Role of saltmarsh systems in estuarine trapping of microplastics

Abstract

Saltmarshes are important natural ecosystems along many temperate (and other) coastlines. They stabilize sediments and act as biofilters for a range of industrial pollutants and, potentially, microplastics. Accumulation of microplastics along estuarine coastlines may be enhanced by the presence of saltmarsh species, as they offer better particle trapping efficiency than adjacent intertidal mudflats under prevailing flood and ebb tidal currents. However, the trapping efficiency of entire saltmarsh systems under varying flow conditions has not been widely assessed. While the effects of saltmarsh systems on water flow, and on sediment transport and trapping, have been relatively well studied, little is known about the contributions of saltmarsh halophytes, resident organisms and the associated saltmarsh sediments to the trapping of microplastics. To address this, a series of flume experiments were undertaken to examine transport and accumulation of Bakelite particles (~ 500 µm) and PVC nurdles (~ 5 mm) as model plastics in sub-sampled saltmarsh and intertidal mudflat monoliths. The results showed that saltmarsh systems influenced the hydrodynamics within and above the canopy, enhancing turbulence and shear stresses. With increasing flow velocities (≤ 0.51 m s −1 ), negligible quantities (2 × 10 −4 mg L −1 ) of sediments and Bakelite particles were eroded and resuspended. The algal biogenic roughness from the mudflat, and the vegetative roughness from the Spartina plants on the saltmarsh, inhibited the transportation of the microplastics within the tested systems. Resident burrowing crabs (Carcinus maenas) promoted the burial, release and transport of microplastics. The results of this study provide evidence of the contributory roles of saltmarsh systems in the sequestration of microplastics and sediment stabilization. Estuarine saltmarsh systems can act as sinks for microplastics with enhanced burial from burrowing crabs under favourable flow conditions. Vegetated intertidal habitats provide diverse support to natural, coastal and estuarine systems and valuable ecosystem services including protection against coastal hazards, regulation of contaminants and pathogens, and provision of habitats for macrobenthos such as bivalves, fishes, gastropods, crustaceans, and other invertebrates [1][2][3][4] . The positive contribution of coastal vegetated habitats to shoreline protection 5 is partly attributed to the combined inputs of the flora and fauna which occupy these habitats. The flora provide an enhanced biogeneic roughness which reduces the hydrodynamic shear stress 6-8 thus decreasing the erosion rate and prompting accumulation/ deposition of sediments. Meanwhile, the fauna offer biostabilization of the sediments through the secretion of biopolymers 9 , generation of armoured surfaces and other mechanisms that minimize the risk of erosion 10,11 . Mangroves, saltmarshes, seagrasses and reed beds occupy most estuarine coastlines that provide these biomorphodynamic and bioengineering functions 3,12,13 . Benthic organisms...

Assessment of the estuarine shoreline microplastics and mesoplastics of the River Itchen, Southampton (UK) for contaminants and for their interaction with invertebrate fauna

“…Otherwise, variability in mp/MeP concentration may be consequent to the dynamic nature of estuaries, with the influence of tidal fluctuations, water circulation patterns, infauna burrowing activity, shoreline topography, pollution events and frequencies that may regulate the resuspension, dispersion and settling of plastics (Krelling et al 2017 ; Ogbuagu et al 2022 ). Ogbuagu et al ( 2022 ) further observed key points for consideration in this discourse: in the saltmarsh-mudflat system of Hythe in Southampton Water, the high retention efficiency of nurdle microplastics (denoted with the pellet morphology in this study) was attributed to the saltmarsh vegetation modulating flow velocity and enhancing sediment accretion, which would in turn limit resuspension of microplastics. Sedimentation rates of organic and anthropogenic material originating from upstream sources, in conjunction with the flood-ebb cycles of the monthly dual spring-neap tides of the study area, may influence the movements of the sediments and result in microplastic accretion over time (NFF 2014 ; Díaz-Jaramillo et al 2021 ).…”

“…Surface horizons, though shallower than the sub-surface, retained much higher proportions of loose microplastics and mesoplastics than below, and this may suggest the occurrence of recent pollution events. Microplastics and mesoplastics retained in the sub-surface horizons up to 8 cm had retention influences from several probable trapping variables: shoreline vegetation comprising mostly S. maritimus , P. australis , J. gerardii , A. pungens and A. hastata influence the sediment accretion (and ultimately, the accumulation of microplastics within) along the Chessel Bay intertidal zone; burrowing and foraging activity by fauna such as the crab Carcinus maenas (Ogbuagu et al 2022 ) and birds like the oystercatcher ( Haematopus ostralegus ) and the turnstone ( Arenaria interpres ) may further distribute the microplastics deeper into the sediment profile. Lourenço et al ( 2017 ) acknowledged the redeposition of microplastics by shorebird faeces to tidal flats of estuarine regions in Portugal, Mauritania and Guinea-Bissau.…”