Plastic debris in the open ocean

Abstract

There is a rising concern regarding the accumulation of floating plastic debris in the open ocean. However, the magnitude and the fate of this pollution are still open questions. Using data from the Malaspina 2010 circumnavigation, regional surveys, and previously published reports, we show a worldwide distribution of plastic on the surface of the open ocean, mostly accumulating in the convergence zones of each of the five subtropical gyres with comparable density. However, the global load of plastic on the open ocean surface was estimated to be on the order of tens of thousands of tons, far less than expected. Our observations of the size distribution of floating plastic debris point at important size-selective sinks removing millimeter-sized fragments of floating plastic on a large scale. This sink may involve a combination of fast nano-fragmentation of the microplastic into particles of microns or smaller, their transference to the ocean interior by food webs and ballasting processes, and processes yet to be discovered. Resolving the fate of the missing plastic debris is of fundamental importance to determine the nature and significance of the impacts of plastic pollution in the ocean.

Fig. 1.

Concentrations of plastic debris in surface waters of the global ocean. Colored circles indicate mass concentrations (legend on top right). The map shows average concentrations in 442 sites (1,127 surface net tows). Gray areas indicate the accumulation zones predicted by a global surface circulation model (6). Dark and light gray represent inner and outer accumulation zones, respectively; white areas are predicted as nonaccumulation zones. Data sources are described in SI Appendix, Table S1. Plastic concentrations along the Malaspina circumnavigation and a latitudinal gradient are graphed in SI Appendix, Figs. S4 and S5.

Fig. 2.

Ranges of surface plastic concentrations by ocean. Nonaccumulation zone (blue boxes), outer accumulation zone (green boxes), and inner accumulation zone (red boxes). The boundaries of the boxes indicate the 25th and 75th percentiles, the black lines within the box mark the mean, and the whiskers above and below the boxes indicate the 90th and 10th percentiles. Data used in this graph are mapped in Fig. 1. An equivalent analysis for a dataset of plastic concentrations not corrected by wind effects is graphed in SI Appendix, Fig. S3.

Fig. 3.

Size distribution of floating plastic debris collected during the Malaspina circumnavigation at calm conditions. (A) Size distribution in abundance (light blue bars) and abundance normalized by the width (in millimeters) of the size class (blue circles). (B) Measured (blue circles) and modeled (red squares) size distributions of normalized abundance in logarithmic scale. Modeled distribution was strictly based on fragmentation of large plastic items. (C) Measured (blue circles) and modeled (red squares) size distributions in normalized volume. Green bars indicate the estimated losses of plastic volume by size class (∆_i). After smoothing the measured distribution with a Weibull function (black line, R = 0.9979, P < 0.0001), losses by size were estimated from its progressive departure from the modeled distribution. Dashed vertical lines through all three graphs correspond to 1-mm and 5-mm size limits. Because plastic presence declined for sizes over 10 cm, modeling analysis was applied up to 10 cm. Note that the largest size class extends from 10 cm to 1 m, the length of the net mouth. Measured size distributions are built from the plastic collected in tows with u_* <0.5 cm⋅s⁻¹ (4,184 plastic items) to avoid wind-mixing effect. An analysis of the effect of wind mixing on plastic size distribution is shown in SI Appendix, Fig. S7, and size distributions for the whole Malaspina dataset (7,359 plastic items) are graphed in SI Appendix, Fig. S10.