REVIEW OF LITERATURE
Microplastics are tiny particles with a size less than 5 mm and do not dissolve in water. These microplastics can be potentially harmful to aquatic organisms especially that these are small particles which makes them less noticeable therefore negative effects on organism are less obvious. If ingested, these particles can prevent the organism from consuming its natural prey which leads to starvation and even death.
SOURCE OF MICROPLASTIC AND ITS ENTRY INTO THE ENVIRONMENT
Microplastics can be categorized according to their origin. There are two general types, the primary and secondary microplastics. Primary microplastics are plastic particles that were created to be small in size for a particular purpose. They are manufactured as microbeads, capsules, fibers or pellets. One of the commonly known primary microplastics are the microbeads. These are typically found in facial wash, cosmetics, toothpastes and scrubs, and they are usually composed of polyethylene. On the other hand, secondary microplastics are the by-products of weathering and fragmentation of larger pieces of plastic (GESAMP 2015). Waves, sunlight, or other physical stress makes the plastic break into smaller pieces and could introduce microplastics into the environment through wastewater and surface run-off.
Plastics that can potentially break into smaller pieces constitute 95% of the waste that accumulates on the shoreline or in the water surface (Bergmann et al. 2015). This accumulation of plastic can be the cause of the introduction of pollution into the ocean.
There are several studies (Fendall and Sewell, 2009; Abu-Hilal and Al-Najjar, 2009; Claessens et al., 2011) which provide a system of identification and characterization of different kinds of microplastics. Though there is a general category that is provided (primary and secondary microplastics), many researchers prefer a more specific classification which then can lead to the source of microplastics. Claessens et al. (2011) presented a wide range of microplastic types such as fibers, granules, films and spherules. While Abu-Hilal and Al-Najjar (2009) characterized microplastics according to the colors such as clear to translucent, colorless, opaque and whitish, black, blue, yellow, red, brown, pink, purple and green. However, Fendall and Sewell (2009) did not assign terms, instead they described microplastics based on their shapes including spherical, elliptical, granular, rods and threads. Still, it is more appropriate to assign terms than just assign random shapes based on the visual characteristics because it is mostly used in studies about microplastics and could be more affiliated to the origin of the microplastics and their routes into the environment.
There is about 40% of the global population living within 100 kilometers from the coasts which makes it easier to transfer microplastic from urban areas to the marine environment through wastewater systems, rivers and other systems that could transport microplastics into the environment. Factors such as typhoon and flash floods can increase microplastic transfer from terrestrial to aquatic ecosystems.
DISTRIBUTION OF MICROPLASTIC IN AQUATIC ENVIRONMENTS
It has been found that microplastics are present in multiple seas worldwide, including the Arctic and Antarctic Ocean. When microplastics enters the sea, they persist and accumulate in different water bodies being transported around the world through winds and currents (Lusher et al. 2015). Since the beginning of mass plastic consumption and production in the 1950s, there have been 311 million tons of global plastic production in 2014 and it is projected to reach 1800 million tons by the year 2050. Plastics leaking to the ocean at a global scale is still unknown but it is suggested that almost 8 million tons of plastic every year leaks into the ocean. It is estimated that there is an approximately 150 million tons of plastic in the ocean in which around 250, 000 tons of these plastics fragmented into 5 trillion plastic pieces and may be floating at the water surface. It is also estimated that the quantity of plastic in the ocean worldwide will double up to 250 million tons by 2025. The occurrence of microplastics in the ocean vary depending on the high levels of industrial activities or high population densities.
Most of the studies regarding microplastics focus on the seawater environment. There is less than 4% of microplastics-related studies that is reportedly associated with freshwater (Lambert and Wagner 2018). With the information being limited, it revealed that the microplastic abundance in freshwater ecosystem is comparable to that of the marine environment (Peng, Wang and Cai 2017)