Impact of polystyrene microplastics on Daphnia magna mortality and reproduction in relation to food availability

Abstract

Microplastics (MPs) in the environment continue to be a growing area of concern in terms of acute and chronic impacts on aquatic life. Whilst increasing numbers of studies are providing important insights into microparticle behaviour and impacts in the marine environment, a paucity of information exists regarding the freshwater environment. This study focusses on the uptake, retention and the impact of 2 µm polystyrene MPs in the freshwater cladoceran Daphnia magna in relation to food intake (algae Chlorella vulgaris), with MP size chosen to approximately match the cell size of the algae. Daphnia were exposed to varied concentrations of MPs and algae. When exposed to a single concentration of MPs Daphnia almost immediately ate them in large quantities. However, the presence of algae, even at low concentrations, had a significant negative impact on MP uptake that was not in proportion to relative availability. As MP concentrations increased, intake did not if algae were present, even at higher concentrations of MPs. This suggests that Daphnia are selectively avoiding eating plastics. Adult Daphnia exposed to MPs for 21 days showed mortality after seven days of exposure in all treatments compared to the control. However significant differences were all related to algal concentration rather than to MP concentration. This suggests that where ample food is present, MPs have little effect on adults. There was also no impact on their reproduction. The neonate toxicity test confirmed previous results that mortality and reproduction was linked to availability of food rather than MP concentrations. This would make sense in light of our suggestion that Daphnia are selectively avoiding eating microplastics.

Keywords: Chlorella vulgaris; Chronic toxicity; Daphnia magna; Eco-toxicology; Life history; Microplastics; Polystyrene.

Figure 1. Uptake of 2 µm polystyrene MPs by Daphnia magna exposed to MPs only (1.46 ×10² mg/L) or MPs with algae (1.46 ×10² mg/L and 1.00 ×10⁻¹ mg/L) over 240 min.

Each point represents the mean ± the standard error.

Figure 2. Excretion of 2 µm polystyrene MPs from the gut of Daphnia magna exposed to MPs only (1.46 ×10² mg/L) or MPs with algae (1.46 ×10² mg/L and 1.00 ×10⁻¹ mg/L) over 240 min.

Each point represents the mean ± the standard error.

Figure 3. Uptake of 2 µm polystyrene MPs by Daphnia magna with and without algae in various volumes (µl) (see Table 1 for actual concentrations).

Each point represents the mean ± the standard error.

Figure 4. Mortality of Daphnia magna expressed as a function of time after chronic exposure to MPs under high and low food conditions for 21 days.

Asterisks denote overlap between two treatments.

Figure 5. Effects of combinations of high and low MPs and algae concentrations on the mean number of offspring on Daphnia magna.

Error bars indicate ± 95% confidence intervals and asterisks denote significant differences compared to the control p < 0.001.

Figure 6. Mortality of neonate Daphnia magna after exposure to different treatments of MPs and algae over 21 days.

Asterisks denote overlap between two treatments.

Figure 7. Daphnia magna reproduction (neonate production) after 21 days’ exposures to a range of MP and algae treatments (algae (low), algae (high), Algae = MP(low), Algae = MP(high), Algae > MP, MP > Algae).

Error bars indicate ± 95% confidence intervals and asterisks denote significant differences compared to the control p < 0.001.

Figure 8. Effect of 21 days’ exposure to different combinations of MPs and algae (Algae (low), algae (high), Algae = MP(low), Algae = MP(high), Algae > MP, MP > Algae) on body length of Daphnia magna.

Each point represents the mean of five replicates ± standard error.