This really should not come as a surprise. Quoting two of the formost researchers on corals reefs:
"Our crystal clear aquaria do not come close to the nutrient loads that swirl around natural reefs. And so when we create low-nutrient water conditions, we still have to deal with the rest of a much more complex puzzle. Much like those who run their aquarium water temperature close to the thermal maximums of corals walk a narrow tight rope, I can't help but think that low-nutrient aquariums may be headed down a similar path." Charles Delbeck, Coral Nov/Dec 2010, pg 127
Veron wrote over 25 years ago: "Imported nutrients are usually transported to reefs from rivers; but if there are no rivers, as with reefs remote from land masses, nutrients can only come from surface ocean circulation. Often this supply is poor, and thus the vast ocean expanses have been refered to as "nutrient deserts". The Indo-Pacific has many huge atolls in these supposed deserts which testify to the resilience of reefs, but the corals themselves may lack the lush appearance of those of more fertile waters. Many reefs have another major supply of inorganic nutrients as, under certain conditions, surface currents moving against a reef face may cause deep ocean water to be drawn to the surface. This "upwelled" water is often rich in phosphorus [2.0 mg/l] and other essential chemicals." J. E. N. Veron "Corals of Austrailia and the Indo-Pacific" pg 30
Get Forest Rohwer's Coral Reefs in the Microbial Seas. It's a very readable introduction to more recent research on corals and lists an extensive bibliography if you choose to dig deeper into the research.
As also pointed out in the link Mcgyver posted corals utilize every source of phosphate and nitrogen possible. The coral holobiont is an assemblage of microbes associated with corals that are symbiotic with corals and help corals utilize and recycle nitrogen both the organic forms and inorganic forms. Phosphate, either organic or inorganic, is essential and is the limiting nutrient corals need to use nitrogen. An example of the impact corals have on the phosphate levels of the ocean it drops from as high as .7 mg/l away from reefs to an average of .13 mg/l on reefs with a few reefs dropping below .05 mg/l
