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Horizontal and vertical distributions of microplastics and their water–sediment partitioning in a macrotidal estuarine embayment: Implications for coastal sequestration potential
Original title: Horizontal and vertical distributions of microplastics and their water–sediment partitioning in a macrotidal estuarine embayment: Implications for coastal sequestration potential
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Scientists studying a tidally active estuary found that microplastics are everywhere in the water, but strong tidal mixing keeps stirring up the seafloor, so these coastal muddy areas don't trap and hold onto plastic pollution as well as calmer waters do. This matters because it means more of the microplastics we produce may keep getting pushed out to the open ocean instead of staying buried in coastal sediments—potentially spreading further into marine food chains that eventually reach our dinner plates.
Estuarine-coastal regimes are critical links between land and the open ocean, yet their role as microplastics (MPs) sinks and the mechanisms governing MP retention remain poorly constrained. We conducted a coupled water-sediment survey in a macrotidal estuarine embayment to resolve MP distributions associated hydrographic and biogeochemical controls. MPs were ubiquitous in surface waters (upper 20-cm; 16,276 N m on average), subsurface waters (five layers between surface and bottom; 7861 N m), and surface sediments (upper 2-cm; 703 N kg), with no clear distance-dependent gradient. Among measured variables, only particulate organic carbon (POC), dominated by detritus with particle densities comparable to MPs, positively correlated with MPs in water column, suggesting that POC transiently traces waterborne MPs. Weak hydrographic stratification and largely uniform subsurface MP distributions indicated strong tidal homogenization of the water column. In contrast, pronounced differences in MP abundance and composition between overlying waters and surface sediments-likely driven by tidally induced unstable burial and frequent resuspension-suggested that sedimentary inventories may not fully reflect contemporary MP inputs. Although sediments showed higher MP concentrations than overlying waters on a per-volume basis, the sediment-water contrast was 10-10 times smaller than that reported for more quiescent depositional environments. Sedimentary MP-C:TOC ratios were very low (0.03%) and remained one to two orders of magnitude lower than waterborne MP-C:POC ratios. These findings suggest that, contrary to expectations, sedimentary MP sequestration and buffering against offshore MP export may be limited in tide-dominated coastal systems. Collectively, this study provides a mechanistic framework for evaluating MP retention and MP incorporation into the marine carbon pool in estuarine-coastal environments.
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