We conducted 12 multi-level net tows that sampled the upper 5 m of the North Atlantic accumulation zone (Law et al., 2010; Maximenko et al., 2012; Lebreton et al., 2012) during daytime hours, from 19 to 22 May 2014, aboard the sailing vessel Sea Dragon (Fig. 1). We used a new collection device capable of sampling surface waters from the air–seawater interface to a depth of 5 m, at 0.5 m intervals. This equipment is composed of eleven frames with 0.5 m height × 0.3 m width fitted with 2.1 m long 150 µm mesh polyester nets. These nets were stacked vertically and secured within an external frame that was dragged in the water from eight towing points, ensuring its stability and perpendicular position in relation to the sea surface, with the top net completely above mean water line (see Fig. 1). Tow durations ranged from 55 to 60 min and were all undertaken while the vessel was travelling at a speed of 1–1.9 kn. The captain, who has 20 years sailing experience, estimated wind speeds and sea state of each sampling period: Beaufort number 1 (N = 3 net tows), 3 (N = 4 net tows), and 4 (N = 5 net tows) (Reisser et al., 2015). After each tow, we transferred the collected contents to a 150 µm sieve and stored them in aluminium bags that were kept frozen during transportation.

We calculated plastic numerical and mass concentrations by dividing the number of plastic pieces and total plastic mass by the volume of filtered seawater of each net sample (pieces m-3 and milligrams m-3). Filtered volume was estimated using frame dimensions and readings from a mechanical flowmeter (32 cm per rotation).

Samples were washed into a clear plastic container filled with filtered seawater, and floating macroscopic plastics were organised into gridded petri dishes for counting and characterisation. Each search for plastic pieces was for at least 1 h per sample, with the aid of thumb forceps, dissecting needles, magnifying glasses, and LED torches. The latter was particularly important for detecting thin transparent plastic fragments, which had low detection probability when not reflecting light. Two thin filaments resembling textile fibres were discarded due to potential air contamination as noted in (Foekema et al., 2013). Once all plastics were counted and characterised, they were washed with deionised water, transferred to aluminium dishes, dried at 60 ∘C, and weighed.

To quantify the variation of plastic concentration with depth and assess the effect of changing sea state on these vertical profiles, we first divided the plastic concentration values of the samples by their corresponding surface concentration values. We then took the average of these normalised concentrations between adjacent nets to estimate normalised plastic concentration values at depths of 0 m (top two nets), 1 m (third and fourth nets), 2 m (fifth and sixth nets), 3 m (seventh and eighth nets), 4 m (9th and 10th), and 4.75 m (11th net). Finally, numerical and mass concentration values from tows collected under the same Beaufort number were grouped and fitted to exponential decay models of the form N=e-λz, where N= normalised plastic concentration, z= depth, and λ= decay rate.

We also predicted normalised plastic concentration depth profiles using the model described in Kukulka et al. (2012): N=ezwbA0-1, where z= depth, wb= plastic rise velocity, and A0=1.5u*wkHs, with u*w= frictional velocity of water, k= 0.4 (von Karman constant), and Hs= significant wave height. We considered wb= 0.0053 m s-1 (plastics' median rise velocity, as estimated in this study), Hs= 0.1, 0.6, or 1 m (typical wave heights experienced at Beaufort numbers of 1, 3, and 4, respectively), and used the wind ranges of Beaufort of 1, 3, and 4 (1–3, 7–10, and 11–16 kn, respectively) to estimate their respective u*w values through the approximation proposed by Pugh (1987): u*w= 0.00012W10, where W10= 10 m wind speed in m s-1. Thus, the considered numerical ranges of frictional velocity of water (u*w) were 0.0006–0.0019 m s-1 for Beaufort number 1, 0.0043–0.0062 m s-1 for Beaufort number 3, and 0.0068–0.0099 m s-1 for Beaufort number 4.

We measured the length of all plastic pieces using a transparent ruler (0.5 mm resolution), and classified them into the following types: hard plastic – fragments of rigid plastic; sheet – fragments of thin plastic, with some degree of flexibility; line – fragments of fishing lines or nets; foam – expanded polystyrene fragments; and pellet – raw material used to produce plastic items (Fotopoulou and Karapanagioti, 2012). We also identified the resin composition of 60 pieces using Raman spectroscopy (WITec alpha 300RA+), and measured the rise velocity of 0–3 plastics from each sample collected.

Our method of rise velocity measurement is an adaptation of an experiment to examine the fall velocity of various types of sediment particles in different fluids (Allen, 1985). Firstly, we made two marks 12.5 cm from the ends of a 1 m long clear plastic tube (diameter = 40 mm). Secondly, we filled the tube with filtered seawater, capped both ends with rubber stops, and locked it in place with a clamp. One of the tube ends was then opened, a plastic piece placed inside, the tube closed again (with no trapped air), and quickly turned upside down and locked in place with a clamp, using a spirit level to adjust its vertical position. Finally, we recorded the time taken for the plastic piece to rise from one mark to the other (distance = 75 cm) using a stopwatch. This was measured four times per plastic piece, and the average was used as the estimation of its rise velocity (wb). Rise velocities of different plastic types were separately plotted against plastic lengths (l), and linear regressions of the form wb=al+b were applied to assess the effect of plastics' characteristics on its rise velocity. We also plotted the rise velocities of plastic pieces collected at different depths to visualise depth patterns.

Finally, we calculated the fractions of plastics of different size classes (0.5–1, 1.5–2, 2.5–3, 3.5–4, 4.5–5, and > 5.5 mm) that were located at the sea surface (depth < 0.5 m) and in deeper layers (depth > 0.5 m) during sampling at Beaufort numbers of 1, 3, and 4. We calculated these fractions using all plastics collected, as well as separated by plastic type.

Plastic numerical and mass concentrations both decreased abruptly from their peak values at the sea surface, where median values were equal to 1.69 pieces m-3 and 1.60 mg m-3 (Fig. 2). Concentration differences between the surface and deeper layers were higher in terms of mass than number of particles. For instance, median mass and numerical concentrations at 0.5–1 m were, respectively 13.3 and 6.5 times lower than their median plastic peaks at 0–0.5 m.

Exponential models fitted well with both numerical and mass concentrations (R2= 0.99–0.84), with depth decay rates (λ) consistently higher for mass than numerical concentration. Furthermore, both numerical and mass concentration decay rates were inversely proportional to Beaufort state (Fig. 3). Depth decay rate of numerical concentration went from 3.0 at Beaufort 1 (95 % confidence interval – 95 %CI = 2.56–3.45), to 1.7 at Beaufort 3 (95 %CI = 1.51–1.88), to 0.8 at Beaufort 4 (95 %CI = 0.62–0.98). Decay rate of mass concentration went from 3.8 at Beaufort 1 (95 %CI = 3.23–4.33), to 2.4 at Beaufort 3 (95 %CI = 1.63–3.14), to 1.7 at Beaufort 4 (95 %CI = 1.50–1.94).

These exponential fits had relatively similar depth decay rates to those predicted by Kukulka's model for Beaufort 3 (λ= 2.36–3.37) and 4 (λ= 0.88–1.28). However, for Beaufort 1 the statistical fit showed much smaller λ (2.56–4.33) than those predicted by Kukulka's model (λ= 141.73–47.2492).

We counted and classified 12 751 macroscopic plastic pieces with lengths varying from 0.5 to 207 mm (median = 1.5 mm; Fig. 4). They were mostly fragments of polyethylene (84.7 %), followed by polypropylene (15.3 %) items. Hard plastics (46.6 %) and sheets (45.4 %) were predominant, with lower presence of plastic lines (7.9 %), pellets (0.05 %), and foams (0.008 %).

Plastic rise velocity ranged from 0.001 to 0.0438 m/s (Fig. 5a). It was directly proportional to plastic length, with the slope of this linear relationship differing among types of plastic (Fig. 5b). While both hard plastics and sheets had a slope equal to 0.002 (95 % CI = 0.0017–0.0026 and 0.0012–0.0023, respectively), plastic lines had a flatter slope of 0.00007 (95 % CI = 0.00002–0.00013), since their rise velocity increased only slightly towards longer pieces. Rise velocities differed among sampled depths, with particles at the surface (0–0.5 m) having a wider range of values and a higher median value than pieces at greater depths (Fig. 5c).

The vertical mixing process was size-selective and affected the size distribution of plastics located at the sea surface (Fig. 6), with the proportion of plastics at depths over 0.5 m generally increasing towards smaller plastic lengths (Fig. 7). For hard plastics and sheets, this trend was observed at all Beaufort numbers sampled. Plastic lines however, only displayed this trend at Beaufort 1, with different size classes showing similar and relatively high underwater proportions at Beaufort 3 and 4.

Data sets produced and analysed in this study are available at figshare (Reisser et al., 2015).