Abundant plankton-sized microplastic particles in shelf waters of the northern Gulf of Mexico

Accumulation of marine debris is a global problem that affects the oceans on multiple scales. The majority of floating marine debris is composed of microplastics: plastic particles up to 5 mm in diameter. With similar sizes and appearances to natural food items, these small fragments pose potential risks to many marine organisms including zooplankton and zooplanktivores. Semi-enclosed seas are reported to have high concentrations of microplastics, however, the distribution and concentration of microplastics in one such system, the Gulf of Mexico, remains unknown. Our study documented and characterized microplastics in continental shelf waters off the Louisiana coast in the northern Gulf of Mexico, using bongo nets, neuston nets, and Niskin bottles. Additionally, we compared the size distributions of microplastics and zooplankton collected using the nets. Plastics were manually sorted from the samples, documented, and measured using digital microscopy. Confirmation of putative plastics was carried out by hydrofluoric acid digestion and a subsample was analyzed using FTIR microscopy. Estimated concentrations of microplastics collected on the inner continental shelf during this study are among the highest reported globally. Total microplastic concentrations ranged from 4.8 to 8.2 particles m⁻³ and 5.0–18.4 particles m⁻³ for the bongo and neuston samples, respectively. Niskin bottles collected smaller plastic particles than the nets and indicated total microplastic concentrations (primarily fibers) from 6.0E4 – 15.7E4 particles m⁻³. Microplastic concentrations were greater than the abundances of all but four of the five most abundant taxa from bongo nets and were not statistically different from the abundances of any of the most numerous taxa from neuston nets. Sizes of microplastics and zooplankton partially or completely overlapped, suggesting the potential for confusion with natural prey. - Highlights: • The 1st survey in the Gulf of Mexico reveals abundant microplastic concentrations. • Microplastic concentrations on inner shelf rival highest levels reported globally. • Bulk water samples revealed extremely abundant microfibers in upper water column. • Microplastics fell within zooplankton size ranges and could be confused with prey. - This study characterized the concentrations and sizes of microplastics from four sites in the northern Gulf of Mexico. Microplastic concentrations were similar to those reported for other semi-enclosed bodies of water and were as high as some of greatest reported measurements, globally. Size ranges of microplastics either partially, or completely overlapped with the sizes of most major zooplankton taxa present in bongo net samples.