Abstract: Globally, legislation has emerged to reduce unsightly marine debris. Microplastics (“MPs”, < 5 mm in size) are less conspicuous, yet they pose a more insidious threat at all levels of the food chain. Mangrove forests filter contaminants like excess nutrients from the water column, and it is hypothesized that mangroves may also sequester MPs before they reach Biscayne Bay (FL, USA). The Biscayne Bay and Southeastern Everglades Ecosystem Restoration (BBSEER) project will deliver freshwater through culverts into the L-31 East Flow-way to restore sheet flow through coastal mangroves, allowing for a comparison of MP concentrations in mangrove stands with and without water deliveries. The objectives of this study are to 1) Determine if MPs decrease in abundance with distance from the canal due to filtering by mangroves, and 2) Evaluate potential MP management solutions.
Water and sediment samples were taken from 3 “zones” (West to East): the L-31E canal (a potential MP source), dwarf mangroves, and fringe mangroves. Samples were dried, sieved, oxidized, and sediments were separated from lighter plastics by density. Samples were stained with Nile Red, a lipophilic dye that fluoresces, and analyzed under blue light. Most plastics were recovered by count from spiked water samples (68.0 ± 34.6% - 100 ± 0%), whereas in sediments, recovery depended on polymer type (10.0 ± 24.5% - 100 ± 0%). MP isolation is ongoing; for plastics > 4 mm in size, 16 particles were found in sediments (15 in the fringe, 1 in the dwarf) and 3 in dwarf water samples from the dry season. Storage of MPs in mangrove sediments suggests that mangroves may prevent MP dispersal downstream.
A management recommendation is proposed to create local land development Best Management Practices that emphasize mangrove buffer zones to trap MPs before they reach the Bay. Stakeholders in the Everglades restoration, waste management, and plastic policy were also interviewed to assess other management alternatives via semi-structured, key informant interviews (25% interview response rate). In addition to developing Best Management Practices, other recommendations included: amending the Clean Waterways Act of 2020 to address storm drains as vectors of MPs; formalizing a Memorandum of Understanding between the County, State, and Federal government to prevent trash in canals; developing a monitoring program for MPs; and creating locally-led recycling co-ops. This project will fill knowledge gaps on MP dynamics in South Florida and bridge interdisciplinary divides to generate local management recommendations for MPs.
