Almost half the plastic manufactured today is heavier than seawater (PlasticEurope 2015). When it enters the sea, it thus sinks to the ocean floor. At the bottom of Tokyo Bay, there is an area that is 80 to 85% covered with plastic waste (Kanehiro et al. 1995).
According to the North American Waste Management, only approximately 66% of plastic waste is lighter than seawater. Plastic waste that is heavier than seawater eventually sinks and is deposited on the ocean floor (van Cauwenberghe et al. 2013, Pham et al. 2014, Woodall et al. 2014). The lighter plastics will also sink to the bottom of the sea as a consequence of attaching marine periphyton.
Fishing nets and fishing lines with weights immediately sink to the bottom, and this abandoned plastic debris covers the ocean floor, causing enormous damage to marine animals and plants. One of these is smothering (Kühn et al. 2015).
Smothering occurs when plastic waste directly covers the marine organisms that inhabit the seafloor. This plastic sediment blocks the sunlight and creates an oxygen-free state (Watters et al. 2010, Van Cauwenberghe et al. 2013, Pham et al. 2014). Furthermore, when plastic is dragged along the bottom by ocean currents or waves, fragile seabed vegetation such as corals and seagrasses will be seriously damaged (Kühn et al. 2015).
In fact, plastic deposited onto reefs smothers corals and causes enormous damage to the coral (Richards & Beger 2011). On Oahu Island in Hawaii, 65% of coral is entangled with fishing lines, and 80% of these are partially or completely dead (Yoshikawa & Asoh 2004).
The plastic debris physically interferes with the coral reef ecosystem. Because these plastics block the sunlight, they disturb the corals’ photosynthetic process (Richards & Beger 2011). They also suffocate the corals, making it difficult for them to access a flow of water and reducing their ability to consume zooplankton, their daily food (Kühn et al. 2015).
Seagrass beds, which are an important feeding ground for dugongs, also become severely damaged by plastic debris. A study has found that the number of seagrass shoots was significantly reduced when a seagrass bed is covered with a net (Uhrin et al. 2005). Blades of seagrass are worn down and are suffocated due to the weight of the net pushing them into the sediment (Uhrin et al. 2005).
Smothering by marine waste reduces the oxygen concentration of the deep sea sediment. For example, in the Nazaré Canyon in Portugal, the oxygen concentration of the water in the sediment has decreased dramatically due to the metabolic processes of the pore water (Mordecai et al. 2011). This has happened because the mass of plastic bags covering the submarine canyon floor has reduced the exchange of pore water in the sediment. This has a fatal impact on the organisms inhabiting the sediment.
When plastic bags were experimentally deployed on a beach for nine weeks, the oxygen concentration of the sediments decreased significantly. The anoxic sediments, of course, reduced the primary productivity of organic matter and significantly lowered the abundance of infaunal invertebrates recorded during the study (Green et al. 2015).
Thus, ecosystem function has long been affected by “smothering” by marine waste. We must be concerned about a loss of biodiversity in such a fragile natural environment (Kühn et al. 2015).