Occurrence of small microplastics in the salp <i>Salpa fusiformis</i> in the Kuroshio region

Abstract We investigated the occurrence pattern of Salpa fusiformis , broad‐spectrum filter‐feeding zooplankton that intercept small particles around the Kuroshio Current, off southern Japan and inferred in situ distribution of near‐surface small microplastics (plastic particles &lt; 330 μ m) based on their gut contents. First, we examined 98 neuston net samples to determine the distribution of S. fusiformis . While no S. fusiformis were found in the daytime samples, they occurred in 75% of the night samples, indicating active diel vertical migration. Their nocturnal density in the surface layer was 0.026–0.82 ind. m −3 and was particularly high around the cold‐core ring within the large meander of the Kuroshio Current. Microplastics were found in 98% of the guts of S. fusiformis (mean: 5.23 ± 4.40 particles/ind.). The in situ density of near‐surface small microplastics estimated from their gut content analysis ranged from 194 ± 138 to 7093 ± 923 particles m −3 . The estimated small microplastics density was negatively correlated with surface current velocity, while small microplastic size and polymer compositions were spatially variable. This suggests that the small microplastic distribution was influenced by oceanographic conditions and location of pollution sources. The maximum small microplastic ingestion rate by S. fusiformis in the surface layer (&lt; 0.75 m depth) was 34.5 particles m −3 d −1 , corresponding to 1.05% of the in situ small microplastic density. Overall, the uptake efficiency of small microplastics from the water column may be substantial owing to the active diel vertical migration and production of large fecal pellets by S. fusiformis .