Abstract
Microplastics (MPs) are plastic particles <5 mm widely distributed in the marine environment and ingested by a broad range in marine organisms. The copepod Calanus finmarchicus is a key species in the marine food web in the North Atlantic Ocean, and it is one of several copepod species known to ingest MPs. Negative impacts are related to the ingestion of MPs such as decreased intake of microalgae feed and possible hazards related to polymer composition and adsorbed environmental pollutants. In the present study, uptake and excretion of polystyrene (PS) MPs (750 particles ml-1) were studied in two separate experiments. In the uptake study, young adult female C. finmarchicus were exposed to MPs in the presence of the microalgae Rhodomonas baltica (7,500 cells ml-1) for up to 96 hours. The 10 m MPs were readily ingested apparently by non-selective filtration. A somewhat lower average number of algae appeared to be ingested in the presence of MPs compared to non-exposed groups, although the difference between the means at each sampling point was not significant at the p≤0.05 level. During the excretion study a 24-hour exposure phase was followed by depuration for up to 72 hours in filtered seawater (FSW) with/without microalgae. The majority the MPs were egested via the faeces during the first hours of depuration in the presence of microalgae feed. In the absence of microalgae, a lack of sufficient data made it difficult to draw any conclusions about MP excretion via the faeces. MP content in the copepods decreased significantly between 1 and 24 hours, and in the presence of algae a significant lower MP content was found in the copepods at 24 hours compared to copepods not receiving feed. In the latter group the MP content did not appear to decrease further towards 72 hours, and, unexpectedly the MP content also increased in the copepods receiving algae in the same period. Although a small amount of particles was assumed to be retained in the copepods over the period, the increase was assumed to partly be due to other factors such as faecal pellet degradation and re-ingestion of MPs. More studies, with a preferably longer depuration phase, should be conducted to reveal more about a possible retention of MPs in the presence and absence of feed. An assumed sedimentation process was probably causing a discrepancy between measured MP removal and retrieved MPs in the copepods and in their faeces. This was complicating interpretations of results, and emphasizes the need of comprehensive measurements of MPs during experiments. Lastly, faecal pellets loaded with MPs could contribute to increased MP exposure, furthermore as pellets, especially those produced in the absence of algae appeared more fragile and therefore are more prone to degrade and thereby release MPs.