Sediment resuspension enhances nutrient exchange in intertidal mudflats

Abstract

Intertidal coastal sediments are important centers for nutrient transformation, regeneration, and storage. Sediment resuspension, due to wave action or tidal currents, can induce nutrient release to the water column and fuel primary production. Storms and extreme weather events are expected to increase due to climate change in coastal areas, but little is known about their effect on nutrient release from coastal sediments. We have conducted in-situ sediment resuspension experiments, in which erosion was simulated by a stepwise increase in current velocities, while measuring nutrient uptake or release in field flumes positioned on intertidal areas of a tidal bay (Eastern Scheldt) and an estuary (Western Scheldt). In both systems, the water column concentration of ammonium (NH4+) and nitrite (NO2−) increased predictably with greater erosion as estimated from pore water dilution and erosion depth. In contrast, the phosphate (PO43−) dynamics were different between systems, and those of nitrate (NO3−) were small and variable. Notably, sediment resuspension caused a decrease in the overlying water PO43− concentration in the tidal bay, while an increase was observed in the estuarine sediments. Our observations showed that the concentration of PO43− in the water column was more intensely affected by resuspension than that of NH4+ and NO2−. The present study highlights the differential effect of sediment resuspension on nutrient exchange in two contrasting tidal coastal environments.

Biogeochemical dynamics in a marine storm demonstrates differences between natural and anthropogenic impacts

“…Marine primary production has been estimated to vary between 40 and 60 Pg of carbon per year worldwide 1 and depends greatly on nutrient input and bioavailability 2 . The regulation of nutrient availability is influenced in large part by sediment resuspension, which plays a critical role in modulating nutrient levels through release and adsorption processes 3 – 6 . While sediment resuspension occurs naturally through processes such as tidal currents and episodic storm events, anthropogenic drivers of sediment resuspension, such as dredging and bottom trawling, are also known for their capacity to directly alter carbon and nutrient dynamics 7 – 9 .…”

“…However, higher observed levels of degraded OM associated with trawl-induced resuspension suggests that trawling may expose OM and nutrients within comparatively deeper layers of sediment 17 . Disturbance from trawling and storms can increase nutrient concentrations in water column can through enhanced mineralization and desorption processes 5 , 6 , 18 . The specific consequences of elevated nutrients is context dependent as it can fulfill an important function for marine primary production but it also increases the risk of hypoxic conditions.…”