Seasonal and In-Depth Distribution of Anthropogenic Microparticles in Hamilton Harbour

This study investigates the seasonal and vertical distribution of anthropogenic microparticles in Hamilton Harbour, a freshwater system connected to Lake Ontario. Sampling was conducted across three different depths, representing the surface (epilimnion, 1 m), seasonal thermocline depth (metalimnion, 5–15 m), and bottom (hypolimnion, 19 m). The sampling period ranged over several months to capture the dynamics of MPs within the water column as seasonal stratification evolved from weakly stratified spring to stratified summer to weakly stratified fall of 2023. The findings reveal variability in MP abundances ranging from 116 to 181 items/L, 133 to 204 items/L, and 48 to 78 items/L for the surface, middle, and bottom layers, respectively. The data indicate that thermal stratification, particularly during the summer months, plays a key role in entrapping MPs within the middle layers, where the highest concentration (204 items/L) was observed. The study also highlights the diversity of MP types, including polyethylene (PE), polyethylene terephthalate (PET), and rubber, with their vertical distribution partly governed by density and hydrodynamic factors such as water mixing. These results demonstrate the importance of vertical distributions, seasonal changes, and particle density when assessing MP pollution in freshwater ecosystems.