Benchmarking the uptake and excretion dynamics of microplastics in the boreal marine copepod Calanus finmarchicus

The marine copepod Calanus finmarchicus is a filter feeder and a keystone species of the Northern Atlantic that, alongside other crustaceans, is known to ingest microplastic (MP). Potential effects of microplastic ingestion include obstruction, abrasion, lower energy budget due to reduced food uptake, and toxicity related to the leakage of chemicals from microplastics into the organism. The magnitude of all these effects is largely determined by microplastic uptake and excretion rates, and particularly by retention or accumulation times inside an organism. However quantitative data of this type are currently unavailable. In the current study, young female C. finmarchicus were exposed 0-96 h to a non-restrictive concentration (750 MP particles/mL) of a comprehensively characterized 10 m spherical MP of general purpose polystyrene in the presence of microalgae (7500 cells/mL). Excretion dynamics were studied over 1-72 h after a 24 h exposure to MP in either the presence or absence of microalgae. Custom made transparent plankton wheels immersed in a water bath were used to expose C. finmarchicus in a semi-static setup to keep MP and microalgal particles in suspension and to maintain constant temperature and light conditions. MP counts were performed in triplicate in exposure media, copepods and their fecal pellets as well as on container walls for a total of 16 sampling time points. Findings show unconventional uptake kinetics for MP in C. finmarchicus when compared to dissolved organic chemicals. Excretion of MP was rapid although there was indication that retention may occur. Excretion dynamics were affected by the presence of microalgae. The current study was designed as a benchmarking exercise with commercial microbeads of uniform size. The findings can therefore not be generalized for MPs of other shapes, compositions, conditions, densities and sizes, but should rather be considered as a reference for future studies using MP with more environmentally relevant characteristics and concentrations.