as much as we read about how tropical reef waters are nutrient poor (in context of oceanography), it is in a relative sense. Oceanographers deem, for example, Caribbean reefs to be nutrient poor in dissolved carbon, nitrogen, and phosphorus based compounds, where the depths of oceans (deeper than continental shelf) act as a holding basin termed a 'carbon sink' because life and materials only sink down wards. That is why reef ecosystems were such wonders (no nutrient in water, very little algae in water or on rocks, what is supporting the bottom trophic levels?) until zooxanthellae was realized to be the base of reef ecosystems. Relatively speaking, nutrient rich waters are those off the west coast areas of 'upwelling' where oceanic currents and weather patterns bring water from the carbon sinks of the depths against the coast, and water flows up to the surface and then away from the coast. When the nutrients come up to the sun light, algae (phytoplankton) ensues and supports the coastal systems. So nutrient rich and nutrient poor are relative terms from different oceanic systems. When we're talking tropical reef waters... nearly all corals (whether lagoon, fore reef, reef crest etc) all pretty much come from nutrient poor waters (naturally speaking). I believe, however, there is merit to water turbidity (not just river run offs). Having logged over 100 dives in the tropics, it's still amazing the amount of stuff in the water column that stings me and makes me itch after a dive. There's certainly a lot more phytoplankton and zooplankton in the water column of nature than those of our tanks, even for nutrient poor waters (relatively speaking in oceanography). Some corals simply depend more/less on feeding than others in conjunction with zooxanthellae. I think in the context of aquarium talk... these terms all get muddled up and used interchangeably in conveying
1) filter feeding needs, and 2) tolerance for crap in the water / chemical warfare from other corals
Last thought... and if I wash my hands in your dirty water...
http://youtu.be/G5kLzAajFZQ