Differentiating littering, urban runoff and marine transport as sources of marine debris in coastal and estuarine environments

Abstract

Marine debris is a burgeoning global issue with economic, ecological and aesthetic impacts. While there are many studies now addressing this topic, the influence of urbanisation factors such as local population density, stormwater drains and roads on the distribution of coastal litter remains poorly understood. To address this knowledge gap, we carried out standardized surveys at 224 transect surveys at 67 sites in two estuaries and along the open coast in Tasmania, Australia. We explored the relative support for three hypotheses regarding the sources of the debris; direct deposition by beachgoers, transport from surrounding areas via storm water drains and coastal runoff, and onshore transport from the marine system. We found strong support for all three mechanisms, however, onshore transport from the marine reservoir was the most important mechanism. Overall, the three models together explained 45.8 percent of the variation in our observations. Our results also suggest that most debris released into the marine environment is deposited locally, which may be the answer to where all the missing plastic is in the ocean. Furthermore, local interventions are likely to be most effective in reducing land-based inputs into the ocean.

Figure 1. The percentage of different types of debris observed in the Derwent Estuary, Tamar Estuary and the east coast of Tasmania.

Percentages are of total number of debris items observed in respective estuaries and coast.

Figure 2. The effect of the number of stormwater drains within 5 km radius of each site has on total amount of debris observed, in the Derwent and Tamar Estuaries.

Each point represents one transect.

Figure 3. The effect of estuary basin on the total amount of debris observed.

The Tamar Estuary has less debris than the Derwent Estuary. Along the x-axis, D = Derwent Estuary, T = Tamar Estuary.

Figure 4. The effect the distance a site is from a public road has on the total amount of debris observed along the east coast of Tasmania.

Each point represents one transect.

Figure 5. Survey sites in the Derwent Estuary, Tamar Estuary and east coast, Tasmania, Australia.

Generated by Kathryn Willis using ArcGIS, [Desktop version 10.2.], (https://www.arcgis.com/features/index.html). LIST Coastline (MHWM) from theLIST ©State of Tasmania (Creative Commons Attribution 3.0 Australia License, https://creativecommons.org/licenses/by/3.0/au/legalcode).