Microplastic particles in ocean water
Worldwide, the spread of plastic debris has been documented through all oceanic ecosystems. Plastic pollution in the oceans is directly correlated with this material being flexible, strong and durable, which is linked to the high amounts of plastics produced, used and easily discarded 2,21. Microplastic fragments have been found in sedimentary habitats, shores, pelagic zones 1,22, deep sea 23 and in living organisms 24, including humans 14.
In our study, microplastic particles in the four range sizes (150 – 500, 501 – 750, 751 – 1000 and 1001 – 5000 µm) were present at all stations (see Figure S1). The size distribution of plastic particles in the seawater samples showed that the smallest size class, between 150 – 500 µm, is more abundant than the larger sizes. Other authors have also reported that smaller microplastic sizes abundance is a common characteristic result of the plastic size distribution among the oceans 25,26. In addition, several studies on microplastic size abundance and distribution have shown a permanent fragmentation of microplastic from larger to smaller, to even into nanoplastic (< 25 µm), occurring continuously in the oceans 6, and in all aquatic environments 15.The main global concern about the predominance of this size class is its risk potential for filter feeders, which tend to confuse it for plankton and end up consuming plastic debris 14. Further analytical chemistry characterization of the type polymers and POP’s present in the samples at each station, was originally intended in this study to cover the smaller fraction and nanoplastic molecular classification. Nevertheless, and as stated below, oil contamination unable this analysis to be achieved.
Modelling of microplastic circulation
Oceanic circulation models suggest the highest concentrations of plastic debris are accumulated along the five main subtropical ocean gyres defined as convergent zones by the Ekman currents 8,27. As such, ocean currents play a major role in the origin source, transportation, distribution and accumulation of plastic debris around the world. In our study area, several station points were detected with large concentrations of microplastic particles mostly in the central to southern part of the study area, which were outside the local small gyres present (see Fig. 4). These findings are coherent with basin-scale microplastic particles transport that explain sources and pathways of microplastic that end up in the Galápagos Archipelago 13 from far South oceanic basins. However, Costa Rica and other countries farther north can be considered as plastic particle origin sources if simulations are not limited to surface currents.
Plastic transport may also depend on the sinking processes that plastic particles undergo when they reach the ocean 5,11. Microplastics present differential buoyancy according to plastic polymers and additives combined for reaching properties of the objects they came from 28. Around 60% of plastic items produced are less dense than seawater3. Biofouling and other interactions with marine biota, degradation, fragmentation or additives leaching may accelerate the sinking process of derived plastic particles 13. The impact of microplastics in the marine environments, however, depends on physical behaviors (migration, sedimentation and accumulation), chemical behaviors (degradation and adsorption) and bio-behaviors (ingestion, translocation and biodegradation) 29. Still, trawl sampling efforts coupled with vessel-based sighting surveys confirm that available data on quantities and characteristics of buoyant plastic particles in the nanoplastic range represent only 13% of the available buoyant plastic mass 2. Therefore, new insights have coupled measured concentrations of ocean plastic of different sizes and types, dispersal models, geo-referenced imaginary and seasonal and intern annual changes to improve the estimations of plastic debris in the upper water column 1.
The Galápagos archipelago and its Marine Reserve lay 1000 km off the coast of the South American coastline and are among the most emblematic wildlife refuges in the world. However, plastic litter and microplastic residues have recently been found even in this isolate group of islands and around its waters. To our knowledge, prior to this study, the levels of this microplastic contamination and its quantification on Galápagos coastlines and across the Eastern Tropical Pacific were barely known and limited to one single study 13.
Microplastic in marine organism of human consumption
Plastic particles in the digestive systems of many species of fish and other marine organisms consumable by humans have been reported and quantified 17,30. Recent studies on plastic size abundance and distribution have shown a continuous fragmentation of microplastic into nanoplastic occurring constantly in the oceans by marine organisms ingesting microplastics and bio-accumulating these particles in their stomachs 3,4.
In the present study, microplastic contamination and consumption by marine organisms were reported through the quantification of microplastic particles in the digestive tract of 240 marine organisms of human consumption including fishes, cephalopod molluscs and crustaceans. Microplastic fragments were detected in 166 out of 240 specimens (69%) from the 16 different species analysed. Moreover, microplastic particles were found in 149 (71%) of 210 fish from 14 different species (in at least eight specimens for each of all fish species analysed). This value is higher than those previously reported 31, which allows to conclude that microplastic debris in the form of fish feed, may accumulate over time and space.
Among all different species analysed in this work, 77% of the carnivorous species presented microplastic pieces in their digestive tract, followed by planktivorous (63%) and detritivores (20%). As previously stated, contamination of microplastic particles of all sizes in the Oceans are easily mistaken with food by marine organisms, especially when they overlap with the size range of their prey 7. From the total 16 examined species, Dosidicus gigas, commonly known as giant squid, reached 93% microplastic prevalence in its digestive tract. It was followed by Alopias pelagicus and Coryphaena hippurus with 87% prevalence each. All three are carnivorous species (see Fig. 3). In a previous study, plastic ingestion in carnivorous species of fish 30 ranged from <1% to 58%. The 77% obtained in our research breaks the normal parameters, showing that tropical Pacific Equator coast has worrying high levels of microplastic pollution in comparison with reports from other Pacific oceanic basins.
On the other hand, planktivorous species are thought to develop mechanisms to avoid consuming microplastic particles 32,33. It has been suggested that planktivorous fish species may have a low risk of plastic ingestion in superficial waters 10. Yet, the 63% prevalence in planktivorous fish analysed in the present work is considerably high when compared to the 5% prevalence found from a previous study 9.
In spite of scientific evidence of plastic entrance to different tissues than those related to the digestive tract 28,34, no plastic was found in the muscle tissue from the 240 marine organisms examined in this study. This may be due to particle size range analysed, which was limited to microplastic >150 µm, and the nanometre range has proven to have greater capacity for tissue translocation 4,35. As briefly mentioned above, our research originally planned to do an analytical chemistry characterization of the type polymers and POP’s present in the samples at each station. Special filter samples were simultaneously collected at each station for this purpose. However, oil contamination was detected in all analysed samples in the lab, which unable to achieve this complementary part of the study and thus, this entire section was excluded from the manuscript. As of today, the cause of the oil contamination remains unknow.
To the best of our knowledge, this is one of the first report quantifying microplastic abundance and circulation in the Tropical Eastern Pacific and around the Galápagos archipelago. This is the first time that microplastic particles are detected and quantified in marine organisms of human consumption in the region.