Idealized modelling of microphytobenthos resuspension by wind and tides in estuaries

This study further explores the relationships between suspended sediment concentration (SSC) and phytoplankton concentration (PC) in tidally-energetic estuaries. A 1DV idealized model is implemented to account for vertical advection of phytoplankton and sediments, their resuspension from the bottom by both tidal and wind shear stresses, vertical mixing processes, and light-induced growth of phytoplankton. Phytoplankton are considered to be comprised by both free (pelagic) phytoplankton and microphytobenthos (MPB). The combined wind-tide shear stress is modelled following the work of Colosimo et al. (2020), which assumes that the time-varying pressure gradients are balanced by the bed tidal friction only. The (scaled) wind-tide shear stress results thus in a linear superposition of both.

The model is applied to the Guadalquivir River Estuary (SW Iberian Peninsula), which is regarded as a highly turbid and reduced-growth environment. Its lower stretches are partially covered by biofilms of MPB. The contribution of these biofilms to PC and the net primary production of the estuary is thought to be larger than that of the free phytoplankton.

Model output from a sensitivity analysis of SSC and PC to wind speed and tidal currents indicate that the contribution of usual winds to MPB resuspension in the deeper parts of the cross-section is below 10% of that of the tides, whose currents may exceed 1m/s. On the contrary, the role of wind is dominant in the tidal flats where it shallows and tidal currents are weaker than 0.1m/s. These results suggest than a significant fraction of the observed biomass in the Guadalquivir estuary, particularly that comprised of MPB, could thus be resuspended by wind action in the shoals where they grew.