Abstract
Worldwide plastic production currently exceeds 335 million metric tonnes per year, and this production is expected to continue to grow exponentially. Much of this plastic ends up as waste in marine and freshwater environments in the form of microplastics (particles < 5 mm). Microplastics are small enough to accumulate within the environment and within biota and, as a result, cause detrimental impacts on organisms and the wider ecosystem. Effects on organisms documented in the scientific literature range from bioaccumulation to organ damage and increased mortality, all of which make microplastic contaminants of increasing concern. Much of the existing research focuses on marine environments, but freshwater microplastic research is also a growing field. Microplastics have been found at alarmingly high concentrations in freshwater systems in a number of countries, but there are still large knowledge gaps regarding the concentrations and effects of microplastics in running-water ecosystems. When I started my PhD studies, there were no published data on freshwater microplastic pollution within New Zealand. Internationally, available data were also limited, with few links existing between microplastic concentrations and urbanisation metrics which could influence pollution. There was also little understanding how microplastic pollution affected important freshwater biota such as stream invertebrates. My two overall PhD research objectives were to determine the extent of freshwater microplastic pollution within New Zealand streams, and to investigate the ecological impacts of microplastic pollution on New Zealand’s stream invertebrate communities in a multiple-stressor context.
In 2019, I undertook a 3-week survey of 52 streams in the five main urban agglomerations in New Zealand to determine microplastic pollution concentrations within the water column of urban streams (Chapter 2, Mora-Teddy and Matthaei 2020). In 2021, I conducted a second survey, in which I revisited the 28 most polluted streams from the first survey (Chapter 3). The second survey aimed to determine whether microplastic pollution was also present within streambed sediments, if macroinvertebrates were ingesting microplastics in situ, and whether microplastic drift concentrations remained consistent with the previous survey. The data collected were also used to determine whether invertebrate kick-net sampling could be a viable method for routinely monitoring freshwater microplastic pollution worldwide (Chapter 4). This idea was inspired by a global lack of regular standardized monitoring programmes and methodology. Finally, I conducted a streamside experiment in 64 flow-through stream mesocosms (Chapter 5) to investigate the individual and combined effects of field-realistic concentrations of microplastics on stream invertebrate communities, using four concentrations and two microplastic sizes. Microplastic treatments were combined with added fine sediment in a full-factorial design, to more realistically simulate urban streams. This is the first multiple-stressor experiment to examine stream invertebrate community dynamics in response to the now ubiquitous pollutant of microplastics. I collected 256 invertebrate drift samples (on 4 occasions 48 hours after each of 3 microplastic additions), plus 64 benthic invertebrate samples 28 days after microplastic addition began. In my thesis, combining field surveys and mesocosm experimentation allowed developing an unusually comprehensive data set on urban stream microplastic pollution as well as microplastic effects on stream invertebrate communities.
Microplastics were common within the drift of New Zealand urban streams, but urbanisation or stream catchment metrics turned out to be poor predictors of microplastic concentrations (Chapter 2, published as Mora-Teddy and Matthaei 2020). These patterns remained consistent in the bed sediments of the same urban streams (Chapter 3). Compared to two years earlier, drift microplastic concentrations within the same streams were lower, but still comparable to those reported globally. This difference could indicate an actual decrease but could also be an artifact of the currently prevalent “snapshot” sampling for microplastics, thus highlighting the need for more regular sampling to allow effectively monitoring microplastic pollution across time and space. Several invertebrate taxa ingested microplastics in situ, including Amphipoda, Potamopyrgus antipodarum, larval Archichauliodes diversus, Austrosimulium spp., Chironomidae, Elmidae, Hydrobiosidae and Muscidae. This was the first time larval Austrosimulium and Muscidae had been observed to ingest microplastics. In Chapter 4, kick-netting generally captured microplastics at higher concentrations than drift or benthic microplastics sampling, regardless whether microplastic items caught in kick-nets were expressed per volume of water (as in drift microplastic samples) or per kg of sediment (as in benthic samples). Consequently, kick-net sampling has the potential to be a time- and cost-efficient tool for monitoring microplastic pollution. In the mesocosm experiment (Chapter 5), all manipulated factors (microplastic size, concentration and fine sediment) had main and/or interactive effects on invertebrate drift responses, whereas very few effects were seen in the benthos. Microplastic size influenced invertebrate drift dynamics at the community and taxon level. Interactions were common in different combinations between sediment, microplastic size and concentration, indicating multiple-stressor relationships between microplastics and fine sediment. Moreover, microplastic ingestion was witnessed in 4 of the 12 taxa analysed (Hydrobiosidae, Deleatidium spp., A. diversus and P. antipodarum). Overall, the findings of my thesis add substantial further evidence that microplastic pollution in freshwaters is a global problem that needs to be monitored regularly and mitigated to reduce its potential harm to freshwater environments.