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Non-native Rhizophora mangle as sinks for coastal contamination on Moloka'i, Hawai'i
Summary
Researchers studied microplastic and pesticide contamination in coastal areas of Moloka'i, Hawaii, comparing zones with invasive mangrove trees to open coastline. They found that mangrove roots concentrated both microplastics (up to 2,004 particles per kg) and the insecticide bifenthrin at high levels, acting as pollution sinks that could pose risks if these trees are removed.
Coastal mangrove forests provide a suite of important services, including sequestration of anthropogenic contamination. Yet, research lags on the environmental fate and potential human health risks of mangrove-sequestered contaminants in the context of mangrove removal for development and range shifts due to climate change. To address this, we conducted a study on Moloka'i, Hawai'i, comparing microplastic and pesticide contamination in coastal compartments both at areas modified by non-native mangroves and unmodified, open coastline. Sediment, porewater, and mangrove plant tissues were collected to quantify microplastic and pesticide concentrations across ecosystem type. Average microplastics were similar between mangrove (8.89 items/kg) and non-mangrove areas (9.01 items/kg) in sediment and porewater, but mangrove roots were a significant reservoir of microplastics (2004 items/kg). Additionally, there was a strong relationship between proximity to urban development and microplastics detected. Six pesticides were detected, most commonly the insecticide bifenthrin, found in most sediment samples (11.3 ng/g), all root samples (243.3 ng/g), and one propagule sample (8.60 ng/g). Other pesticides detected with appreciable concentrations include the neonicotinoid insecticide imidacloprid and the legacy insecticide transformation product, p,p’-DDE. The other detections, all at concentrations < 1 ng/g, were p,p’-DDT, trifluralin, and permethrin. The high concentrations of bifenthrin in roots compared to lower concentrations detected in sediment suggest that mangrove roots strongly accumulate some pesticides, indicating mangrove roots as a sink for organic contaminants. Future research should focus on expanding this work to other Hawai'ian Islands and other locations where mangroves have been introduced to further examine the observed trends. Researchers should also investigate the fate and cycling of pesticides and microplastics adhered to mangrove roots, which will better inform future non-native mangrove removal efforts on Moloka'i and elsewhere.