02 production uncoupled


Jan 24, 2005
Cologne, Germany
According to my observations plants do photosythesize strongly even with very low available N. This usually comes along with foliage that is of light green colour and little to no plant growth.

I wonder why many growth studies are based solely upon 02 production.
Likewise DO data seems highly suspicious to me if N or any other nutrient was in short supply when measurements were taken since each plant not only has its particular CP for light and nutrients but also for the non limiting point.

If research doesn't test under non limiting conditions DO data seems of no value. Dry weight measurements are the way to go!

Best regards,

Tom Barr

Staff member
Jan 23, 2005
Relative growth rates and O2 are well correlated though........

O2 and carbon fixing is very well linked.
But respiration is involved also, but there are fairly simple ways around that.

N can be low, but as long as it is not strongly limiting, the rates of growth and O2 evolution are pretty well correlated.

Example, at 20ppm of NO3, the growth rate is say 100% max.
At 2ppm, the rate of growth is perhaps 20%, maybe even less.

Plants will still grow, but not as fast, some species do pretty poor and then you have dose more frequently.

A way around that is sediment nutrients, but then it's no longer "lean", since now there is a very rich N source. So you still get say 80-90% even with no to virtually no N in the water column.

Fish waste also can add some N.

Plants adapt to lower levels and spend more energy and resources acquiring nutrients when they are lower. As long as the levels are fairly stable, things are okay.

There's more wiggling room with higher levels and the plants use less energy to take up the nutrients.

This is a common relationship for virtually all nutrients.


Pick your nutrients and apply it to the graph.
This is standard stuff.

Here's an example of water column and soil ferts:

You can see, for corn, that adding both enhances the total growth, not just one or the other.

Probably more true for aquatic plants.
Corns not exactly aquatic, but even it benefits from both sources.

Tom Barr