Risk Assessment for Microplastic Exposure in the Estuarine Copepod Acartia tonsa

Plastic marine debris is an emerging pollutant of concern both regionally and globally, with microplastics in particular receiving increasing attention. Microplastic marine debris may present serious hazards to individual marine organisms; however, a risk assessment framework has the potential to uncover impacts of microplastics at population, community, and ecosystem scales. Here, we present a risk assessment framework for understanding microplastics effects on an ecologically important estuarine zooplankton species: the copepod, Acartia tonsa. We are using this approach to test whether microplastic marine debris exposure impacts copepod survival in Delaware Bay. Our risk assessment framework includes: (1) determination of spatial and temporal distribution of both microplastics and A. tonsa in Delaware Bay based on field sampling and a high-resolution hydrodynamic numerical model; (2) development of concentration-response functions for mortality and growth upon time-integrated microplastic exposure of A. tonsa in the laboratory; and (3) quantification of microplastic exposure risk to A. tonsa by analyzing microplastic exposure based on results from distribution modeling (1) and adverse responses functions (2). Baywide MPMD concentrations averaged between 0.19 and 1.24 pieces m^-3 depending on size fraction and sampling month, with upper bay stations located in or near the estuarine turbidity maximum having higher MPMD concentrations than lower bay stations; a result which is consistent with copepod distributions. Model results suggest that dynamic surface currents and tidal oscillations can lead to high spatial and temporal variability of MPMD in Delaware Bay, with MPMD concentrations varying by a factor of 1,000 within a tidal cycle at our sample locations. Laboratory experiments with A. tonsa suggest size-dependent and time-dependent mortality functions for microbead exposures. Collectively, we are using these model and laboratory data in ecological risk models at the individual and population levels.