Observing the marine environment from space
Scientific instruments have extended our senses, and perhaps the biggest eye-opener of all has come with our ability to observe the sea surface from space. Marieke Eleveld and co-authors have used the Medium Resolution Imaging Spectrometer (MERIS) sensor for observing patterns in estuarine turbid waters.
Estuaries are found along many of the world’s diverse coastlines and provide many ecosystem services, ranking at the top in total monetary value per hectare per year. They host important ecosystem functions, such as biogeochemical cycling and movement of nutrients, maintenance of biodiversity and biological production, and mitigation of floods. Variation in transport of fine sediments, the main component of suspended particulate matter (SPM) is at the heart of many of these functions.
In their new publication, Eleveld et al. (2014) explain which information is captured in remotely sensed SPM signals. Spectral absorption and scattering have been used to derive SPM concentrations from remote sensing reflectance for the Westerschelde estuary. Averaged SPM from data collected at similar tidal phases reveal the impact of tidal variability (see Fig. 1), while other variability is suppressed (averaged out). Likewise, seasonal composites quantified SPM variability caused by bulk meteorological drivers such as wind waves, and river discharge. This method is portable to other estuaries and coasts.
Figure 1. Composites of average (geometric mean) SPM for semi-diurnal tides. The SPM concentrations range from blue (low, 3 mg/l), to dark red (high, 100 mg/l). These tidal composites show (a) high concentrations at low water, and (c) low concentrations at high water with a larger distance to the sediment bed. They highlight (b) advection of North Sea surface waters at flood and (d) estuarine surface waters at ebb.
Eleveld et al. (2014) also expose biases in the SPM observations due to tidal aliasing and weather (Fig. 2). Sun-synchronous satellites always sample some sort of coupling between tidal constituents, but implications for SPM are location-specific. In the case of the Westerschelde, high water co-occurs with spring tide, and low water co-occurs with neap during sun-synchronous satellite overpass.
A sampling bias for fair weather causes underestimation of SPM for wind exposed sites in regions where change in weather is determined by the passage of fronts. This bias proved to be dominant for the wind exposed mouth of the Westerschelde estuary. Different biases may occur for other regions.
These biases – which are inherent to optical remote sensing from platforms in a sun-synchronous, near-polar orbit – concern well-known ocean colour sensors such as SeaWiFS, MODIS and MERIS. New ocean colour sensors on geostationary platforms, or identical ocean colour sensors on two platforms in the same orbit, as anticipated for Sentinel-3A and B, could be complementary in resolving tidal SPM signal.
More details can be found in our article:
Figure 2. We expose the biases in the SPM observations due to tidal aliasing and weather for optical remotes sensing from platforms in a sun-synchronous, near-polar orbit.
- Eleveld, M.A., van der Wal, D., van Kessel, T. (2014). Estuarine suspended particulate matter concentrations from sun-synchronous satellite remote sensing: Tidal and meteorological effects and biases. Remote Sensing of Environment 143, 204–215. doi:10.1016/j.rse.2013.12.019