Spatial and Temporal Variation of Microplastics
Oceanographic modeling indicates that a significant proportion of floating debris reaching the oceans is accumulated in “gyres” . Subtropical gyres are large-scale systems of wind-driven surface currents which flow clockwise in the northern hemisphere and counterclockwise in the south. This flow is caused by the Coriolis effect, a force which tends to move wind and water currents to the right in the northern hemisphere and to the left in the south, thus creating cyclonic movements in the atmosphere and oceans . The earliest attempts to mathematically reproduce the probable pathways of marine debris were conducted using a global set of trajectories of satellite-tracked drifters. A probabilistic model eliminates the bias in spatial distribution of drifter data due to heterogenous deployments [16,46]. A study conducted for over 6000 plankton tows undertaken between 1986 and 2008 in the North Atlantic Ocean and the Caribbean Sea showed that plastic constituted 60% of the samples . In this study, distinct spatial patterns of plastic were found. About 83% of total sampled plastic was found in subtropical latitudes, with the highest concentration mapped to the North Atlantic gyre (20,328 ± 2,324 pieces/km2) [6,36]. An even higher plastic concentration of 33,271 pieces/km2 has been found in the North Pacific gyre . The findings of such large quantities have led to the North Pacific gyre being labeled as “plastic soup” and described as the “great Pacific garbage patch” .
Plastic particles found in the marine environment are composed of a variety of polymers. Depending upon their density, composition and shape, the plastic particles can be fully buoyant (that is, float on the surface), be neutrally buoyant or sink to the seabed . Density plays an important role in the location of plastic particles within the water column in marine bodies. Low-density microplastics have been predominantly found on the sea-surface layer [20,21]. However, a study involving the collection of microplastics from the North Atlantic found microplastics with a density greater than that of water to be floating on the surface waters . The average density of seawater is 1025 kg/m3 . In another study, 90% of the microplastics recovered from the western North Atlantic Ocean had an average density less than the density of seawater, with densities ranging from 808 to 1238 kg/m3 . The occurrence of higher density microplastics on the surface of oceans may result from the powerful upward and downward movements of water due to temperature difference at different depths or the presence of air bubbles or pockets in denser microplastics which tend to increase their buoyancy . However, PVC (with a density of 1150-1700 kg/m3) and polyamide (with a density of 1120-1380 kg/m3) may transport to various marine regions as a result of wind and tidal currents, rather than due to density variation. Similarly, some microplastics, like polyethylene, which possess a density of about 920-970 kg/m3, undergo an increase in density as a result of weathering. Several studies have shown that the growth of fouling organisms like biomass can sink the buoyant microplastics [20,26,31,42]. Plastic debris in marine bodies rapidly accumulate microbial biofilms, which allow the colonization of algae and invertebrates on the plastic surface, thus increasing the density of plastic particles .
High-density microplastics that are composed of materials like polyester, PVC and polyamide are found in largest quantities in the benthic zones. The determination of microplastic quantities on the seabed is constrained by the cost and difficulties in sampling . Some countries such as the Netherlands and Scotland have initiated “fishing for litter” schemes and submersible video recordings to document the microplastic quantity on the seafloor [6,43,44]. However, microplastics may eventually escape the lower detection limits of these sampling methods . When high-density microplastics enter the seas, they tend to remain in suspension owing to high flow rate, tidal fronts or a large surface area . These dense microplastics tend to sink to the seabed when the momentum is lost .
The slow emergence of global legislation governing the indiscriminate disposal of plastics has led to an increase in microplastics entering the marine bodies, corresponding to the higher production rates of plastic [25,42,45]. While progressive fragmentation over time and the popularity of plastic scrubbers have increased the volume of microplastic debris in the ocean, it has caused a reduction in the average size of plastic litter over time [6,42]. During the analysis of samples from the North Sea and Northwest Atlantic, microplastics concentration in the 1980s and 1990s were found to be substantially higher than those in the 1960s and 1970s .