The existence of a reef system composed by crustose coralline algae and scleractinian corals, under the influence of the world's largest freshwater plume, led us to investigate how much light is available for reef-building organisms at the bottom of the Amazon shelf. We analyzed ocean color data from MODIS Aqua to assess the plume dynamics and estimate the benthic light availability over the Amazon shelf. These methods may be applied to other river-dominated shelves. Four zones were detected regarding the bottom light regime. The seasonal changes of the dynamic forcing over the shelf reflect little in the light parameters over the Amazon shelf. Three zones presented constant light regimes along the year: dark coastal zone under the permanent influence of the plume; dim-light zone in the deeper northern shelf; and high-light zone in the shallower southern shelf. Only one zone presented a seasonally changing benthic light regime, and it was located under the seasonally variable edge of the plume over the northern mid-to-outer shelf. The minimum light level estimated in the deeper dim-light zone was ∼2 μE m−2 s−1 which is sufficient for reef-building organisms to thrive. The cause of the incipient living coverage and low vitality of crustose coralline algae, previously reported in the northern shelf, cannot be attributed to a lack of light, but might be related to sedimentation processes associated to the plume. Thus, the potential, in terms of light, for reef builders to develop along this and other river-influenced margins is much greater than previously anticipated.

Depth range
20- 100 m

Mesophotic “mentions”
3 x (total of 6469 words)

Classification
* Presents original data
* Focused on 'mesophotic' depth range
* Focused on 'mesophotic coral ecosystem'

Fields
Oceanography

Locations
Brazil - Eastern Brazil

Platforms
Land-based