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One idea relates to the physical qualities of plastics.
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Scientists think that about 10 percent of all plastic, which includes plastic bags and bottles, ends up in the ocean. The attributes that make plastic a useful material for a large number of products are its light weight and the strong chemical bonds in its internal structure, which make the material durable. Because plastic by itself is less dense than water, it floats along the ocean surface where it is continuously exposed to ultraviolet light from the Sun, which has the effect of loosening its chemical bonds. Ocean waves smash these weakened pieces of plastic against each other, and they are broken down into smaller and smaller pieces, eventually creating vast numbers of microplastics (pieces less than 5 millimeters across) that disperse throughout the water.
Due to the widespread increase in plastic production, there is six times as much plastic in the oceans as there was 40 years ago. Based on the amount of manufactured plastic, scientists have estimated that there are many trillion pieces of plastic in the ocean. To obtain a more precise idea about the amount, scientists conducted a study in 2010-2011 that involved sampling the ocean surface at 141 locations. After six months they concluded that only 7,000 to 35,000 tons of plastic were present on the ocean surface. It is widely believed, however, that tens of millions of tons of plastics have entered ocean waters. Moreover, the scientists primarily found larger pieces of plastic, which means that the quantity of microplastics found diverged from expectations to an even greater degree.
Scientists have attempted to explain the apparent disappearance of vast amounts of plastic from the surface of the ocean. Plastic is hydrophobic-it repels water, like oil does-which by itself contributes to its buoyancy (ability to float). However, this hydrophobic quality attracts single-celled organisms, such as diatoms and bacteria, which attach themselves to the plastic's surface and replicate. Over time the resulting biofilm (layer of organisms) alters the density of the plastic and reduces its degree of hydrophobia, increasing its chances of sinking. Organisms residing in the biofilm release certain chemicals that attract other organisms, including barnacles, aquatic insects, and algae, which make their homes on the plastic. This adds to the plastic's weight and further increases its chances of sinking. In one experiment scientists submerged plastic food bags for three weeks, measuring the amount of biofilm and buoyancy of the bags each week. The bags sank deeper during each week of the experiment. Severe coastal storms and vertical movement of water associated with differences in temperature or salinity may be another reason why microplastics move into deeper waters. Scientists have found as many as 40 pieces of microplastic per 50 milliliters of sediment in locations sampled along the ocean floor.
Some of the microplastics are likely ingested (eaten) by ocean organisms. Many marine invertebrates (animals lacking a backbone) are filter feeders-they draw water into their bodies and ingest tiny bits of food contained within it. These animals do not discriminate among food items and can easily take in plastics along with the rest of their meal. Thus, scientists have found microplastics in the bodies of marine organisms, including zooplankton, crabs fish, and marine worms. Although the plastic by itself can remain in or pass through the animals' bodies without significantly affecting them, many plastics come with pollutants left over from the manufacturing process or picked up as the plastics travel through the oceans, and these chemicals are often harmful to the animals.
Copepods-tiny, free-swimming organisms that eat algae-are among the filter feeders that have been found to ingest microplastics. But then, in an apparent attempt to avoid taking in even more of these materials, copepods switch to feeding on smaller algae, ones that are even tinier than microplastics. As a result of this diet change, copepods' energy intake can drop by as much as 40 percent. These copepods lay smaller eggs that are unable to hatch successfully. Microplastics may also get stuck in the antennae of copepods and other zooplankton, where they interfere with the organisms' ability to sense food in the surrounding waters, and also in copepods' joints, limiting their ability to move and catch prey.
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