Nitrate Accumulation in Aquaculture –

The Problem and Potential Solutions Andrew J. Ray, Ph.D., Assistant Professor of Aquaculture Production

Aquaculture Division, Kentucky State University Land Grant Program

Recirculating aquaculture systems (RAS) are growing in popularity because of some key features including enhanced biosecurity, high levels of control, the ability to grow marine animals away from the coast, and the ability to grow warm-water species indoors practically anywhere. The term recirculating refers to the fact that very little water is exchanged, less than 1% per day, although many managers exchange far less than this. System types vary, but include clear-water RAS which have external biofilters that contain a microbial community on plastic biomedia, and biofloc systems which have a dense microbial community suspended in the water column. Either approach, or hybrids between the two, rely on microbes to detoxify ammonia.

Animals break down proteins in the feeds, about 20-25% accumulates in the animal tissues and the remainder is excreted into the water as ammonium and organic nitrogen. Ammonium and ammonia exist in equilibrium in water; higher pH, higher temperature, and lower salinity all favor the ammonia compound which is highly toxic. To remove ammonia, the microbial process of nitrification is usually encouraged in RAS. Nitrification is performed by microbes which adhere to the surfaces of tanks, pipes, plastic biomedia, and biofloc particles. The process of nitrification is a set of two oxidation reactions, where microbes get energy from the oxidation of ammonia to nitrite, and then nitrite to nitrate. These two steps are performed by two different groups of microbes, each of which has to establish a population in the system before it can be effective. Once the process is established, it is usually reliable unless some other problem occurs, such as over-feeding or animal mortality.

The end product of nitrification is nitrate, and as feed is continually added to RAS, nitrate continues to accumulate. Nitrate is much less toxic than either ammonia or nitrite; however, high concentrations reduce animal growth and can decrease survival. For example, concentrations over about 400 mg NO3-N/L have been shown to depress the growth rate and survival of marine shrimp (Litopenaeus vannamei). In intensive biofloc

aquaculture systems, concentrations of 400 mg NO3-N/L have been observed after using water for roughly three shrimp culture cycles. Salinity has an effect on the toxicity of nitrate, as it does with ammonia and nitrite. At higher salinities, these compounds are less toxic to aquatic animals.

Several options exist for dealing with nitrate, including water exchange, phytoremediation, denitrification, and heterotrophic assimilation. Water exchange can be a practical solution if the water used is inexpensive. The amount of water exchange needed depends on animal stocking density, feed rate, and nitrification efficiency, among other factors. However, approximately 7-10% exchange per month will maintain a dilute nitrate concentration in most RAS. In cases where water is expensive, such as in systems that use artificial sea salt, water exchange may not be a cost-effective solution.

Figure 1. A recirculating biofloc aquaculture system used to produce marine shrimp at KSU. After roughly three crops of shrimp in this system, nitrate concentration can become toxic to shrimp.

Many plants do well taking up nitrate from an aquatic medium. The practice of aquaponics has become very popular recently. Fish, most notably tilapia, are grown in a tank and water is typically pumped through filters to take out solid waste and perform some level of nitrification. The water then travels through containers where it makes contact with the roots of traditionally terrestrial plants. With the proper ratio of fish to plants, nitrate can be significantly reduced or eliminated by the plants. Some of the issues with aquaponics include high cost of construction and operation relative to terrestrial plant production, low fish production compared to plant production, and there are few examples of brackish or salt water aquaponics. Brackish-water plants are being evaluated at the KSU Aquaculture Research Center so they may be used for phytoremediation of saline water. These plants include saltwort (Salicornia sp.), Cordgrass (Spartina sp.), and Scurvygrass (Cochlearia sp.).

Figure 2. An example of an integrated aquaponics approach at KSU, where cordgrass is grown directly in a tank with marine shrimp. An LED light fixture illuminates the plants in a purple color. The plants are floating on a polystyrene sheet, and black plastic shields the water from the light so that algae does not grow.

Figure 3. An example of de-coupled aquaponics at KSU where cordgrass is being grown in troughs using water from a biofloc aquaculture system with high nitrate levels.

Another way to use phytoremediation is to perform de-coupled aquaponics. In this case the fish or shrimp are separated from the plants, they are not grown in the same system, which reduces the need for pumping and may reduce problems associated with differences in plant and animal tolerances. For instance, cool-weather plants can be grown with warm-water fish or shrimp effluent water. Water can be used in RAS, and when the nitrate concentration becomes elevated, the water can be moved to isolated plant grow beds. Such beds may be located in greenhouses for seasonal plant production. In coupled or de-coupled aquaponics, plant beds typically need to be aerated so the plant roots don’t create anoxic zones. Another concern is root fouling where a thick biofilm develops on the roots and may suffocate the plants. A small number of animals, such as shrimp or tilapia, can be placed in the plant growing bed to help clean the roots.

In addition to vascular plants, algae can also be used to take up nitrate. In aquaculture systems exposed to light, microalgae can play an important role in nitrate removal. These microscopic organisms take up nitrate directly into their cells, then to ultimately remove nitrogen, the algae must also be removed. This can be accomplished by using settling basins or filter-feeding fish, among other strategies. Macro-algae such as Ulva sp. have been used in RAS as a means of removing nitrate as well. These submerged seaweeds must also be removed periodically from the system to take out nitrogen.

The process of denitrification can be thought of as nitrification in reverse, with the final product being nitrogen gas that is then released to the atmosphere. Denitrification is a series of reduction reactions performed by microbes in an anoxic environment. Anoxic refers to very low oxygen concentrations (below about 0.5 mg/L); anoxic environments can be found in aquaculture systems in areas such as settling basins and pond bottoms. In these areas solids settle out and decompose which consumes oxygen and the process of denitrification is documented to occur in both areas. In denitrification filters, oxygen is sometimes stripped from incoming water, this can be achieved by diffusing nitrogen gas through the water. Like nitrifying bacteria, denitrifying microbes require substrate. This can be provided by the sediment in settling basins and pond bottoms, or can be created with plastic bio-media as in nitrifying biofilters.

The process of denitrification is slower than nitrification and therefore requires larger filters. Care must be taken when facilitating an anoxic environment, as ammonia can be created and more concerning, sulfide can be produced. Sulfide is extremely toxic to aquatic animals, a very small amount can quickly kill fish and shrimp. Usually sulfide has a noticeable aroma of “rotten eggs.” De-coupled denitrification should be considered, where water is treated separately from the animals in case toxic compounds are generated.

Heterotrophic assimilation is another process by which nitrate concentration may be reduced. Heterotrophic microbes have been well documented in their ability to take up ammonia; however, less attention has been given to the fact that some groups can also remove nitrate from water. Adding sugar to the water of RAS can activate heterotrophic microbes. Some managers do this constantly, throughout the animal growth phase, and try to maintain a C:N ratio above about 12:1. This ratio is calculated using the carbon and nitrogen content of feed and the carbon content of sugar. If this high C:N can be maintained then nitrate will not generally accumulate at all. Rather, ammonia is diverted directly into microbial biomass.

The drawbacks to heterotrophic assimilation include excess solids generation, the extra cost of adding sugar, and reduced dissolved oxygen. The process is aerobic, meaning that DO concentration must be maintained at reasonable concentrations (above about 4 mg/L); however, by adding sugar the microbes can rapidly consume oxygen. Again though, by de-coupling the process from the aquaculture system and treating water with high nitrate concentrations separately, managers can prevent harming the animals. Water can be aerated in a container and sugar added to lower nitrate concentration. Solids will need to be removed though.

In conclusion, nitrate can be a limiting factor in the continual reuse of water for RAS. There are several distinct strategies that can be used to help remediate nitrate concentration; each has benefits and drawbacks. System managers should consider these carefully, and in many cases, the solution to nitrate accumulation may prove to be a combination of strategies put together in a cost-effective manner.

Funding for the aquaculture research at KSU discussed in this article has been provided by the USDA’s National Institute of Food and Agriculture and by the Kentucky Agricultural Development Fund.