# Daylength/Photoperiod for Aquatic Plants



## dwalstad (Apr 14, 2006)

Here's a recent question about photoperiods for growing plants in a freshwater refugium:

"BTW, what do you people suggest for a photo period? I think most of the reefers run lights on all the time. They mostly grow multicellular algae. Would higher plants need something different? I am currently leaving the lights on 24 hrs."

If you can spare the electricity, I would keep the lights on 24 hr per day to maximize plant growth in your refugium. I see that you have floating plants, so CO2 won't limit the growth of your plants. Plus, with 45 tanks filled with guppies hooked up to your system, the plants in your refugium will never want for nitrogen and other nutrients. 

This isn't the first time a question about the effect of photoperiod on plant growth has been asked. Several times I've seen answers (even from aquatic botanists!) that aquatic plants need a "rest period". This implies that aquatic plants somehow need to recover from a _hard day of photosynthesis_. Well, the following paper says otherwise.

REFERENCE: Pilon J and Santamaria L. 2002. Clonal variation in morphological and physiological responses to irradiance and photoperiod for the aquatic angiosperm _Potamogeton pectinatus_. Journal of Ecology 90: 859-870.

It took me a couple days to decipher this incredibly comprehensive paper. The bottom line is that _Potamogeton pectinatus_, which is a rooted aquatic plant with worldwide distribution, grew better with a 22 hr daylength than a 13 hr daylength. This was true for 3 different strains of the plant species whether it was exposed to low intensity or high intensity light. The increase was dramatic and statistically significant in all cases. For example, a _P. pectinatus_ strain from Russia increased its biomass from 180% with 13 hr daylength to 300% with 22 hr daylength when it was grown under high light intensity. When it was grown under low light intensity, it increased biomass 90% under 13 hr daylength and 230% under 22 hr daylength.

The investigators found no significant difference in plant growth for plants exposed to 13 hr and 16 hr daylength. Thus, we must not split hairs here over a 3 hr increase in daylength. Nor do I recommend that hobbyist keep the lights on in their aquariums for 22 hr/day. My point is that aquatic plants keep photosynthesizing and growing as long as the lights are on.


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## DarioDario (Nov 14, 2008)

oooh very interesting... I'm always kinda put off to look over and see that my tank hasn't turned on yet or that it had to turn off sooner then I want. I would be a little worried about Algae exploding if the plants weren't able to keep up. Any experts willing to chime in on this?


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## Sunstar (Sep 17, 2008)

Where the plants might enjoy the light, I wonder if the animals within would not prefer it to be dark at a semi regular time.

I agree with you dario, I just looked over at my one tank and it is still in "fishy night" 

on the other hand, I have a vase on my desk that contains shrimp and a few other plants. I leave the light on MUCH longer, I dare say at least 16 hours at times. kinda tacky within, I don't fertilise it much as I managed to wipe out the young shrimp once, but the plants bubble away happily and send out runners.

Edit:

I am trying "moon lights" by using christmas LED's. I would like something brighter but this is working to enjoy longer albeit different observing time.


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## brenmuk (Oct 7, 2008)

Hi Diana,

Interesting sounding paper. How did they control for other factors such as plant nutrients did they always ensure that available nutrients were always in excess?

Would you say in the case of a planted fish tank using the el natural method that there comes a point at which increasing the photo period can cause problems?


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## rlking (Sep 8, 2008)

Hi Diana, 
Thank you so much for answering my question. Even though I recieved answers on the other post indicating higher plants needed a rest period I decided to leave the lights on 24 hours a day. 
Actually I left on a short vacation and was torn between leaving the lights on, turning them off or buying a timer. Since I had been leaving the lights on I decided to keep things as they were and make changes when I got back. When I returned everything looked fine with all plants except the water lettuce showing significant growth. Still no ammonia! With your new information I will continue to keep them on. 
Thanks again
Rod


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## dwalstad (Apr 14, 2006)

brenmuk said:


> Hi Diana,
> 
> Interesting sounding paper. How did they control for other factors such as plant nutrients did they always ensure that available nutrients were always in excess?
> 
> Would you say in the case of a planted fish tank using the el natural method that there comes a point at which increasing the photo period can cause problems?


The investigators used uniform conditions (river clay/sand substrate) and weekly water changes of tapwater for all plants in the experiments. Since the plants seemed to be growing well during the 60 day experiment, whether or not nutrients were in excess may be a moot point.

Now, please don't assume that I am recommending that people run their NPT tanks with lights continuously on. That was not my goal. Nor would I ever keep the lights on in my tanks 24 hr/day.

This letter was directed to Rod with his refugium (separate tank with plants only for water purification). He will be better off running his refugium tank 24 hr/day than 13 or 16 hr/day.

It is also a matter of principle. I've heard too many hobbyists talk about aquatic plants needing a "rest period". There's no scientific literature backing up this claim.


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## zer0zax (Mar 25, 2007)

Very interesting stuff! I always thought that plants needed a rest period, if they couldn't breathe in oxygen and expel Co2 at night, they would run out of gas and die, not completing the photosynthesis cycle. I think it is hard to set rules in stone regarding aquariums. Through science a lot of things have been discovered, but we all have so many different variables, lighting, water parameters, plants...

Would the plants run low on available Co2 if they photosynthesize 24/7? I would think they would "use up" the nutrients from mulm, but then again they would maybe make more mulm as the system is ran 24/7 instead of 8-16 hours a day. 

I was also curious about running a refugium or sump. Would it be better to run the main tank and fuge on separate lighting schedules, or the fuge 24/7? I will have to read that thread! I always thought that it would be better to run them on separate schedules, one provides oxygen to the other, and the other provides Co2 to the one. If the fuge can successfully take up excess nutrients 24/7 though, then it really would be a nutrient sink instead of two aquariums ran back to back. Guess I have my answer! Keep these thoughts and articles coming Diana! Very good question you brought up, Rlking. I need another cup of coffee, to many grinding gears, great stuff!!


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## dwalstad (Apr 14, 2006)

zer0zax said:


> Would the plants run low on available Co2 if they photosynthesize 24/7?
> 
> I was also curious about running a refugium or sump. Would it be better to run the main tank and fuge on separate lighting schedules, or the fuge 24/7? I always thought that it would be better to run them on separate schedules, one provides oxygen to the other, and the other provides Co2 to the one. If the fuge can successfully take up excess nutrients 24/7 though, then it really would be a nutrient sink instead of two aquariums ran back to back.
> 
> ...


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## essabee (Oct 11, 2006)

Diana rightly pointed out that topic under discussion has to do with "Plants-only" tanks and not NPT.
Truly the topic should have been in an aquatic plants board instead of the _El Naturale_

If you keep your mind on the summer photo-period in outer temperate and inner torrid areas, 22 hours are natural. Then Nature too must have ingrained into plant to make hay (photosynthesize) while the sun shines. It would be quite natural to expect plants to have the ability to photosynthesize for such periods with natural increase of their tissues weight. I am not surprised with the scientific confirmation of this fact.

As to rest period for plants, or any other living organism, the mode is highly individualistic. Look at your your pet dog, if you have one, the numerous spells of alternate intense activity and sleep all through the day and night. Then take the case of the dolphin - where alternately only half the brain appears to sleep to keep their breathing functions active. Plant tissues follow their circadian clock for various functions and the rest spells that different tissues follow need not synchronize. So don't expect plants to sleep in the way we do. Also why is darkness necessary for rest, does it confer any advantage?


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## rlking (Sep 8, 2008)

Thought I'd add a little backgroung information about the refugium. I raise guppies and have about 30 tanks (300 gal) hooked up with a 150 gal sump/refugium. I wanted a low maintenance but effective filter. I origianlly planned on planting the refugium with submerged aquatic plants. I used a soil substrate and planted it with Hygro augustifolia, Java Moss, Watersprite, Crypts lucens, and Java Fern. I added floating plants at Diana's suggestion (Frogbit, duckweed and Water Lettuce). 
The fuge has been running for about 3 weeks now and everything seems to be working. I started out with about 100 guppies in the system and have been adding them a little at a time. I now have over 300 gupps in the tank. I've been taking ammonia levels daily. I have yet to detect any. So Far so good. 
Rod
PS
As the system matures I plan on adding another 15 tanks to the system.


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## zer0zax (Mar 25, 2007)

RlKing- I read your thread about the refugium, very nice! I think you are cycling everything up in a good way, adding fish slowly, monitoring nutrients, and adding more plants. With this many tanks connected, it would not be fun to have everything stocked all at once, only to find out that the refugium can't handle it! I have never had a sump or refugium before, so I am living vicariously through your experiences! I think it is a good idea to cycle everything slowly, the plants have time to adapt before the fish load is increased, and as you add more plants they will have time to adapt while the original plants handle the load. Nice work!

Have you thought about growing emersed swords or trees at all? You have a ton of options since you have plenty of light and an open-top tank! Keep the updates rolling


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