The changes that the Amazon River promotes in the tropical North Atlantic ocean water make an unfavorable environment for reef development. Every second, 175 million liters of water mixed with sediments are brought to the ocean. The result is low sunlight penetration, variability in nutrient concentration, salinity and pH, extensive moody bottoms and significant changes in temperature and oxygenation levels towards the bottom - conditions not associated with reefs. The plume generated by the Amazon River has 1.3 million km2 and flows mostly to the North, reaching areas as far as the Caribbean Sea.
Against all the odds, 39 scientists from nine Brazilian and one North-American universities mobilized two expeditions to the mouth of the river in other to map the bottom of the ocean in the outer shelf. Two previous studies - from the 1970s and 1980s - reported the findings of single samples of carbonatic structures. None suggested the existence of a reef system underneath the river plume. Surprisingly, the researchers found reefs in a complex as extensive as 1,000 km, in depths from 10 to 120 meters, with rhodolith beds, live calcareous algae, sponges, corals and hydrocorals colonies formed from 13,000 years ago till the present. The system is an habitat for 73 species of fishes and six species of lobsters.