Wind driven transport of macroplastic debris in a large urban harbour measured by GPS-tracked drifters

The transport pathways of floating plastic debris in Toronto Harbour, Ontario, Canada, were assessed using a series of GPS-tracked drifter bottles. The drifter trajectories were largely controlled by winds, and they could traverse the 2 km wide harbour within a day. The average ratio of drifter speed to wind speed (the wind factor) is consistent with values of 2-5 % used in modelling dispersion of marine debris. However, significant variability in wind factors meant some drifters travelled 2-5 times faster than expected in small waterbodies (Toronto Harbour), and as much as 7 times faster in large waterbodies (Lake Ontario). Importantly, based on our calculated wind factor equations and the coincident accumulation of our drifters with real plastic debris, we can justify the use of wind factors when studying plastic debris transport. Most (75 %) of the drifters that were released in the harbour, stayed within the harbour, accumulating downwind. However, 14 of all 66 drifters escaped Toronto Harbour, where ∼70 % escaped through the West Gap while ∼30 % escaped via the Outer Harbour. One drifter made a 290 km journey across Lake Ontario in a period of 14 days, demonstrating that Toronto is a potential source of plastic debris throughout Lake Ontario.