Anthropogenic marine debris found in peri-urban and urban mangroves on Penang Island, Malaysia
The abundance diversity parameters and intertidal sorting of accumulated anthropogenic marine debris (AMD) were evaluated in the urban and peri-urban mangroves along the coast of Penang Island, Malaysia. Debris items were counted and classified along 80 m transects in progressively higher water marks from as close to the seaward side as practical, within a 100 m2 quadrat sampling frame. Abundance, diversity, evenness, and patterns of accumulation in relation to tidal levels were compared between four sites which were sampled at different periods over 2 months and constrained by possible changes in their previous individual wind fields, and neap-spring tidal development. Plastics made up most of the AMD with plastic bags, sheets, and fragments being the most common items found at all sites. More AMD was retained in the urban sites compared to peri-urban sites, consistent with their larger resident population density. Diverse debris forms were consistent with the type of land use and population livelihood in each area, with a clear separation between the most urbanised and peri-urban sites on the NE and NW coasts and the urban and peri-urban sites on the SE and SW coastlines on the island. At the low tidal levels close to the edge in the urban sites, we observed the largest differences in abundance, diversity, and evenness over a similar parameter compared to the transects away from the edge, owing to sorting and edge effect. These patterns of change and differences across transects and sites, suggested: 1) an independence of accumulated material with neap-spring phases, for a constant wind field; 2) a mangrove canopy and root structure away from the edge that efficiently retains deposited materials with little sorting through the main body of the canopy; and 3) materials deposited closer to the edge are increasingly lost to the water body due to the edge effect.