Tidal phase-driven variability in marine debris magnitudes from the estuary of the Banjir Kanal Timur river, Semarang

This study investigated the impact of tidal dynamics and river discharge on marine debris deposition from the Banjir Kanal Timur estuary, Semarang, into the Java Sea, while also estimating mismanaged plastic waste (MPW) volumes and comparing them with field observations. Marine debris measurements and analyses were conducted twice a month, employing well-established research methodologies alongside the NOAA Marine Debris Program approach. The results indicate that during the flood phase at the Banjir Kanal Timur estuary, plastics dominated in abundance (93.5%–98.2%) and weight (88.0%–95.1%), with food wrappers (36.8%–40.3% abundance; 27.1%–34.0% weight) and plastic bags (28.1%–34.4% abundance; 31.8%–38.2% weight) as the most prominent contributors. During the ebb phase, plastics remained dominant in terms of abundance (88.6%–92.9%) but contributed less to total weight (32.7%–50.8%), as heavier materials, such as rubber (up to 37.1% by weight), processed wood (7.8%–14.7% by weight), and glass (8.9%–10.1% by weight), became more prevalent. River discharge predominantly influenced marine debris abundance during the flood phase (t = –4.221, p = 0.002), while both river discharge and tidal changes primarily regulated debris weight during the same phase (F = 55.378, p < 0.001). In contrast, during the ebb phase, neither river discharge nor tidal fluctuations significantly affected marine debris abundance (F = 1.836, p = 0.214), whereas only river discharge exerted a significant positive influence on debris weight (t = 7.512, p < 0.001), with tidal effects remaining negligible (t = 0.193, p = 0.851). Comparison between projected plastic debris deposition and field observations indicates that the observed debris represented only 1%–2% of the projected MPW. • Plastics dominate in marine debris abundance during all tidal phases. • Flood phases reflect the dominance of lightweight and buoyant debris, especially plastic items. • Heavier non-plastic items substantially influence the debris cumulative weight in ebb phases. • River discharge plays a crucial role in determining marine debris composition.