Plastic pollution has a major global problem, and a lot has been written about plastic pollution in the ocean. Plastic pollution can cause environmental harm in many ways, however plastic ingestion by animals is most common. Over the past few years, almost 600 marine vertebrate species have been recorded to have ingested plastic. The results of this ingestion can be fatal, as it leads to obstruction, laceration and perforation of the gastrointestinal tract, as well as malnutrition, immune impairment and contamination. Despite extensive knowledge of plastic ingestion in marine animals, not a lot is known about the impact of plastic pollution and ingestion in freshwater species.

How Is Plastic Ending Up In Our Water Systems?

Plastic pollution from discarded bottles, polystyrene products, grocery bags, synthetic clothing fiber and other waste has the ability to break down into smaller pieces of plastic as it weathers, making it easy for these pieces to be transported through our water courses, especially after high rain events. The plastic can be broken down into pieces so small it cannot be observed, known as microplastics. These microparticles can end up in human drinking water, and also bioaccumulates in the food chain. The same pathways that introduce plastics into marine systems – such as land-based activities – also introduce plastics into freshwater systems.

Given the well-documented impacts of plastic pollution on marine animals, it is expected that freshwater organisms would be similarly affected, assuming the same levels of plastic are present in freshwater habitats. Until recently, the plastic volume in freshwater systems and the impact thereof on aquatic animals have been relatively understudied.

Freshwater Habitats At Risk

Freshwater ecosystems are biologically diverse, but are facing many threats. The main ecological stressors include flow modification, introduction of invasive species, impacts related to climate change, degraded water quality coupled with increased turbidity, and habitat degradation. These threats have resulted in large declines of many freshwater animals, including fish, mammals, reptiles, amphibians, molluscs and crayfish. It is therefore crucial that ecologists better understand how plastic pollution affects freshwater habitats and the animals that reside there.

One particularly at-risk group of animals are freshwater turtles. Over 50% of all turtle species are considered threatened on IUCN’s Red List of Threatened Species. Although the threat of plastic ingestion is well documented in marine turtles, for the 352 non-marine turtle species the impacts and rates of ingestion are still unclear, as it is rarely reported and has never been comprehensively reviewed.

Studying Plastic Ingestion in Freshwater Turtles

A study recently published in Scientific Reports now offers the first comprehensive review of plastic and human-caused litter ingestion by freshwater turtles, and compares the number of relevant diet studies (i.e. studies that could detect ingested plastics) on non-marine turtles to those on marine turtles. The study also presents one of the few exiting datasets on plastic ingestion frequency in wild turtles. The data set includes a non-native population of the Red-eared Slider (Trachemys scripta elegans) in California. This turtle species serves as a proxy for the potential threat of plastic ingestion faced by the sympatric Northwestern Pond Turtle (Emys marmorata), which is declining in numbers.

In order to study publications, the authors used ISI Web of Science to identify studies that potentially documented plastic ingestion in free-living turtles. Two bodies of literature were reviewed: first all journal articles examining the diets of non-marine turtles published prior to January 2021 were complied, and then journal articles published in the past 10 years on the diet of marine and non-marine turtles were compiled.

The researchers also did a field study on plastic ingestion in turtles, using the Davis Arboretum waterway in Yolo Country, California. This 2.4 km stretch of waterway is semi-urbanized and home to an endemic population of Northwestern Pond Turtles, and a non-native population of Red-eared Sliders. Captured turtles were humanely euthanized and dissected to investigate the contents of their stomachs.

Results of the Literature Review

The study discovered that all 7 marine turtle species have been documented ingesting plastic, however only 5 of the 352 freshwater turtle species have been documented ingesting plastic. It was challenging to identify plastic and litter ingestion literature for non-marine turtles, as common terms like “plastic”, “litter” and “trash” were absent from the keywords or titles of the dietary studies.

Results of the Field Study

Although remnants of plastic and non-plastic litter were discovered in the captured turtles, there was no signal of plastic ingestion negatively affecting turtle body condition. The ingested plastic was not isolated, but rather contained within the food content of the stomach, and no plastic appeared to be blocking material transfer within the gastrointestinal tract.

Summary of the Study

The literature review and field study on plastic ingestion by non-marine turtles indicate that these turtles are ingesting plastic and non-plastic litter. Although the first (of five) global review articles on plastic ingestion by marine turtles were published 35 years ago, no such reviews exist for non-marine turtles. This implies that plastic ingestion has been severely understudied in non-marine turtles compared to marine species, and remains poorly assessed. Additional studies are therefore required to determine whether plastic ingestion poses a large threat to non-marine turtles (and other freshwater species). The need for further research will become more pressing as human-caused pollution in freshwater systems increases.

Habitat degradation and exploitation (such as poaching of eggs) of turtles are recognized as leading threats to all turtles worldwide. It is important that conservation efforts are focused on the drivers of declines in turtle populations. All threats, including plastic ingestion, should be taken into account to allow for informed conservation of the many turtle species.

A Call to Action

The authors of the study have invited the freshwater biology community to prioritize acquisition and publication of plastic ingestion data. They acknowledge the need for consistent reporting methods and terminology to be used in future studies. They have requested other researchers to replicate their study, especially by conservationists who remove non-native and invasive turtle species from the wild through lethal methods. If all removed non-native species are deposited at a reputable museum collection, these turtles can be dissected to study plastic ingestion. The findings from these turtles should be published. The authors also made some best-practice recommendations, such as terminology to be used and methods to be followed to determine the presence of plastics in the gastrointestinal tracts of turtles.