Microplastic exposure to zooplankton at tidal fronts in Charleston Harbor, SC USA

Abstract

Zooplankton, fish, marine mammals and birds forage at flood-tide front aggregation zones that form at the mouths of the Ashley and Cooper Rivers in Charleston Harbor, South Carolina USA where semi-buoyant particulates and phytoplankton accumulate. Increased trophodynamics and the potential for the accumulation of semi-buoyant microplastic particles at aggregation zones may present a pathway of increased exposure for microplastic to enter the estuarine trophic system via ingestion by zooplankton. Optical and fluorescence microscopy were used to enumerate microplastic fiber and fragment concentrations, with melting point analysis applied for confirmation, in water sieved (sized 43–104 μm) from the upper meter of the water column at positions across tidal fronts. The Cooper River front contained significantly higher microplastic water concentrations, dominated by fragments, on the riverside and within the frontal zone versus the seaward side, indicating an input of microplastic from land-based sources. No significant increase in abundance of zooplankton was found within the Cooper River or Ashley River fronts. Microplastic in the gut of field-collected zooplankton was observed using fluorescence microscopy. Only zooplankton from the Cooper River were observed with ingested microplastic. Absence of detection in zooplankton from the Ashley River may be due to fibers being more prevalent than fragments in surface waters at that front. On average, 1% of the filter-feeding zooplankton observed contained ingested microplastic particles suggesting zooplankton may be selectively feeding. Although the fraction was small, it represents significant exposure to microplastic for pelagic fish species that may be feeding at flood-tidal fronts in Charleston Harbor.