Thesis – Microplastic Contamination in Moreton Bay: Distribution and Sources

Abstract

Microplastic pollution is an escalating concern for estuarine and coastal ecosystems, yet sediment contamination within Moreton Bay, a Ramsar-listed embayment adjoining Australia’s fastest-growing metropolitan corridor, remains poorly quantified. This study assessed the distribution, abundance and polymer composition of microplastics in surface sediments across twelve sites spanning a gradient from the major river mouths to the outer, more marine-influenced bay. Sediments were processed through density separation and characterised using attenuated total reflectance Fourier-transform infrared (FTIR) spectroscopy. Microplastic concentrations ranged from 120 to 682 particles per kilogram of dry sediment, with a mean of 375 particles/kg. A pronounced spatial gradient was observed, with sites near the Brisbane and Logan River mouths containing approximately 3.1 times more microplastic than outer-bay sites. Polyethylene and polypropylene together accounted for 65 per cent of identified polymers, implicating land-based packaging and stormwater-borne inputs. These findings establish a baseline for Moreton Bay and support catchment-scale management consistent with the National Plastics Plan.

Introduction

Global plastic production has grown almost exponentially since the mid-twentieth century, now exceeding 400 million tonnes annually, and a substantial fraction of this material ultimately enters aquatic systems (Jambeck et al., 2015). Once in the environment, larger items fragment through ultraviolet weathering, mechanical abrasion and biological activity into progressively smaller particles (Andrady, 2011). Microplastics, operationally defined as plastic particles smaller than 5 mm, have since been documented in virtually every marine compartment, from surface waters to deep-sea sediments (Cole et al., 2011; Thompson et al., 2004). Because dense polymers and biofouled particles tend to settle, sediments are increasingly recognised as a long-term sink that integrates contamination over time and provides a more stable medium for monitoring than the water column (Van Cauwenberghe et al., 2015).

Moreton Bay is a large, semi-enclosed embayment of approximately 1,500 square kilometres adjoining the Brisbane metropolitan area in South East Queensland. It has been listed as a Wetland of International Importance under the Ramsar Convention since 1993 and is protected in part through the Moreton Bay Marine Park. The bay supports internationally significant populations of dugongs, green turtles and migratory shorebirds, alongside commercial and recreational fisheries of considerable economic value. It also receives freshwater and associated pollutant loads from several catchments, principally the Brisbane, Logan, Pine and Caboolture Rivers. South East Queensland is among the fastest-growing regions in the country, with a population exceeding 3.8 million and continued urban expansion placing mounting pressure on receiving waters (Australian Bureau of Statistics, 2023). These characteristics render Moreton Bay both ecologically valuable and potentially vulnerable to land-derived microplastic inputs.

Despite growing national attention to plastic pollution, quantitative data on microplastic contamination in Moreton Bay sediments remain scarce. This study addresses that gap with three aims: first, to quantify microplastic concentrations in surface sediments across twelve sites; second, to characterise the spatial distribution of contamination relative to major riverine inputs; and third, to identify the dominant polymer types in order to infer the likely sources.

Literature Review

Microplastics are commonly categorised by origin. Primary microplastics are manufactured at a small size, such as the microbeads formerly used in personal care products and industrial resin pellets, whereas secondary microplastics arise from the fragmentation of larger items (Andrady, 2011; Cole et al., 2011). In urbanised catchments, secondary particles derived from packaging, synthetic textiles and tyre wear typically dominate, and stormwater and wastewater outfalls are recognised as major delivery pathways to coastal waters (Browne et al., 2011).

Sediments accumulate microplastics through a combination of particle density, biofouling and aggregation with organic matter, all of which increase settling velocity (Van Cauwenberghe et al., 2015). Estuarine and nearshore depositional zones consequently record higher concentrations than the overlying water, particularly close to input sources. Fibres and fragments are the morphologies most frequently reported in sediment studies, reflecting the breakdown of textiles and rigid packaging respectively.

Australian investigations have confirmed that microplastics are widespread in national waters. Reisser et al. (2013) reported plastic particles at every site sampled in waters around Australia, with polyethylene and polypropylene dominating the polymer profile. Urban estuaries elsewhere in the country, such as the Derwent Estuary in Tasmania, likewise show elevated sediment concentrations associated with population density and industrial activity (Willis et al., 2017). Nevertheless, South East Queensland estuarine sediments have received comparatively little attention, and no systematic, spatially resolved baseline exists for Moreton Bay. Establishing such a baseline is a prerequisite for detecting change and for evaluating the effectiveness of catchment interventions.

Methodology

Twelve sampling sites were established across Moreton Bay to capture a gradient from the mouths of the four principal contributing rivers to the outer, more marine-influenced eastern bay near Moreton and North Stradbroke Islands. Site placement was informed by prevailing circulation patterns and by the location of major riverine and stormwater discharges. At each site, triplicate surface sediment samples (upper 5 cm) were collected using a stainless-steel Van Veen grab during a single dry-season campaign, so as to minimise the confounding influence of episodic flood inputs.

In the laboratory, sediments were oven-dried at 60 degrees Celsius to constant mass. A known dry mass of each sample was subjected to density separation using a saturated zinc chloride solution (density approximately 1.6 g/cm3), which permits buoyant recovery of denser polymers such as polyvinyl chloride and polyethylene terephthalate in addition to the lighter polyolefins. The supernatant was vacuum-filtered onto glass-fibre filters, and suspected microplastics were enumerated and categorised by morphology (fibre, fragment, film or bead) under a stereomicroscope, following established methodological guidance (Hidalgo-Ruz et al., 2012).

Polymer identity was confirmed for a representative subset of particles using FTIR-ATR spectroscopy, with spectra matched against reference libraries at a minimum acceptance threshold of 70 per cent. To control for airborne and procedural contamination, all solutions were pre-filtered, glassware was rinsed with filtered water, cotton laboratory coats were worn, and procedural blanks were processed alongside every batch. The concentrations reported below are blank-corrected, consistent with international monitoring guidance (GESAMP, 2019). Figure 1 summarises the analytical workflow from field collection to polymer identification.

Results

Microplastics were detected in every sample. Concentrations ranged from 120 particles/kg dry sediment at the outermost eastern site to 682 particles/kg at the lower Brisbane River mouth (Table 1). The mean concentration across all sites was calculated as the sum of the site means divided by the number of sites: mean = 4,500 / 12 = 375 particles/kg.

A clear spatial gradient was evident. The five sites nearest the river mouths (S1 to S5) averaged 565 particles/kg, whereas the five outer-bay sites (S8 to S12) averaged 184 particles/kg, an enrichment of approximately 3.1 times (565 / 184 = 3.1). The Brisbane River mouth and the adjacent inner-western embayments recorded the highest values, consistent with the Brisbane River being the largest freshwater and catchment-runoff contributor to the bay. Fibres were the most abundant morphology overall, accounting for roughly half of all recovered particles, followed by fragments, with films and beads comparatively rare.

FTIR analysis identified seven polymer categories (Table 2). Polyethylene was the most common at 38 per cent, followed by polypropylene at 27 per cent; together these two polyolefins accounted for 65 per cent of identified particles. Polystyrene, polyethylene terephthalate, polyvinyl chloride and polyamide made up most of the remainder, with a small residual fraction of spectra unable to be matched owing to weathering or additive interference.

Discussion

The concentrations recorded in Moreton Bay are broadly comparable to those reported for other urbanised Australian estuaries. They fall within the range documented for the Derwent Estuary, where sediment contamination has been linked to surrounding population density and historical industrial discharge (Willis et al., 2017), and they are consistent with the dominance of polyethylene and polypropylene observed in waters around Australia more generally (Reisser et al., 2013). The strong landward-to-seaward gradient supports the interpretation that the microplastic burden of the bay is predominantly land-derived and delivered through riverine and stormwater pathways, rather than originating from diffuse oceanic sources.

The polymer profile reinforces this conclusion. Polyethylene and polypropylene are the most heavily produced packaging polymers globally and are characteristic of secondary fragmentation from consumer waste, while the presence of polyethylene terephthalate and polyamide is consistent with textile fibres entering the system through wastewater. The predominance of fibres among recovered particles echoes findings from other urban catchments and points to laundry effluent and the breakdown of synthetic textiles as an important, and frequently underappreciated, contributor.

These findings carry ecological significance for a system that sustains filter-feeding invertebrates, seagrass-associated fauna and iconic megafauna. Microplastics are readily ingested across trophic levels and have been documented in Australian marine organisms, including reef-building corals on the Great Barrier Reef (Hall et al., 2015). In a bay that supports dugong and green turtle populations dependent on seagrass meadows, the accumulation of microplastics in depositional sediments raises concerns regarding direct ingestion, the transfer of sorbed contaminants, and possible effects on benthic community structure.

Limitations

Several limitations should be acknowledged. The sampling represented a single dry-season snapshot and therefore did not capture temporal variability or the pulse inputs associated with wet-season flooding, which can substantially elevate riverine plastic loads. FTIR-ATR analysis reliably characterises particles larger than approximately 100 micrometres but under-represents the smallest, and often most numerous, size fractions. Density separation may not recover all particles bound within aggregates, and visual sorting introduces a degree of operator subjectivity despite spectroscopic confirmation. The reported concentrations should therefore be regarded as conservative estimates.

Management implications

The results have direct relevance for environmental management in South East Queensland. Because contamination is concentrated near the river mouths, interventions targeting catchment and stormwater sources, such as gross pollutant traps, improved litter capture and reduced single-use plastic consumption, are likely to deliver the greatest benefit within the bay. The findings support the objectives of the National Plastics Plan (Commonwealth of Australia, 2021) and provide a quantitative baseline against which future monitoring can assess progress. Incorporating microplastics into routine sediment-quality assessment, alongside established frameworks such as the Australian and New Zealand water quality guidelines (ANZG, 2018), would strengthen the evidence base for regional decision-making.

Conclusion

This study provides a spatially resolved assessment of microplastic contamination in Moreton Bay sediments. Microplastics were ubiquitous, averaged 375 particles/kg dry sediment, and displayed a pronounced gradient of decreasing abundance from the river mouths towards the outer bay. The dominance of polyethylene and polypropylene, together with the prevalence of fibres, identifies land-based packaging and textile waste, conveyed by rivers and stormwater, as the principal sources. These results establish a baseline for ongoing monitoring and reinforce the case for catchment-scale management to protect a wetland of international importance. Future work should extend sampling across seasons, incorporate smaller size fractions, and examine the uptake of microplastics within the food web of the bay.

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