Yes, these studies have been done.
The phosphorus atom is one of living matter's basic building blocks. It is present in every living creature and in every reef aquarium's water. Unfortunately, it is often present in excess in reef aquaria, and that excess has the potential to cause at least two substantial problems for...
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For those interested in knowing how much phosphorus is being exported by macroalgae, this
free PDF article in the journal
Marine Biology has some important information. It gives the phosphorus and nitrogen content for nine different species of macroalgae, including many that reefkeepers typically maintain. For example, Caulerpa racemosa collected off Hawaii contains about 0.08% phosphorus by dry weight and 5.6% nitrogen. Harvesting 10 grams (dry weight) of this macroalgae from an aquarium would be the equivalent of removing 24 mg of phosphate from the water column. That amount is the equivalent of reducing the phosphate concentration from 0.2 ppm to 0.1 ppm in a 67-gal. aquarium. All of the other species tested gave similar results (plus or minus a factor of two). Interestingly, using the same paper’s nitrogen data, this would also be equivalent to reducing the nitrate content by 2.5 grams, or 10 ppm in that same 67-gal. aquarium.