Is Zooplankton an Entry Point of Microplastics into the Marine Food Web?

We found a low concentrationof microplastics in zooplanktonand their fecal pellets, suggesting a low risk of ingestion and biotransferof microplastics via zooplankton. Microplastics (MPs) overlap in size with phytoplanktonand canbe ingested by zooplankton, transferring them to higher trophic levels.Copepods are the most abundant metazoans among zooplankton and themain link between primary producers and higher trophic levels. Ingestionof MPs has been investigated in the laboratory, but we still knowlittle about the ingestion of MPs by zooplankton in the natural environment.In this study, we determined the concentration and characteristicsof MPs down to 10 & mu;m in zooplankton samples, sorted calanoidcopepods, and fecal pellets collected in the Kattegat/Skagerrak Sea(Denmark). We found a median concentration of 1.7 x 10(-3) MPs ind(-1) in the zooplankton samples, 2.9 x10(-3) MPs ind(-1) in the sorted-copepods,and 3 x 10(-3) MPs per fecal pellet. Most MPsin the zooplankton samples and fecal pellets were fragments smallerthan 100 & mu;m, whereas fibers dominated in the sorted copepods.Based on the collected data, we estimated a MP budget for the surfacelayer (0-18 m), where copepods contained only 3% of the MPsin the water, while 5% of the MPs were packed in fecal pellets. However,the number of MPs exported daily to the pycnocline via fecal pelletswas estimated to be 1.4% of the total MPs in the surface layer. Ourresults indicate that zooplankton are an entry point of small MPsin the food web, but the number of MPs in zooplankton and their fecalpellets was low compared with the number of MPs found in the watercolumn and the occurrence and/or ingestion of MPs reported for nekton.This suggests a low risk of MP transferring to higher trophic levelsthrough zooplankton and a quantitatively low, but ecologically relevant,contribution of fecal pellets to the vertical exportation of MPs inthe ocean.