Seasonal Distribution of Microplastics in Hamilton Harbor’s Water

Microplastics (MPs) are plastic particles smaller than 5 mm and known to have prolong negative impacts on ecosystem and human health. MPs often are transported to freshwater and marine systems, and it is important to track their occurrence in water bodies. The distribution of these contaminants in water is affected by various parameters including particles’ buoyancy, surface composition, biofilm formation, and flow hydrodynamics. One hydrodynamic parameter that can affect the MPs vertical distribution is lake stratification during hot months. Temperature-induced lake stratification is shown to be an important factor in the depth-wise distribution of organic matter and oxygen on water column. This study focuses on the impact of lake stratification and thermocline formation on the vertical distribution of MPs in Hamilton Harbor. Thermocline formation in a lake could cause a dramatic difference in the temperature, which changes the density of water between top and bottom layer where it calls metalimnion, and thermocline is a part of this stratified section. As a result of thermocline formation, hydrodynamic behavior of MPs could be affected, where the distribution can be affected by partitioned mixing. To conduct this research, large-scale water samples were taken from July and October at different depths, and pretreatment methods prior to microscopy were done to extract maximum MPs from the water samples. Finally, microscopy was carried out and water residues on filters from near-surface layer (1 m), thermocline layer (6.5 to 9 m), and near-bottom samples (1 m above the bottom layer) were counted based on their size and shape. The thermocline zone had a higher MP concentration during the warm season than either the surface or bottom layers, according to initial findings, indicating the important impact of thermal stratification on MP distribution. Fibers were the most dominant particles in all layers of water during both warm and cold months. Particles were divided into four categories based on their size, and large-size particles (larger than 300 µm) outnumbered smaller particles from different categories. Findings of this research prove that thermocline could entrap particles in the middle layer during hot months.