The key question is, when there is no CO2 injection, and CO2 levels are low, does having higher light increase growth rates? The answer is yes, though the increase in growth is no where as much as when there is injected CO2.
This is demonstrated in the experiment below by Andersen (1999) Interactions between light and inorganic carbon stimulate the growth of Riccia fluitans, University of Copenhagen.
Figure (above) shows how 1 gram of Riccia develops over two weeks at given lighting and CO2 levels.
The other outcome of the experiment showed that the increase in growth rates when either CO2 or lighting was increased is subjected to diminishing returns. This means that when light or CO2 is lacking, small increments gave a big boost to growth, however, increasing from a medium to high level still gives a boost to growth, but in a marginally smaller way.
It also demonstrates, in a counter-intuitive way, that having high light even in low tech planted aquarium does have a significant impact on growth. While this may cause more algae issues for an unbalanced planted aquarium, I find that using high light on low tech tanks allow growing of more difficult species that usually don't adapt well to low tech conditions.
The chart below compares photosynthesis in a tank with high CO2 vs low CO2.
When CO2 levels are low, photosynthesis levels are limited by low CO2 levels; thus the maximum amount of light that can be utilised is lower compared to that of a high tech planted tank. The photosynthesis curve thus taper off at a lower point compared to a tank with high levels of CO2 as light levels increase.
However, plants can channel energy to different functions; a lack of nutrients for example, may cause the plant to dedicate more energy to root growth, a lack of light stimulates stem elongation and more energy in light absorbing pigments and chlorophyll. At low CO2 levels, plants invest more energy in enzyme production to aid CO2 uptake & fixation.
Therefore, the light compensation point (level of light at which net energy needs of the plant is met) is actually higher for low tech tanks compared to CO2 injected tanks. For the latter, CO2 is easy to attain so plants require less light to have surplus energy to grow. This gives rise to the odd combination that at the extreme end of low light, high tech tanks can survive with lower absolute light levels compared to low tech tanks, while being able also to utilize much higher lighting levels if given. High tech planted tanks operate within a much larger range then compared to low tech planted tanks; which cannot have too low light, nor make use of too high lighting.
This is detailed in the chart above. The red line is the light compensation level of photosynthesis. Only above this level do we get net growth. Below this level, plants starve.
This means that having more light in low tech tanks WILL improve growth rates, even though the marginal impact is smaller than compared to in a CO2 enriched tank.
Using higher lighting in low tech tanks is one way to grow more difficult species that otherwise do very poorly in low CO2 environments.
The main downside of using higher lighting is that one may face more algae issues. I use 100 Umols of PAR effectively in my low tech tank setups, but for those looking to growth plants not usually suitable for low tech tanks, I would recommend a range between 60-80 Umols at substrate level.
For high tech planted tanks, growth rates also eventually taper off as light levels are increased even with CO2 enrichment. There is little marginal gain to growth rates by increasing PAR beyond 600-800 umols.
This is mainly theory crafting, as other than outdoor farm tanks receiving sunlight, most tanks will never get so much light. For outdoor farm tanks, having 6% - 70% shade cloth will cut down PAR from sunlight from 2000 to around 600-800 Umols.
Using soil and higher lighting is one way to cheat past the limitations of low tech / non CO2 injected planted tanks. In this tank, the Monte carlo carpet grows in significantly over a 3 month period.
To learn more about reading PAR tables, click here.
To learn more about algae control, click here.