Abstract
Plastics are pervasive pollutants that are being produced at an increasing rate to meet consumer demands. Most plastics do not break down easily, and when they do, smaller fragments are created, including secondary microplastics. Microplastic contamination in lakes has been recorded worldwide, and the ingestion of microplastics has been documented in zooplankton, macroinvertebrates and fish. Microplastic ingestion and exposure can cause varying deleterious effects on these species; however, these impacts require further study, particularly within realistic freshwater food webs. To date, most studies have focused on marine systems, been laboratory-based, included only one or two trophic levels, and/or used unrealistically high concentrations of microplastics. In the present study, I aim to address these knowledge gaps. In addition, this is the first experiment investigating the effects of microplastics on New Zealand lake food webs.
The experiment was conducted in outdoor 1,200-L mesocosms and involved multiple trophic levels. Each plastic-treated mesocosm included three different types of microplastics: two sizes of spherical fluorescent orange polyethylene primary microplastics (45 - 53 μm and 355 - 425 μm in diameter) and 300 - 600-μm secondary polyethylene microplastics. Plastic treatments had a field-realistic combined concentration of 1.5 particles/L. Mesocosms were established to represent a shallow coastal lake food web, with the introduced European perch (Perca fluviatilus) as the top predator. In a 2 x 2 design, the presence of perch was crossed with the addition of microplastics, with seven replicate mesocosms per treatment combination.
Of the macroinvertebrates examined in plastic-treated mesocosms, microplastic ingestion was observed in only one instance, by a Chironomus sp. larva. No microplastic ingestion by zooplankton was observed, likely due to the microplastics in this experiment being larger than in past experiments that have reported microplastic ingestion by zooplankton. Of the perch in plastic treatments, 28.5% ingested the added plastics; however, perch selected against plastics relative to a wide range of invertebrate prey. Microplastic exposure did not result in any observed changes in perch mass, mean prey items consumed, refuge use, or time spent swimming. There were differences in the prey taxa consumed by perch in plastic-treated mesocosms compared to control mesocosms; however, this was likely due to changes in zooplankton and macroinvertebrate communities.
Both perch presence and microplastic exposure were associated with changes in the abundance of macroinvertebrate and zooplankton taxa. In the presence of microplastic, the cased caddisfly Triplectides spp. had a significantly lower abundance. In addition, the impacts of microplastics on total zooplankton and the cladoceran Ceriodaphnia changed over the two sampling dates, where the abundance of both appeared to be negatively impacted by microplastics in week eight, the final sampling date. These changes in zooplankton and macroinvertebrate taxa also contributed to the near-significant compositional differences in invertebrate (combined macroinvertebrate and zooplankton) communities between control and plastic treatments in the final week of the experiment. These findings indicate that microplastic exposure may negatively impact freshwater invertebrate communities, even at low, field-realistic concentrations. Consequently, current microplastic concentrations may be having an impact within New Zealand lakes.