ENFERMEDAD DE PARKINSON

Overall, microplastic concentrations in sediments were highly variable among sites, with an average of 141.5 ± 19.6 (SE) microplastics kg-1 _{sediments across all sites} (Figure 4.5). Fibres were slightly more abundant than particles, with an average of 78.9 (± 13.4 SE) fibres kg-1 _{of sediment, compared to particles with an average of 61.4 (± 9.3} SE) particles kg-1 _{of sediment. Microplastic counts within the sediments were found to} differ between sites, and were recorded the lowest in the estuary, with an average of 6.15 particles kg-1 _{sediment (± 3.19 SE) (Table 4.2, Figure 4.5).}

Microplastic particles that were counted ranged from 1-5 mm in size, with fibres sometimes slightly larger (one fibre was 32 mm in size). Other materials within the 1-5 mm size range, such as glass and aluminium, were also found within the sediments. For example, a total of 81 clear glass spheres were observed in pre-wet sediment samples, which doubled to 170 in post-wet samples (not standardised). These clear glass spheres were sometimes encased in bitumen or concrete with yellow paint, and therefore

71

assumed be parts of the reflected paint used on roads as have also been reported by (Horton et al., 2017a).

Proximity to storm drains did not affect microplastic loads within the sediment (Table 4.2). Similar to the pilot study, there was no significant difference found between microplastic loads within the sediments 1 m away from the storm drain compared with 27 m away (Table 4.2), and this effect was consistent between seasons (Table 4.2). In addition, there was no significant interaction between the plastic loads in sediments collected in the pre and post-wet season and the distance from the drain (Table 4.2). In comparison to water samples, where the highest plastic loads were found within the estuary site, plastic loads were the lowest in the sediments in this location (Figure 4.6).

Figure 4.5: Microplastic abundance within sediments at different locations within the freshwater section of the river, and in the Estuary. Sediments were collected outside storm water

effluents, 3 and 27 meters away along three different transect lines. Whiskers represent a 95% confidence. * indicates outliers (1.5 x Inter Quartile Range).

72

Figure 4.6: Microplastic abundance compared between the pre and post-wet season within the freshwater and estuary in the water surface samples. Plankton tows were performed outside the same storm drains where sediment samples were collected. For water samples, fibres were excluded. Boxes represent the median, with the upper and lower quartiles. Whiskers represent a 95% confidence. *

indicates outliers (1.5 x Inter Quartile Range).

4.4 Discussion

4.4.1 Microplastic loads within the river and bay after seasonal rainfall flushes

For this chapter, I aimed to quantify the microplastic loads in a Queensland river system by sampling microplastic concentrations close to storm drains and coastal waters elevated after rainfall events. For the first time, this study quantifies microplastic loads within a small urban river system in tropical Queensland, Australia. Results reveal that rainfall (687 mm) during the wet season did not lead to a measurable change in plastic

73

loads within the river or bay. Moreover, there was no seasonal change in microplastic abundance in any other location of the river or bay, and there was no clear gradient of microplastic abundance leading out of the river after the wet season. While these patterns are inconsistent with results from other freshwater systems in Australia

(Hitchcock and Mitrovic, 2019) and around the world (Castaneda et al., 2014; Fischer et al., 2016), these findings are consistent with the results in chapter 3, where wet-season rainfall had negligible effect on macro plastic loads in the river. Unlike my study site, much of the current literature investigating microplastics in freshwater systems focusses on large, continuously flowing rivers (e.g. (Castaneda et al., 2014; Morritt et al., 2014; Yonkos et al., 2014). I suggest that the presence of weirs and a dam in Ross River, which constrict water flow for at least half of the year, likely underpins the differences between my results and other studies of freshwater rivers.

As rainfall was below the overall average for Townsville, which is typically approximately 1000 mm per year (Australian Bureau of Meteorology, 2017), it is also possible that seasonal differences in microplastic loads could occur during years with higher rainfall and increased outflow from the river. Indeed, at the time of sampling after the wet season and a heavy rainfall event, all of the weirs were flowing but the dam was not spilling. After several years below average rainfall in the area and little water movement (Townsville BOM), hydrodynamic models suggest that plastic particles will biofoul and sink to the bottom of the river, where they are unlikely to become re-suspended unless there is significantly water movement from rainfall or wind (Kooi et al., 2018). This is consistent with my findings of high microplastic loads within river sediments. The only place that did not have high plastic loads in the sediment was within the estuary, where sediment resuspension due to large tidal flow and water movement (Allen et al. 1980) and would prevent particles from accumulating (as per modelling study (Kooi et al., 2018). In addition, due to slow water flow, the river system has significant aquatic plant life that grows on the surface of the water. During years with limited rainfall, this plant life accumulates along the weirs. While sampling, floating microplastic particles were observed to be trapped in the vegetation. This has been recorded previously in freshwater models and studies (Kooi et al., 2018; Williams and Simmons, 1996), and it means that microplastic counts reported in this study are likely underestimates, as the vegetation prevents release of microplastics into the open river water and sediments and therefore cannot be efficiently sampled using a plankton

74

tow net (Kooi et al., 2018). Regardless, in the event of a cyclone or other large scale storm, much of the plastic accumulating within the river vegetation is likely to be washed out to sea.