Microplastics Alter Nitrogen Cycling and Reduce the Abundance of Meiofauna in Estuarine Sediments

Abstract

Since 1950, global plastic production has increased from 2 to over 400 million tonnes annually, yet there are still no unified global regulatory frameworks that address this issue. Although microplastics are recognized as emerging contaminants in marine environments, little is known about their impact on meiofauna and their associated biogeochemical processes. This study investigated how meiofauna, oxygen dynamics, and the nitrogen cycle respond to polyethylene microplastics (µPE) in estuarine sediments from the west coast of Sweden. Three different concentrations of weathered polyethylene were added to sediment cores. Concentrations of µPE significantly reduced total meiofauna abundance by up to 45%. In particular, the reduction in nematode abundance (65%) led to a shift in meiofauna´s community composition from nematode dominated to foraminifera dominated. Furthermore, this shift led to a reduction in meiofauna mediated bioturbation, resulting in a significant decrease of oxygen penetration depth (OPD). While µPE enhanced coupled nitrification- denitrification (Dn), the shallower OPD presumably reduced denitrification efficiency, promoting benthic nitrous oxide (N2O) release. High µPE addition also significantly increased the rate of dissimilatory nitrate reduction to ammonium (DNRA). These results suggest that meioturbation may regulate benthic biogeochemical processes and enhance Dn efficacy by promoting deeper OPD. Further research on how plastic pollution affects marine biogeochemistry in coastal and open ocean is needed.