The current state of microplastic pollution in the world's largest gulf and its future directions

Highlights

• Current knowledge on microplastics (MPs) in Gulf of Mexico (GoM) is reviewed.

• MPs count in water ranged from 12 to 381 particles L⁻¹.

• MPs occurrence ranged from 8 to 100% of biota samples analysed.

• Irregular shaped fragments are abundant than other shaped MPs.

• Studies regarding MPs pollution in GoM are still limited.

Abstract

Microplastics can have several negative consequences on a variety of organisms, and their prevalence in marine ecosystems has become a major concern. Researchers have recently focused their attention on the world's largest gulf, the Gulf of Mexico (GoM), to determine and assess the impact of microplastic pollution on various environmental compartments (i.e., water, sediment, and biota). This paper critically reviews the analytical methodologies as well as summarizes the distribution, accumulation, sources, and composition of microplastics in a handful of studies (n = 14) conducted in the Gulf of Mexico (GoM) covering countries like the USA (n = 10) and Mexico (n = 4). Current quality control measures with respect to sampling and microplastic extraction are summarized. Of 14 studies reviewed, 47% primarily focused on examining sediments for microplastics, with biota and water comprising 35% and 18%, respectively. The abundance ranged from 31.7 to 1392 items m⁻² and 60–1940 items kg⁻¹ in sediment, 12–381 particles L⁻¹ in water, and 1.31–4.7 particles per fish in biota. Irregular shaped fragments were the most abundant, followed by fiber, film, foam, hard, and beads etc. Different polymer types of microplastics have been found, including polyethylene, polypropylene, polystyrene, polyamide, nylon, and rayon etc. According to published research, 46 out of 100 fish thriving in this region are susceptible to microplastic ingestion. Although microplastic concentration in the GoM is among the highest found worldwide, the determination of microplastic contamination is still a growing field of research and methodological discrepancies largely limit the realization of establishing a baseline information on the microplastic abundance of the GoM. In this respect, considerable efforts must be dedicated towards evaluating their distribution and exposure levels; thereby, major challenges and future research directions are briefly discussed.

Introduction

Global plastic production has increased nearly 310 times in the last few decades, from 1.5 million tons in 1950 to 370 million tons in 2019 (PlasticsEurope, 2020). The growing problem of waste accumulation, combined with insufficient waste management, has resulted in the uncontrolled release of large amounts of plastics into a variety of environments (Geyer et al., 2017), from land to sea, with the proportion of plastics in solid waste reaching 242 Mt globally in 2016. In the year 2010, a discharge of about 4.8–12.7 million tons of plastic from land into the ocean has been estimated (Jambeck et al., 2015). Marine plastic debris was first reported in the 1970s in the literature (Carpenter and Smith, 1972; Colton et al., 1974). Since then, numerous studies have revealed the widespread presence of plastic debris in the world's oceans (Hurley et al., 2018), ranging in size from meters to micrometers. Microplastics (particles <5 mm) have been found in the ocean (Hurley et al., 2018), coastal beaches (Yaranal et al., 2021), atmosphere (Can-Güven, 2021), lake (Dusaucy et al., 2021), and river (Kumar et al., 2021), and pose a greater risk than macroplastics.

Microplastics, which come in a variety of polymers, shapes, and sizes, can be produced as tiny plastic particles (primary microplastics) or obtained from the breakdown of larger plastics via physical (UV), chemical (hydrolysis), and biological degradation (secondary microplastics). The amount of plastic and microplastic floating in our oceans, according to Eriksen et al. (2014), is around 5.25 trillion pieces, or 65,269,000 tons. Owing to their small size and ubiquity in all ecosystems, microplastics exposure to biota and humans is highly probable (Smith et al., 2018; Mercogliano et al., 2020). Several studies have documented microplastics in a wide range of taxa, including zooplankton, fish, shrimp, seals, bivalves, sea turtles, stranded whales on continental coastlines, and humans (i.e., placenta) (Nelms et al., 2018; Abbasi et al., 2018; Donohue et al., 2019; Zhao et al., 2018; Garcia-Garin et al., 2021; Ragusa et al., 2021). Microplastic ingestion can have deleterious physiological consequences on species' growth, reproductive success, and behavior (Ferreira et al., 2016; De Sá et al., 2018). Moreover, microplastics have been shown to act as vectors for persistent organic pollutants such as DDT, heavy metals, and microorganisms (Guo and Wang, 2019; Tourinho et al., 2019; Kutralam-Muniasamy et al., 2021). They may also absorb and transfer other pollutants from their surroundings to organisms. Taking together, microplastic pollution is becoming more widely recognized as a serious global issue due to its wide-ranging effects on marine biota and ecosystems.

The growing concerns over the environmental impacts of microplastics have been a response to the dramatic increase in the assessment of their distribution, quantity, and characteristics to address effective management strategies. Furthermore, the Gulf regions are given special attention, considering that they receive discharges from the continental landmass, port areas, tourism activities and other river systems. The Gulf of Mexico (GoM) is the world's largest Gulf, connecting the Atlantic Ocean and the Caribbean Sea, with a surface area of 615,000 square miles, a width of 810 miles, and a depth of 4384 m. It is located in North America, bounded to the south by the United States, to the east by Mexico, and to the west by Cuba. Plastic waste has been found and accumulated throughout the beaches of GoM in recent decades (Colton et al., 1974; Plotkin et al., 1993; Wessel et al., 2019). Only recently did microplastic studies in the GoM begin to appear in the literature, and there has been a surge in research on microplastic identification and quantification (for literature search see section 2). Microplastic pollution has been found in a variety of environmental resources from coastal ecosystem sinks, including estuary beach sediments (Wessel et al., 2016), oyster reefs (Scircle et al., 2020), and coastal seas (Di Mauro et al., 2017). However, when it comes to the impact of microplastic pollution on GoM, there is limited field data on the concentrations and types of microplastics in the main environmental compartments (i.e., sediment, water, and biota). To extend research on microplastics in the GoM, we must first comprehend and analyze existing information to identify gaps and challenges, particularly in light of recent advancements in the literature. Meanwhile, several laboratory methodologies for investigating microplastics have been developed, limiting the representativeness and comparability of the results. Several recent studies on microplastic pollution have focused on different parts of the world, including Latin America (Kutralam-Muniasamy et al., 2020), China (Fok et al., 2020), and the Persian Gulf (Al-Salem et al., 2020). To the best of our knowledge, no comprehensive assessment of the current state of knowledge on microplastics and GoM has ever been published. Hence, the goal of this review is to investigate, assess, and give a strong basis of what is currently known in order to better comprehend the progress and state of microplastic contamination in the GoM. The main objectives are to: (1) summarize the occurrence and distribution of microplastics in water, sediment, and biota in all studies conducted in this region; (2) review and critically analyze methodological approaches from sampling to characterization, including quality control measures; and (3) discuss microplastics sources and transport in the GoM. Finally, we present an overview of the challenges ahead as well as future directions.