Pascale Tremblay, Jean François Maguer, Renaud Grover, Christine Ferrier-Pagès
The Journal of Experimental Biology 2015 218: 1223-1234; doi: 10.1242/jeb.115303
Many interesting points and observations about nutrient usage in this one, but going with this quote:
…heterotrophy is an important process for the acquisition of nutrients, especially nitrogen, and the build up of the coral biomass, and may explain the particular resistance and/or resilience of some coral species to stress.
The further away they are from a literal, heterotrophic food source, the more environmental stress they are likely to be affected by since dissolved nutrients can be so unavailable.