The Fate and Transport of Microplastics within Estuaries

Abstract

Microplastics are an emerging and environmentally problematic suite of contaminants. Their fate, transport and impacts are poorly understood. The so-called “missing plastic problem”, in which global surface ocean plastic aligns with only a single year’s estimate of inputs, may be partially solved by considering if microplastics are retained within estuaries, by way of an “estuarine filter”, similar to that which exists for suspended sediment and other contaminants. To investigate the fate of microplastics in estuaries, a series of field and laboratory studies were conducted, using Southampton Water, U.K., as a study site. These investigated environmental controls on microplastic abundance in estuarine waters; the intertidal trapping of microplastics in a salt marsh system; and the settling and resuspension of microplastics. The sea surface microlayer (SML) was sampled, to better understand this key interface between atmosphere and oceans, and the role it plays in the estuarine filter. Environmental controls were considered during a four-month water sampling campaign, which determined that within Southampton Water, the partially-mixed nature of the estuary meant that there were limited controls exerted by environmental variables such as weather conditions and river flow. The strong tidal currents in Southampton Water meant that abundances were highly variable with time and space. Intertidal trapping was investigated using high-resolution salt marsh creek sampling, which found a significant decrease in microplastic abundance when comparing abundances in flood tide samples to ebb tide samples, during both neaps and springs. Laboratory settling column and flume studies were utilised to investigate settling and resuspension. Limited settling was seen after an extended period, but flocculation of microfibres was not observed. During resuspension experiments, microplastic fibres were suspended at velocities similar to those that moved fine unconsolidated cohesive sediments. Nurdles were suspended at velocities that eroded sand of a smaller grain size than the nurdles themselves, but were also were observed to be buried by moving sediment.
The findings within this thesis support the hypothesis of an estuarine filter for microplastics, indicating that salt marshes and other low-energy intertidal areas are significant sinks. Both burial and resuspension might occur depending on the tidal cycle, so that whether an estuary acts as a filter depends on the balance of these processes, driven by tidal asymmetry. However, results also suggest that the hydrodynamic regime of estuaries is a key control of microplastic abundance and distribution. Southampton Water’s partially-mixed, ebb-dominant system likely transports microplastics out of the estuary. There are significant global implications from this research, including to microplastic budgets and estimates of global ocean inputs, and to the potential risks posed to estuaries and intertidal wetlands.

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Identifiers

ORCID for Andy Cundy: orcid.org/0000-0003-4368-2569

ORCID for Charlotte Thompson: orcid.org/0000-0003-1105-6838

ORCID for Ian Williams: orcid.org/0000-0002-0121-1219

ORCID for Andrea Russell: orcid.org/0000-0002-8382-6443

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