized by shape, color, and size, and the polymer type of a subset of representative particles was determined using Fourier Transform Infrared (FTIR) spectroscopy. MP abundance ranged from 300 to 1600 particles/m3 in surface seawater from seaweed-farming sites and from 500 to 800 particles/m3 at non-farming sites. MP abundance in surface sediment ranged from 10 to 170 particles/kg dry weight (DW) at seaweed-farming sites and from 30 to 110 particles/kg DW at non-farming sites. Mixed-effects analysis showed no significant effects of site category, sampling campaign, or their interaction on MP abundance in either matrix. Fibers and fragments dominated the particles, accounting for more than 60% of MPs, with blue and black particles and those smaller than 1 mm occurring most frequently. Based on the FTIR-analyzed subset results, polypropylene (PP) was the most dominant MP polymer (57%). Overall, MPs were found widespread across the surveyed coastal area, and their distribution was consistent with a mixed coastal contamination signal rather than a distinct site-category pattern. These findings provide a short-term baseline for MP monitoring in an important Indonesian seaweed-production region, highlighting the need for broader spatial coverage, hydrodynamic measurements, and direct comparisons with potential source materials to improve future source assessments and mitigate ecological risks.