Drivers and Consequences of Ingesting Plastic Marine Debris for Marine Mammals

Abstract

Plastic pollution is so ubiquitous in the oceans that it has infiltrated marine ecosystems, with plastic ingestion documented across all trophic levels. This includes marine mammals: the whales, dolphins and porpoises, seals and sealions, walruses, otters, and polar bears. In the case of large, macroplastic debris it is largely assumed that marine mammals mistakenly consume plastic because it visually resembles the physical characteristics of common prey items; for example, a plastic bag or sheet for the gelatinous bodies of squid and jellies. However, some whales dive exceptionally deep to forage, below the photic zone where there is little to no light to see. How then, are deep-diving whales mistaking plastic for food? Smaller microplastics – either intentionally produced or the product of biological, physical, and chemical degradation within the environment are also consumed. Marine mammals are most likely to consume microplastics via contaminated prey in a process called trophic transfer. Whether or not these ingested particles translocate and deposit in various organs outside the gastrointestinal tract in this taxon is unknown. Furthermore, an understanding of the consequences of microplastic exposure to marine mammals is lacking owing to ethical and logistical constraints of studying marine mammals as well as the complicated toxicological nature of microplastics as a suite of contaminants. Therefore, this dissertation aimed to identify drivers and consequences of plastic marine debris in marine mammals. In Chapter 1, I introduce the issue of plastic pollution in the oceans, exploring sources and fate in the marine environment. I summarize known impacts of plastic on marine organisms with a particular focus on marine mammals. For Chapter 2, I utilized opportunistically obtained tissue samples from twelve different species to investigate whether or not microplastics were translocating within the bodies of marine mammals. Using a complement of Raman spectroscopy and pyrolysis gas chromatography with mass spectrometry, myself and my co-authors demonstrated the occurrence of microplastics, ranging in size, mass concentration, and particle count concentration from 24.4 – 138 µm, 0.59 – 25.0 µg/g, and 0.04 – 0.39 particles/g, respectively, in four tissues (acoustic fat pad, blubber, lung, and melon). Chapter 3 built upon these findings to explore what some of the consequences of microplastic exposure may be on marine mammals using an ex vivo blubber model and an in vitro skin cell model. Coupled with glycerol assays, gene expression analyses, protein-protein interaction networks, and functional pathway analyses, I found species-specific, treatment-specific, and tissue-specific responses to microplastic exposure impacting metabolism. Further I identified two key genes – ANGPTL4 and IRS2 – of importance in the lipolytic response to microplastic exposure in common bottlenose dolphins. In Chapter 4, I aimed to identify a sensory mechanism for consumption of plastic in deep-diving cetaceans who forage in the aphotic zone, presumably relying solely on echolocation to locate prey without the assistance of vision. By ensonifying plastic objects and common deep-diver prey items, myself and colleagues demonstrated that the acoustic target strength between the two were similar. This suggests that consumption of plastic by deep-diving odontocetes is driven by a misperception of acoustic signals. I close with Chapter 5, a synthesis of this body of work where I summarize key findings, explore persistent knowledge gaps, highlight limitations of the work, and suggest future research. Findings from this work extend across multiple disciplines, informing conservation, policy, toxicology and sensory ecology. This dissertation contributes to the field by demonstrating a mechanism of microplastic exposure (translocation), illuminating previously unconsidered sensory mechanisms for consumption (echolocation) and adds to the expansive body of literature teasing apart the complicated impacts of microplastic exposure in a group of species where there exists a paucity of information.