Microplastic contamination in the Avon/Ōtākaro River, Christchurch, New Zealand.

Microplastics (< 5 mm in diameter) are detrimental to the health of aquatic organisms and freshwater ecosystems. Freshwater research in New Zealand has been limited to date with no investigations being conducted into the concentrations of microplastics in urban freshwater systems in Christchurch. Thirty river sediment samples were collected from six zones along the Avon/Ōtākaro River, resulting in the identification of 156 microplastic particles. The mean abundance was 5.2 particles per 100 g-1 of sediment. The frequency of polymers was PET (41) > PP (33) > PS (29) for the lower reach of the river. Polyester (15) and nylon (14) were present primarily in higher population locations in the middle and upper reaches of the Avon/Ōtākaro River. Fragments (47%) were the most abundant morphotype. Microplastic size on average was between 100 – 300 µm (43%). Red (27%) and blue (26%) were the most commonly identified colours. Microplastic accumulation zones were influenced by flow rate, land elevation and tributary infiltration. In stormwater drain sediment samples, microplastics were present at a lower mean abundance (3.3 particles per 100 g-1 of sediment). From the fifteen samples, 45 microplastics were identified from stormwater drain sediments and thirteen polymers were isolated. The frequency of detection for polymers was nylon (8) > rubber (6) > PP (6) in all reaches of the Avon/Ōtākaro River. Fragments (66%) and microplastics between 500 – 1000 µm (34%) were most abundant. White (20%) was the most commonly identified colour. A range of external coatings were identified on microplastics including polymers used in transportation (ABS, rubber, PC), additives, building materials, and recreational activities. There was a relationship between land use and the type of microplastic isolated from stormwater drain sediments. Overall, there was no relationship between river and stormwater drain sediment microplastics. These results indicate land-based sources such as land overflows and road runoff as well as stormwater drains influence river microplastic accumulation. The implications of this study provided baseline data for the Avon/Ōtākaro River which can be utilised for future Christchurch research and as a reference for future New Zealand freshwater microplastic research.