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Marine Litter in the Mediterranean Sea

The fate of floating marine debris has been a topic of conversation for decades (Ryan, 2015), long before the current issue of plastic pollution came to the fore. With some of the first mentions of plastic appearing in academic journals in the 70’s, with Carpenter and Smith (1972) and Colton et al (1974) discussing the occurrence of plastic particles in the Sargasso Sea. Both discuss how an increase of plastic concentration within marine environments would change its classification to hazardous (Carpenter and Smith, 1972; Colton et al., 1974). With Carpenter and Smith (1972) suggesting that mortality and intestinal blockage was a potential outcome of ingestion of plastic by marine organisms.

Ten years later Bean (1987) published a paper regarding legal strategies for reducing plastics and reiterated the point made over a decade earlier that persistent plastic could in fact be classed as hazardous waste. Now, over 50 years after the first warnings of the implications of plastics in the marine environment they have been found from the remote reaches of Antarctica (Barnes et al., 2010) to Greenland (Cózar et al., 2017). An approximation has been made that the world’s oceans hold over 5 trillion pieces of plastic (Eriksen et al., 2014).

Marine organisms are suffering mortal consequences from plastic ingestion and debris entanglement (Gregory, 2009; Gall and Thomson, 2015) just as Colton et al (1974) warned. Plastics in the marine environment are even putting some species at risk of extinction (Gall and Thompson, 2015). With plastic also being found in crustaceans, bivalves and fish species (Devriese et al, 2015; Van Cauwenberghe and Janssen, 2014; Bråte et al, 2016), all species consumed by the general population, marine plastic is also posing risks to human health (Barboza et al, 2018). Wright and Kelly (2017) discussed plastic transport pathways into the human body and it’s potential toxicity to humans, as well as commenting that tissue damage, fibrosis and carcinogenesis could all occur when plastic is introduced into the human body.

Topographic map of the Mediterranean basin.
The continuous beaching of litter causes build up until the litter is present in high concentrations such as seen here.
Marine litter is having an increasingly large effect on marine mammals.

The Mediterranean Sea has been modelled as having one of the highest concentrations of marine debris (Lebreton et al., 2012). Suaria et al (2016) reported that between 5-10% of the world’s plastic can be found in the Mediterranean Sea, which is roughly 5-30 thousand tonnes of plastic. Suaria and Aliani (2014) undertook a large scale survey within the central and western areas of the Mediterranean, using visual survey methods to search for natural and anthropogenic debris. They found that anthropogenic objects made up 78% (1095 objects) of the sightings, with 82% (898 objects) of those sightings being plastic (Suaria and Aliani, 2014). 92.8% of the plastic objects sighted were less than 50cm in size (Suaria and Aliani, 2014).

In the eastern Mediterranean, a visual survey carried out by Constantino et al (2019) over 32 days in the Levantine sub-basin, covering 1784km in 137 transects found an average of 232±325 items/km2. Over 90% of the items counted were made of plastic and the data values reported from this survey are higher then the results of surveys carried out in central and western areas of the Mediterranean (Constantino et al, 2019). Surveying for litter in the Mediterranean is not a new undertaking either, in 1988 the report of another visual survey was published determining that plastic containers were the most prevalent litter type found and that the litter concentration of the surveyed area was ~0.012g/m2.

Litter has also been found in benthic surveys. Galil et al (1995) collected 277 items using a beam trawl during a survey in the eastern Mediterranean, with 36% of those items being plastic. A remotely operated vehicle was used by Consoli et al (2018) to study an area off of the coast of Sicily, they found 56 litter items in a ~60,000m2 area, 73% of these items were made from plastic. Another trawl survey carried out by Pasquini et al (2016) in the northern and central Adriatic sea found a mean of 913±80 items/km2 at the 67 stations investigated, with plastic comprising 80% of the items collected and 62% of the weight. This survey provided evidence that caused the Adriatic basin to be ranked as one of the most polluted marine areas in the world (Pasquini et al, 2016).

Beaches too are suffering from plastic pollution. Pasternak et al (2016) conducted nineteen surveys on eight beaches in Israel over the course of three years and recorded a total of 69,122 items, 90% of which were plastic. Beach cleans carried out on a single beach in Jisr, Israel, semi-weekly for four months, counted 3305 pieces of litter, 78% of which were plastic (Portman, 2017). Prevenios et al (2018) calculated that on average 142±155 items/100m/15 days and reported that sea transport was one of main mechanisms affecting litter removal and accretion on beaches.

Marine litter is having an increasingly large effect on marine organisms.
Marine litter mixes with other marine debris to create large floating rafts.

It has been found that during the summer that tourists cause a 40% increase in litter generation, accounting for more than 75% of the annual summer season waste (Galgani, 2014). Seasonal atmospheric forcing patterns then have a strong influence on where that litter accumulates on the sea surface (Shchekinova, and Kumkar, 2018). Of the current data available it is yet unclear if there are any clear trends making themselves apparent  regarding litter accumulation (Galgani, 2014). With regards to the benthos García-Rivera et al (2018) found that in the Valenciana and Tramontana areas the litter concentrations has not changed, whereas in the Alboran Sea, the concentrations have decreased. This has been factored to a clean-up effort being undertaken by local commercial trawlers (García-Rivera et al, 2018). However, despite this being an ongoing study, currently with eleven years worth of data, the author’s stressed that they do not have enough data to conclude that the decrease in litter is significant (García-Rivera et al, 2018).

There are multiple transport pathways for anthropogenic litter to reach the marine environment . Sources include but are not limited to; shipping activities; riverine inputs; intensive recreational, fishing and industrial activities; improper waste management and beach dumping (UNEP, 2009; Pasquini et al, 2016; Suaria et al, 2016). Once the litter is part of the marine environment it is then dispersed by atmospheric forcing and oceanographic processes (Prevenio et al, 2018). Due to the Mediterranean’s only outflow of water being limited, the extreme coast and catchment population density of 427 million residents not including the tourists (Pasternak et al, 2016), the Mediterranean Sea is accumulating vast amounts marine litter (Suaria et al, 2016).

The accumulating litter is not present in a vacuum and is affecting the Mediterranean’s host of marine organisms (Suaria and Aliani, 2014). 22 stranded Loggerhead turtles (Caretta caretta) found on the Tuscany coastline were examined and found to have ingested 62.37g of marine litter altogether, with a mean and standard deviation of 16.5±29.1 pieces of plastic per turtle (Campini et al, 2013). Codina-Garcia et al (2013) studied the digestive tract contents of seabirds that had been caught in longliners operating off of the Catalan coastline and found that 66% of the 171 seabirds studied had ingested plastic. This is not a new occurrence either, in 2002 Shoham-Finder et al (2002) reported that plastic bags found in the stomach contents of a Risso’s dolphin were partly responsible for the specimen’s poor physical condition.

Apart from the harm to marine organisms there are also the socioeconomic repercussions to consider. The tourism industry suffers as consumers don’t want to holiday in areas ruined by the presence of litter, exacerbating the loss of the recreational value of beaches and coastline (Balance et al, 2000; Newman et al, 2015).  Recreational and industrial vessels are also at risks of incurring a range of costs if marine litter entangles with their propulsion and/or other working mechanisms (Sheavly and Register, 2007; Newman et al, 2015). The removal of said item/s costs money, as well as wasting time and losing profits if it is a shipping vessel; plus if the issue can’t be dealt with on site, hiring a rescue crew increases the cost tenfold (Sheavly and Register, 2007; Newman et al, 2015).There’s also the issue of potential harm to human health marine litter may present to consider as well (Wright and Kelly, 2017).

An example of a propeller becoming entangled with a discarded fishing net.
This bird is now encumbered by a plastic bag, it is unknown if the bird will be able to escape or how long it will survive if it can not.

Marine litter is surveyed and studied in a variety of ways. Visual surveys (Suaria and Aliani, 2014) and net tows (Cózar et al, 2015) are used to sample the surface waters whereas trawls and remotely operated vehicles are used to sample the benthos (Pasquini et al, 2016; Consoli et al, 2018). Beach cleans are used to monitor the coastline (Pasternak, 2016; Portman and Brennan, 2017) and autopsies of organisms to gather ingestion data (Shoham-Finder et al, 2002; Campini et al, 2013; Codina-Garcia et al, 2013). The surveys are carried out over a variety of temporal and spatial scales (Ryan et al, 2009).

This has led to a hodgepodge of data sets that cover a variety of areas and topics that are not statistically powerful by themselves and are too sporadic to show even part of the marine litter lifecycle (Ryan et al, 2009; Moller et al, 2016). There is no standardised methodology or way to standardise the multiple data sets that have been collected from the Mediterranean Sea. Models make an effort to bridge the gap between the window of viewing given by the in situ data and the bigger picture at play but there are still large discrepancies between what is modelled and what is actually happening (van Sebille et al, 2015).

To overcome these issues, the feasibility of using earth observation data to monitor marine litter is being explored. Potential avenues include synthetic aperture radar, optical satellite imaging and lidar (Maximenko, 2017). Aoki (2013) and Arii (2014) have used SAR data to monitor debris and Aoyama (2016) developed a method that used optical satellite images to detect marine debris. All stipulated however that accurate in situ data is necessary for validation purposes and the process is only feasible if the area covered by the debris is large enough to be seen clearly within the satellite images (Aoki, 2013; Arii 2014; Aoyama, 2016). Using earth observation to track marine litter will also increase the current comprehension of surface currents and air-sea interaction (Moller  et al, 2016; Maximenko, 2017). If successful, marine litter could be tracked on a global scale and in a much more cost effective and sustainable way then the current methods being employed.

References

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