Variation in microplastics composition at small spatial and temporal scales in a tidal flat of the Yangtze Estuary, China

Highlights

• More microplastics were found during neap than spring tide period.

• Microplastic particles were larger during the neap than the spring tide period.

• No variation in the abundance of microplastics on the semidiurnal scale.

• Microplastics were more abundant in the vegetation zone than in the mudflat.

• Variation in microplastics abundance was driven by local hydrology.

Abstract

Microplastics are small, degrade slowly, and easily persist in the water column because they are close to neutrally buoyant. Understanding the distribution of microplastics is fundamental to evaluating the ecological risks that they cause and to identifying ways to control microplastics pollution. Most of the existing research on the distribution of microplastics in the coastal zone has focused on large spatial and temporal scales. To build on past work, we investigated variation in microplastics in a tidal flat of the Yangtze Estuary on small spatial (sediment depth, mudflat vs. vegetation zone) and temporal (fortnightly and semidiurnal) scales. Microplastics were more abundant in surface (0–2 cm) sediments during neap versus spring tide cycles, likely indicating increased deposition during periods with calm waters and increased suspension when water was more turbulent, but did not vary at greater depths in the sediment. Individual microplastics particles were also larger during neap versus spring tide periods. In contrast to the variation between spring and neap tide periods, we found no variation in the abundance of microplastics on the semidiurnal scale. Microplastics were also more abundant in the transect in the vegetation than at slightly lower elevations in the adjacent mudflat. Across all samples, the abundance of microplastics was negatively correlated with the strength of hydrological processes such as submergence time and flow velocity. Our results showed that sampling of microplastics in the intertidal environment needs to consider variation among spring and neap tide cycles, and also among different intertidal habitats that may differ only slightly in elevation. We encourage coupling sampling with direct measures of hydrological processes so that variation in microplastics abundance and size can be rigorously linked to hydrological processes.

Introduction

The production of plastics has increased rapidly since mass production began in the 1940s, with wide applications of plastics in commercial, industrial and other industries (Cole et al., 2011; Li et al., 2016). As demand for plastics and its production increased, more and more plastic waste entered the environment. Of particular interest and concern are small plastic fragments <5 mm in size, called microplastics. Microplastics are small, chemically stable, and difficult to degrade naturally (Andrady, 2011). A fundamental characteristic of microplastics is that they are close to neutrally buoyant, and so are easily transported throughout aquatic and marine environments (Hidalgo-Ruz et al., 2012). A variety of processes, such the leaching of additives, biofouling and incorporation within marine aggregates, can alter the buoyancy of microplastics once they are in the water, encouraging eventual deposition into benthic habitats (Wang et al., 2016). Numerous studies have examined microplastics pollution in the ocean (Lusher et al., 2014), rivers (Mani et al., 2015), lakes (Hu et al., 2018; Xiong et al., 2018), and even Arctic sea ice (Obbard et al., 2014). Microplastics have also been widely detected in many organisms, including fish, benthic fauna, and birds, where they not only harm the growth of these organisms, but may also enter the human body through the food chain (Li et al., 2015; Rochman et al., 2015; Zhao et al., 2016; Lourenço et al., 2017; Su et al., 2018). Another concern is that microplastics can be a carrier of some toxic chemicals such as POPs and heavy metals that absorb onto the plastic particles (Bakir et al., 2014; Holmes et al., 2014).

Estuarine and coastal ecosystems may be particularly vulnerable to microplastics pollution. These habitats are located in the transition zone between land and sea, which is often the area with highest human population density. Plastic waste generated by human activities is transported to the coastal zone from terrestrial habitats by rivers, and from the ocean by currents and tides (Lima et al., 2015; Peng et al., 2017). At the same time, estuarine and coastal ecosystems provide habitat for many plants, birds and benthic organisms (Day Jr. et al., 1989). Therefore, a number of studies have examined the abundance of microplastics in coastal zone waters, sediments and organisms around the world (Claessens et al., 2011; Zhao et al., 2015; Cheung and Cheung, 2016; Jabeen et al., 2016).

Studying the distribution of microplastics provides the fundamental information needed for controlling microplastics pollution and assessing its ecological risks. The degree of microplastics pollution in coastal zones is closely related to the intensity of regional human activities (Claessens et al., 2011; Nor and Obbard, 2014; Blumenröder et al., 2017). In addition, physical processes in the coastal zone may transport, suspend or bury microplastics, and therefore affect the spatial and temporal distribution of microplastics pollution. These physical processes mainly include wind and hydrological processes (Vianello et al., 2013; Kim et al., 2015). Hydrological processes are the most important physical processes in the coastal area, playing a key role in the retention and transportation of inorganic substances (Wang et al., 2016). Meanwhile, because surface sediments in shallow marine environments are highly dynamic, being reworked by biota and hydrological processes, the abundance of microplastics in the surface sediments is likely to vary not only over space but also over time as materials are deposited onto the sediments, turned over in surface sediment layers, and re-suspended into the water column. In terms of the spatial distribution of microplastics, many studies have shown that the abundance of microplastics in coastal sediments is closely related to regional runoff, waves, tides and currents (Lee et al., 2013; Kim et al., 2015; Yu et al., 2018). In particular, sites with stronger hydrodynamic processes tend to have lower abundances of microplastics (Claessens et al., 2011; Vianello et al., 2013). In addition, some studies suggested that salinity is also an important factor affecting the spatial distribution of microplastics in coastal zones (Lima et al., 2015). Finally, microplastics abundance in the coastal zone is higher in the dry season than the rainy season, indicating that precipitation plays an important role in transporting microplastics in or out of coastal habitats (Lee et al., 2013; Zhu et al., 2018).

Most of the existing research on the distribution of microplastics in coastal habitats has focused on large spatial and temporal scales. A number of studies have called for the establishment of a large-scale monitoring database of microplastics in order to provide a scientific basis for managing them (Blumenröder et al., 2017; Peng et al., 2017). In order to accomplish this, we also need to know how the abundance of microplastics varies on small spatial and temporal scales, so that large-scale sampling programs can standardize their sampling in a manner that minimizes variation due to small-scale processes.

More generally, the importance of spatial and temporal scale cannot be ignored in environmental research. When studies are based on different scales, different processes and patterns can be detected and different results are obtained (Schneider, 2001). Therefore, the distribution of microplastics on small spatial and temporal scales is likely to be driven by factors other than those that determine distribution patterns at large spatial scales, such as variation in human activity. In particular, it is likely that local variation in hydrological processes is increasingly important in determining the distribution of microplastics on small spatial-temporal scales.

We worked in a tidal flat in the Yangtze Estuary to test three hypotheses. First, the abundance of microplastics would vary as a function of intertidal elevation, spring versus neap tidal cycles, and among days within a tidal cycle. Second, the variation among samples could be explained by variation in local hydrological processes. Third, by focusing on small spatial and temporal scales, our study would identify different drivers of microplastics abundance than have studies that focus on large spatial and temporal scales. To test these hypotheses, we quantified hydrological processes and the abundance, size and composition of microplastics in two intertidal zones on a fortnightly and semidiurnal temporal scale, and correlated microplastics abundance with different hydrological processes.