Microplastics as novel sedimentary particles in coastal wetlands: A review

Background/Question/Methods

: Plastic debris has been recognized for its negative effects on the marine environment including wildlife entanglement and bioaccumulation of toxins. However, coastal wetlands are often neglected in marine debris studies. Interactions of plastics with natural particles are also largely understudied across all ecosystems but are becoming the focus of an emerging field on plastic cycling. As studies accumulate, it is important to move beyond presence-or-absence studies and consider the context in which they fit – particularly, the cycling of plastic particles within broader sedimentary processes. To better understand the transport and fate of plastic particles in the environment, the question of whether microplastics (<5mm) behave as a type of sedimentary particle in coastal wetlands was posed. This review evaluates debris distributions in coastal wetlands, incorporation of microplastics into natural aggregates (“heteroaggregation”), plastic degradation within sediments, and persistence of plastic in the sedimentary record to highlight research needs and opportunities. Google Scholar and Web of Science were the primary search engines used to compile literature on diverse plastic polymers across global coastal wetlands. Coastal wetlands are hereto defined as intertidal ecosystems at the transition of fresh and marine domains with a focus on organic, intermittently exposed soils rather than sandy beaches or subtidal environments. These include salt marshes, mangrove swamps, and mudflats in lagoons, bays, and estuaries.

Results/Conclusions

: Polyethylene and polypropylene were the most common polymers identified in coastal wetlands. Sedimentation rates and presence of vegetation were generally positively correlated with retention of macroplastics (>2.5cm), but patterns for microplastic distribution are unclear. Heteroaggregation can alter sediment properties such as settling velocity, water holding capacity, and soil stability which could either increase or decrease microplastic bioavailability. Moreover, abiotic degradation of plastics is stifled under the shady, low tidal energy environments of vegetated coastal wetlands, and the rate of biodegradation by microbes likely cannot keep up with inputs of plastics. Some studies have found microplastics buried deep below the soil surface, and although only one sampled in a coastal wetland, the depositional nature of wetlands may effectively store plastics long-term. Thus, the ubiquity, persistence, and interactions of plastic particles within wetlands warrants their consideration as novel sedimentary particles. Furthermore, coastal wetlands may act as plastic reservoirs. Coupled with the momentum of global “green” movements, these findings have broader implications for ecology in that they may advance the fields of coastal sedimentology and anthropogenic soils and contribute a new perspective to the Anthropocene debate.