Microplastic contamination in east Antarctic sea ice

Highlights

• 14 polymer types are identified in microplastic samples from Antarctic sea ice.

• Microplastic concentrations in sea ice are higher than in Southern Ocean seawater samples.

• Relatively large particle sizes suggest local pollution sources.

Abstract

The durability of plastics in the marine environment has led to concerns regarding the pervasiveness of this debris in remote polar habitats. Microplastic (MP) enrichment in East Antarctic sea ice was measured in one ice core sampled from coastal land-fast sea ice. The core was processed and filtered material was analyzed using micro Fourier-Transform Infrared (μFTIR) spectroscopy. 96 MP particles were identified, averaging 11.71 particles L⁻¹. The most common MP polymers (polyethylene, polypropylene, and polyamide) were consistent with those most frequently represented in the majority of marine MP studies. Sea-ice MP concentrations were positively related with chlorophyll a, suggesting living biomass could assist in incorporating MPs in sea ice. Our preliminary results indicate that sea ice has the potential to serve as a reservoir for MP debris in the Southern Ocean, which may have consequences for Southern Ocean food webs and biogeochemistry.

Introduction

Despite their small size, microplastics (MP, plastic particles < 5 mm) have become pervasive even in the most remote marine habitats. Within the past few years a wealth of MP research has been performed in many mediums with high particle concentrations reported (Isobe et al., 2017; Jiang, 2018). Ocean surface waters, deep-sea sediments, and even marine organisms themselves have been analyzed for the presence, quantity, size, and polymer types of MP particles (Andrady, 2011; Bergmann et al., 2017; Germanov et al., 2018; Jiang, 2018). Studies suggest that high MP concentrations in the ocean environment have far reaching implications for biogeochemical processes, biota, and marine ecosystems (Arthur and Baker, 2011; Cole et al., 2011; Koelmans et al., 2014).

MP research remains a relatively young field, evidenced by the fact that particles in polar regions have been discovered only within the last six years (Lusher et al., 2015; Obbard et al., 2014). The possibility of plastic accumulation in polar regions was widely overlooked due to the lack of nearby urban populations and local pollution sources. Yet, while polar seas are more remote, MP concentrations have been found to rival those of more urbanized and heavily populated areas (Cózar et al., 2017; Jiang, 2018). In the Antarctic, dense concentrations of MPs were recently reported in the surface waters of the Pacific and Indian Ocean sectors of the Southern Ocean, with levels comparable to those in Northern Hemisphere oceans at 3.1 × 10⁻⁵ particles L⁻¹ (Isobe et al., 2017). The highest concentrations (9.9 × 10⁻⁵ particles L⁻¹) were those found nearest the Antarctic coastline and accounted for 86% of all MPs recorded during the survey (Isobe et al., 2017). Similarly, sediment samples from the region have revealed MP contamination levels comparable to those found in sea beds worldwide (Munari et al., 2017; Waller et al., 2017), indicating that ocean currents as well as sea ice may be providing means of transport for MP particles (Obbard, 2018).

Annually, sea ice covers roughly 35 million km² of Earth's surface throughout the seasons (approx. 8% of global oceans), making it one of the largest biomes on the planet (Parkinson and DiGirolamo, 2016). In the Southern Ocean, sea ice extends outward from the continent, covering roughly 17–20 million km² in austral winter through spring (NSIDC, 2018). Both land-fast ice (sea ice fastened to a coastline or ice shelf) and pack ice (free-drifting in response to winds and currents) can be found in the Southern Ocean. As opposed to multi-year ice common in the Arctic, the majority of ice in the Antarctic is first-year ice, with about 80% of the sea-ice melting each season (Thomas and Dieckmann, 2003). To date there are three studies identifying MPs in sea ice, all conducted in the Arctic. Initially, MPs were discovered unintentionally in archived ice cores dating as far back as 2005 (Obbard et al., 2014). Recently, Arctic sea ice was confirmed as a primary sink for MP litter after mean concentrations of 2.3 × 10⁴ particles L⁻¹ were detected in pack ice and 6.3 × 10³ particles L⁻¹ in fast ice (Peeken et al., 2018), respectively. An additional field study in the Baltic Sea showed sea-ice MP concentrations ranged from 8 to 41 particles L⁻¹ of melted ice (Geilfus et al., 2019). To our knowledge, the current study is the first to determine MP concentrations in Antarctic sea ice.

MPs remain a new and emerging field of study in the marine environment, and as a result, there is a lack of consistency in sampling techniques for particle extraction and quantification. MP particles are pervasive in all marine realms, and sample processing remains heavily dependent upon the medium analyzed (e.g. sediments, animal tissues, or sea water samples). Hydrogen peroxide treatments, density separations, acid or alkaline digestions, enzymatic treatments, mesh sieving and pre-filtrations have all been used to reduce biological material prior to spectroscopic analysis of MP samples (Bergmann et al., 2017; Valeria Hidalgo-Ruz et al., 2012; Masura et al., 2015). Known methods for MP isolation can be time-consuming, costly, or lead to MP loss. After sample processing is complete, Fourier-Transform Infrared (FTIR) spectroscopy remains one of the best techniques for accurate polymer identification and quantification (Käppler et al., 2016). This spectroscopic method uses infrared light to measure light absorption through particles, leading to spectral fingerprints characteristic of individual polymeric chemical structures (Berthomieu and Hienerwadel, 2009). However, even with the use of focal plane array detector (FPA)-based μFTIR the identification and quantification of MP particles can be laborious and time consuming, as several data management and analysis steps may be needed (Primpke et al., 2017).

With so few studies performed, there are currently no robust or universal methods for analyzing MPs in seawater, let alone sea-ice cores. Of those performed on sea ice, one study used visual sorting to identify plastic particles (Obbard et al., 2014), the second processed samples with hydrogen peroxide (H₂O₂) and filtration (Peeken et al., 2018) prior to FTIR analysis, and the third used the Nile Red technique to visually inspect particles that were suspected to be MPs (Geilfus et al., 2019). However, Antarctic sea ice harbors some of the highest accumulations of biological material anywhere in the marine environment (Iida and Odate, 2014), with sea ice contributing a small but significant portion of total Southern Ocean productivity (Arrigo, 2014). While diatoms dominate these communities, sea ice also hosts other algae, bacteria, heterotrophic protozoans, small metazoans, and high concentrations of extracellular polymeric substances (EPS), also known as exopolysaccharides (Garrison, 1991; Krembs and Deming, 2008). Micro-organisms, such as bacteria and algae, excrete these high-molecular weight compounds in the ice (Meiners et al., 2004) which we hypothesize may bind with MP particles in the same manner as they bind with trace metals and particulate materials (Lannuzel et al., 2014; van der Merwe et al., 2009). Therefore, productive biological communities and dense chlorophyll concentrations in Antarctic sea ice could make MP analysis with FTIR more difficult. Overlapping particles covering the filter can cause complicated spectral profiles due to overlapping spectra of mixture, making an automated analysis approach nearly impossible.

Sea ice could provide an important temporal reservoir for retaining MPs in the surface layers of the Southern Ocean, allowing them to remain bio-available for consumption. This study provides methods for comprehensive sea-ice MP isolation prior to μFTIR analysis, aims to determine whether MP litter is present in Antarctic sea ice, and opens up questions surrounding the impacts of MP debris in Antarctic sea ice. An archived ice core sampled off Casey Station was analyzed to assess the quantity and composition of MPs in East-Antarctic fast ice. In the absence of standard procedures, two methods (hydrogen peroxide treatment, HP; pre-filtration, PF) were performed to compare effectiveness of sample processing and accuracy with polymer identification using FTIR (Primpke et al., 2017). Here we present a workflow for sea ice associated MP analyses and the first data on MP contamination in East-Antarctic sea ice.