Temporal patterns in the abundance, type and composition of microplastics on the coast of the Río de la Plata estuary

Highlights

• Microplastics (MPs) were more abundant in February in water and in April in sediment.

• The polymers identified were polyethylene (PE) and polypropylene (PP).

• The wind from the NE direction was related to a greater accumulation of MPs in the sediment.

• MPs abundance was negatively related to the larger sizes of sediment particles.

• The wind from the NW direction was associated with a lower abundance of MPs in the water column.

Abstract

The objective was to establish possible temporal patterns in the microplastics (MPs) abundance in the water and in the intertidal sediment in an urbanized location of the Río de la Plata estuary (Argentina), in relation to environmental factors. The site was sampled monthly for a year (February 2019–January 2020). The presence of MPs was recorded, being more abundant in February in the water (110 MPs m⁻³) and in April in the sediment (613 MPs m⁻²). The types of MPs found were fibers, fragments, pellets, film and foam; while the polymers identified were polyethylene (PE) and polypropylene (PP). Regarding the environmental variables, the predominance of wind from the NE direction was related to a greater accumulation of MPs in the sediment, while wind from the NO direction was associated with a lower abundance of MPs in the water. Also, MPs abundance was negatively related to the larger sizes of sediment particles.

Introduction

The mass production and broad use of plastic products generates an input of waste into the environment that can be very slowly degraded or not degraded at all (Andrady, 2015; Barnes et al., 2009). The fraction of plastic particles <5 mm are called microplastics (MPs) (Arthur et al., 2009), and are considered emergent pollutants since their detection in the environment is relatively recent (compared to other pollutants), and knowledge of their effects are largely unknown. However at the present time, studies into the sources, chemical interactions, presence in biota, environmental fate, potential effects and impacts of MPs are increasingly abundant (Crew et al., 2020; Hirt and Body-Malapel, 2020; Pequeno et al., 2021; M. Wu et al., 2020).

In the oceans, the presence of MPs has been well documented, yet knowledge of their dynamic in estuaries is still scarce (Browne et al., 2010; Costa et al., 2011). These contaminants in estuaries and coastal environments have a great variability (Frere et al., 2017; Moore et al., 2011), and their study needs to take into consideration their spatial distribution, but especially the temporal variability (Stanton et al., 2020), as it is usually considered for other more studied pollutants like nutrients (Norén and Naustvoll, 2010).

The proximity of water bodies to urbanized environments is a factor that determines the reach of MPs pollution in coastal areas (Galgani et al., 2015; Yonkos et al., 2014). Since superficial sediments in these areas are highly dynamic, it is likely that the abundance of MPs varies temporally as they get resuspended into the water column, modulated by environmental factors. Physical processes, such as tides and winds, can help transport, resuspend and sediment MPs altering their spatial and temporal distribution, both in the sediment and in the water column (F. Wu et al., 2020). The abundance of MPs in coastal areas can be higher in the dry seasons than in rainy seasons, indicating that precipitations can have a dilution effect on MPs concentrations (Lee et al., 2013; Zhu et al., 2018); however, according to some studies, storm events can increase the abundance of MPs on the coast (Gündoğdu et al., 2018; Moore et al., 2011; Veerasingam et al., 2016).

Hydrodynamic variables, wind intensity and direction distribute the sediments in estuaries by suspending, transporting and depositing particles according to their size and density; smaller particles are deposited where water flow is slower (Dalrymple et al., 1992). Hence the proportion of finer particles in the sediment can be an indicator of depositional environments where MPs are expected to be more abundant (Browne et al., 2010). The transport of MPs in the coastal sector may also vary according to their density. This is a characteristic that is complex to analyze since the buoyancy of the MPs depends on the polymer, degradation processes and can also vary with the formation of the biofilm on them (Andrady, 2017; Zhang et al., 2017).

Generating a large scale monitoring database of MPs is important to build basic information for management protocols (Blumenröder et al., 2017; Peng et al., 2017). To standardize MPs monitoring methods, it is necessary to know their abundance and temporal distribution at local and regional spatial scales, and to understand how the local variations in hydrological processes affect the distribution of MPs in smaller spatiotemporal scales (F. Wu et al., 2020).

Due to the fact that the coastal areas of many estuaries are highly populated by large cities, as well as industrial and port activities and wastewater discharge, the vulnerability of these ecosystems to MPs is very high, transforming coastal environments into “hot spots” for these pollutants (Cole et al., 2011). The Río de la Plata estuary is the second largest watershed in South America (Mianzan et al., 2001), constituting an important water resource providing with different ecosystem services to the region, such as drinking water, fishing and recreational activities, but also receives the input of agricultural runoff and industrial and urban point sources of pollution (Gómez et al., 2012; Gómez and Cochero, 2013). In the coast of the estuary, MPs have been found suspended in the water column, in the digestive tract of fish and in the golden mussel, Limnoperna fortunei (Pazos et al., 2017, Pazos et al., 2018, Pazos et al., 2020). Also, in the country there are records about MPs in Bahía Blanca estuary. In this ecosystem MPs has been found in water column, fish, oysters (Crassostrea gigas) and crabs (Neohelice granulate) (Arias et al., 2019; Fernández Severini et al., 2019; Villagran et al., 2020).

Despite records of the presence of MPs in the estuary of the Río de la Plata, it is still unclear how the MPs abundance relates to the natural environmental factors of the estuary. The aim of the study was to explore how environmental variables such as rainfall, winds (direction and intensity), tidal height and granulometry influence the temporal distribution of MPs (over a year) in the water column and intertidal sediment at a coastal site with urban land use. We postulate in this research that the variation in winds, rainfall, tide amplitude and sediment grain size modulate the distribution patterns of MPs in the water column and in the intertidal sediment.