Does plastic waste kill mangroves? A field experiment to assess the impact of macro plastics on mangrove growth, stress response and survival

Celine E.J Van Bijsterveldt,Bregje K Van Wesenbeeck,Sri Ramadhani,Olivier V Raven,Fleur E Van Gool,Rudhi Pribadi,Tjeerd J Bouma

doi:10.1016/j.scitotenv.2020.143826

Abstract

The value of mangroves has been widely acknowledged, but mangrove forests continue to decline due to numerous anthropogenic stressors. The impact of plastic waste is however poorly known, even though the amount of plastic litter is the largest in the region where mangroves are declining the fastest: South East Asia. In this study, we examine the extent of the plastic waste problem in mangroves along the north coast of Java, Indonesia. First, we investigate how much of the forest floor is covered by plastic in the field (in number of items per m2 and in percentage of the forest floor covered by plastic), and if plastic is also buried in the upper layers of the sediment. We then experimentally investigate the effects of a range of plastic cover percentages (0%, 50% and 100%) on root growth, stress response of the tree and tree survival over a period of six weeks. Field monitoring showed that plastic was abundant, with 27 plastic items per m2 on average, covering up to 50% of the forest floor at multiple locations. Moreover, core data revealed that plastic was frequently buried in the upper layers of the sediment where it becomes immobile and can create prolonged anoxic conditions. Our experiment subsequently revealed that prolonged suffocation by plastic caused immediate pneumatophore growth and potential leaf loss. However, trees in the 50%-plastic cover treatment proved surprisingly resilient and were able to maintain their canopy over the course of the experiment, whereas trees in the 100%-plastic cover treatment had a significantly decreased leaf area index and survival by the end of the experiment. Our findings demonstrate that mangrove trees are relatively resilient to partial burial by plastic waste. However, mangrove stands are likely to deteriorate eventually if plastic continues to accumulate.

Highlights

Even though mangroves are widely valued for the ecosystem services they provide, forests are rapidly declining despite restoration efforts
We used the coastline of Demak regency in Central Java as a model site to study the effect of plastics on mangroves, as this area is exemplary for many densely populated mangrove areas found in South East Asia
The plastics visibly accumulate in sediment ridges on the seaward edge of the mangrove stands, where the flow velocity of the entering water drops significantly due to the drag caused by prop roots and pneumatophores

Summary

Introduction

Even though mangroves are widely valued for the ecosystem services they provide, forests are rapidly declining despite restoration efforts. Mangrove forests provide multiple provisioning, regulating and recreational ecosystem services, but they are most valued for their role in coastal protection (Barbier et al, 2011). In spite of this global recognition of their importance, anthropogenic influences such as land use change, cause continued mangrove decline worldwide (Alongi, 2002; Thomas et al, 2017). M. Ellison, 2000; Primavera and Esteban, 2008), and achieving such an understanding of the local hydrodynamics and sediment balance has increasingly been recommended to improve mangrove settlement success The most notable anthropogenic pollution that might stress mangroves is plastic waste (Smith, 2012)

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