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Discussion Starter · #1 ·
I've been working a lot this summer on deploying DO, temperature, light, Chlorophyll-a, and Phycocyanin sensors in various reservoirs around the state for different projects. I had the opportunity to setup and complete an experiment looking at phosphorus release rates from sediment cores while under varying Dissolved Oxygen conditions. One of the tools I used was a DO/temperature sensor that gives real-time data through an LCD screen at 1 minute increments. Our typical sensor has a solid white housing and to retrieve data you have to plug it into a computer. I was curious to see how DO changed in a 24hr period of time in my aquarium so I placed this sensor in my 10g at 1:30pm today. I'll post the data tomorrow! I'll try to incorporate photoperiod and CO2 injection into the graph.

DO is measured in mg/L which equals ppm.
Temperature is measured in degrees Celcius.



 

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Discussion Starter · #3 ·
Very interesting! Thanks for sharing the results.

I would also be curious to see how an airstone affects O2 levels...if you have one of those laying around :)
Results tomorrow! This was just a snapshot of one reading about 2 hours into my photoperiod. I just walked over now and checked DO reading. It says 10.58 mg/L at 26.16 °C.

I don't currently have an airpump to try that out, but I'm guessing the surface agitation would degas much of the trapped O2 from plant photosynthesis and bring it closer to 100% saturation levels for the given temperature. On a side note, I'm using an Aquaclear 20 HOB filter.
 

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Very interesting possibilities here, but I’m having difficulty understanding what is going on in the graph.

Just focusing on actual ppm and temperature, I’m seeing a range of roughly 7-11ppm in DO and a temperature range of roughly 23-26 oC, correct?

We are also seeing a dramatic increase in DO when CO2 is on. Is the implication that this is because plants are now photosynthesizing and not respiring? How heavy would you characterize your plant mass to be and is it a high-light setup? It would also be interesting to see what the pattern is if there is no surface agitation which, I assume, would give us a clearer picture of actual plant impact upon O2.

I’m also curious about the ppm readings, e.g.; at 26 oC, the maximum ppm (100% solubility) should be about 8.2ppm at sea level pressures, but you are recording 11ppm? Further, DO ppm should rise and fall inversely to temperature, but you are recording the opposite.

All of this, if true, leads me to believe that you may be experiencing super-saturation during photosynthesis (which can harm livestock) which is briefly possible with very rapid O2 infusion or am I just not reading this graph correctly?

Lot of questions, but since you have access to this instrument, it offers the potential for a great deal of insight. Not least of the possibilities would be to use your instrument to calibrate some of the O2 test kits, such as the Salifert kit.

As a side note: does your temp vary by that >5 oF every day?
 
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Discussion Starter · #6 · (Edited)
Very interesting possibilities here, but I’m having difficulty understanding what is going on in the graph.

Just focusing on actual ppm and temperature, I’m seeing a range of roughly 7-11ppm in DO and a temperature range of roughly 23-26 oC, correct?

We are also seeing a dramatic increase in DO when CO2 is on. Is the implication that this is because plants are now photosynthesizing and not respiring? How heavy would you characterize your plant mass to be and is it a high-light setup? It would also be interesting to see what the pattern is if there is no surface agitation which, I assume, would give us a clearer picture of actual plant impact upon O2.

I’m also curious about the ppm readings, e.g.; at 26 oC, the maximum ppm (100% solubility) should be about 8.2ppm at sea level pressures, but you are recording 11ppm? Further, DO ppm should rise and fall inversely to temperature, but you are recording the opposite.

All of this, if true, leads me to believe that you may be experiencing super-saturation during photosynthesis (which can harm livestock) which is briefly possible with very rapid O2 infusion or am I just not reading this graph correctly?

Lot of questions, but since you have access to this instrument, it offers the potential for a great deal of insight. Not least of the possibilities would be to use your instrument to calibrate some of the O2 test kits, such as the Salifert kit.

As a side note: does your temp vary by that >5 oF every day?
Yes, the DO and Temp had the ranges you mentioned. We had the windows open all day so the temperature fluctuated just from that. It is possible to get DO above 100% saturation. This is pretty common in fast moving streams with a lot of aeration, but it is also possible in our aquariums due to the high oxygen production by plants. I don't plan on using this sensor for calibrating other test kits. It is actually getting deployed into a reservoir soon if we are unable to retrieve our sensors from the bottom of a research lake.

Plant mass is pretty heavy, and yes you see a drastic increase in DO because photosynthesis is producing oxygen. I'd say this 10g is Medium-High light. Compared to the people who have sumps/overflows, this HOB doesn't produce much surface agitation. I'm willing to bet my aquarium has lower overall DO at night than those with more agitation. I might get airstone to reduce the downward swing at night.

Here's a couple critical points recorded during the 24 hour period:

Highest Temp: 2020-09-12 @21:05:00, Temp:26.20, DO:10.73, DO Sat%:132.67
Lowest Temp: 2020-09-13 @07:25:00, Temp:22.81, DO:7.35, DO Sat%:85.35
Highest DO: 2020-09-13 @12:02:00, Temp:23.92, DO:11.06, DO Sat%:131.14
Lowest DO: 2020-09-13 @02:38:00, Temp:24.68, DO:5.97, DO Sat%:71.78

For clarity on the 2nd graph with the aqua and yellow portions, the aqua indicates the time blocks that CO2 is on, and the yellow indicates when the lights are on. You can see clear points in the DO where photosynthesis starts and stops.
 

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Thanks. I'm going to have to think about it. That's a rather dramatic saturation excess just based upon plants. Then there is the inverse relationship of DO to temp, where they seem to track each other nearly perfectly, but should be moving inversely, at least to some degree. How would you describe your fish load - light, medium, heavy? That should be countering the effects of photosynthesis at least as much as it contributes to a drop during respiration.

I can see heavy saturation in a fast moving stream that is riddled with rocks, etc., but a box of water with moderate gas exchange is troubling to me.
 

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Discussion Starter · #8 ·
Thanks. I'm going to have to think about it. That's a rather dramatic saturation excess just based upon plants. Then there is the inverse relationship of DO to temp, where they seem to track each other nearly perfectly, but should be moving inversely, at least to some degree. How would you describe your fish load - light, medium, heavy? That should be countering the effects of photosynthesis at least as much as it contributes to a drop during respiration.

I can see heavy saturation in a fast moving stream that is riddled with rocks, etc., but a box of water with moderate gas exchange is troubling to me.
Yes, when you only look at DO vs Temp, it has an inverse relationship with saturation points (i.e. cold water has higher DO saturation, and hot water has lower DO saturation). I think the change in temperature might affect the DO curve slightly if I could keep it consistent for the 24 hours. I can test again and bug my wife not to open the windows for a day to keep the house temp the same. I have the heater in the aquarium set to 72F/22C, so the house getting up to 78F/26C was inevitably going to raise the water temp. You can see at 7:30am when my wife closed the windows and turned on the furnace, as there is almost a V shape to the temp graph.

The graph correlations you are seeing between DO and Temp are totally coincidental because the house got hot during the day and colder at night. This does mimic the real world though as lakes and reservoirs do the same thing in the Littoral Zones where aquatic plants grow. Warmer temps during the day with higher DO because of photosynthesis, and colder temps at night with lower DO due to respiration.

In this 10g I have 2 cherry barbs, 6 pygmy cories, 2 dwarf gourami.
 

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By any chance, can you take accurate pH measurements at the points before and after lights on and off? It would be interesting to see if CO2 is moving by such significant rates. Maybe it has a sensitivity to tank size.

I use a Salifert O2 kit to gauge relative levels and have been able to calibrate it ... crudely. My setup is quite a bit different and my tank is 3x yours. I run CO2 24/7 and a surface skimmer 24/7 with steady temps 24/7. I can move that kit O2 reading down by about 30% by turning off the skimmer, but I can't move the O2 by comparing lights on to lights off (remains stable). I have a heavy plant mass with high light. I estimate that the skimmer maintains O2 at about 100% saturation given my temps.
 
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Discussion Starter · #10 ·
By any chance, can you take accurate pH measurements at the points before and after lights on and off? It would be interesting to see if CO2 is moving by such significant rates. Maybe it has a sensitivity to tank size.
I don't have a pH sensor available to use. The one we use looks like this...



Bump:
I estimate that the skimmer maintains O2 at about 100% saturation given my temps.
If you give water more surface agitation, it will try to meet an equilibrium point with O2 in the air, which is that 100% saturation point you are looking at.

For example, I just completed a sediment core incubation study where 3 of my core samples were being aerated by an aquarium airpump 24/7, but since there were no plants in the core tubes and they were in the dark, there was no photosynthetic activity to add in more oxygen. The cores were in a constant 20C environment, so 100% DO Saturation would be 9.1 ppm (mg/L). My cores sat at about 8.3 ppm DO for 10 days straight. My other cores got N2 gas injected into the water and they had DO levels around 0.10 ppm.
 

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The swings you are seeing are much greater than I would have expected, particularly as temps are dropping. As we’ve been discussing, it must be primarily related to a significant reduction in agitation as compared to what I have.

I would be concerned with the extreme points that you reach at max photosynthesis and max respiration. My understanding of stress issues with fish is that below ~8ppm O2 and many tropical fish begin to struggle. Then, above ~130% O2 saturation, gas bubble disease may creep in causing it’s own share of damage, but may not show up directly at lower supersaturation levels, instead resulting in secondary problems such as susceptibility to other vectors.

I think that the best take-away from your interesting study, albeit obviously limited, is that ensuring good gas exchange is very important from both the CO2 standpoint and the O2 standpoint, which we don’t often consider in view of O2 contribution from photosynthesis.

As I mentioned, given your tanks’ ability to show such wide swings, it would be very interesting to add an estimate of the CO2 swings that might be occurring vis-à-vis these O2 recordings.

Thanks for sharing all of this.
 

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I would be concerned with the extreme points that you reach at max photosynthesis and max respiration. My understanding of stress issues with fish is that below ~8ppm O2 and many tropical fish begin to struggle.
O2 saturation at 79°F is 8.1 ppm, and at 83°F it is 7.8 ppm. Stress from hypoxia occurs at lower levels, ~3 ppm, not at saturation.
 

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Really interesting results! The tendencies in the saturation curve seems somewhat expected to me, but I didn't expect the O2 to reach these levels of supersaturation. I think if the temperature was constant and at the upper range (26oC), you would get even higher O2 saturation, due to increased plant photosynthesis, although it would also increase respiration and this may compensate the increase in O2 production.

Anyways, it would be nice if we had the same kind of data from different aquariums to compare, but I know these oxygen meters are not cheap.
 

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Discussion Starter · #15 ·
Really interesting results! The tendencies in the saturation curve seems somewhat expected to me, but I didn't expect the O2 to reach these levels of supersaturation. I think if the temperature was constant and at the upper range (26oC), you would get even higher O2 saturation, due to increased plant photosynthesis, although it would also increase respiration and this may compensate the increase in O2 production.

Anyways, it would be nice if we had the same kind of data from different aquariums to compare, but I know these oxygen meters are not cheap.
Oh yeah, I would love to test a similarly stocked aquarium that uses and overflow/sump to compare. I'm willing to bet the DO Sat% stays lower during peak photosythesis and stays higher at night due to increased surface agitation. Basically, the curve gets flatter.

Increasing temperature would lower the O2 saturation level, but would increase overall biological growth. I guess there is only one way to find out if the curve changes based on a constant temp at the upper range...ANOTHER TEST!

I will set the heater to maintain a constant 78F/26C and record another 24 hours.
 

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O2 saturation at 79°F is 8.1 ppm, and at 83°F it is 7.8 ppm. Stress from hypoxia occurs at lower levels, ~3 ppm, not at saturation.
My temps run a few degrees below 79F, but my fish begin to struggle when I start to fall much below a crudely estimated 8ppm O2 due to the heavy CO2 concentration. So, I try to maintain the good gas exchange 24/7 to ensure full saturation in the 75-77 degree area. Of course, this reading is based upon a crude reagent test (Salifert), so I might have more room that I believe I have. I might be able to acclimate them, but I’ve bounced off of this ceiling many times while trying.

The fish don’t die, but are clearly stressed at these estimated levels and stress is not good long term.
 

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Discussion Starter · #17 ·
OK! Using the program associated with the sensor, I can only make the graph range consistent for Temperature (22C - 27C). Here are both graphs with Day 1 being the first one I reported with a large range in temperature due to house temperatures changing. Day 2 had the heater set to 77F/25C, so the temperature stayed more consistent.



 

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So, you've measured the tank having a ~4 oC point temp swing and with a ~1/2 oC point swing and DO readings appear to be unaffected, right? Wouldn't you expect at least a half-point compression from those DO extremes?

I remain interested in seeing how this turns out in terms of conclusions.
 

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Discussion Starter · #19 ·
So, you've measured the tank having a ~4 oC point temp swing and with a ~1/2 oC point swing and DO readings appear to be unaffected, right? Wouldn't you expect at least a half-point compression from those DO extremes?

I remain interested in seeing how this turns out in terms of conclusions.
I'm not going to make any scientific conclusions as I don't really have time to dive in and see if the data points are statistically different. The graphs appear to be very close to one another. The overall shape of the DO curves look almost identical.

My wife is an Ichthyology PhD student and she was interested in the idea that supersaturated O2 can harm fish. She found a paper from 1913 that said they did an experiment where they supersaturated water to about 400% Saturation and it showed no ill effects on the fish.
 

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My wife is an Ichthyology PhD student and she was interested in the idea that supersaturated O2 can harm fish. She found a paper from 1913 that said they did an experiment where they supersaturated water to about 400% Saturation and it showed no ill effects on the fish.
Look up "Gas Bubble Disease." It is similar to the "bends" in humans.

Here is a quote - legitimate or not - that I found in the first article that appeared:

"Other situations where supersaturation can occur include ponds with aquatic plant growth, because photosynthesis can increase the total dissolved gases. This is why aquarists recommend keeping keep pond oxygen levels below 125 percent."

A rapid temp increase can also cause supersaturation, although I don't know if 4 oC is enough.

It all points to a strong reason to ensure good gas exchange.
 
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