Photoperiod vs Intensity

Harris Tiu

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Mar 16, 2019
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Hi All,

As I understand, intensity levels are as follows:
Low light is 20-30 micromols PAR,
Medium light is 40-60 micromols PAR,
High light is 90< micromols PAR.

My 2 questions are what the ideal photoperiods are at these levels.

Would the ideal photoperiod for all these be 8-10 hours?

Or would it be better to taper down the light duration when we ramp up the light intensity?
like say:
11 hours at low light,
9 hours at medium light,
7 hours at high light?

Thanks
 

Allwissend

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Interesting question. I don't believe anyone really has the evidence needed to support an 'ideal' as in optimal photoperiod for the growth of many plant species at different light intensities. That won't stop many for making claims. But think we have to leave out the part about ideal out, besides even if it's ideal to grow the plants it may not be good for your tank if you also end up growing algae.

Second remark is that light intensity / PPFD varies quite a lot with water column depth and light distribution. You may have 100 µmol s-1 m-2 at the bottom at the tank in the center, but only 60 at the edges, and 400 µmol s-1 m-2 at the surface. A plant may start at 100 when young but within a week be exposed to 400 for the same duration... besides constant trimming, intuition says it would hurt the plant to change the light duration as it gets closer to the surface just because it is getting exposed to higher PPFD. It's also true that many times green hair algae start at the top of the stem resting on the water surface where water flow is low and light levels are high.

Photosynthesis is also rate limited where the amount of light that hits the plant that second is more relevant than the total amount over the day. The summary is that while there is some room for play, 11 hours of low light may not be the same as 7 hours of high light. At low light levels there simply is not enough energy to induce photosynthesis, there is a light compensation point where photosynthesis = respiration but as the light increases photosynthesis becomes limited by other factors and ultimately chlorophyll becomes damaged and the rate drops. These thresholds are not only species dependent but also dependent on environmental factors (temperature, CO2 etc).

In my experience this far plants look better with high light for short time (exception being low light plants) and it's easier to avoid algae. Kasselmann argues for a "minimum lighting period of 12h light period" in her book . It's not a bad thing for the aquarium as a whole to have plants producing oxygen for longer periods of time but it may be necessary to reduce the light intensity to prevent algae growth and the reality plenty people grow a wide variety of plants without 12h.

I think the takeaway is that there is no solid data on aquatic plants species yet and you should aim to find the optimum for yourself and your aquarium... Heavy trimming session might not be fun if you need to do them every 2 days ;)
 
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Harris Tiu

Member
Mar 16, 2019
43
1
8
36
Makati
Interesting question. I don't believe anyone really has the evidence needed to support an 'ideal' as in optimal photoperiod for the growth of many plant species at different light intensities. That won't stop many for making claims. But think we have to leave out the part about ideal out, besides even if it's ideal to grow the plants it may not be good for your tank if you also end up growing algae.

Second remark is that light intensity / PPFD varies quite a lot with water column depth and light distribution. You may have 100 µmol s-1 m-2 at the bottom at the tank in the center, but only 60 at the edges, and 400 µmol s-1 m-2 at the surface. A plant may start at 100 when young but within a week be exposed to 400 for the same duration... besides constant trimming, intuition says it would hurt the plant to change the light duration as it gets closer to the surface just because it is getting exposed to higher PPFD. It's also true that many times green hair algae start at the top of the stem resting on the water surface where water flow is low and light levels are high.

Photosynthesis is also rate limited where the amount of light that hits the plant that second is more relevant than the total amount over the day. The summary is that while there is some room for play, 11 hours of low light may not be the same as 7 hours of high light. At low light levels there simply is not enough energy to induce photosynthesis, there is a light compensation point where photosynthesis = respiration but as the light increases photosynthesis becomes limited by other factors and ultimately chlorophyll becomes damaged and the rate drops. These thresholds are not only species dependent but also dependent on environmental factors (temperature, CO2 etc).

In my experience this far plants look better with high light for short time (exception being low light plants) and it's easier to avoid algae. Kasselmann argues for a "minimum lighting period of 12h light period" in her book . It's not a bad thing for the aquarium as a whole to have plants producing oxygen for longer periods of time but it may be necessary to reduce the light intensity to prevent algae growth and the reality plenty people grow a wide variety of plants without 12h.

I think the takeaway is that there is no solid data on aquatic plants species yet and you should aim to find the optimum for yourself and your aquarium... Heavy trimming session might not be fun if you need to do them every 2 days ;)

Allwissend,

Thanks for the response.

I was wondering if I used 8-10 hours for all the lighting levels I mentioned, then total PAR received per day would increase with intensity.
In contrast, there is also the idea that the relationships arent linear, where the highlight plants needs for PAR are much higher. Meaning, 50PAR at 9 hours is the same with 100PAR at 4.5 hours but highlight plants need more light, hence 7 hours would be more suitable for high light plants.

However, as you said, observe and adjust to what looks good.

My plan is to test with low light plants at 30PAR starting with 6 hours of light, when I see growth, I will increase to 8 hours, then try to get to 12 hours by increasing by 30 minute blocks every 4 weeks.

Then with high light plants at 90PAR, starting with 6 hours of light, when I see growth, Will increase to 7 hours, then try to get to 12 hours by increasing by 30 minute blocks every 4 weeks.

If the plants start closing up, I stop.
If I get algae, I stop and reduce the hours.


Thanks
 
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