with a focus on the Persian/Arabian Gulf and Red Sea
Introduction
Micro-plastics (henceforth referred to as ‘MPs’) are defined as pieces of plastic which are less then 5mm in diameter (Arthur et al., 2008) and can be categorised as primary or secondary (Deng et al., 2020). Primary MPs enter the marine environment already 5mm in size (Wang et al., 2019) and come from a range of sources including microbeads from cosmetic products, microfibers from textiles, city dust and plastic pellets (Boucher & Friot, 2017). Secondary MPs result from the degradation of macro-plastics by physical, chemical and biological processes (Deng et al., 2020). In the marine environment this can occur by mechanical abrasion with sediments and fragmentation in sea swash (Chubarenko & Stepanova, 2017; Efimova et al., 2018). This is driven by UV radiation making plastics brittle and susceptible to fragmentation in the first place (Duis & Cors, 2016). It has been estimated globally that eight million tonnes of plastic enter the oceans every year (Jambeck et al., 2015). Given that only 1% of this plastic pollution is estimated to remain floating on the water surface (Law, 2017; Martin et al., 2020), it has been suggested that many plastics entering the marine environment are degraded to MP’s and sequestered in sediments (Cauwenberghe et al., 2013).
This is most likely to occur in blue carbon habitats in coastal ecosystems such as salt marshes, seagrass meadows and mangroves (Alongi, 2020) which have a high capacity for carbon accumulation in their sediments (Duarte et al., 2013). Mangrove ecosystems are transitional intertidal systems located mainly in the tropic and sub-tropic regions (Bibi et al., 2019; Hamilton & Casey, 2016) between 30°N and 30°S latitude (Deng et al, 2020). The mangrove habitat consists predominately of mangrove trees which are woody plants with specialised aerial roots known as rhizophores (arc-shaped, buttress or stilt roots) or pneumatophores (straight, pencil-like roots) (Sadeer et al., 2019). These aerial roots act as a respiratory system which facilitates gas exchange and overcomes the high saline conditions and anaerobic soils (Sadeer et al., 2019; Inoue et al., 2019). Mangrove roots also grow laterally to anchor the trees in the loose sediment (Inoue et al., 2019). The aerial root systems and above ground biomass of mangrove habitats can greatly dissipate wave energy (Vanegas et al., 2019; Sánchez-Núñez et al., 2020), e.g. areas of rhizophores have reduced wave height by 80% in laboratory experiments (Hashim & Catherine, 2013), and reduce sediment resuspension (Duarte et al., 2013). This increases the burial rate and sequestration of carbon in mangrove sediments, giving them a high capacity as carbon sinks (Breithaupt et al., 2012; Lee et al., 2014) and driving them to be one of the most productive ecosystems in the world (Lee et al, 2014; Sadeer et al., 2019; Deng et al., 2020). However, there is growing concern and evidence to suggest that MP’s may be similarly accumulated in mangroves through these processes and thus the need to quantifying MP accumulation and the impacts on mangrove ecosystems is receiving much attention in current marine research.
Sources and types of MP’s in mangrove sediments
The presence of MPs in mangrove sediments has been recently characterised in many studies, particularly in China (Duan et al., 2020; Wang et al., 2020; Zhang et al., 2020). Fibres are one of the most dominant MP type found in mangrove sediments, followed by fragments (Deng et al., 2020). For example, fibres made up 51% and 69% of to the total MPs found in mangrove sediments at two locations in South Africa (Govender et al., 2020), on average 72% across seven mangrove habitats in Singapore (Mohamed Nor & Obbard, 2014) and 50% and 91.2 % at two locations in China (Wang et al., 2020). MP fibres mainly come from the domestic washing of synthetic textiles and are discharged into the marine environment in effluent from waste water (Napper & Thompson, 2016; Boucher & Friot, 2017). This has been identified as a main source for the high percentage of fibres in mangroves in South Africa (Govender et al., 2020). In the Persian Gulf, fibres made up very high percentages of the total micro plastics found in coastal environments: 88% in sediments across five mangrove sites (Naji et al., 2017) and >57% in sediments across another five mangrove sites (Naji et al., 2019) all along the Northern coast of the Persian gulf; 93.8% in Qatar coastal surface waters and beach sediments (Abayomi et al., 2017). The sources of fibres in mangrove sediments of the Persian Gulf have been suggest again as effluent from waste water (Naji et al., 2017) as well as weathering of fishing gear such as ropes and nets (Martin et al., 2020 ). Maritime activities have also been suggested as a main source of fibres or threads in mangrove sediments from the weathering of fishing gear in Southern China (Cole et al., 2011; Li et al., 2020).
MPs in Mangrove Biota
The percentages of fibres found in the mangrove sediments of the Persian Gulf are reflected in the types of plastic found to be ingested by mangrove inhabiting organisms. For example, Abbasi et al., (2018) report that nearly all MPs encountered in the tissues of species of fish and prawn were fibrous particles. Specifically, these were detected in the guts (gastrointestinal tracts), skin, muscle, gills and liver of demersal and pelagic fish species and in the exoskeleton and muscle of prawn species. Similarly Naji et al., (2018) report that 58% of the MPs found in the soft tissues of molluscs in mangroves along the North coast of the gulf, were fibre particles. In both cases, higher concentrations of MPs were found in higher trophic level predatory species, suggesting MPs may be accumulated in species through trophic transfer. This is also consistent with the findings of Al-Slalem et al., 2020, who found higher concentrations of MPs in higher trophic level fish species such as the yellowfin seabream (Acanthopagrus latus). This trend is particularly concerning given that these types of predatory species can be consumed by humans e.g. the yellowfin seabream is of high commercial value in the State of Kuwait, where Al-Slalem et al., (2020) conducted their study (Al-Husaini et al., 2015). It is clear that the high percentages of fibres found in the sediments of the mangroves in the Persian Gulf is associated with the subsequent levels of fibres found to be taken up by biota. There is evidence that the polymer type and colour of MPs in mangrove sediments also corresponds to that found in mangrove biota (Deng et al., 2020) suggesting that the amount and type of MPs taken up by biota may be dependent on that which can be sequestered in mangrove sediments. This highlights the importance of quantifying the extent of MP accumulation in mangrove sediments as well as the factors affecting the processes which accumulate them. Martin et al., (2020) noted that the size of MPs in sediments generally remain uniform suggesting that fragmentation is likely to occur prior to burial and MP sequestration of sediments is therefore size-dependant. The details of the study are explored in the following section.
Sequestration of MP’s in Mangroves of the Red Sea and Persian Gulf
In the Red Sea and Persian Gulf there lower than expected concentrations of plastics have been found floating on the sea surface given the nature of the estuarine circulation and currents which characterise them (Martí et al., 2017; Martin et al., 2019a; Martin et al., 2020). It was therefore hypothesised by Martin et al., (2020) that there are significant plastic removal and sequestration processes by the many mangrove habitats located along the Red Sea and Arabian/Persian Gulf. One of the key findings in the study is the consistency of plastic deposition in the mangrove sediments with the history of plastic production e.g. half (3920 ±940 items m-2) of the total (7840 ±1630 items m-2) abundance of MP’s were found in the upper 5cm of surface sediment, with the accumulation of MP’s starting in sediment dated to the 1930s. This is consistent with the onset and increase of industrial production of plastics (Boucher & Friot 2017; Martin et al., 2020). Furthermore, the study found an exponential increase in the plastic burial rate (8.5 ± 1.2% year-1) since the 1950s. This is a crucial link as it demonstrates not just the presence of MP’s in mangrove sediments, but the accumulation of MP’s as their production has increased and ability of mangrove habitats to retain plastics in their sediments over long time frames, acting as major sinks for plastic pollution. Other studies seem to lack this extensive historical characterisation of MP’s in mangrove sediments as samples are only taken from the top 1-10cm of surface sediment (Naji et al., 2017; Duan et al., 2020; Wang et al., 2020; Zhang et al., 2020) whereas Martin et al., (2020) used corers 1.4-1.7m long to sample sediments which could date back much further. It is suggested that studies looking to quantify the extent of plastic retention in mangrove sediments should sample sediments across much higher depth ranges.
It is generally agreed so far that the sequestration of MPs in mangrove sediments occurs through the same process in which they act as carbon sinks. The aerial roots, particularly pneumatophores, are agreed to play a key role in acting as a sieve for plastics entering mangrove systems and aid their fragmentation (Li et al., 2019; Martin et al., 2019b; Govender et al., 2020). Though there are increasing amounts of studies addressing the characterisation of MP sequestration in mangrove sediments and biota, there seems to be no studies addressing the effect on mangrove vegetation such as anoxia of roots and seeds in sediments. Furthermore, there is concern that the nature of mangroves to act as sinks for MPs, may allow them to turn into sources of MPs for adjacent marine environments (Govender et al., 2020). Both these areas could be addressed in future research.
References
Abbasi, S., Soltani, N., Keshavarzi, B., Moore, F., Turner, A. and Hassanaghaei, M., 2018. Microplastics in different tissues of fish and prawn from the Musa Estuary, Persian Gulf. Chemosphere, 205, pp. 80-87. https://doi.org/10.1016/j.chemosphere.2018.04.076.
Abayomi, O., Range, P., Al-Ghouti, M., Obbard, J., Almeer, S. and Ben-Hamadou, R., 2017. Microplastics in coastal environments of the Arabian Gulf. Marine Pollution Bulletin, 124(1), pp. 181-188. https://doi.org/10.1016/j.marpolbul.2017.07.011.
Al-Husaini, M., Bishop, J., Al-Foudari, H. and Al-Baz, A., 2015. A review of the status and development of Kuwait’s fisheries. Marine Pollution Bulletin, 100, pp. 597-606. https://doi.org/10.1016/j.marpolbul.2015.07.053
Alongi, D., 2020. Global Significance of Mangrove Blue Carbon in Climate Change Mitigation. Sci, 2(3). https://doi.org/10.3390/sci2030067.
Al-Slalem, S., Uddin, S. and Lyons, B., 2020. Evidence of microplastics (MP) in gut content of major consumed marine fish species in the State of Kuwait (of the Arabian/Persian Gulf). Marine Pollution Bulletin, 154. https://doi.org/10.1016/j.marpolbul.2020.111052.
Arthur, C., Baker, J. and Bamford, H., 2009. Proceedings of the International Research Workshop on the Occurrence, Effects, and Fate of Microplastic Marine Debris, September 9-11, 2008, University of Washington Tacoma, Tacoma, WA, USA.
Boucher, J. and Friot D., 2017. Primary Microplastics in the Oceans: A Global Evaluation of Sources. Switzerland: IUCN.
Breithaupt, J., Smoak, J., Smith III, T., Sanders, C and Hoare, A., 2012. Organic carbon burial rates in mangrove sediments: Strengthening the global budget. Global Biogeochemical Cycles, 26(3). https://doi.org/10.1029/2012GB004375.
Cauwenberghe, L., Vanreusel, A., Mees, J. and Janssen, C., 2013. Microplastic pollution in deep-sea sediments. Environmental Pollution, 182, pp. 495-499. https://doi.org/10.1016/j.envpol.2013.08.013.
Chubarenko, I. and Stepanova, N., 2017. Microplastics in sea coastal zone: Lessons learned from the Baltic amber. Environmental Pollution, 224, pp. 243-254. https://doi.org/10.1016/j.envpol.2017.01.085.
Cole, M., Lindeque, P., Halsband, C. and Galloway, T., 2011. Microplastics as contaminants in the marine environment: A review. Marine Pollution Bulletin, 62(12), pp. 2588-2597. https://doi.org/10.1016/j.marpolbul.2011.09.025.
Deng, H., He, J., Feng, D., Zhao, Y., Sun, W., Yu, H., et al., 2020. Microplastics pollution in mangrove ecosystems: A critical review of current knowledge and future directions. Science of The Total Environment, 753(20). https://doi.org/10.1016/j.scitotenv.2020.142041.
Duarte, C., Losada, I., Hendriks, I., Mazarrasa, I. and Marba, N., 2013. The role of coastal plant communities for climate change mitigation and adaptation. Nature Climate Change, 3, pp. 961–968. https://doi.org/10.1038/nclimate1970.
Duan, J., Han, J., Zhou, H., Lau, Y., An, W., Wei, P., et al., 2020. Development of a digestion method for determining microplastic pollution in vegetal-rich clayey mangrove sediments. Science of The Total Environment, 707. https://doi.org/10.1016/j.scitotenv.2019.136030.
Duis, K., and Coors, A., 2016. Microplastics in the aquatic and terrestrial environment: sources (with a specific focus on personal care products), fate and effects. Environmental Science Europe, 28(22). https://doi.org/10.1186/s12302-015-0069-y.
Efimova, I., Margarita, B., Bagaev, A., Kileso, A. and Chubarenko, I., 2018. Secondary Microplastics Generation in the Sea Swash Zone With Coarse Bottom Sediments: Laboratory Experiments. Frontiers in Marine Science, 5. https://doi.org/10.3389/fmars.2018.00313.
Govender, J., Naidoo, T., Rajkaran, A., Cebekhulu, S., Bhugeloo, A. and Sershen, 2020. Towards Characterising Microplastic Abundance, Typology and Retention in Mangrove-Dominated Estuaries. Water, 12. https://doi.org/10.3390/w12102802.
Hamilton, S. and Casey, D., 2016. Creation of a high spatio‐temporal resolution global database of continuous mangrove forest cover for the 21st century (CGMFC‐21). Global Ecology and Biogeography, 25(6), pp. 729-738. https://doi.org/10.1111/geb.12449.
Hashim, A. and Catherine, S., 2013. A Laboratory Study on Wave Reduction by Mangrove Forests. APCBEE Procedia, 5, pp. 27-32. https://doi.org/10.1016/j.apcbee.2013.05.006.
Inoue, T., Kohzu, A., Shimono, A., 2019. Tracking the route of atmospheric nitrogen to diazotrophs colonizing buried mangrove roots. Tree Physiology, 39(11), pp. 1896-1906. https://doi.org/10.1093/treephys/tpz088.
Jambeck, J., Roland, G., Wilcox, C., Siegler, T., Perryman, M., Andrady, A., et al., 2015. Plastic waste inputs from land into the ocean. Science, 347(6223), pp. 768-771. https://doi.org/10.1126/science.1260352.
Law, L., 2017. Plastics in the Marine Environment. Annual Review of Marine Science, 9, pp. 205-229. https://doi.org/10.1146/annurev-marine-010816-060409.
Lee, S., Primavera, J., Dahdouh-Guebas, F., McKee, K., Bosire, J., Cannicci, S., et al., 2014. Ecological role and services of tropical mangrove ecosystems: a reassessment. Global Ecology and Biogeography, 23(7), pp. 726-743. https://doi.org/10.1111/geb.12155.
Li, R., Zhang, L., Xue, B. and Wang, Y., 2019. Abundance and characteristics of microplastics in the mangrove sediment of the semi-enclosed Maowei Sea of the south China sea: New implications for location, rhizosphere, and sediment compositions. Environmental Pollution, 244, pp. 685-692. https://doi.org/10.1016/j.envpol.2018.10.089.
Li, R., Yu, L., Chai, M., Wu, H. and Zhu, X., 2020. The distribution, characteristics and ecological risks of microplastics in the mangroves of Southern China. Science of The Total Environment, 708. https://doi.org/10.1016/j.scitotenv.2019.135025.
Marti, E., Martin, C., Cozar, A and Duarte, C., 2017. Low Abundance of Plastic Fragments in the Surface Waters of the Red Sea. Frontiers in Marine Science, 4. https://doi.org/10.3389/fmars.2017.00333.
Martin, C., Agustí, S. and Duarte, C., 2019a. Seasonality of marine plastic abundance in central Red Sea pelagic waters. Science of The Total Environment, 688, pp. 536-541. https://doi.org/10.1016/j.scitotenv.2019.06.240.
Martin, C., Almahasheer, H. and Duarte, C., 2019b. Mangrove forests as traps for marine litter. Environmental Pollution, 247, pp. 499-508. https://doi.org/10.1016/j.envpol.2019.01.067.
Martin, C., Baalkhuyur, F., Saderne, V., Cusack, M., Almahasheer, H., Krishnakumar, P., et al., 2020. Exponential increase of plastic burial in mangrove sediments as a major plastic sink. Science Advances, 6(44). https://doi.org/10.1126/sciadv.aaz5593.
Mohamed Nor, N. and Obbard, J., 2014. Microplastics in Singapore’s coastal mangrove ecosystems. Marine Pollution Bulletin, 79(1-2), pp. 278-283. https://doi.org/10.1016/j.marpolbul.2013.11.025.
Naji, A., Esmaili, Z., Mason, S. and Vethaak, A., 2017. The occurrence of microplastic contamination in littoral sediments of the Persian Gulf, Iran. Environmental Science and Pollution Research, 24, pp. 20459–20468. https://doi.org/10.1007/s11356-017-9587-z.
Naji, A., Nuri, M., Amiri, P. and Niyogi, S., 2019. Small microplastic particles (S-MPPs) in sediments of mangrove ecosystem on the northern coast of the Persian Gulf. Marine Pollution Bulletin, 146, pp. 305-311. https://doi.org/10.1016/j.marpolbul.2019.06.033.
Release of synthetic microplastic plastic fibres from domestic washing machines: Effects of fabric type and washing conditions. Marine Pollution Bulletin, 112(1-2), pp. 39-45. https://doi.org/10.1016/j.marpolbul.2016.09.025.
Sadeer, N., Mahomoodally, M., Zengin, G., Jeewon, R., Nazurally, N., Rengasamy, K., et al., 2019. Ethnopharmacology, Phytochemistry, and Global Distribution of Mangroves―A Comprehensive Review. Marine Drugs, 17(4). https://doi.org/10.3390/md17040231.
Sánchez-Núñez, D., Pineda, J. and Osorio, A., 2020. From local-to global-scale control factors of wave attenuation in mangrove environments and the role of indirect mangrove wave attenuation. Estuarine, Coastal and Shelf Science, 245. https://doi.org/10.1016/j.ecss.2020.106926.
Vanegas, C., Osorio, A. and Urrego, L., 2019. Wave dissipation across a Rhizophora mangrove patch on a Colombian Caribbean Island: An experimental approach. Ecological Engineering, 130, pp. 271-281. https://doi.org/10.1016/j.ecoleng.2017.07.014.
Wang, T., Li, B., Zou, X., Wang, Y., Li, Y., Xu, Y., et al., 2019. Emission of primary microplastics in mainland China: Invisible but not negligible. Water Research, 162(1), pp. 214-224. https://doi.org/10.1016/j.watres.2019.06.042.
Wang, G., Shan, E., Zhang, B., Teng, J., Wu, D., Yang, X., et al., 2020. Microplastic pollution in intertidal sediments along the coastline of China. Environmental Pollution, 263. https://doi.org/10.1016/j.envpol.2020.114428.
Zhang, L., Zhang, S., Guo, J., Yu, K., Wang, Y., Li, R., 2020. Dynamic distribution of microplastics in mangrove sediments in Beibu Gulf, South China: Implications of tidal current velocity and tidal range. Journal of Hazardous Materials, 399. https://doi.org/10.1016/j.jhazmat.2020.122849.