Synergistic and additive effects of microplastic, nickel and pyrene on survival, reproduction, and egestion of a tropical copepod

Coastal tropical ecosystems provide livelihood for millions of people but are at the same time exposed to an increasing intensity of diverse anthropogenic stressors, including pollution. Nevertheless, the combined effects of pollutants on marine ecosystems are poorly understood, particularly regarding lower trophic levels (plankton) and tropical ocean. We exposed the tropical copepod Centropages velificatus to 4-5 concentrations of a heavy metal (nickel), an oil compound (pyrene) and microplastic (PET), either alone or in combination, and measured their egestion, reproduction, and mortality rates. Microplastic alone did not have any effect on pellet or egg production of copepods, whereas nickel reduced egg production rate at concentrations ≥ 1 µg L⁻¹ and pyrene reduced both egg and pellet production rates at concentrations ≥ 1 nM. The addition of nickel and pyrene to PET - microplastic resulted in a reduction similar to one caused by nickel or pyrene alone, suggesting an additive effect. In contrast, a combination of nickel and pyrene had a synergistic effect, with a strong reduction in survival, egg and pellet production. Our results suggest that combinations of contaminants that are commonly found in tropical coastal waters have detrimental effects on copepods – the crucial link in the pelagic food web – at lower concentrations than suggested by single stressor studies. This can have an influence on the food web productivity - the basis of fisheries that local communities rely on.