Macro/micro nutrients in seawater

[Fred Dulley]

Hi gang at UKAPS.
Nothing to do with the aquaria and a little bit off-topic I know but I wanted to pick your brains with regards to this termonology.
I'm currently undertaking my 2nd year of Ocean Science at Plymouth Uni.
During our Chemical and Biological Oceanography lectures, the lecturer (who loves to work in mols per litre) insists on classing N+P+Si as micronutrients. I've always understood them to be macro nutrients (most certainly N+P). I've been reading a few sources online that say they are macro nutrients (corroborates with my opinion).
Surely even though it's seawater, N+P are still macro nutrients and not micro nutrients?

We've also come across Redfield Ratio...

Taken from one of Tom Barr's posts

I've had a few discussion with folks, mostly in the UK over this one. I will not get into the issue with the difference between the mass ratio, which is what is used, versus what Redfield actually shown in his paper based on averages of marine phytoplankton, it was atomic ratios, atoms, not mass (eg ppm etc). So 106 carbon atoms to 16 nitrogen atoms to 1 P atoms and so forth. To convert to mass, you need to factor in molar weights, N/P will be 16N's* 14 g/mol/ 1P* 30.97 g/mol= 7.2 for a ratio for N;P for algae based on weight or about 10:1 for ratio for NO3/PO4.

From what I can see...it's OK if you're talking purely about number of atoms. But if you want to apply it via a concentration based on mass (ppm for example), then necessary calculations must be carried out. Is that right?

Is the Redfield Ratio right if you're talking about no. of atoms. Or is it completely wrong?

All your inputs are appreciated (especially Tom and Clive )

 

[ceg4048]

Hi Fred,
When talking about Marine ecosystems, it's important to understand the differences with regard to freshwater systems. In the oceans, by an astronomical margin, the dominant forms of life, by mass and population, are the autotrophic community composed of phytoplankton and zooplankton. So their composition and mode of existence will differ from freshwater macrophytes. As you mentioned, it's also important to be clear regarding atomic ratios in terms not only of composition of the organism but also in terms of what the organism consumes. Composition and consumption will also be a function of the species being studied. So for example if you're focused more on the 10,000 or so species of Diatomic algae then Silicon will be an important element but perhaps not so important if Cyanobacteria is a major focus of the study.

Redfield's 1963 equation for planktonic respiration looks like this:
106 CO2 + 16 HNO3 + H3PO4 + 122 H2O = (CH2O)106 (NH3)16 (H3PO4) + 138 O2

So you can see that in this equation the N and P values are relatively low relative to Carbon, Hydrogen and Oxygen, but this is a reaction equation, not a compositional one. If you're studying Oceanography then you have to consider that the material is within the context of reactions and this may be the reason the lecturer has classed those elements in the micronutrient category. These reactions also have best relevance in terms of molar expression. Mass concentration would have less relevance in this context.

In our expression of tank horticulture, we are much more tunnel vision. We are much less interested in the web of ocean reactions and we concentrate more on the narrow band of reactions that maximizes the success of a very small percentage
of reactions. Our applied technology is based on the application of mass quantities of nutrients under very special and isolated conditions, so it's really not comparable. Redfield's ratios and studies do not apply to farmers for these reasons.

However, Redfield's studies are fundamental to any basic Oceanography course because he made some of the first observations of marine biochemistry. The basis of his research had a lot to do with the stoichiometric analysis of the biochemical functions of these autotrophs that were collected. He didn't have the tools that are available now and so he was not able to completely separate the quantities of elements that the organisms are made of from their metabolic products. So for example suppose I were studying how much carbon and oxygen you were made of but didn't take into account the CO2 in the gas spaces of your lungs or in you bloodstream or in your kidneys and intestines. These would all count as metabolic products, not compositional products.

Here's an extract from a ScienceWeek article which may shed some light:

1) An interesting empirical observation in biology is the relationship between the elemental composition of organisms and ecosystems. All organisms are composed primarily of a mixture of six major elements: hydrogen, carbon, nitrogen, oxygen, phosphorus and sulfur. But the proportion of these basic ingredients varies between organisms -- and such variations can lead to interesting properties within ecosystems.

2) For example, in the oceans most of the biomass comprises small drifting organisms (plankton) that are rich in nitrogen. These organisms are essentially functionally similar ensembles of metabolites, often encased in a shell formed from the most readily available ingredients. Much plankton is consumed by other plankton with similar chemical compositions. The result is that on average, the nitrogen-phosphorus (N-P) ratios of plankton in the oceans are remarkably similar throughout the world, averaging approximately 16:1 by atoms. When these organisms or their body parts sink into the ocean interior, their energy-rich bodies are consumed by bacteria which, in aerobic conditions, oxidize the organic matter to form dissolved inorganic nutrients, especially CO2, NO3(-) and PO4(3-).

3) In 1934, Alfred Redfield (1890-1983) wrote a now classic paper in which he proposed that the NP ratio of plankton (16:1) causes the ocean to have a remarkably similar ratio of dissolved NO3(-) and PO4(3-). This hypothesis suggested that, devoid of life, the chemical composition of the oceans would be markedly different. The concept of Redfield ratios has been fundamental to our understanding of the biogeochemistry of the oceans ever since.

4) The basic problem with Redfield ratios is that they are empirical. The ratios were originally derived from measurements of the elemental composition of plankton, and the NO3(-) and PO4(3-) content of seawater from a few stations in the Atlantic, but were subsequently supported by hundreds of independent measurements. Yet there is no known reason why the average NP ratio of plankton should be 16:1. Why not 6:1? Or 60:1? If one looks at the elemental composition of individual species of phytoplankton grown under nitrogen or phosphorus limitation, the NP ratio can vary from around 6:1 to 60:1. Redfield understood this problem, but did not try explain it, except to note that the NP ratio of inorganic nutrients in the ocean interior was an average, and that small-scale variability around the mean was to be expected.

5) Despite many reports that the elemental composition of organisms in a region of the ocean does not conform to Redfield ratios, or that the elemental composition of marine phytoplankton grown in cultures is not 16:1, Redfield's fundamental concept remains valid. It cannot be rationalized by reductionist arguments, nor refuted by anecdotal observations. The fact that the NO(3-) PO4(3-) ratio in the interior of all major ocean basins is remarkably similar to the NP ratio of plankton is due to the residence times of these two elements in the ocean (roughly 10^(4) years), relative to the ocean's circulation time (roughly 10^(3) years). As the residence times exceed the mixing times by an order of magnitude, it should not be surprising that the NO(3-) PO4(3-) ratios in the ocean interior are remarkably constant.

6) The specific elemental composition that is the Redfield ratio is truly an "emergent" property that reflects the interaction of multiple processes, including the acquisition of the elements by plankton, the formation of new biomass and the remineralization of the biomass by bacteria in the ocean interior, as well as losses of nutrients from the ocean because of burial in the sediments (for example, phosphorus in apatite), or outgassing to the atmosphere (for example, production and loss of N2, due to denitrification).

Hope this helps!

Cheers,

 

[Fred Dulley]

Thanks ever so much, Clive.
You are a wealth of knowledge.