So here's another water curiosity....

[AllSignsPointToFish]

My aquacultured gulf live rock has been in the tank for about a week and a half. The ammonia and nitrite have decreased to undetectable amounts, as expected, and the nitrates have increased to around 40 ppm and are holding steady. However, the phosphates are hovering somewhere between undetectable and 0.25 ppm according the the API colorimetric test.

I would have thought that the phosphates would have leveled out at higher concentrations after the die off in the live rock, so I'm not sure what to think.

How can the phosphates be so low after recuring live rock? I just added the Chaetomorpha to the sump last Friday, but I am seeing some macroalgae on the live rock making a comeback. I do have a few coral colonies alive in the tank as well as a dozen or so bivalves of various sorts and a few sponge-like organisms.

I suppose it could be the test, but all reagents are unexpired and the test results have been very consistent.

 

[Randy Holmes-Farley Verified Community Expert]

The ratio of nitrate to phosphate that would come from degradation of a typical marine organism (say, macroalgae) might be on the order of 0.1 ppm phosphate for each 10 ppm of nitrate.

So I don't think your ratio is all that far off of expectations, especially with the test kit uncertainty and the uncertainty of what organics are actually degrading.

Also, a fair amount of phosphate may bind to the rock and sand, and so go undetected.

I discuss that ratio in many articles, but give the specific numbers here:

http://www.reefedition.com/phosphate-in-the-reef-aquarium-by-randy-holmes-farley/

from it:

Phosphate Export by Organisms: Macroalgae

As mentioned above, growing and harvesting macroalgae can be a very effective way to reduce phosphate levels (along with other nutrients) in reef aquaria. In my reef system, where I have large, lit refugia to grow the macroalgae Caulerpa racemosaand Chaetomorpha sp., these algae are clearly a significant phosphate export mechanism. Aquaria with large amounts of thriving macroalgae can avoid microalgae problems or excessive phosphate levels that might inhibit coral calcification. Whether the reduction in phosphate is the cause of the microalgae reduction is not always obvious; other nutrients can also become limiting. But to reef aquarists with a severe microalgae problem, the exact mechanism may make no difference. If rapidly growing macroalgae absorb enough phosphorus to keep the orthophosphate concentrations in the water column acceptably low, and at the same time keep microalgae under control, most reefkeepers will be satisfied.

For those interested in knowing how much phosphorus is being exported by macroalgae, this free PDF article in the journalMarine 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.

 

[AllSignsPointToFish]

Fantastic! I would definitely agree that my test results agree with the ratio you posted when the margin of error is taken into account. I guess I kept expecting the phosphates to creep up at some point, but if I can get my macroalgae established pretty soon, I may be able to control the phosphates. That is awesome news.

On a related topic, is there a guide somewhere for common types of macroalgae that one might encounter on aquacultured gulf live rock?

 

[Randy Holmes-Farley Verified Community Expert]

On a related topic, is there a guide somewhere for common types of macroalgae that one might encounter on aquacultured gulf live rock?

Probably....but I've not seen it.

If you look at the types sold, that might be a clue: http://live-plants.com/

 

[AllSignsPointToFish]

Thanks for the cool link, Randy.

I have some sort of macroalgae that kind of looks like Christmas tree worms, but it's green with conical leaf sections. I've been trying to identify it, but I've had no luck so far.

 

[Randy Holmes-Farley Verified Community Expert]

Thanks for the cool link, Randy.

I have some sort of macroalgae that kind of looks like Christmas tree worms, but it's green with conical leaf sections. I've been trying to identify it, but I've had no luck so far.

Try posting a picture in the Marine Plants section here for ID.