Zooplankton responses to environmentally relevant microplastic conditions at low food availability

The global use of plastic has drastically increased over recent years. Aquatic ecosystems are pools for plastic build up that break down into microplastics (<5 mm) (MP). Despite this, there remains a high uncertainty on how these MPs transfer across trophic levels and impact aquatic organisms including zooplankton. Most research to date has used MPs that are not environmentally relevant in terms of type, concentrations, shapes, sizes, etc. Therefore, the purpose of this thesis was to use life table experiments to determine if MPs at environmentally relevant conditions impacted the life history and lifespan of two species of Daphnia, Daphnia magna and Daphnia pulex, at low algal food availability. We focused on commonly used plastic types of polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC) at environmentally relevant concentrations of 500, 2500, and 5000 particles/L. Results show that zooplankton exposed to environmentally MPs at low food availability had no significant negative effects on total reproduction, reproductive timing, or total lifespan. Despite long term chronic exposure to environmentally relevant MPs, these findings suggest that the impact on aquatic organisms may not be as apparent as hypothesized from previous experiments that used unrealistic MP conditions. I did find that PP had a significant positive impact on D. magna lifespan under medium and high concentrations, however, additional research is needed to understand this impact. While MPs at environmentally relevant conditions may not have direct negative impacts on zooplankton life history, they still have the potential to impact aquatic ecosystems. These potential impacts include biomagnification in food webs, increase toxicological exposure for zooplankton if metals bind to MPs, and changes in food web interactions. In conclusions, this research helps develop a better understanding on the limitations and understanding we have for MPs research in scientific literature, beyond unrealistic laboratory conditions.