Investigating Plastic Contamination of Galapagos Mangrove Systems

Abstract

Plastic pollution within the marine environment is increasing, yet there is limited understanding of its accumulation within ecologically and socially important mangrove ecosystems. By conducting a meta-analysis of existing data, this thesis first reveals high contamination across all environmental compartments within mangrove ecosystems sampled globally, often exceeding proximate marine environment abundances. Despite the vulnerability of the Galapagos Archipelago, a UNESCO World Heritage Site, and an oceanic island where high plastic contamination of beaches has been recorded, there is a lack of data on plastic contamination in its mangrove ecosystems and only limited data on coastal seawater microplastic abundances. This thesis aimed to address these knowledge gaps by: i) determining the influence of sampling methodology on microplastic abundance recorded within coastal seawater, ii) characterise island scale distribution of microplastic within coastal seawater informing on plastic sources and iii) quantify plastic pollution within three distinct mangrove ecosystems.

Sampling San Cristobal Island coastal seawater using combined methodologies, of 1 litre whole seawater grab samples filtered to 1.2 μm and sea surface plankton tows with a net mesh size of 200 μm, revealed a four-order magnitude higher microplastic abundance in grab samples than plankton tows; in combination with 44% of microplastic particles smaller than 200 μm in grab samples, this suggests previous assessments using plankton nets underestimated the risk of this pollutant to marine species. Targeted grab sampling revealed high seawater concentrations within the harbour/town area with a mean of 19.7 ± 1.9 microplastics L-1, exceeding the Pacific Ocean average by 2.8-fold. Microplastic concentrations within three distinct mangrove habitats on San Cristobal Island were also found to have a higher average abundance of 33.06 ± 14.02 microplastics L-1. Sampled using hand-pump methodology filtered to 70 μm, this contamination within mangrove seawater was ~four orders of magnitude greater than previous proximate coastal seawater. Visual transect survey methodology revealed further contamination within the mangrove ecosystem, with an average macroplastic abundance of 1.93 ± 0.69 items m-2, comprising items larger in size than those found on proximate beaches. Understanding the abundance, dynamics and impacts of micro-and macroplastic abundance within mangrove ecosystems is required to comprehend the risk to the associated biota and the broader global impacts on mangroves, including their potential for carbon sequestration.

The outflow pipe within the harbour town emerged as a local point source of plastic pollution, using grab sampling methodology, contradicting the theory that plastic pollution within the Galapagos originates solely from allochthonous sources. However, the microplastic composition disparity between the harbour and mangrove seawater, in combination with island microplastic spatial distribution, indicates allochthonous sources of plastic pollution from mainland South America contribute to mangrove seawater microplastic contamination; 40% of microplastic particles were anthropogenic cellulose fibres in the harbour/town seawater versus 68% microplastic particles were white PVC/PVF/additive fragments in mangrove seawater. Consequently, local and international mitigation strategies are required to address these high levels of plastic pollution within the Galapagos, especially those found in mangrove seawater.