The Indonesian Throughflow plays an important role in the global ocean circulation and climate. Existing studies of the Indonesian Throughflow have focused on the Makassar Strait and the exit straits, where the upper thermocline currents carry North Pacific waters to the Indian Ocean. Here we show, using mooring observations, that a previous unknown intermediate western boundary current (with the core at ~1000 m depth) exists in the Maluku Sea, which transports intermediate waters (primarily the Antarctic Intermediate Water) from the Pacific into the Seram-Banda Seas through the Lifamatola Passage above the bottom overflow. Our results suggest the importance of the western boundary current in global ocean intermediate circulation and overturn. We anticipate that our study is the beginning of more extensive investigations of the intermediate circulation of the Indo-Pacific ocean in global overturn, which shall improve our understanding of ocean heat and CO₂storages significantly.

The issue of plastic pollution in the marine environment is a matter of great concern. Our research presents pioneering findings on sub-surface microplastics in the Indonesian archipelagic waters. Data on microplastic presence in sub-surface water in the Indonesian Archipelagic Water is crucial for expanding our understanding of microplastic distribution from the surface to the bottom layers of the ocean, a research area that has been relatively overlooked. We discovered microplastic particles at 5 m depth below the surface through simultaneous pump method. The highest concentration of microplastics was discovered in Ambon, followed by North Java and North Sulawesi. In contrast, the lowest abundance of microplastics was detected in the Maluku Sea. These findings are consistent with prior studies establishing a relationship between human activity–as indicated by population density–and microplastic pollution. However, our results indicate that levels of microplastics in Ambon were significantly higher than those found in other sampled areas, particularly compared to North Java, which boasts the highest population density in Indonesia. This disparity is likely due to a faster seawater flushing rate and shorter water residence time in North Java relative to Ambon. The origins and pathways through which these microplastics are introduced into Indonesian archipelagic waters remain uncertain. Based on particle composition (52.73% fibers and 51.38% size <500 µm), it appears that they may result from either larger plastic items being broken down during transportation over long distances by prevailing currents from input from coastal areas or as a byproduct of local activities. Additional research is necessary to gain a comprehensive understanding of microplastic in water columns. This includes investigating the fate of microplastics and examining their impact on marine organisms within this treasured ecosystem. Moreover, it is important to develop methods for mapping the worldwide distribution of microplastics.

As the world’s largest archipelagic state, Indonesia is increasingly exposed to microplastic contamination across its aquatic environments. This review synthesizes evidence from 497 publications reporting microplastic occurrence in environmental matrices across the Indonesian archipelago, including freshwater and marine waters, sediments, and aquatic biota. The analysis examines reported abundance levels, dominant polymer types, spatial patterns of contamination, and methodological approaches used in microplastic research conducted in Indonesia. The literature reveals a pronounced geographic imbalance in research coverage. More than 60% of studies were conducted in western Indonesia, with a strong concentration on Java Island, whereas large parts of eastern Indonesia remain poorly documented. Reported concentrations vary widely among environmental matrices and sampling designs, reflecting differences in both environmental conditions and analytical practices. Notably, studies that did not apply rigorous quality assurance procedures or confirm polymer composition using spectroscopic techniques tended to report markedly higher concentrations, suggesting that methodological limitations may contribute to inflated contamination estimates. To place these observations into a comparable framework, risk band thresholds were derived from the abundance dataset using a quantile-based statistical approach. Overall, this review provides an integrated baseline of microplastic contamination across Indonesian environments and identifies critical geographic and methodological gaps that should guide future monitoring strategies and environmental risk assessments.

Microplastic (MP) pollution in the Indonesian Throughflow (ITF)—the sole low-latitude conduit linking the Pacific and Indian Oceans—has significant implications for cross-basin contaminant redistribution, yet systematic three-dimensional observations remain scarce. We present a standardized three-dimensional bottle-sample dataset from 22 stations spanning the Western (Lombok–Makassar–Sulawesi) and Eastern (Maluku–Lifamatola–Banda) ITF channels, collected during the IMPOLSE 2025 joint cruise. Samples were taken at three depth layers—surface (~50 m), thermocline (160–200 m), and deep (600–1000 m)—using Niskin bottles mounted on a CTD rosette. MP abundance ranged from 375 to 5,344 particles m-3, substantially exceeding reported open-ocean concentrations. Fibers dominated across all layers (70–80%), consistent with land-based textile inputs. Kruskal–Wallis analysis revealed significant depth-layer differences (H = 13.1, p = 0.001), with surface concentrations exceeding both thermocline (p = 0.005) and deep layers (p = 0.008). Exploratory Generalized Additive Models (GAMs) indicated depth-dependent physical associations: mixed-layer state and stratification were associated with surface MP variability; geographic and water-mass gradients shaped thermocline-layer variability; and thermocline depth and temperature were associated with deep-layer MPs. The Eastern channel showed weaker vertical attenuation (thermocline-to-surface ratio T/S = 0.76) than the Western channel (T/S = 0.59), consistent with more intense tidal mixing in the Maluku–Lifamatola–Banda system. These results provide a standardized observational framework for ITF-mediated MP transport and establish a baseline for estimating cross-basin MP flux to the Indian Ocean. The presence of MPs at thermocline and mesopelagic depths suggests potential ecological exposure of vertically migrating organisms and downstream food webs.