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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.

Rhodolith beds can contribute large amounts of CaCO3 to coastal environments, playing an important role in Carbon sequestration. The spatial distribution of rhodoliths is a consequence of their sensitivity to light intensity, currents, wave action, sedimentation rates, and salinity, whereas mesophotic, low sedimentation quiescent conditions are optimal. Although there is evidence of a clear relationship between hydrodynamics and rhodolith morphology and mobility, there are few studies relating wave action and rhodolith mobility. We aim to assess the mobility potential by waves, of the rhodoliths of the Abrolhos Shelf (Brazil), that contains the largest rhodolith field in the world. Based on the local wave climate and its propagation through the shelf, the rhodolith mobility potential was evaluated. Rhodolith mobility potential distribution is directly related to shelf morphology and incoming waves characteristics. Despite being restricted to deeper areas of the shelf (>20 m), rhodoliths in the area are mobilized by wave action, especially from the more energetic waves associated with storms. Waves can mobilize rhodoliths in the area, with mobility potential distribution being controlled by shelf morphology and seasonal wave climate. Largest areas of mobilization are observed during energetic waves conditions associated to the passage of cold fronts.

Coastal waters influenced by the Amazon River are highly dynamic and productive environments, although few studies have focused on the seasonal changes in oceanographic processes, except for the Amazon plume. The aim of this paper is to evaluate the effects of short- (daily) and medium-term (seasonal) variations in meteorological and hydrodynamic conditions on hydrological variables in a sub-tidal zone located to the southeast of the Amazon estuary, in an area influenced by other rivers. The study was undertaken between November 2008 and September 2009. The month with the lowest fluvial discharge (November, dry season) was characterized by strong tidal currents and high significant wave heights, the highest salinity and pH, and the lowest dissolved nutrient concentrations and turbidity. During the equinoctial spring tide (March, rainy season), when discharge was increasing, similar significant wave height and tidal current speeds were recorded, with high tide elevations, and the highest concentrations of chlorophyll a and nitrate. The maximum fluvial discharge (June, rainy season) was characterized by the lowest hydrodynamic energy, salinity and pH, but also the highest silicate concentrations. During the other equinoctial spring tide (September, dry season), when discharge was decreasing, tides and tidal currents were similar to those recorded in March, whereas phosphate and nitrite concentrations were the highest recorded in the study. Overall, coastal circulation and hydrological variables shifted seasonally, responding primarily to atmospheric, hydrographic, and astronomical factors, including winds, waves, fluvial discharge, tides, and tidal currents. The study also confirmed that coastal areas to the southeast of the Amazon plume are highly dynamic and productive.