SRAC Publication No. 468

Southern Regional Aquaculture Center

VI April 1996 PR

Carbon Dioxide in Fish Ponds

John Hargreaves and Martin Brunson*

The primary sources of highest when dissolved carbon dioxide in fish oxygen concentrations ponds are derived from are lowest. Thus, dawn respiration by fish and is a critical time for the microscopic plants evaluating pond water and animals that com- quality from the stand- prise the fish pond point of both dissolved biota. Decomposition of oxygen and, to a lesser organic matter is also a extent, carbon dioxide. major source of carbon In addition, there is dioxide in fish ponds. some evidence to sug- While producers are gest that the toxicity of rightly concerned with carbon dioxide is maintaining adequate enhanced by low dis- concentrations of dis- A pond “breathes” solved oxygen concen- solved oxygen, knowl- in and out once a day trations. Fish are able edge of the “flip-side” to rid themselves of of the oxygen equation carbon dioxide through is also important. erally opposite that of dissolved the gills in response to a differ- oxygen (Figure 1). During the day, ence in carbon dioxide concentra- Fish ponds can be thought of as algae take up or “fix” carbon tion between fish blood and the “breathing” over a 24-hour peri- dioxide that is free in the water surrounding water. If environ- od. During the day, when the sun and carbon dioxide concentration mental carbon dioxide concentra- is shining brightly, oxygen is sup- is therefore lowest (often 0 mg/L) tions are high, the fish will have plied to the pond by photosynthe- during late afternoon, when dis- difficulty reducing internal carbon sis of algae and other aquatic solved oxygen is highest. During dioxide concentrations, resulting plants (the “inhale”). During the the night, the respiration of pond in accumulation in fish blood. night, photosynthesis ceases, and organisms produces carbon diox- This accumulation inhibits the the algae, sediment and fish con- ide, which accumulates to a maxi- ability of hemoglobin, the oxygen- sume oxygen (the “exhale”), pro- mum (usually around 10 to 15 carrying molecule in fish blood, to ducing the characteristic fluctuat- mg/L) at dawn. bind oxygen, and may cause the ing pattern of dissolved oxygen The problem with the potential fish to feel stress similar to suffo- concentration well known to fish cation. farmers. The daily pattern of car- toxicity of carbon dioxide can be bon dioxide concentration is gen- related to the daily fluctuating The density of the algae bloom pattern of dissolved oxygen and has an important effect on the carbon dioxide concentrations. magnitude of daily fluctuations of * Mississippi State University Carbon dioxide concentrations are oxygen and carbon dioxide. important interrelationships between carbon dioxide, pH and total alkalinity. Knowing pH and total alkalinity will allow the estimation of carbon dioxide. Esti- mation of carbon dioxide by pH measurements is plagued by diffi- culty in obtaining an accurate pH measurement. Litmus paper, drop counting test kits, and various probes with meters have been used with varying success. The selection of measurement devices for pH is largely a situation in which “you get what you pay for.” For example, pH pens are inaccurate, particularly if not cali- brated correctly, and do not com- pensate for changes in tempera- ture. Some scientific supply hous- Figure 1. The daily cycle of oxygen and carbon dioxide in a fish pond. es now sell narrow-range litmus paper which allows for low-cost, Oxygen and carbon dioxide con- water temperatures increase the rapid estimation of pH. centrations in ponds with a light metabolism of all pond organisms algae bloom will not fluctuate and therefore respiration rates are Graphical estimation very much between early morn- high. It is also a time of year when technique is easy ing and late afternoon, analogous feeding rates are high. The to “shallow breathing.” In ponds decomposition of wastes generat- A simple graphical technique for with a thick, dense algae bloom, ed by large quantities of organic estimating carbon dioxide concen- fluctuations are more extreme, matter added to fish ponds in the tration is presented in Figure 2. analogous to “deep breathing.” summer requires large quantities The first step is to determine the Carbon dioxide problems are of dissolved oxygen and produces total alkalinity of the pond water therefore more likely as the thick- large quantities of carbon dioxide. using a standard test kit. Next, ness of the bloom increases. During the summer, carbon diox- determine the pH from a water ide concentrations are lower than sample collected without splash- Carbon dioxide problems during winter, but dissolved oxy- ing or bubbles. Draw a straight most likely in summer gen concentrations are often criti- line up from the pH value to the cally low. Fortunately, summer is curved line representing the total Over an annual cycle, carbon also the time of year when ponds alkalinity value closest to that of dioxide concentrations are maxi- are aerated frequently. In addition the pond (Step 1). Now extend mum during winter and mini- to supplying critical dissolved another straight line to the left- mum during summer. However, oxygen, vigorous aeration will hand axis, indicating the free car- carbon dioxide is rarely a prob- drive off some proportion of the bon dioxide concentration (Step lem in winter because dissolved carbon dioxide produced in the 2). The straight line extending oxygen concentrations are usually pond. across from 20 mg/L represents a well above saturation levels. “critical” concentration, above Occasionally during the winter Measure pH to estimate which carbon dioxide may be a fish may appear to swim listlessly carbon dioxide problem. Therefore, using the near the surface as if they were chart, a set of “critical” pH values “under the influence,” possibly Carbon dioxide can be measured can be determined for ponds with due to elevated carbon dioxide directly with standard test kits. different total alkalinity (Table 1). levels. Such a condition may arise Alternatively, measurement of pH In general, water can hold more after a period of extremely calm can be used to estimate carbon carbon dioxide as temperature and cloudy weather, but quickly dioxide concentration because car- declines, although differences in passes once sunny or windy bon dioxide acts as an acid in temperature are less important weather returns. water. As carbon dioxide is than differences in total alkalinity added during the night, pH will Summer is the time of year when and thus, for practical purposes, decline. (Conversely, when carbon application of some kind of tem- carbon dioxide is most likely to dioxide is removed during the be a problem in fish ponds. Warm perature correction is not neces- day, pH will increase.) There are sary for estimation of carbon diox-

2 ide. Figure 4 on page 6 is a blank graph that can be photocopied and used on the farm. The graphic and the table indicate that carbon dioxide is more likely to be a problem as alkalinity increases. However, alkalinity provides “buffering capacity” to pH changes caused by carbon dioxide and pH is therefore unlikely to fall to such critical levels. The pH of most ponds at dawn is usually between 7.5-8. Carbon dioxide in ponds with low alkalinity (20 to 50 mg/L as CaCO3) may cause the pH to fall to the lower limits of the range for optimum fish growth and produc- tion. The potential for carbon dioxide problems can be evaluated by a simple, “quick-and-dirty” method. Collect a bucket of water and measure the pH. Put an air- stone in the bucket and run air through the water for about 30 minutes. If the pH increases by more than one pH unit, then car- bon dioxide may be a problem.

Carbon dioxide is an unusual problem in fish ponds In general terms, carbon dioxide is rarely a cause for concern in fish ponds with sufficient alkalini- ty. There are a few specific circum- stances or scenarios in which car- bon dioxide may be a problem, such as the period following the crash of an algae bloom or the application of an algicide, such as copper sulfate. Large quantities of organic material derived from dead plankton are quickly decom- posed, reducing oxygen and Figure 2. A graphical technique for estimation of carbon dioxide concentration. increasing carbon dioxide. Again, emergency aeration practices Table 1. Critical pH values at a given level of alkalinity. serve the dual role of supplying oxygen and reducing carbon diox- Total alkalinity (mg/L as CaCO3) Carbon dioxide may be a problem ide. at a pH value less than: Under certain circumstances, car- 50 6.7 bon dioxide can be a problem in 75 6.9 ponds deeper than 4 or 5 feet, 100 7.0 such as in so-called combined 125 7.1 watershed/levee ponds. Deep ponds may “stratify” or develop 150 7.2 layers of relatively lighter,

3 warmer, oxygen-rich water over- Chemical treatment is depth of 4 feet and a carbon diox- laying layers of relatively more a temporary solution ide concentration of 20 mg/L is dense, cooler, stagnant (and car- 3.45 x 20 x 10 x 4 = 2,760 pounds bon dioxide rich!) water. In ponds Carbon dioxide can be removed or approximately 1.4 tons. Treat- that have not been aerated or by chemical treatment of pond ment of the same pond with sodi- mixed for several weeks during water with liming agents such as um carbonate would require 5,184 warm and relatively calm weath- quicklime, hydrated lime or sodi- pounds or 2.6 tons. Clearly, large er, strong sustained winds or vig- um carbonate (Table 2). These lim- quantities of liming materials are orous aeration can cause ponds to ing agents chemically react direct- required to chemically treat a car- “roll over” and mix deep water ly with carbon dioxide, resulting bon dioxide problem. with surface water, thereby in reduced carbon dioxide and increasing carbon dioxide concen- increased alkalinity and pH. At best, treatment with liming tration throughout the water col- Agricultural lime will not chemi- agents represents a temporary umn. During the summer, when cally remove carbon dioxide from solution. Once carbon dioxide is carbon dioxide is most likely to be pond waters. consumed by reaction with lim- a problem, ponds are typically ing agents, additional carbon aerated through the night. In order to calculate the amount dioxide may accumulate because Although deep ponds may stratify of a particular liming agent to treatment of ponds with liming and destratify daily, water cur- apply to a pond, the following agents does not address the root rents established by aeration and generalized formula can be used. cause of a presumed carbon diox- wind blowing over the water sur- Alternatively, consult Figure 3 for ide problem. In ponds receiving face usually keep the water col- a quick, graphical estimation of feed at very high rates (>100 umn well-mixed and, as a result, quicklime treatment requirements lbs/acre per day) or in which carbon dioxide problems rarely for a given pond size. rapid decomposition occurs fol- occur. lowing an algae crash, treatment factor from table with a liming agent does not Carbon dioxide may accumulate x carbon dioxide concentration affect the rate of carbon dioxide when fish are held at high density, (mg/L) production and thus represents a such as in live-cars, hauling tanks x pond area (acres) temporary, “band-aid” solution. or crowded in front of aerators x average depth (ft) during low oxygen episodes. Even Perhaps a more serious conse- = pounds of liming agent to though carbon dioxide levels may quence of chemical treatment of add rise dramatically, the problem can carbon dioxide problems is relat- usually be alleviated by aeration, ed to pH, which may exceed 10 in which adds oxygen while driving For example, the amount of poorly buffered (low alkalinity) off some carbon dioxide. hydrated lime required to treat a waters following treatment with 10-acre pond with an average certain liming agents (such as quicklime and hydrated lime). High pH causes a shift towards a greater proportion of the more Table 2. Liming agents able to remove carbon dioxide from pond toxic form of ammonia. Conse- waters. quently, a well intended applica- tion of certain liming agents to Liming Agent Chemical Factor Comments “treat” what is thought to be a formula carbon dioxide problem can result Quicklime CaO 3.45 -caustic (protect skin and eyes) in a very stressful environment -potential for high pH for fish. -relatively low solubility The bottom line Hydrated lime Ca(OH)2 4.57 -caustic (protect skin and eyes) -potential for high pH Application of chemicals to treat -relatively low solubility a carbon dioxide “problem” is -relatively inexpensive likely to be of limited, temporary benefit. Aeration and mixing are Sodium carbonate Na2CO3 6.48 -safe the most effective available -low potential for high pH mechanical methods for the man- -relatively high solubility agement of carbon dioxide and -quick reaction with carbon dissolved oxygen. Vigorous aera- dioxide tion accelerates the diffusion of carbon dioxide out of water and

4 Figure 3. A graphical technique for estimation of quicklime required to neutralize carbon dioxide. mixing will prevent or minimize A clear determination of a carbon The toxicity of carbon dioxide the establishment of a carbon dioxide problem is required prior increases as dissolved oxygen dioxide-rich layer of water near to any treatment. If a carbon diox- concentration declines. Often, the the pond bottom. Maintaining a ide problem is suspected, other problem can be traced to some- moderate plankton density (Secchi water quality variables (particu- thing other than carbon dioxide. disk visibility between 6-12”) will larly dissolved oxygen and maximize the biological uptake of ammonia) should be evaluated carbon dioxide. before attempting any treatment.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from the United States Department of Agriculture.

5 Figure 4. Blank graph for use by pond managers.

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