Macro to Micro: spatial distribution of anthropogenic litter in the Whau urban freshwater catchment

Abstract

Anthropogenic pollution of freshwater ecosystems is considered a major environmental issue. Microplastics have become ubiquitous in lotic systems according to global research, and macrolitter appears to be following this path. Few studies address multi-scale litter contamination of small urban streams, with none based in New Zealand. This study quantifies the abundance, composition and spatial distribution of macrolitter and microplastics in 18 small headwater streams of the Whau catchment, Auckland. Variation in macrolitter was investigated between riparian and benthic habitats, and variation in microplastic between sediment and the water column. Relationships between macro and micro plastics were explored, as well as the role local stream characteristics play in driving the spatial distribution of litter. Microplastics were extracted from water and sediment samples using H2O2 digestion of organic matter, density separation using NaI, polymer staining with Nile red dye and fluorescence microscopy analysis. Procedural blanks were incorporated to account for sample contamination. Macrolitter abundance ranged between 1 to 7.9 items m⁻², with an average of 3.4 items m⁻². Slightly higher abundances of macrolitter were found in riparian zones compared to benthic. Macrolitter material composition was substantially dominated by plastic. Microplastic concentrations were found at an average of 169 particles m⁻³ and up to 1,293 particles m⁻³ in water samples. Sediments comprised an average of 1,058 particles kg⁻¹ and a maximum of 3,979 particles kg⁻¹. 63 – 500 μm particles and fragments were most common in both water columns and sediments. Overall, pollution in Whau streams was higher than other New Zealand urban streams, and comparable to global findings. Macroplastics were not found to have a strong relationship with microplastics, and local stream characteristics could not adequately explain litter distributions. Nevertheless, some trends were found that inform understandings of anthropogenic pollution in lotic systems. Streams were shown to be pathways of litter retention, accumulation, transport and eventually deposition into marine environments. Heavily channelised streams are speculated to reduce retention of macrolitter, without hindering microplastic mobility. This study confirms the presence and variety of anthropogenic litter in the Whau catchment of New Zealand, and improves understandings of composition and dispersal of litter in small-scale freshwater environments.