Linking ocean colour features in the Western Iberian margin to wave-induced sediment resuspension and coccolithophore patches

Two ocean colour features in the western Iberian margin (western Europe) were described in present work. They were identified in 10 years (2002-2012) of MODIS-AQUA normalized water-leaving radiance (nLw) at 555 nm and their causes were investigated using complementary satellite, in-situ and model data. One feature was a band of high nLw555 (>0.8 mW cm-2 mu m- 1 sr- 1) with exceptionally large widths (>20 km) along the coast. Over the 10-year period, this wide coastal band of high nLw555 only occurred during winter, following better the increases in wave height, than the increases in precipitation and river discharge. The band seemed particularly associated with episodes of large wave heights (>6 m), under weak thermal stratification and northerly winds. Findings suggest that the band is the signature of exceptional offshore extensions of the more reflective turbid coastal waters, as a result of wave-induced sediment resuspension along the coast. The other feature consisted in areas of equally high nLw555, but separated from the coast by more than 40 km. These offshore areas of high nLw555 shared spectral signatures of phytoplankton, namely coccolithophores, typically formed during spring following water column re-stratification and reached larger areas after winters with deeper oceanic mixed layers. Less commonly, they also occurred from summer to autumn. Results indicate they are mainly the signature of enhanced coccolithophore abundances that also create turbid reflective waters due to their calcite plates. The used methodology separated the two features in time and therefore seems to allow for satellite monitoring of sediment resuspension and coccolithophore blooms off west Iberia and similar coastal environments, which is particularly relevant for detecting events of sediment transport, potentially linked to water quality problems, and long-term changes in phytoplankton composition.

Automated summary

In the western Iberian margin, two ocean color features were identified: a coastal band of high nLw555 due to wave-induced sediment resuspension, and offshore areas of high nLw555 attributed to coccolithophore blooms. These features were detected and separated in time, allowing for satellite monitoring of sediment resuspension and phytoplankton blooms in similar coastal environments.