Investigating Hydrodynamic and Environmental Influences on the Concentration of Marine Debris in a Nearshore, Finescale Convergence Phenomenon

Abstract

Plastic pollution is now pervasive across marine environments, from remote oceanic gyres to densely populated coastlines, yet the mechanisms governing its accumulation and dispersion across spatial scales remain incompletely understood. While large-scale convergence zones such as subtropical gyres are well known to concentrate floating debris, far less attention has been paid to the efficiency and dynamics of finescale coastal convergence processes. Understanding how these nearshore systems operate is critical for linking offshore plastic transport to coastal exposure and identifying effective mitigation strategies. Here, we document an exceptionally efficient nearshore convergence system at Makai Pier (Oʻahu, Hawaiʻi), where micro- and macroplastics and organic debris repeatedly aggregate in finescale (meter-scale) surface patches. Using a two-year time series of semi-quantitative surveys calibrated with targeted quantitative sampling, we quantify microplastic densities, concentration factors, and environmental influences on feature formation. Microplastic densities within surface features reached ~7,600 particles m⁻², among the highest reported to date and several orders of magnitude greater than typical open-ocean convergence zones, including the North Pacific Garbage Patch. Concentration factors ranged from 10² to >10⁵ during above-average wind and/or north-swell conditions. In the semi-quantitative time series, wave height emerged as the dominant predictor of concentration factor variability, with heights greater than 7 ft associated with concentration factors of 10² to >10⁴ on about 95% of survey days. We infer that wind and bathymetry likely modulate feature persistence and retention. The adjacent swash zone consistently exhibited elevated microplastic concentrations relative to background waters, indicating an additional nearshore concentrating mechanism and a potential source of material to surface features. Polymer composition and particle size distributions were consistent with remote, gyre-derived debris, highlighting connectivity between offshore and coastal plastic pathways. These results demonstrate that finescale nearshore convergence systems concentrate large quantities of plastic with some predictability, offering efficient, low-carbon opportunities for targeted debris removal.