When ocean circulation meets plastic...

As mentioned in the last blog, there is more than one 'garbage patch' in the North Pacific Ocean. The formation of these 'garbage patches' is due to ocean circulation systems, which act to accumulate and retain debris.

Howell et al. (2012) provide an excellent synthesis of ocean circulation interactions with marine debris, which I will be reviewing here. Circulation in the North Pacific Ocean is primarily driven by prevailing winds, which create two major gyres, the cyclonic subpolar and the anticyclonic subtropical. Between the two gyres lies a transition zone. At this boundary of the two frontal systems, the cooler, plankton-rich waters of the subpolar gyre meet the warmer, plankton-poor waters of the subtropical gyre. Therefore, a proxy indicator for this transition zone is the chlorophyll front. As the ocean circulates in these patterns, so too does the plastic within it. The result is 3 observed accumulation zones, which we commonly know as 'garbage patches'. The schematic from Howell et al. below shows the major ocean circulation systems and the garbage patches.   

Figure 1: Schematic of North Pacific ocean circulation and areas of concentrated marine debris. Source: Howell et al., 2012. 

The two major ocean gyres can be seen demonstrated in the wind stress (A and B) and surface current (C and D) maps below. The transition zone is indicated by the chlorophyll front (E and F).   

Figure 2: Maps of the North Pacific in February and August 2000-2007, showing wind stress fields (A and B), surface currents (C and D), and the transition zone chlorophyll front, a proxy indicator for the North Pacific transition zone (E and F). Source: Howell et al., 2012.

The Eastern Garbage Patch

This is usually what the media is referring to when talking of the Great Pacific Garbage Patch. In this region between Hawaii and California (visible in figure 2 at 130^o W, 30^o N), the anticyclonic surface currents of the subtropical gyre are at a minimum, meaning that the plastic carried there ends up in a 'dead zone'. The scale of the debris and the region is uncertain, however in 2001 Moore et al. estimated 334,271 pieces of debris per km².

The Western Garbage Patch

This area near Japan can be seen in the wind stress curl and surface current maps in figure 2 as a tight recirculation gyre at 130^o E, 30^o N.

The North Pacific Subtropical Convergence Zone

This convergence zone is at the southern edge of the transition zone. Denser waters from the north sink under warmer waters from the south to form a front. The same mechanisms forming the front also leads to an aggregation of organic and inorganic matter. Active organisms can easily overcome the weak vertical flow of water, however buoyant passive matter (i.e. plastic) is more easily retained. Once in the subtropical convergence zone, it's uncertain what happens to the plastic. As a resilient and non-biodegradable material it's likely the plastic remains there for decades.     

Pichel et al. (2007) found that in spring and early summer there is a high density of marine debris in the subtropical convergence zone. Using aerial surveys of areas within the subtropical convergence zone where high concentrations of debris were expected to be found, they observed over 1800 individual pieces of debris (including two large net bundles each over 10m in diameter!). A significant correlation was found between the density of debris and sea-surface temperature, chlorophyll-a, and the gradient of chlorophyll-a. Using this information Pichel et al. developed the Debris Estimated Likelihood Index (DELI), and produced the map below, where pink is the likeliest place to find debris. This map, which is valid only for spring and early summer (due to shifting of the convergence zone), shows high expected concentrations in the subtropical convergence zone above the Hawaiian islands (the grey) compared to surrounding areas. 

Figure 3: Map of Debris Estimated Likelihood Index in the subtropical convergence zone above Hawaii. Source: Pichel et al., 2007. 

Ocean currents cause plastic accumulates at such a rate, that in samples from the North Pacific Central Gyre near the Eastern Garbage Patch the mass of plastic was approximately 6 times the mass of plankton!