Mangrove Forests of Biscayne Bay, FL, USA may Act as Sinks for Plastic Debris

Abstract

Mangrove forests of Biscayne Bay in southeast Florida, USA can sequester pollutants from freshwater inputs. This “filtering” of water minimizes point source discharges through canals, but mangroves may also play an important role in the cycling of plastic pollution that reaches the Bay. The objectives of this study are to determine: 1) The composition of debris in the Bay’s mangroves and 2) How the structure of mangrove forests affects debris distribution. Debris was hypothesized to be more abundant further into the forest due to trapping by vegetation, and mesoplastics (5 mm – 2.5 cm) would be positively correlated to macroplastics (> 2.5 cm) due to fragmentation. Nine transects were surveyed and debris was recorded by size and potential origin. 94.5% of all debris was plastic of which 57.8% were fragments. Negative binomial generalized linear regression was used to relate total plastic and mesoplastic densities to distance from shoreline, elevation, basal area, prop root and pneumatophore cover, and mangrove seedling abundance. Plastic increased with distance from shore and basal area, although the latter was just above the p-value cut-off of 0.05 for mesoplastic (p-value = 0.0513), and was weakly negatively related to prop root coverage. Total plastic was weakly negatively related to red mangrove seedlings and pneumatophore coverage, although these relationships were less clear. Mesoplastic and macroplastic were positively correlated (p-value < 0.05). Selected mangrove forests of Biscayne Bay appear to be sinks for plastic debris, where it accumulates in the interior forest from which it is unlikely to escape.