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Sediment Oxygen Demand in Shrimp Ponds
Ajitsinha (email@example.com): Can pond probiotics reduce sediment oxygen demand?
Dallas Weaver (firstname.lastname@example.org), a PhD aquaculture consultant that specializes in water chemistry, feeds and closed systems, answers that question and many more:
In general, no. Unless the probiotic decomposes the organic waste before incorporation into the sediment, it won’t impact the flux of carbon into the sediment. If it does decompose the organics before it gets into the sediment, it uses oxygen from the water column. You don’t win in terms of oxygen levels in the water.
Ajitsinha (email@example.com): Please explain the “flux of carbon into the sediment”. Also, will the addition of molasses shift some nitrogen mineralization from the sediment into the water column?
Dallas Weaver (firstname.lastname@example.org): Organic material (referred to as carbon) tends to move into the soil by a normal mixing process called “bioturbation”. It is this organic material input that provides the source of oxygen demand in the soils and when there is too much organic material relative to the rate that oxygen can diffuse into the soil, this organic material can provide the carbon for nitrate destruction. When the nitrate/nitrite is gone, sulfate will be reduced resulting in hydrogen sulfide formation.
Adding molasses stimulates bacterial growth in the water column, and the bacteria support an ecology of organisms that convert the ammonia into protein. The molasses provides the energy for this food chain just like sunlight provides the energy for the algae growth (which also converts ammonia, nitrate, nitrite into protein). Hopefully some of this protein then flows through the feed chain to your shrimp. If anything, adding molasses to the water will decrease the rate of bacterial mineralization of ammonia to nitrite and nitrate.
Part of the impact of molasses is the growth of nonpathogenic bacteria which support a whole food chain of bacteria eating organisms. Many of these organisms that are feeding on nonpathogens will also eat the potential shrimp pathogens, which is a good outcome.
Patricio Bucheli (email@example.com): Dr. Weaver, your explanation on how molasses works as an “energy source” is very clear. Regarding that fact, do you know how we can measure or define the amount of molasses to use?
Dallas Weaver (firstname.lastname@example.org): I have produced protozoans, rotifers and other small live food items using molasses and other carbohydrates as the energy sources. I needed very high productivity per unit volume. Photosynthesis is limited to about 10 grams of carbon fixation per square meter per day. I needed about 100 to 300 grams. I was producing 3 to 5 kilograms of live rotifers a day. In these systems, I used a mix of dry yeast and fish meal as feed for the organisms and added carbohydrate (molasses, sugar, dextrose, lactose, acetate, fine cellulose or cornstarch as the energy source) to drive the microbial ecology, along with some trace nutrients (vitamins and minerals) and lipids. These systems were heavy oxygen burners (some were equipped with pure oxygen) and required a lot of aeration per unit volume. With the large amount of carbon being used in the system, the pH tended to be low (down around 7.0, I also had active pH controllers on the system, both high and low) and the carbon dioxide was fairly high. With the low pH, I could let the ammonia run at about 1 to 2 parts per million and adjusted the carbon/nitrogen (C/N) ratio of the feed/carbohydrate mix to control the ammonia. I use a computer program to do all the C/N calculations and then provide the operators of the systems with information on changes in the ammonia readings and their rate of the change.
In a pond situation where much of your energy is from sunlight and your microbial ecology doesn’t flow as directly into your target organism, the problem is a little more difficult. I would be tempted to use a small amount of molasses for the probiotic impact and not add a lot more except when the ammonia or nitrite approach detrimental levels at your pH (> 20 µg/l as N, for unionized ammonia). You want to help the growth of nitrifying bacteria and elevate the nitrate levels in the system (this will help oxidize the organics in the sediments and prevent hydrogen sulfide formation) by maintaining a total ammonia nitrogen (TAN) level greater than 0.3 ppm total ammonia nitrogen, but less than 3 ppm, depending on the pH. I would also do molasses feeding in the morning to help minimize the diurnal pH swing in the pond.
I hope this helps a bit in understanding the dynamics of these systems. You have to think through all the details to keep this complex microbial ecology from crashing.
D. Ramraj (email@example.com): The low and medium density shrimp ponds in the tropics are typically photoautotrophic systems with high diurnal fluctuations in pH, alkalinity and dissolved oxygen. How effective will the addition of a carbon source be in stimulating the heterotrophic bacteria in lower density ponds? How will it affect the use of probiotics?
While the water column tends to be photoautotrophic, pond sediments become anaerobic a short time after stocking. Given the size of the ponds and the aeration methods it may not be possible to reverse this condition until harvest. Even if the column shows acceptable dissolved oxygen (DO) levels, the shrimp (which are benthic) would probably be stressed due to high sediment DO demand.
Is there a way to reduce the sediment oxygen demand? Will older ponds (after several crops) tend to have more oxygen demand in their sediments? Will probiotics be effective in reducing the organic load and sediment DO demand if they are used in pond sediment prior to stocking?
We have noticed improved production, growth rates and carrying capacities in ponds that were cultured after lying fallow for long periods. Can this be due to lowering of sediment organic load and the sediment DO demand?
Dallas Weaver (firstname.lastname@example.org): One of the more harmful impacts of an anaerobic bottom is the potential for generating hydrogen sulfide. If you can keep the nitrates higher, this will help suppress the sulfide forming bacteria and have some impact on the oxygen consumption rate of the pond bottom. The nitrate oxidizes some of the organic carbon in the bottom.
To get higher nitrates in the water column you need to either add nitrate as NaNO3, a calcium form, and/or get more nitrification in the water column. To achieve the latter, you have to decrease the water exchange to almost zero, if you want some of the nitrification to occur in the water column. Nitrifying bacteria are slow growing and water exchanges of greater than 7-10% a day can eliminate water column nitrification activity.
When the nitrate concentration is higher than the oxygen concentration, the nitrate will diffuse into the sediments and bacteria will use it as an electron acceptor to oxidize the organic material in the sediments.
Organic material tends to build up in the pond bottoms at high feed rates. Fallowing, draining and then plowing the pond can really oxidize a lot of material fairly fast. You have to get the pond bottom dry enough to plow—so that the soil has air spaces between the particles—but not so dry that the soil is lifeless. To get oxygen into the pond bottom, it’s all about moisture and texture control.
Given that decomposing bacteria are everywhere, the chances of your ponds being free of decomposing bacteria are very small. Probiotics probably won’t have any significant impact on sediment oxygen in used ponds.
I have often speculated about, but have never tested, the concept of creating a small, deep, shaded, aerated pond to grow a good culture of nitrifying bacteria by feeding the pond ammonia or urea and maintaining a concentration of 1 to 3 ppm TAN. Towards the start of a new crop, you could add some of this water to the pond along with some ammonia to feed the bacteria. This would give nitrification a head start. One of the problems with a pond is that for the first six weeks, or so, you may have very little ammonia in the water as the algae growth keeps up with the production from the feed and shrimp (assuming the pond is starting with new water). Towards the end of the cycle, the ecology is starting to recycle the ammonia as the algae reaches its maximum density and you may start seeing ammonia and the nitrifying bacteria, but a little too late. With a doubling time measured in several days to a week, without a large number initially, the nitrifying bacteria numbers won’t increase fast enough to keep up with the production of ammonia.
D. Ramraj (email@example.com): Nitrate levels tend to be lower than DO levels in ponds with phytoplankton blooms and frequent water exchanges. Most nitrate compounds will dissolve in the water column and may not be available to the sediment. The challenge is to deliver the nitrate into the anaerobic sediments. Is there a nitrate compound that can settle to the bottom and release nitrate slowly?
Dallas Weaver (firstname.lastname@example.org): There is no way to build up nitrate with a lot of water exchange. With slow growing bacteria, the exchange rate is often the controlling factor.
Having the nitrate in the water column is fine, it will diffuse into the sediments. If the concentrations are higher than the DO, the mass flow into the sediments can be faster than the oxygen. In a way, the nitrification of ammonia stores oxygen from the water column in the form of nitrate, which then transports the oxygen into the sediments and oxidizes the organic carbon in the sediments. This allows you to have a higher carbon flux into the sediments without hydrogen sulfide formation.
Ajitsinha (email@example.com): Assuming that we are able to manage the nitrate level in water column, what is its desirable level?
Dallas Weaver (firstname.lastname@example.org): It depends on what you are growing. With shrimp, the nitrate toxicity is very low—on the data that I have seen. In recycle systems operating at 50+ ppm, I had no nitrate problem with Penaeus vannamei. That range would give a lot more oxidizing power to the sediments.
Ajitsinha (email@example.com): In our shrimp ponds the nitrate levels are normally less than 0.1 ppm. The secchi disc readings are less than 20 cm. Algae is abundant and appears to be consuming the nitrate. Under these conditions what can I do to raise the level of nitrate?
Dallas Weaver (firstname.lastname@example.org): You are in a difficult position.
What’s your ammonia reading? Do you have hydrogen sulfide in the sediments? What is your morning and afternoon pH? Also what is your equilibrium pH (take a small sample and aerate it for at least two hours, until the pH stabilizes)? What is your measured alkalinity?
Ajitsinha (email@example.com): We do not monitor hydrogen sulfide. One pond water sample had a pH of 8.8. It stabilized at 8.5 after aeration. Below you will find summarized values (minimum/maximum/average) for some of the parameters in our 11 ponds over a period of over 150 days.
Alkalinity = 70 to 110 ppm (as CaCO3)
Ammonia = 0.35 to 0.05 ppm
Nitrite = 0.18 to 0.05 ppm
Nitrate = 0.08 to 0.015 ppm
Total Phosphorus = 0.48 to 0.35 ppm
pH = 7.5 (at 7 a.m.) to 9.2 (average 8.00)
Diurnal pH Variation = 1.0 to 0.30
Secchi Disc Reading = 10 to 40 cm (average 20)
Source: The Shrimp List (a mailing list for shrimp farmers, “firstname.lastname@example.org”). Subject: [shrimp] Sediment Oxygen Demand. January 2–13, 2008.
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