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Plasticrusts are a new type ofplastic pollution in the form ofplastic debris, covering rocks in intertidal shorelines which vary in thickness and in color and are composed ofpolyethylene as determined byfourier-transform infrared spectroscopy (FTIR) analysis.[1][2] They were first discovered on the South coast of the volcanic island ofMadeira in theAtlantic Ocean in 2016 and have additionally been found onGiglio Island,Italy.[1][2][3] They are considered a sub-type ofplastiglomerate and could possibly have negative effects on surrounding fauna by entering the food web through consumption bybenthic invertebrates.[1][2][3]
Plasticrusts are formed by waves smashingplastic debris against rugose rocks.[1][2][3] A report concluded that "plasticrust abundance may depend on the local levels of nearshoreplastic debris, wave exposure and tidal amplitude."[2][3] This was based on the abundance of plasticrusts compared to the other listed factors at each location where plasticrust was detected or searched for.[2][3]
Plasticrust has so far been identified at two locations, the coasts ofMadeira in theNE Atlantic Ocean andGiglio Island in theTyrrhenian Sea,Italy, shown below in three maps.[1][2]
In Madeira, the plasticrust was found for the first time in 2016, on the south coast of the island, specifically at 32o44'31" N, 16o42'27" W.[1] The Madeira location is an isolated and off-shore island in the NE Atlantic Ocean, and is thus exposed to open waves and a tidal amplitude of 2.6 meters (8.5 feet).[2] Additionally, it has been confirmed that the plasticrust has persisted since then and that the percent coverage of the plasticrust has actually increased, based on a sampling performed in January 2019.[1]
The Giglio Island plasticrusts were found on ocean facing mid-intertidal rocks in a wave exposed rocky intertidal habitat in the Franco Promontory in westernGiglio Island,Tuscan Archipelago,Tyrrhenian Sea,Italy on October 19, 2019.[2] This location, in contrast toMadeira, is a nearshore island found among other, larger, islands and the Italian mainland. Because of this, it is relatively wave sheltered. Additionally, wind speed and wave height are lower in the Tyrrhenian Sea than in the NE Atlantic and tidal amplitude is less wide at 0.45 meters (1.5 feet).[2]
Three bays were also studied in Giglio which were wave-sheltered instead of wave exposed, including Campese Bay East: 42.3657, 10.8747, Campese Bay West: 42.3700, 10.8813, and Allume Bay: 42.3528, 10.8802. However, no plasticrust was found at these locations, and this information combined with the abundance of plasticrust inGiglio relative to the abundance of plasticrust inMadeira was the basis for determining that plasticrust abundance is related to wave exposure.[2] Similarly, the difference in tidal amplitude between the two locations was the basis for the conclusion that tidal amplitude influences plasticrust abundance.[2]
Although plasticrust has so far only been identified in two locations, according to a study, their findings of plasticrust and pyroplastic, a similar type of plastic pollution, are evidence that plasticrust, among other new plastic debris types, is not a local phenomenon.[2]
The plasticrust samples from both locations were studied usingfourier-transform infrared spectroscopy analysis.[1][2] The results of the analyses at each location determined that the plasticrust at both locations is composed ofpolyethylene, one of the most frequently usedplastics worldwide.[citation needed]
The plasticrust identified inMadeira was found in two colors, blue and white. They had a thickness of 0.77±0.10 mm and a percent coverage as of sampling in January 2019 of 9.46±1.77.[1][3] FTIR analysis did not determine the density of thepolyethylene that composed the plasticrust however, the plasticrusts off the coast ofMadeira likely originate from packaging materials, such as plastic bags, which are made of low densitypolyethylene. The evidence for this being that pieces ofmarine litter found inMadeira's coastal waters often have domestic origin.[1]
TheGiglio Island plasticrusts had similar properties to those found inMadeira. They were white and blue, and they had a thickness of 0.5 to 0.7 mm.[2][3] Additionally, they covered an area of 0.46±0.08 mm2 and had a percent coverage of 0.02±0.01%.[2][3]
The research done on plasticrusts at both locations suggest that plasticrust may harm surrounding fauna throughmicroplastic particles entering thefood web, specifically via consumption bybenthic invertebrates such as barnacles, snails, and crabs.[1][2]
It's been suggested that plasticrusts inMadeira may be consumed by intertidal snails (Tectarius striatus), which are found around and on top of the plasticrust. They usually eatdiatoms and algae, however another species,common periwinkle, has been shown to not be able to distinguish between algae with adherent microplastics and clean algae, so according to the initial study "we cannot discard its potential grazing on plasticrusts".[1]
In Giglio, gastropods were not found at the same location as the plasticrusts as they were in Madeira. However,marbled rock crabs, which eat algae and invertebrates, have been observed at the same elevation along the shoreline that plasticrusts were found.[2][3]
There are three other types of plastic debris pollution which are similar to plasticrust, includingplastiglomerate, pyroplastic, and anthropoquina.[3]
Plastiglomerate is melted plastic mixed with substrate to create new fragments of material with greater density.[4][3] First discovered in 2014, it is also described as "an indurated, multi-composite material made hard by agglutination of rock and molten plastic".[4][5] It subdivides into two types,in-situ andclastic. The in-situ type is where the plastic is adhered to rock outcrops, similar to plasticrusts. The clastic type is wherebasalt, coral, shells, woody debris, and sand are cemented with a matrix of plastic.[4] They are often referred to as a new type of rock, but they are not defined as such due to the fact that they are anthropogenic in origin and rocks by definition from naturally.[3][5]
Pyroplastics are considered a sub-type ofplastiglomerates which are formed from the melting or burning of plastic.[3][6] Similar to plasticrusts, they can be found adhered to outcrops of rock, and along the shoreline as distinctclasts which show various degrees of weathering.[6] Pyroplastics were also found on Giglio Island with the plasticrusts.[2][3]
Anthropoquinas refer tosedimentary rocks that contain objects with an anthropogenic source, which are referred to as technofossils, such as plastic fragments or metal nails, along with natural materials such asmollusk shells andsiliciclastic grains.[3][7] Like plasticrusts, which are created by sea waves, the formation of anthropoquinas is not contributed to by human activity.[1][2][3] They are formed, like all sedimentary rocks, throughcementation.[7]
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