|dc.description.abstract||A discrepancy between high plastic production rates and low recycling rates contributes to a ubiquitous plastic pollution problem. If this discrepancy persists, it is estimated that approximately 12 billion tons of plastics will accumulate in the environment by 2050. Although many countries have issued policies to limit the utilisation of single use plastics, the COVID-19 pandemic has increased demands for plastics and overwhelmed waste management systems. Thus, plastic pollution will persist, especially in marine environments where most mismanaged plastics accumulate. Plastics in the marine environment slowly disintegrate into microplastics (<5 mm) and adversely affect many animals when ingested. Thus, microplastics have recently been identified as an emerging contaminant of concern internationally, resulting in an exponential growth in the number of microplastic studies within the last decade. Studies in some ecologically important animals such as jellyfish, however, are preliminary. For example, jellyfish are claimed to ingest microplastics via trophic transfer and have been promoted as bioindicators for plastic pollution despite limited evidence. Moreover, although microplastics in the field are covered by biofilms, all jellyfish and microplastic experiments have used virgin microbeads that might underestimate ingestion rates. This thesis, therefore, tested three hypotheses: 1) that jellyfish would ingest microplastics and they would be adversely affected by microplastic ingestion (chapter 2), 2) that jellyfish would mainly accumulate microplastics via indirect ingestion (i.e. trophic transfer) and biofilms would promote ingestion rates (chapter 3), 3) that jellyfish would be useful bioindicators of microplastic pollution and treated wastewater would be a significant source of microplastics in an estuary (chapter 4).
Medusae of Aurelia coerulea were exposed to 2,000 polystyrene microbeads L-1 and determined numbers of microbeads ingested. In addition, impacts of microbead ingestion on respiration rates and histology of their gut tissues were assessed (Chapter 2). No tissue damage was observed and respiration rates were unaffected by ingestion of microbeads. Importantly, the medusae ingested less than 0.2% of microbeads offered, egested microbeads within eight hours and stopped ingesting the microbeads after 16 hours, suggesting that the medusae may recognise virgin microbeads as non-food items. I, therefore, exposed the medusae to microbeads with photosynthetic biofilms, microbeads with heterotrophic biofilms and virgin microbeads (Chapter 3). Medusae ingested more microbeads with photosynthetic biofilms than microbeads with heterotrophic biofilms or virgin microbeads. The results highlight that the use of aged microbeads in experiments is important as the ingestion rates may be underestimated if virgin microbeads are used. Although jellyfish are claimed to acquire microbeads via trophic transfer, no studies had tested whether trophic transfer is a dominant pathway as jellyfish can also ingest microplastics directly from their surrounding water. Thus, I exposed ephyrae of Aurelia coerulea to aged microbeads (to test direct ingestion) and to Artemia nauplii fed aged microbeads (to test trophic transfer), and quantifued numbers of microbeads in the gastrovascular cavities (Chapter 3). I found that the ephyrae ingested 35 times more microbeads via trophic transfer than direct ingestion, suggesting that trophic transfer is the primary pathway by which jellyfish acquire microbeads. Furthermore, I investigated whether jellyfish in the field are susceptible to microplastic ingestion and whether jellyfish can be bioindicators of microplastic pollution. Water samples and medusae of Chrysaora cf pentostoma were collected nearby and distant from treated wastewater diffusers in two estuaries (the Gold Coast Broadwater and the Tweed River Estuary) that receive contrasting amounts of wastewater, to test whether microplastics in the guts of medusae represented those in the environment (Chapter 4). Only 83% of the medusae sampled contained microplastics and types and colours of microplastics in the gastrovascular cavities of jellyfish differed to those in the surrounding water. Thus, medusae are not good bioindicators of microplastic pollution because not all medusae acquire microplastics and the microplastics they accumulated did not reflect those in their environment. I also tested whether the released treated wastewater would have significant effects on microplastic concentrations and compositions in the receiving waters of the estuaries as wastewater treatment plants are claimed to be one of the significant sources of microplastics. I found no significant difference between microplastic concentrations and compositions nearby and distant from wastewater releases in either estuary. Thus, treated wastewater had no detectable impacts on microplastic concentrations and compositions in the receiving waters. Results from both laboratory and field experiments (Chapter2; Chapter 3; Chapter 4) strongly indicated that jellyfish accumulate relativly small amounts of microplastics and are poor bioindicators for microplastic pollution.||en_US