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Light Compensation Point and optimal PAR levels

tyrophagus

tyrophagus

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Joined
1 Apr 2010
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Location
Swindon
I not sure this information is available. This is what I understand so far.

We use PAR to assess lighting for plants rather than lumens, lux etc. We see lumens and plants use PAR. What I am unable to find is the optimal PAR level for the average planted aquarium.

Light compensation points (LCP) vary for different plants so "high light plants" have a higher LCP meaning they require more light to meet their minimum requirements for survival. Different aquariums will have different plants so it's impossible to have 1 value for PAR that suits all Aquaria.

Trawling the forum I found suggestions that 30 - 80 umol is the range for the LCP for most water plants. Tom Barr when working on the Estimative Index used 600 umol of light to max out the nutrient requirements of the plants he used.

Does anyone know what the PAR level is at the substrate in their tanks? What should I aim for in terms of PAR taking as a given that CO2 and nutrients are optimum?

I have on order a PAR meter so this information would be useful. I have two TMC Growbeam LED lighting tiles and I have a few issues with my plants. I'm hoping that if I can set my lights to an appropriate level I can rule out either too much light or too little light. Ideally I would like to provide light at a level that exceeds the LCP for the plants I have.

So what should I aim for? 100umol to start?
 
James I have a few problems with my crypt wendetii green and my hydrocotyle sibthorpoides. Especially the latter is showing yellowing of older leaves and pinholes. The new growth is relatively good and nice and green. I plan to introduce a spray bar soon to maximize my flow and co2 distribution. However I have plants swaying all over the tank and bubbles of co2 in the areas that the plants are affected. I don't think its co2 but I can't be sure.

The LEDs are currently set at 35% output and I have recently reduced this from 45% as I was concerned I was driving photosynthesis to high and causing damage to the plants. (but are my lights to low)

It's only old leaves that are affected.

Are there any references for PAR requirements of different aquatic plants?
 
Yellowing and pin holes points to ferts and co2, see james difficiency guide, what are you dosing?
 
co2 is about 5 - 10 drops per second (to fast to count) and drop checker is yellow day and night (4dkh water), solenoid comes on 2 hours before lights, goes off 2 hours before lights out. drop checker goes lime green overnight.

dosing EI for a 40 gallon tank. 1/2tsp kno3 + 1/8tsp kpo4 + 2tsp mgso4 every other day with trace every other day.

50% or more water change weekly.

I have only just started dosing magnesium in the last 10 days. Thames water does not measure magnesium levels in my supply as its not a legal requirement. I thought this might be a mg deficiency.

If Clive has taught me one thing though its get your light and co2 right first. 🙂
 
tyrophagus said:
...Does anyone know what the PAR level is at the substrate in their tanks? What should I aim for in terms of PAR taking as a given that CO2 and nutrients are optimum?
Click to expand...
It's very easy to rule out if a plant is receiving light energy levels below LCP. If the plant grows at all then it is by definition receiving spectral energy higher than LCP. Energy below LCP results in death. Energy at LCP yields zero growth. Anything higher than LCP yields a non-zero growth rate. Excursion above LCP yields increased growth rates with increased PAR.

There is a difference between growth and health however. It's entirely possible to have positive growth rates yet to have poor health. It's also possible to have a very low growth rate and yet to have excellent health.

I agree with Garuf that yellowing and holes cannot be attributable to low spectral energy, and that this is most likely a nutrient/CO2 issue.

Since no one has yet catalogued growth rates as a function of PAR/CO2 it's going to be very difficult to determine optimum PAR (which doesn't exist, really). As we know from Barr's measurements typical low light scenario is one in which there is in the neighbourhood of 100 umoles at the surface declining to about 30 umoles at the substrate. But we also know that there are many low light non-CO2 tanks having this or weaker spectral profiles yet, since nutrients and CO2 are available, few issues arise.

As you mentioned, there is very little data on LED PAR values, only the data reported to be in a PFK article, so it is very necessary to determine these data for our equipment. Even if the measured PAR turns out to be on the low side though, that still cannot explain yellowing and pinholes. That can only be explained if the PAR readings turn out to be unexpectedly astronomical. It's entirely possible that the LED output profile looks dim you you but yet packs a whopping PAR punch.

Cheers,
 
40 micromols is good starting point. Even for the most veteran skilled aquascaper.

the plants could go lower certainly............but few folks have too little light unless they just missed the boat entirely like a few Soccer refs recently....... 🙄 and are newbies that have not looked into much about how to grow plants.

Regards,
Tom Barr
 
crypt wendetii green and hydrocotyle sibthorpoides needs completely different growth conditions, the first takes most of the nutrients from the substrate the latter from water column, the light/co2 levels only influence the growth rate and the leaves size, so check your nutrients levels first.
 
Hi all,
The other thing that you have to think about with PAR and the photosynthetic compensation point is the effect of plant canopy architecture. Plants are very efficient at maximising light interception by the canopy, but it isn't an altruistic thing, competition is between both individual leaves and different plants. This is nature not "red in tooth and claw" but "green in leaf and stem". If a plant gains a competitive advantage by sacrificing older leaves and building new leaves at the top of the canopy, this is what it will do. By this I don't mean the plant thinks "oh a new leaf here would be better than that old leaf there", but growth rate, morphological and physical plasticity are all dependent upon the genes hard wired into that plant and plant species by natural selection over millennia. All the plants we have (in the case of hybrids and cultivars their parents) are winners in the "Race of life".
There is a difference between growth and health however. It's entirely possible to have positive growth rates yet to have poor health. It's also possible to have a very low growth rate and yet to have excellent health.
Click to expand...
This is very true, plants which are efficient at harvesting light in low PAR situations will typically have a suite of adaptations in common - slow maximal growth rates, dark green leaves (they contain more chlorophyll) and low nutrient requirements.

I recently bought a new light (2nd hand via UKAPS) which doubled the light output and one very noticeable effect has been I have a lot more biomass in the aquarium, in this case light availability was limiting plant growth (although I practice nutrient depletion and would have said that plant growth was nutrient limited before the addition of the light.) I can't measure the PAR at the bottom of the tank, but I'd be reasonably confident that I'm now maintaining light levels at the tank bottom (by an occasional thin through of the floating plants and upper canopy stems/leaves) at pretty much
the same level as it was before the new light, the only difference is that there is now more energy in the ecosystem and this has allowed a thicker canopy to grow.

cheers Darrel
 
ceg4048 said:
tyrophagus said:
...Does anyone know what the PAR level is at the substrate in their tanks? What should I aim for in terms of PAR taking as a given that CO2 and nutrients are optimum?
Click to expand...
It's very easy to rule out if a plant is receiving light energy levels below LCP. If the plant grows at all then it is by definition receiving spectral energy higher than LCP. Energy below LCP results in death. Energy at LCP yields zero growth. Anything higher than LCP yields a non-zero growth rate. Excursion above LCP yields increased growth rates with increased PAR.
Click to expand...

Clive it's so obvious when you point it out. So my plants are all growing which means the light is above the LCP. The light does look dim to me, I would have expected it needed to be brighter (but thats lumens not PAR) The fact I have some yellowing and pinholes especially on the hydrocotyle means there is a nutrient deficiency.

OK. So if I'm dosing EI (my levels are posted above) and my lights are hopefully not to strong (I'll measure this when I get a meter but they are at 35% output currently) then the culprit must be CO2 - either the ppm is to low or the flow is not right.

agree?
 
Yes, I agree with this. There is something bizarre happening in this tank, but the symptoms are undeniable. Flow/distribution is a strong suspicion here but I have to ask the obvious...as silly as it sounds, could there be a possibility that you have the KNO3/KH2PO4 powders swapped so that you are dosing only 1/8th tsp KNO3? Not saying you are silly but just attempting to turn every stone... 😉

I think I mentioned this before, but what I find most bizarre is that the plants are rooted in Aquasoil are they not? This product is high in N so we really ought not to see an N shortage unless possibly the substrate is "Pre-owned" as they say at the Mercedes dealership.

Cheers,
 
clonitza said:
crypt wendetii green and hydrocotyle sibthorpoides needs completely different growth conditions, the first takes most of the nutrients from the substrate the latter from water column, the light/co2 levels only influence the growth rate and the leaves size, so check your nutrients levels first.
Click to expand...

I have no issues with either using water column or sediment ferts, so I really do not think that has much dependence here.
I've grown these both going on some 10 + years without any issues of any sort, regardless of location of the ferts.

This is true for most all aquatic plants and can be generalized.
Research also suggest it where nutrient rich water column exist and are stable.

Ask Tropica and Ole, Troels etc, Nina Cedergreen, Tom Madsen etc.
In the aquarium, I've found little, if any difference.

Regards,
Tom Barr
 
tyrophagus said:
OK. So if I'm dosing EI (my levels are posted above) and my lights are hopefully not to strong (I'll measure this when I get a meter but they are at 35% output currently) then the culprit must be CO2 - either the ppm is to low or the flow is not right.

agree?
Click to expand...

Well, less light means it is easier to manage CO2 and less stress to fish, more stability to the entire balance/system.
But CO2 is the typical strongest suspect for most issues.

Plants will use leaves as storage organs, much like Darrel is suggesting. They will sacrifice an old leaf to make a newer one closer to the light and also closer to thr air/CO2 above. It's a race to get to the light/CO2 basically.

If they have plenty of CO2, then there's less need and stress, they also have a lot more O2, which reduces stem elongation. Low O2+ high CO2, poor current= stem elongation in many submersed species.

This is due to ethylene entrapment. More current= more diffusion of ethylene gas out, more O2 from fast growth= less ethylene. Higher CO2 will still have some negative effect but the good O2/current/growth counter act it for the most part.

Ethylene is like any gas under water, it is very slow to diffuse out of the plant like it does normally in emergent/terrestrial systems. So it makes an excellent signal for submersed flooded conditions for the plant to respond. If those conditions change and the O2 levels decrease, light decreases, current decreases........then the plant needs to be able to respond rapidly.

Otherwise all those lower leaves as well as the tops will die.
Many fast growing weeds do this, which accounts for most of the aquatic plants we grow 😳
Nothing but weeds.

Still focus strongly on all aspects of CO2 and anything you can think of.
CO2 burns me and it burns Amano as well, so I know it's going burn everyone else at some point.

CO2 ppm's change faster and over a wide range than any other parameter(seconds/minutes). Light? Months/years. Nutrients? Days to weeks.........but CO2 is also a bear to measure correctly and many assume it's easy to measure/simple etc.

So think about what you can do to reduce the effects(lower light is a good place to start) and then what other things you can do and measure to adjust CO2 "just right". It's part luck for some, and bad luck for others........ so then you have to go looking for the issues and see what you can do to improve it.

Needle wheels seem to have many good traits.
High grade solenoids
Dual stage high grade Regs
Good current/mixing, high flow through rates for the reactors, needle wheels
High O2 while the CO2 is on.
High grade check valves, check them for sticking often
Good tubing for CO2
High grade needle valves
Good connections, check with soapy water.
Progressively slow adjustment.

I think many wing CO2 adjustments and do not see the subtle effects over time.
They get impatient and rush. You miss seeing the effects and the signs of poor CO2, some are slight, subtle. The CO2 might just be a little off, but if you only know high and low, you do not know just slightly low or slightly high.
The plants will tell you and they will look really nice, they make the best test kit.

Fish should not be stressed in this process, so the slow way is best. Excel, Easy Carbo etc can be used if you have not gotten there quite yet, but you should wean off that and then tweak the CO2 slowly from there.



Regards,
Tom Barr
 
ceg4048 said:
Yes, I agree with this. There is something bizarre happening in this tank, but the symptoms are undeniable. Flow/distribution is a strong suspicion here but I have to ask the obvious...as silly as it sounds, could there be a possibility that you have the KNO3/KH2PO4 powders swapped so that you are dosing only 1/8th tsp KNO3? Not saying you are silly but just attempting to turn every stone... 😉

I think I mentioned this before, but what I find most bizarre is that the plants are rooted in Aquasoil are they not? This product is high in N so we really ought not to see an N shortage unless possibly the substrate is "Pre-owned" as they say at the Mercedes dealership.

Cheers,
Click to expand...


Clive they are in Ada Amazonia, fresh from the amazon so to speak! I'm not switching them round I give more nitrate than phosphate. The crypts are actually not to bad, have only minor yellowing with green veins, it's the hydrocotyle that's the problem. It's biomass has halved. The new leaves look fine and it's growing. I was hoping it was Mg but I suppose that's to early to tell as I have only been adding it for a 2 weeks. Somethings wrong because I still get a film on the surface and my co2 is at approximately 8 drops per second.

I have a spray bar on order, non-gucci but then I can change that later.

Thanks for the advice everyone :thumbup:
 
Thanks Tom. I find the management of a planted aquarium fascinating and I really appreciate the science that is put forward on this forum in an easy to understand way. What's missing from the article section is a plant physiology primer. You can find the info if you search but there's no single reference.

Perhaps one day, we all have busy lives...
 
Clive they are in Ada Amazonia
Click to expand...
If I knew 🙂, thought it was just plain gravel :lol:

Tom thank you for the insight on CO2 & water flow, doing aquaristics only since last year I still have lots and lots to learn so every post you mates write it's like a breath of fresh air 🙂.
 
Hi all,
I'll try and put together a "plant physiology primer" before the end of the summer. I have lots of powerpoints etc. so it will just be a case of cut, pasting and referencing the relevant bits.

If any-one does have questions about photosynthetic processes I point them towards SAPS http://www-saps.plantsci.cam.ac.uk/qanda_photo.htm, it is for schools but has lots of good stuff on it.

There is a brilliant powerpoint of photosynthesis here (by prof John Gray of Cambridge University here):
http://www-saps.plantsci.cam.ac.uk/summer/2005/edinburgh2.ppt. It is a thing of beauty as well as a great resource.

cheers Darrel
 
I think that would be very useful. Its one thing trawling the net for information and another to have someone knowledgeable summarise the information for you. Look forward to reading it Darrel. That powerpoint presentation is a thing of beauty although a lot of it is nice pictures to me 🙂
 
ceg4048 said:
Yes, I agree with this. There is something bizarre happening in this tank, but the symptoms are undeniable. Flow/distribution is a strong suspicion here but I have to ask the obvious...as silly as it sounds, could there be a possibility that you have the KNO3/KH2PO4 powders swapped so that you are dosing only 1/8th tsp KNO3? Not saying you are silly but just attempting to turn every stone... 😉

I think I mentioned this before, but what I find most bizarre is that the plants are rooted in Aquasoil are they not? This product is high in N so we really ought not to see an N shortage unless possibly the substrate is "Pre-owned" as they say at the Mercedes dealership.

Cheers,
Click to expand...

Clive believe it or not I may have a light issue. I took some PAR readings today after my meter arrived and I have 40 at the surface dropping to 18 midwater then about 12 at substrate level.
I'll take some more readings when I can and perhaps produce a PAR chart for the LED lights.

How slowly should I increase the light intensity? I was thinking of about 3 PAR a day at the substrate until it reaches 30.
 
Hi mate,
At that rate you will have almost tripled the energy levels in less than a week. I'd bring the substrate reading up by about 6 umoles and leave it there for a week while tweaking the CO2. Then repeat.

I thought the reported numbers seemed too high, but maybe the number at full throttle is 600 but the attenuation isn't as linear as we thought. It'll be interesting to see your numbers. :geek:

Cheers,
 
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