The State of Microplastic Pollution in the San Pedro Bay

A recent study revealed that surface-only sampling underestimates microplastic concentrations in marine environments, emphasizing the need for comprehensive research on their distribution and transport to better manage plastic pollution (1). This study, which was published in Environmental Research, explored the concentration and distribution of microplastics in coastal marine environments (1).

Microplastics are small pieces of plastic less than 5 mm in size (2). They can come from a variety of sources, which include anthropogenic practices (2). The main question scientists are exploring is how to detect and monitor these plastics in the environment better, to preserve the environment for future generations.

Samiksha Singh, who works at the Department of Environmental Sciences at the University of California, Riverside and the Gauteng City Region Observatory, in Johannesburg, South Africa, pursued this topic because microplastic distribution in coastal marine environments remains poorly understood (1).

Recently, Singh sat down with Spectroscopy to discuss her team’s findings and methodology, which involved using pyrolysis gas chromatography–mass spectrometry (Py-GC–MS) to analyze tire and road particles and micro-Fourier transform infrared spectroscopy (μFT-IR) to process the particle spectra.

Samiksha Singh works at the Department of Environmental Sciences at the University of California, Riverside and the Gauteng City Region Observatory, in Johannesburg, South Africa. Photo Credit: Samiksha Singh

What were the primary objectives of your study on microplastic (MP) distribution in the marine environment of San Pedro Bay, and how did these objectives address existing research gaps?

The primary objective of the study was to determine the concentration, character, and distribution of microplastics (MPs) in the near coastal marine environment, through the water column and surficial bottom sediments. This is a novel approach because most studies have been limited to sampling microplastics at the ocean surface.

Can you elaborate on the sampling methods used for collecting MPs across the water column and benthic sediment? How did these methods ensure a comprehensive sampling assessment?

Samples were collected once in the wet and dry season at seven locations, three nearshore and four offshore. Samples were collected at the surface layer of the water column using a Manta trawl at mid-water depth (7–10 m below the surface) using a bongo net and at the bottom of the water column using an epibenthic sled. Sediment samples were also collected at each site using a Van Veen Grab. These methods ensured comprehensive sampling through the water column ranging from approximately 9 m at the shallowest site to approximately 350 m at the deepest site. The nearest sampling location was 1 km from shore while the site furthest offshore was 14 km. The range of sites and the multiple collection methods used through the water column and sediment ensured comprehensive sampling, providing a significant understanding of microplastic distribution in this environment.

Your findings indicate significant differences in MP concentrations between nearshore and offshore environments. What factors do you think contribute to these spatial variations?

Higher microplastic concentrations were found in the nearshore compared to offshore environments. This significant difference could be attributed to land-based anthropogenic activity contributing to elevated concentrations of microplastics in the nearshore environment. In addition, the co-location of this study, with the outfalls of the Los Angeles, San Gabriel, and Santa Ana Rivers, coupled with the fact that rivers are a conduit of plastic pollution to the marine environment, likely contributed to the significantly higher microplastic concentrations in the nearshore environment.

The study highlights that surface-only sampling strategies underestimate MP concentrations. What challenges did you face in sampling deeper water layers and sediments, and how were they addressed?

Surface-only sampling may underestimate the concentration of microplastics because floating microplastics may not remain near the surface as they are subjected to photodegradation, wind mixing, waves, and biofouling, which can cause these particles to sink and/or be mixed deeper into the water column. Additionally, seasonal ocean stratification or ocean mixing influences the movement of microplastics through the water column. The selective loss of particles from the surface to the rest of the water column is also affected by size, polymer density, and the rise and settling velocity of particles. Cost and time constraints, both for sample collection and analyses, are one of the major challenges when trying to conduct comprehensive microplastic sampling beyond the surface. Having a robust sampling methodology, sampling plan, and the right equipment can decrease these challenges by ensuring timely and effective sample collection.

You identified polyethylene and polypropylene as the dominant polymers in the water column. What insights do these findings provide about the sources and transport dynamics of MPs in coastal environments?

Polyethylene and polypropylene were the most abundant polymers in the water column of this study. They are also the most abundantly manufactured polymers accounting for about 74% of global plastic production. These polymers are most abundant in the marine environment globally because of their extensive use in single-use plastics such as plastic bags, bottle caps, and containers. The abundance of polyethylene and polypropylene microplastics may also be enriched at the sea surface due to their low density. This indicates that the microplastics found in the marine environment come from land-based sources and are transported by many mechanisms, including by river systems through rainfall and runoff.

The seasonal variation in sediment microplastic concentrations suggests environmental factors at play. What role do you think wet and dry seasons, as well as coastal ocean currents, play in MP distribution?

Seasonality can play a role in influencing microplastic concentration. During the wet season, after a rainfall event, it can be expected that there will be an increase in microplastic concentrations because of the elevated supply of terrestrial microplastics transported by urban storm runoff. Upwelling during the early dry season and stratification during the late dry season may influence where certain microplastics, depending on size, shape and density, are found. Our collaborators at the Southern California Coastal Water Research Project are exploring the process of currents and seasonal cycles on microplastics by implementing microplastic transport in their hydrodynamic model of San Pedro Bay. Stay tuned!

The prevalence of tire and road wear particles throughout the study area is concerning. How significant is the contribution of urban runoff to MP pollution, and what mitigation strategies might help address this issue?

Urban runoff is a significant contributor to microplastics in the marine environment. Land-based sources of microplastics like tire and road wear particles enter the marine environment through urban runoff and river systems. These particles are very small, so once they enter river channels, mitigation is probably off the table. So, source reduction approaches are the focal area for mitigating the influx of microplastics to the marine environment. For tire and road wear particles specifically, there are many opportunities to decrease supply, from changes to build infrastructure and tire formulations to decrease their primary emission and subsequent transport, as well as optimization of removal near the zone of production through street cleaning and runoff trapping operations.

Given the complexity of MP distribution across the marine environment, what recommendations would you make for improving future monitoring programs and mitigating the impact of plastic pollution in coastal systems?

Field-based sampling is challenging given the time, logistical, technological, and monetary demands. More money is needed for repetitive sampling across seasons and during extreme events. Comprehensive sampling is needed not only to understand the current situation but also to show future changes in microplastic concentrations, which will be difficult to detect without substantial sampling efforts. This increased data collection will aid in informing numerical fate and transport models and help identify locations of interest. Future monitoring programs will need to include more sampling locations and robust water column sampling as well as investigations into smaller microplastics that have greater implications for human and organism health. Moving away from net-based sampling to more time-efficient sampling and in turn analysis is critical for the continued study of microplastics, which, at the moment, is both time, resource, and human resource intensive. Understanding the changes in microplastics and the flux of microplastics through the environment is of utmost importance to develop a composite understanding of the problem and in turn assist in mitigation efforts and policy development.

Would you be able to address for our readers the seriousness of MPs to plant and animal life in the ocean ecosystem?

Microplastic pollution is a serious issue affecting not only the environment but humans, animals, and whole ecosystems as well. Their ubiquitous nature, small size, and varying morphologies make it difficult to completely understand all the implications of microplastics. In addition, the additives, plasticizers, and toxins on these particles from production or sorbed onto the particle as it moves through the environment are concerning. Microplastics and their toxic chemicals have the potential for biomagnification and bioaccumulation through the food chain. More research is rapidly being performed on these topics, but many questions remain on just what microplastics will mean for environmental and human health.

References

1. Singh, S.; Gray, A. B.; Murphy-Hagan, C.; et al. Microplastic Pollution in the Water Column and Benthic Sediment of the San Pedro Bay, California, USA. Environ. Res. 2025, 269, 120866. DOI: 10.1016/j.envres.2025.120866
2. National Oceanic and Atmospheric Administration, What Are Microplastics? NOAA.gov. Available at: https://oceanservice.noaa.gov/facts/microplastics.html (accessed 2025-02-09).