Where’s the plastic gone?

Featured image: Plastic pollution in Ghana. Photo courtesy Wikimedia Commons/ Muntaka Chasant, CC BY-SA 4.0 license.

Paper: The global biological microplastic particle sink

Authors: K. Kvale, A. E. F. Prowe, C.-T. Chien, A. Landolvi & A. Oschlies

Scientists estimate that about 4% of the plastic waste generated globally ends up in the ocean, much of it in the form of microplastics. These tiny plastics, smaller than the width of a pencil, are a major pollution problem: because of their small size, they are extremely difficult to remove and can be transferred up the food chain to species that humans eat. Furthermore, harmful chemicals have been shown to adsorb onto microplastics, so consumption of microplastics may have indirect health impacts.  While scientists have put together a “plastic budget” for the ocean by estimating inputs of plastic to the ocean and fragmentation rates of larger plastics into microplastics, models based on observations of the amount of plastic waste in the ocean suggest that there is less plastic in the surface ocean than expected based on these budgets. The authors of this study used a model to test two possible explanations for this ‘missing’ plastic, zooplankton ingestion and sinking to the sea floor with marine particles, and find that these biological pathways can account for 100% of the observed “missing” surface microplastic, even in simulations where these processes are modeled as being inefficient.

The first of these potential explanations occurs via zooplankton, or tiny animal plankton, eating microplastics and subsequently pooping them out! Zooplankton fecal pellets are dense and sink very quickly, so they are a perfect vehicle to transport microplastics to the deeper ocean. The other potential explanation is removal of microplastics through sinking marine snow, which is made up of small biological particles that have aggregated together. These relatively large aggregates can include microplastics and can remove them from the surface ocean via sinking.  Below is a picture of some marine snow sinking to the sediments on the seafloor.

Source: NOAA

Many plastics are buoyant and may return to the upper ocean once these aggregates and fecal pellets are deposited on the seafloor and/or decomposed, but the fraction of microplastics that do this is unknown. The preference of zooplankton grazing for microplastics and the distribution of microplastic buoyancies are also not well known, as this can be affected by the type of zooplankton, the type of plastic, the age of the plastic, and more. To account for all of these uncertainties, the authors used three different simulations with varying values for each of these variables. They then compared these results to a model simulation that doesn’t include zooplankton ingestion or microplastics sinking out with particles to see how these effects change the amount of microplastic in the upper ocean.

The authors found that both of these mechanisms can significantly reduce the amount of microplastics in the surface ocean by transporting them to the deep ocean. Overall, they found that zooplankton fecal pellet export of microplastics is likely more substantial than marine snow aggregation. This is especially true in areas where there is little food available for zooplankton, as fewer biological particles (food for zooplankton) means less sinking of microplastic with those particles. This also suggests that zooplankton substitution of microplastics for food may be substantial in these regions. The authors also found that microplastics in this model mainly accumulated along the coasts in both the surface ocean and deep ocean. Some accumulation occurred in the model in areas that are teeming with life and that are known for productive fisheries. This is in contrast to larger plastics, which typically accumulate in the ocean gyres (think North Pacific Garbage Patch).

The authors also found there could be significant changes in these biological processes due to climate change. Climate change is predicted to increase respiration rates in the ocean due to increases in temperature, which means more particle decomposition and thus more microplastics re‑released to the upper ocean from either zooplankton fecal pellets or biological particle aggregates. In the model, these changes lead to higher microplastic concentrations on average in the surface ocean. Additionally, plastic pollution rates are predicted to increase with increasing global population, which may further increase microplastic concentrations in the surface ocean. This could have consequences for the amount of microplastics that end up in our food, as zooplankton feed in the surface ocean and are then eaten by fish, which may transfer these plastics up the food chain to marine species that humans consume.

Though this model result shows us that these biological pathways could be important in shaping the spatial distribution of microplastics, it also has its limitations. The authors discuss potentially incorporating more details into the model moving forward as more data becomes available, including various particle sinking rates, plastic types, microplastic particle sizes, abiotic degradation of plastics, higher trophic levels, and colonization of plastics by organisms. All of these factors are thought to affect the microplastic distribution, and thus including them in the model will help to make the model more accurate.

While the problem of microplastics in the ocean remains challenging, figuring out where they accumulate could be key to controlling their effects. This model suggests that zooplankton consumption and marine snow aggregation may play a large role in shaping the spatial patterns of microplastics, opening the door for further study of these processes in the field. Continued research on this topic is crucial for understanding the potential pathways for both animal and human consumption of microplastics.

Where’s the plastic gone? by Lillian Henderson is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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