interesting co2 info

[aspen]

here's a post about co2 and fish.

http://daah.info/index.php?board=9;action=display;threadid=234

i had no idea that co2 inhibited a fish's ability to use o2. live and learn. it makes you wonder about adding all of those high levels of co2 to our planted tanks.

rick

(i edited to make reference to the new post url.)

 

[tjudy]

I went and found that post on the discus forum. Any chance we can get a reference to where that person found the information? A 50% reduction in O2 uptake sounds very high, for no other reason than the amounts of CO2 we inject in our tanks are relatively small compared to the amount of O2 that passively diffuses in at the water surface.

The uptake of gases by fish is based upon passive duffusion. The 'rules' of passive diffusion are that the concentration of one solute in a solution do not affect the passive diffusion of another solute. Though I cannot say that I know for sure, it seems improbable that the 50% reduction in O2 uptake by fish would be a result of an increased level of CO2. UNLESS there is a direct correlation between increasing CO2 levels and a simultaneous decrease in O2 levels; but my understanding is that does not happen.

This leads me to suspect that any negative effect of CO2 on the uptake of O2 must happen not at the diffusion site (gills), but in some other part of a fish's physiology; or that the increasing CO2 is indeed decreasing dissolved O2 in some way.

Another possibility could be that it is not the CO2, but rather the decreasing pH that is causing a reduction in dissolved O2 gas.

I will get a student researching this right away.... I know of three or four who need some extra credit... :twisted:

 

[tjudy]

My students are hard at work answering this question. We have had an interesting resource uncovered:

Paul Maslin, Fish Respiration Last Updated February 22, 2000
http://www.csuchico.edu/~pmaslin/ichthy/fshrsp.html

Paul Maslin has written an interesting piece on the dissolved oxygen in aquatic ecosystems. He has a section specifically on the effects of pH. Apparently, a decrease in pH negatively affects the amount of DO water can hold directly. He also describes how some fish can be very susceptible to asphixiation in low pH, because of their physiology.

The respiration of oxygen and carbon dioxide is based upon partial pressures of the gases in the water and in the bloodstream. Apparently, fish hemoglobin will not load completely with oxygen at lower pH. This means that the fish is in double jeopardy: low DO to begin with and a reduced ability to absorb it.

I do not think that the question is completely answered yet. Obviously, different types of fish are affected by pH differently. Also, the level of DO has to get low enough to have a detrimental effect, and a fish's gills do not remove the majority of oxygen in the water anyway; so there is likely a lot of room for reduced DO levels.

The research continues.... :wink:

 

[aspen]

great post ted! i'm working on it, check out the thread again.

rick

 

[tjudy]

:?

Here is an ominous quote regarding the effect of pH levels on US streams:

6.0 -6.5 Freshwater shrimp absent. Unlikely to be directly harmful to fish unless free carbon dioxide is high (in excess of 100 ppm).

The site does not actually say what will happen to fish, but it at least gives a CO2 number (100ppm).

Here is the site:
www.epa.gov/region4/water/drinkingwater/science9-12.htm

I doubt that it really has any significance to our application, but it indicates that there is a link between low pH, excess CO2 and fish health.

 

[aspen]

ted, could you edit that link please? it comes back as an error, and i can't find the document that you are referring to.

rick

 

[tjudy]

try it now...

 

[aspen]

thanks ted, that seems like a really good article, esp to give us an idea of water quality, and how our tanks should be kept, and that COULD be wrong when we have water problems. but...

there are some questions that i have though:

i have heard that discus have been found in water that has a ph as low as upper 2's and low 3's. this would lead me to believe that the indicators that they are using are incorrect, unless this would only apply to north american conditions and fauna.

a co2 level of 100 ppm?? first, i'm not sure that it is possible to keep levels in water that high, without a thick level of scum on the top of a rotting swamp. (i can't think of another case, really.)

and fish will start to die at that level? well, imo, co2 levels above 30 ppm are lethal to fish. now they do stipulate 'free co2' as opposed to maybe dissolved co2? any thoughts on that?

rick

 

[tjudy]

That article is really all about water quality in North American ponds. THe fish involved are not use dto low pH. I do not think the article is all that useful to us on the whole, but the part where it detriment to fish in high CO2 is aclue. 100ppm CO2 is possible in ponds undergoing eutrification, especially at night when the plants stop taking it in.

I spent my summers on a farm that had pigs. Water ran off from the pig pens into a pond. The pond would be covered with duckweed all Summer. The other plants would all die and rot. I would test that water's pH on a regular basis (ok.. I'll admit that I was a weird kid), and the pH swing from day to night was pretty incredible. 7 during mid-day and 5 at night. The only plants were 'scum' algae and duckweed. I pulled rakes all the way across the pond to find other plants... none. I assume, at this point 23 years later, that the swing had to be from CO2.

No fish in that pond at all, by the way... plenty of frogs though.