I’ll discuss my “hot takes” on the reefing industry in this new series. In this first installment, I will cover N-DOC testing and the importance of measuring organic nutrients for our tanks.
What is an N-DOC test?
N-DOC is a relatively new test offered by Triton. This test aims to measure the organic nitrogen and carbon in your aquarium. N = nitrogen, DOC = dissolved organic carbon. This method accompanies an ICP test to compile inorganic nitrogen and phosphorus data with the organic data provided by N-DOC into one comprehensive view of nutrients in your aquarium. Triton has suggested values of where to keep your organic carbon and nitrogen concentrations based on data compiled from their users (more on this later).
Why Organic Molecules?
Without getting too deep down the chemistry rabbit hole – every test kit on the market that measures nutrient concentrations measures their inorganic values. This only gives us a small picture of the overall nutrient levels in our aquariums. Even if inorganic nutrients are low, organic concentrations could be elevated, leading to algae issues or cyanobacteria blooms due to imbalances between nitrogen and phosphorus. Thus, N-DOC allows aquarists to better view their nutrient levels by enabling them to see organic values.
Example of organic vs inorganic species.
The Hot Take
So far, this doesn’t sound too controversial – there’s a test that gives users more data – that sounds great – what’s the big deal? The question is how to utilize that data for your aquarium. Here’s where my own opinions come into play.
Forget about the Redfield ratio – its application to aquariums is one of the most significant pieces of misinformation in the hobby and was created from a game of telephone gone horribly wrong. To defend this argument, I will go into the history of the Redfield ratio and how the data was gathered.
The Redfield ratio comes from a paper published by Oceanographer Dr. Albert Redfield in 1934 titled “On The Portions of Organic Derivatives in Sea Water and Their Relation to the Composition of Plankton.” In this paper, Dr. Redfield showcased that oceanic samples across the world had very similar ratios of nitrate to phosphate. He then compares these ambient oceanic concentrations to those found among global plankton populations. In his manuscript published in 1958, he expanded this correlation to include all marine nitrogen, phosphorus, oxygen, and carbon sources.
Dr. Redfield found that planktonic biomass had a very similar quantity of C, N, P, and O as the surrounding ocean. These findings laid the groundwork for much of our current understanding of global nutrient cycling. It was suggested that nutrient concentrations inside plankton were nearly identical to surrounding oceanic concentrations due to the worldwide regulation of nutrients through organisms that fixed and produced the various compounds at a continual and equalized rate.
In his final works, Redfield illustrated that the ratio found in plankton biomass was 106 C to 16 N to 1 P atoms. Eventually, this data made its way into the aquarium hobby. At one point, I heard that it was initially deployed in Europe, where it was utilized as a standard to limit the amount of algae growth that occurred in high-end freshwater planted tanks. The logic was that freshwater plants may be similar enough to plankton to where they could thrive with the suggested nutrient values and outcompete algae. This eventually carried over to saltwater systems and, over time, became the standard it is today.
So, what’s my beef with the Redfield ratio?
- The data is an average across the entire ocean and not specifically focused on areas where coral grows and is located. Reefs have been shown to have distinct nutrient values compared to the open ocean.
- The biomass data was gathered from a variety of plankton species, and there’s an assumption that zooxanthellae would have an identical or similar rate of nutrient uptake. A large amount of data suggests the opposite and illustrates that zooxanthella endemic to coral thrives off much lower nutrient concentrations than described in the Redfield ratio.
- The 1958 paper that finalized the original ratio of 106:16:1 is based on an atomic ratio, not weight, so those that read this as “16 ppm nitrate to 1 ppm phosphate” have not converted atomic mass to ppm – if one were to do so the actual ratio would be 10:1 ppm.
- Even if you decided to apply this 10:1 ratio to your tank, it’s still not a complete look at your nutrients. The 1958 and modern ratios consider ALL oceanic sources of nitrogen, phosphorus, carbon, and oxygen, which means organic and inorganic sources. Our test kits only read inorganic sources. Phosphate testers are even more limited and only consider one inorganic phosphate source: orthophosphates and their associated subspecies. Thus, even if you were to get your tank to reside within a 10:1 ratio of NO3 to PO4, you are still very likely outside of the Redfield ratio due to organic sources of nitrogen and phosphorus as well as inorganic polyphosphates remaining untestable with traditional at-home methods.
The oceanic data set is not specific to reefs or tropical regions. The plankton biomass data does not consider zooxanthella. The ratio is presented in atomic weight, not ppm, like our test kits read. Additionally, the ratio is based on all species of N and P, not just the inorganic species our test kits pick up. Thus, there’s no way to verify you are within a 10:1 ratio of nitrogen to phosphorus in your tank without advanced testing such as ICP and N-DOC analysis. Even if you did get organic and inorganic values within a 10:1 range, there’s still no data to support that’s preferable to corals. The ratio’s application makes negative sense once viewing the actual data.
Modern renditions of the Redfield ratio.
Gruber, N., & Deutsch, C. A. (2014, November 27). Redfield’s evolving legacy. Nature News. https://www.nature.com/articles/ngeo2308
Analyzing N-DOC
N-DOC is very exciting since our test kits only show us a small portion of the picture. The only issue I have with it lies in Triton’s recommendations. They stated in an initial newsletter that they determined ideal nutrient concentrations by compiling a large amount of user data and determining which tanks were the healthiest. Beyond these statements, nothing is publicly released about their methodology for determining tank health. To me, this is suspicious. We should make decisions about our systems with concrete, specific data. If Triton was solely analyzing ICP and N-DOC data to determine the health of a system, this leaves out many factors that could be crucial to system health, such as:
- Overall biodiversity at the micro and macro levels.
- The concentrations of other organic compounds not registered by the N-DOC test.
- All of the above as a function of time concerning coral health and growth rate.
Additionally, the sample size and any other statistics about the data are unknown. For all we know, it may have been constructed from a sample size of 10 and does not contain statistically significant data. While I highly doubt this is the case, I am hesitant to follow their recommendations blindly. Triton may indeed be on the right path, and I won’t outright deny their newly suggested ratio, but I would certainly need to see their methodology to be convinced.
Editor’s note – we will reach out to Triton, and welcome any statistics and data they can provide for clarity, and make them available for future publication.
On a more positive note, there’s a lot to like about this service. It can serve as a jumping-off point for the community to experiment with pushing organic nitrogen and carbon concentrations in various directions to see what anecdotally works for each of our systems. Now is a prime opportunity for those with blue thumbs to step in and do community testing to determine what is best for corals. I have my ideas, but very little current research reflects a “perfect ratio” for corals regarding nutrients. In a future article, I will touch on these ideas and break down the current data surrounding these topics.
Read Dr. Redfield’s papers here:
- https://www.researchgate.net/publication/344709447_ON_THE_PROPORTIONS_OF_ORGANIC_DERIVATIVES_IN_SEA_WATER_AND_THEIR_RELATION_TO_THE_COMPOSITION_OF_PLANKTON_This_is_the_text_from_Alfred_Charles_Redfield_paper
- https://ebme.marine.rutgers.edu/HistoryEarthSystems/HistEarthSystems_Fall2008/Week4b/Redfield_AmSci_1958.pdf
