Microplastics with and without chemical additives modestly affected phytoplankton and zooplankton in a large in-lake mesocosm study

Microplastics are complex contaminants, potentially posing both a physical and chemical risk to aquatic organisms. To better understand the physical and chemical impacts of microplastics, we conducted a large in situ pelagic mesocosm experiment in a freshwater boreal lake at the International Institute for Sustainable Development-Experimental Lakes Area. An equal mixture of polyethylene, polystyrene, and polyethylene terephthalate fragments with and without chemical additives were added to mesocosms as a single pulse and were contrasted with a control treatment with no added microplastics. Plankton communities were monitored for 62 days following microplastic additions. Total phytoplankton biomass was not affected by either microplastic treatment; however, a shift in phytoplankton community composition was detected in the microplastic treatment without additives on Day 62. Total zooplankton and cladoceran abundance marginally increased over time in both microplastic treated mesocosms, and diversity was lower in the additive treatment. There was a negative impact on Tropocyclops extensus egg production in microplastic treatments with and without additives, and the abundance of early-instar Chaoborus was temporarily higher in mesocosms containing microplastics without additives. Overall, the impacts of microplastics were relatively small, irrespective of the presence or absence of chemical additives, on natural pelagic phytoplankton or zooplankton communities over the 62-day exposure.