Multi-proxy reconstruction of bottle-cap rafting using biofouling communities, stable isotopes and drift modeling

Floating plastic debris can provide long-lived substrates for attached organisms, but reconstructing the drift history of small consumer items remains difficult. Here we report a colonized plastic bottle cap collected in the northwest Pacific. The cap hosted a tube-building polychaete and an associated assemblage including benthic foraminifera. We combined (1) a census of the fouling community, (2) chamber-level δ¹⁸O and δ¹³C measurements from two Rosalina globularis tests, and (3) Lagrangian drift simulations driven by surface currents to constrain the cap's likely trajectory and timescale. The assemblage comprised nine taxa and 307 individuals, and was strongly dominated by spirorbid tubes (Spirorbis sp.; 76.5%). For specimen #021, δ¹⁸O-derived temperatures were 26.0 °C for the pooled early chambers (p-f-4), 26.8 °C for f-3, 29.9 °C for f-2, 23.3 °C for f-1, and 22.3 °C for the final chamber; the final-chamber estimate was close to the in situ sea-surface temperature at collection (21.7-21.8 °C). The final chamber of specimen #005 yielded an estimated temperature of 27.0 °C but should be treated as a reference value because of its very small carbonate mass. Drift simulations suggested that trajectories reaching the sampling site within ∼1-3 months most frequently originated from the Philippine region and were transported northward by the Kuroshio system. This multi-proxy approach illustrates how benthic biofoulers, including foraminifera, can help reconstruct the dispersal history of small plastic items and highlights the potential for occasional long-distance transport of coastal benthic taxa on tiny rafts.