Estuarine suspended particulate matter concentrations from sun-synchronous satellite remote sensing: Tidal and meteorological effects and biases

Optical data from a sun-synchronous satellite were used to investigate how large-scale estuarine suspended particulate matter (SPM) concentrations were affected by tidal and bulk meteorological drivers, and how retrieved SPM is biased by tidal aliasing and sampling under clear sky conditions. Local absorption and scattering properties were used to derive surface SPM maps from 84 cloud-free ENVISAT MERIS FR reflectance images of the Westerschelde estuary (51° 30′ N, 3° 30′ E) for the period 2006-2008, and validated with in situ SPM at fixed stations (r = 0.89 for geometric means). The distinctly different SPM maps were categorized for different tidal and seasonal conditions. Resulting composites reveal spatial patterns in SPM as a function of semi-diurnal tidal phase, fortnightly tidal phase, or season. For the estuary proper, tidal and seasonal effects on the variation of SPM are similar in magnitude. Observed controls for surface SPM are distance to shallow source area, tidal current velocity, and advection of North Sea and estuarine surface waters. Turbidity maxima appear only during favourable tidal and meteorological conditions. For the Westerschelde, the bias introduced by sun-synchronous sampling causes low water image acquisitions to uniquely coincide with spring tides, and high water images with neap tides. Cloud-free images were associated with low wind velocities. Simulations from a mud transport model confirmed the overestimation of geometric mean SPM from the tidal aliasing, and underestimation from fair weather. This resulted in a net relative error of − 8% at the wave-exposed mouth, but biases cancelled out in the upper estuary. We argue that local biases should be considered when interpreting water quality products for estuaries and coasts around the world.