Microplastic and biogeochemical releases from plastic, metal, cement, and fiber coastal restoration materials

Coastal areas often include ecologically sensitive habitats and dense human populations, which can contribute to ecosystem degradation and the need for coastal restoration. Typically, coastal restoration requires the placement of a foreign material to serve as a base substrate or shoreline stabilizer, but the potential for these materials to contribute to plastic pollution or impact biogeochemical cycles is not well understood. This research investigated common coastal restoration materials to address two key knowledge gaps, 1) can plastic restoration materials be a source of microplastics?, and 2) can alternative restoration materials release nutrients, metals, or alter microbial respiration? Two polyethylene plastics and five non-plastic alternative materials were studied in experimental laboratory microcosms, including a one-year incubation to quantify microplastic release, a 5-week nutrient and metal release study of fresh and post field-deployed materials, and a one-week respiration (CO₂ production) incubation with site sediment and water. Laboratory results indicated the two plastics studied released microplastics at a rate of 395 ± 100 and 158 ± 56 g^-1 y^-1, for Naltex® and Vexar®, respectively. Jute, made of plant fiber, released 4.7 and 37 times more dissolved inorganic nitrogen and soluble reactive phosphorus, respectively, than the control. The biopolymer, BESE-elements®, released 60 and 32 times more dissolved organic carbon and CO₂-C, respectively, than the control. Fresh galvanized metal gabion wire released iron, manganese, lead, and 28 times more zinc than the control, while cement materials showed minimal effects. Restoration practitioners, resource managers, and permitting organizations should consider microplastic and biogeochemical impacts when choosing materials.