Plastics remain in the ocean environment because of their stability and resistance to degradation or biological processing. they do break up into smaller pieces either mechanically or by the action of ultraviolet light (UV).
The process of polymer (plastic) degradation by light is called photodegradation. Natural degradation of plastic begins with photodegradation, which then leads to thermooxidative degradation. UV provides the activation energy required to initiate the incorporation of O2 atoms into the polymer.