Introduction

Gothenburg canal near Drottningstorget, 06.02.16, both natural debris and plastics drifting on the water

Plastic as a production and packaging material has the strong advantage to be relatively cheap and to have an excellent user convenience “- combining unrivalled functional properties with low cost” (WEF, 2016, p.6) as the report the New Plastics Economy puts it. But additional to the assumed low costs for producers and consumers appear external costs, as typical for many economic activities. Externalities occur first from production emissions, contributing to global warming, and second from pollution of natural systems where the plastic is not taken care of properly after its use phase. In addition, the material is lost from the cycle if plastic is littered instead of recycled or at least incinerated for energy re-generation. Consequently plastic materials are not that cheap any more if external costs on the environment are taken into account. About a third of plastic packaging is not collected suitably but instead released into surrounding ecosystems, causing substantial costs to the economy “by reducing the productivity of vital natural systems such as the ocean” (WEF, 2016, p.6). Due to its extreme durability, plastics have very a long lifetime and are estimated to persist for centuries (Engler, 2012). The impacts of plastic on the ocean in general as well as on marine ecosystems will be assessed further in the following. (WEF, 2016)

Abundance

Sources

So the first question to be answered is where does it come from; that is what different sources are there for marine plastic debris? One source is the fishing industry with its many small and big vessels that often simply dump their waste including old gear such as fishing nets into the ocean (Sheavly & Register, 2007). According to Hammer et al. (2011) stems debris from ocean-based sources from basically all kinds of ships and ocean activities that exist (merchant ships, ferries and cruise liners, military and research vessel, boats used for recreational purposes, offshore oil and gas platforms, and of course the already mentioned fishing vessels).

Though most debris (80%) comes from land-based sources which include everything that is carried to the coast from inland by rivers and everything that is transported by wind or water level changes into the sea (Jambeck et al, 2015). Also, some waste is dumped into the ocean on purpose in nation states or regions where there is lack of proper waste management and of knowledge. However, small plastic particles pose an even bigger threat to ecosystems than big pieces of plastic, as the so-called microplastics (mostly described as smaller than 5 mm) are often bioavailable and accumulate in the food chain (Wright et al, 2013; Moore, 2008). Their sources are on the one side the weathering down of bigger plastic debris into smaller and smaller fragments through solar radiation and wave movements etc. (Andrady, 2011; Mani et al, 2015; Kershaw et al, 2011). On the other side, plastic particles can also derive directly from sources like industry, cosmetic products or clothing in a very small size. If coming from industry, pellets are spilled accidentally during ship transport or emitted with waste waters from production processes (Duxbury, 1992). Next, microplastics can be found in several cosmetics such as toothpaste or facial cleansers where the particles are used for their scrubbing effect (Fendall & Sewell, 2009; Gregory, 1996). Otherwise originates a huge amount of fibres as a rub-off of synthetic clothing from washing machines every day (Browne et al, 2011; Katsnelson, 2015).

Transport routes and distribution

Microplastics are relatively evenly distributed at coasts and often it is not possible to link measured concentrations to urban or industrial areas (Claessens et al., 2011). Thus sea currents distribute particles all around the globe, though in varying concentrations (Andrady, 2011; Derraik, 2002; Sherman & van Sebille, 2016). But the role of rivers as transporters of plastic is significant, as they carry their plastic load from inlands to the oceans (Claessens et al., 2011). That is why most plastic particles can be found near the cost and in the so-called ocean-gyres (Cole et al, 2011). These are vast patches in between the continents where ocean currents concentrate floating particles due to their flow conditions (Cole et al, 2011). The most commonly known one of these gyres is the Great Pacific garbage patch in between North America and Southeast Asia (Kaiser, 2010). 

Figur 1: Schematic plastic flows from Ryan et al, 2009

Impacts

Threats to marine biota 

Plastic items in the oceans pose an often fatal risk to a growing number of marine species. Sea turtles, whale species and seals are reported to suffer most from getting entangled into debris objects which makes them starve, strangle or suffocate to death eventually. Then, ingestion of plastic particles occurs most excessively for ocean-feeding birds and is probably known the longest for albatrosses who feed plastics to their chicks as well. The smaller the items are, the smaller also the species that swallow them. So plankton can ingest microplastics, small fish eat that plankton and plastic particles, bigger fish eat the smaller fish and plastics, which is eaten by marine birds and so forth. Even turtles for instance seem to mistake plastic bags for jellyfish, so they feed on them. (Hammer et al, 2012; Wright et al, 2013; Wilcox et al, 2015; Gregory, 2009)

Other impacts

Hammer et al. (2012) estimate that 70% of all marine debris sooner or later sinks to the sea floor. Therefore the impact of particles on the bottom of the ocean also needs to be addressed. The impact of plastic accumulated on the sea floor is a hindered gas exchange between the ground sediments and the water layers on top of it, which might lead to anaerobic milieus and affects the biota that live in and on the ocean bed (Moore, 2008). A further point of concern is the spread of invasive species via plastic items as biota encrusted to floating particles can easily enter alien habitats (Gregory, 2009).

Contaminants

As Van Cauwenberghe and Janssen formulate it are “threats to human health through the consumption of microplastics present in seafood […] becoming apparent” (2014, p. 69). Their research proved that microplastics were present in both blue mussels and oysters and a conclusion is that humans who eat shellfish are thus exposed to some plastic-associated ecotoxicity as well. Yet they stated that further studies are necessary to evaluate to what extent chemical contaminants are conveyed to humans by seafood and to estimate the risk related to that (Van Cauwenberghe & Janssen, 2014).

More in detail, plastics carry toxic additives that determine their properties for the intended use which can be transferred to marine biota through plastic ingestion (Engler, 2012). Additionally, plastic particles adsorb chemical substances that the oceans contain in low concentrations and accumulate these on their surface (Seltenrich, 2015). The toxics are mostly durable and can cause severe disturbance in the hormone system (endocrine disruptors) as well as accrue over the food chain (Engler, 2012). PCB (polychlorinated biphenyls) and BPA (bisphenol A) are among the most well-known ones of these substances. Consequently, humans are exposed to this threat with potentially accumulated concentrations of toxics when consuming seafood (Seltenrich, 2015).