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121
01/07/2014, 05:50 AM
I would like to test for oxygen under shallow sand over different periods of time.

The sand will be in clear pots (each with a 7cm diameter), with each containing various depths of sugar sized sand (4, 3, 2 and 1cm deep).

I found that 1g of sugar sized sand can hold about 0.25ml of water. If I tried to use a syringe to take a 1ml water sample from the bottom of the pots, it would pull in oxygen rich water and kill the anaerobic bacteria. Is it best to buy a probe? What's the best technique for such shallow depths?

disc1
01/07/2014, 06:54 AM
I think most of the bacteria you're looking for are facultative anaerobes. They don't die from contact with oxygen. But there are certainly probes out there to measure DO if that's what you want to do.

121
01/07/2014, 12:34 PM
I thought about probes, but the probe sensor would get a film on it quite fast if buried under the sand without flow. If I pull the probe to clean it, it would oxygenate the bottom and kill the test.
Taking a sample with a test kit syringe (with the long pointy attachment) would work but surely that action would also oxygenate the sand by pulling oxygenated water into such shallow sand samples.

I think by dividing my sandbed into stacked trays (as apposed to one bed) I could increase denitrification x5 with 3-4cm depths. If I could go as low as 1cm per level, I figure I could increase it by x10+. The only way to know is to test to see if the bottom of each depth is deprived of oxygen. I'm stating the obvious, but once the oxygen is depleted, the anaerobes will use NO3 instead. That's why I want to test for dissolved oxygen.

I want to see just how shallow I can go.

disc1
01/07/2014, 02:16 PM
I think you're going around by Laura's house to find out about denitrification. Just the absence of oxygen alone does not create denitrification. I think you have a huge misconception between something that CAN happen and something that WILL happen. How do you even know you've got the right bacteria in these trays? Are you seeding them with something that is known?

A MUCH better test would be to measure nitrate over time in a controlled environment with and without the little trays of sand. You can set them up in seawater and add a known amount of nitrate and see if it goes down. But just simply showing that you've created an anoxic environment does not imply that you will have any specific bacterial activity.

121
01/07/2014, 02:54 PM
I think you're going around by Laura's house to find out about denitrification. Just the absence of oxygen alone does not create denitrification. I think you have a huge misconception between something that CAN happen and something that WILL happen. How do you even know you've got the right bacteria in these trays? Are you seeding them with something that is known?

That should be stage two. The first thing is to find a minimum depth with sugar sized sand that will accommodate denitrification. Flow will affect the depth. Mesocosm has stated how there are millions of bacteria stains present in our aquariums. A tray could be inoculated with the right strain if it came to that. My main objective is to find how shallow I can go for denitrification first.

A MUCH better test would be to measure nitrate over time in a controlled environment with and without the little trays of sand. You can set them up in seawater and add a known amount of nitrate and see if it goes down. But just simply showing that you've created an anoxic environment does not imply that you will have any specific bacterial activity.

My NO3 is 50ppm atm. I have anoxic environments but I can increase my surface area to the max if I know how shallow I can go. If I go too shallow then it will be in vain due to the sand bodies being over oxygenated (and not using NO3). If I know the bottom of a said shallow sample is deprived of DO then I can go ahead with that depth and increase the surface area.

The test is to maximize NNR (natural nitrate reduction) without using water changes to specifically reduce NO3.

disc1
01/07/2014, 03:22 PM
If all you want to know is what the oxygen level is at the bottom of the tray then just sample it and don't worry about what you will kill or not kill. If the first stage is just to find out how deep oxygen goes then you only need to talk about oxygen at that point.

If the goal is to talk about denitrification, then talk about denitrification. Just finding a lack of oxygen doesn't mean that there will be any denitrification.

bertoni
01/07/2014, 03:35 PM
Measuring oxygen under the sand is going to be tedious. I tried it for a class I took once. It's hard to know how much of your sample actually is interstitial water and how much has been sucked out of the water column, but I was definitely getting a significant amount of interstitial water with a syringe. A very slow draw is best.

121
01/08/2014, 07:42 AM
I think you're going around by Laura's house to find out about denitrification. Just the absence of oxygen alone does not create denitrification. I think you have a huge misconception between something that CAN happen and something that WILL happen.

I was think about this comment. Why bother making oxygen free zones then? The absence of oxygen is the only way to create natural denitrification.

brandon429
01/08/2014, 07:57 AM
rob toonen did this exact experiment in about 2003 if I recall, it can be google found for sure. ten ten gallon tanks doing dsb comparisons. I dont recall the outcome but my own sandbed is pretty cool it has massive gas bubbling that can be easily seen. I dont know what the nitrate is I havent seen that param tested since about 2003.

disc1
01/08/2014, 08:00 AM
I was think about this comment. Why bother making oxygen free zones then? The absence of oxygen is the only way to create natural denitrification.

I think you misunderstand. You do need an anoxic environment to carry out this type of denitrification, but just having a low oxygen environment alone does not mean that it's going to happen.

For example, think about the gas in your car. You have to have gas in your car in order for it to run, but just looking at the gas gauge and seeing that the tank is full does not mean that the car is going to start. You also need to have a good battery and the engine needs to be in running condition.

If you have an anoxic zone but don't have the other pieces needed for denitrification then all you have done is create a low oxygen area. If you want to know if denitrification is happening, then you need to look for denitrification.

121
01/08/2014, 08:10 AM
What I failed to mention in my OP was that the pots will be in my sump so the conditions won't be sterile to begin with.

What other pieces are you suggesting that are needed for denitrification apart from DO limited zones?

disc1
01/08/2014, 08:42 AM
What I failed to mention in my OP was that the pots will be in my sump so the conditions won't be sterile to begin with.

What other pieces are you suggesting that are needed for denitrification apart from DO limited zones?

Well, the right bacteria have to be living there for a start. There has to be enough diffusion for them to have a steady stream of nitrate. There are other factors. The point is that there is more to it than just lack of oxygen.

dkeller_nc
01/08/2014, 11:18 AM
Back to the original question - you can get micro DO probes that have a very fast response time and might be suitable for your experiment. I'm not sure what they would cost, but here's one example from a german company:

http://www.presens.de/products/brochures/category/sensor-probes/brochure/oxygen-microsensors.html

I should also mention that getting accurate DO values by syringe sampling is very problematic unless one has a gas chromatograph or an infrared analyzer. The problem is that with a traditional DO probe that looks like a pH sensor, one has to put the syringe sample into a cup or other container, then place the probe into it. Gas exchange in a small volume like this is incredibly rapid - on the order of a few seconds. So exact technique will heavily influence the data.