Microplastics in surface waters of the Gulf of Gabes, southern Mediterranean Sea: Distribution, composition and influence of hydrodynamics

Highlights

• First report of microplastic distribution in Gulf of Gabes's surface waters.

• Dominance of fragments and films, PE and PP among collected items.

• Presence of an “atypical” offshore station (shape, colour and size).

• Hydrodynamic modelling in agreement with observed microplastic distribution.

Abstract

The Mediterranean Sea has been described as one of the most affected areas by marine litter in the world. Although microplastics and their effects have been investigated in this area, most of the currently available studies have been limited to the northwestern part of the basin. This study constitutes a first attempt to determine the abundance, characteristics and composition of microplastics in near surface waters of the Gulf of Gabes (southern Mediterranean Sea, Tunisia). Samples were collected using a 200 μm-mesh size trawl net along two transects. The study revealed an average concentration of 63,739 items/km² where fragments and films were the most frequent microplastics. Polyethylene, reformulated polyethylene and polypropylene were the most abundant plastics identified among the samples (86–100%). The influence of hydrodynamics on microplastics in the Gulf of Gabes was investigated through the use of a Lagrangian tracking model to simulate the dispersion of particles in water. Modelling results seem to be in agreement with the reported distribution and characteristics of microplastics in this area.

Introduction

Because of their unique properties, plastic materials are used in a wide range of application sectors and in our daily lives. As a result, plastic yearly production has exceeded the 340 million tons mark (PlasticsEurope, 2018). Although it exits a great variety of types, applications and life cycles for plastics, most of them are designed for a short service time or single use applications, such as plastic bags, with low value recovery and difficulty to degrade (Wang et al., 2016). The sharp contrast between the remarkable durability of plastic materials and their short service time has led to the accumulation of plastic wastes in the environment. Plastics have been reported in the ocean, in lakes, in rivers and even in aquatic and terrestrial animals due to ingestion (Dümichen et al., 2017). In particular, marine plastic litter is estimated to increase by one order of magnitude by 2025 when considering the level of expected future demand and discharges of plastics (Jambeck et al., 2015). When plastics debris undergo environmental impacts like sunlight, oxidizing atmosphere and mechanical stress, polymer degradation occurs, yielding small polymer particles (Dümichen et al., 2017).

Microplastics (MPs), according to the National Oceanic and Atmospheric Administration (NOAA), are plastic particles smaller than 5 mm in size (Masura et al., 2015). MPs have either been intentionally manufactured to be small (primary MPs) or have fragmented from larger plastic items (secondary MPs) (UNEP, 2016). MP pollution in the marine environment is the cause of several hazardous and ecologically damaging effects. Owing to their small size, these particles are considered bioavailable to organisms throughout the food web (Galloway et al., 2017). In the accumulation zones, their concentrations are in the same range than zooplankton concentrations, thus increasing the risk of ingestion by zooplankton predators (Collignon et al., 2012). In addition, toxic chemicals, such as bisphenol A, polycyclic aromatic hydrocarbons, pesticides and polychlorinated biphenyls have been frequently found in oceanic plastic debris (Hirai et al., 2011). These substances, added to plastics during production or adsorbed from the environment, are an environmental concern since they extend the degradation times of plastic and may, in addition, leach out, introducing potentially hazardous chemicals to the biota (Cole et al., 2011).

The first reports on MPs in surface waters already date back to the early 1970s (Carpenter and Smith, 1972). At that time, these reports have got little public impact. According to the results of 24 expeditions (2007–2013) across all five sub-tropical gyres, coastal Australia, Bay of Bengal and the Mediterranean Sea, almost 270,000 tons of plastic would be currently floating in the oceans giving an estimation of a minimum of 5.25 trillion particles weighting 268,940 tons (Avio et al., 2017).

The Mediterranean Sea is a semi-enclosed basin subjected to anthropogenic disturbances, especially along the coasts, and has been recently proposed as one of the most impacted regions of the world with regards to MPs (Cózar et al., 2015; Faure et al., 2015; Pedrotti et al., 2016; Suaria et al., 2016). In recent years, many studies have been undertaken to assess and better understand MP pollution in this area. High amounts of plastic debris have been reported on the seafloor (Pham et al., 2014) and surface waters of Mediterranean Sea (Collignon et al., 2012; Suaria and Aliani, 2014; Ruiz-Orejόn et al., 2016, 2018; Suaria et al., 2016; Van der Hal et al., 2017; Schmidt et al., 2018).

Hydrodynamic conditions such as driving forces, winds and currents influence the transport of MPs as well as their abundance, distribution and weathering processes (Veerasingam et al., 2016). MP transportation modelling should therefore be dependent upon necessary assumptions, as well as the quality of hydrodynamic forcing data. Combined hydrodynamic-Lagrangian transportation models, with different degrees of realism, have been recently proposed to forecast the transport and distribution of floating debris in the Mediterranean Sea (Lebreton et al., 2012; Eriksen et al., 2014; Mansui et al., 2015; Ourmieres et al., 2018). Most of the currently available studies on the abundance of floating micro-sized particles, either based on sampling/observations or numerical simulations have, however, been limited to the northwestern part of the Mediterranean basin.

The Gulf of Gabes (GG), located in the south-east of Tunisia, is identified as one of the eleven consensus eco-regions within the Mediterranean Sea according to its physical, biogeochemical and biological characteristics (Ayata et al., 2018). This shallow and phytoplankton bloom area is strongly influenced by hydrodynamics, mainly driven by tides and anticyclonic winds (see review by Béjaoui et al., 2019). Despite the importance of the GG at the Mediterranean Basin level, to our knowledge, no data about MPs in surface waters are currently available in this region.

In this context, the objectives of the present study are 1) to investigate, for the first time, the distribution and the physico-chemical characteristics of MPs in surface waters of the GG, and 2) to assess the potential influence of hydrodynamics on the MP distribution through the use of a Lagrangian tracking model to simulate the dispersion of MP particles in water. In a lesser extent, the potential impact of biological activity is also examined. By providing the first data about MPs in surface waters of the GG, it is expected that this study will contribute to MP management in this area.