Aquaponic systems have a variety of benefits when it comes to innovative and sustainable food production. In addition to the positive effects of increased CO2 concentration and reduced energy consumption, aquaponics offers two main advantages for nutrient cycling. Firstly, the combination of aquaculture and hydroponic systems prevents the release of nitrogen and phosphorus-rich effluents into already polluted groundwater. Secondly, it allows for the fertilization of crops with an organic solution instead of mineral-based fertilizers that deplete natural resources. Moreover, aquaponics can achieve comparable plant growth to conventional hydroponics, and even surpass soil-based production. This is likely due to increased CO2 concentrations and changes in the root zone. Furthermore, tomatoes grown aquaponically have been found to have equal or superior mineral content and nutritional quality compared to conventionally grown tomatoes.
Although aquaculture effluents offer the benefits of recycling and the use of organic fertilizers, utilizing them also presents challenges in monitoring nutrient levels in the solution. This is because it is more difficult to regulate the nutrient composition of a solution derived from the biological breakdown of organic matter compared to tracking the concentration of nutrients in a precisely measured hydroponic solution containing mineral compounds. Additionally, the nutritional requirements of plants change throughout their growth stages, and meeting these requirements is vital for achieving maximum yields.
Understanding the functioning of nutrient cycles in aquaponics is crucial for optimizing the recycling rates of phosphorus and nitrogen in order to produce plant biomass from aquaculture effluents. Factors such as fish species, fish density, water temperature, type of plants, and microbial community can all impact these recycling rates.
Origin of Nutrients
The primary sources of nutrients in an aquaponic system consist of the fish feed and the water added to the system, which includes magnesium, calcium, and sulfur. When it comes to fish feed, there are two main types: fishmeal-based and plant-based feed. Fishmeal has traditionally been used in aquaculture and relies on fish meal and fish oil for its lipids and proteins. However, concerns about the sustainability of such feed have been raised, leading to increased attention on plant-based diets. A meta-analysis conducted by Hua and Bureau (2012) found that incorporating plant proteins into fish feed in high proportions can affect fish growth. Plant proteins can impact feed digestibility and levels of anti-nutritional factors. In particular, phytates, which are a form of phosphorus found in plants, do not benefit certain fish species such as salmon, trout, and others. The impact of varying fish feed composition on crop yields is not well-understood.
The classical formulation of fish feed includes 6-8 main ingredients. It consists of 6-8% organic nitrogen, 1.2% organic phosphorus, and 40-45% organic carbon. For herbivorous or omnivorous fish, it contains approximately 25% protein, while for carnivorous fish, the protein content is around 55%. The lipids in the feed can be derived from either fish or plants.
After adding the fish feed to the system, a significant portion is consumed by the fish for growth and metabolism, or it is expelled as soluble and solid feces. The remaining feed decomposes in the tanks. Consequently, the uneaten feed and metabolic byproducts dissolve partly in the aquaponic water, allowing the plants to directly absorb nutrients from the aquaponic solution.
In the majority of cultivation systems, nutrients can be included to enhance the aquaponic solution and provide a more suitable balance for the plants. This holds true even when the system is connected, as iron or potassium (which are commonly deficient) can be added without causing any harm to the fish.
Fish Feed Leftovers and Fish Faeces
The fish should ideally consume all of the given feed, but sometimes a small portion (less than 5%) is not consumed and instead breaks down in the system. This adds to the nutrient load of the water, leading to a decrease in dissolved oxygen and an increase in carbon dioxide and ammonia, among other substances. The composition of the leftover fish feed is influenced by the composition of the feed itself.
The composition of fish feces is dependent on the fish’s diet, which in turn affects the water quality. Various factors such as fish species, feeding levels, feed composition, fish size, and system temperature influence nutrient retention in fish biomass. Higher temperatures increase fish metabolism, resulting in more nutrients being present in the solid portion of the feces. The excreted nutrient proportion is also affected by the diet’s quality and digestibility. To maintain a balanced system and maximize crop yields, careful consideration should be given to the digestibility of the fish feed, the size of the feces, and the settling ratio. While prioritizing fish needs, the feed components can also be chosen to meet the plant’s requirements if it doesn’t impact the fish.
Microbiological Processes
Solubilisation
The process of solubilization involves breaking down complex organic molecules found in fish waste and feed leftovers into nutrients in the form of ionic minerals that can be absorbed by plants. This breakdown is mainly carried out by heterotrophic bacteria, whose full identification is still ongoing. Research has begun to unravel the intricacies of these bacterial communities. In aquaculture, the most commonly observed bacteria include Rhizobium sp., Flavobacterium sp., Sphingobacterium sp., Comamonas sp., Acinetobacter sp., Aeromonas sp., and Pseudomonas sp. A notable example of bacteria’s importance in aquaponics is the conversion of insoluble phytates into phosphorus (P), which is made available for plants through the production of phytases, found particularly in γ-proteobacteria. Other nutrients, aside from phosphorus, can also become solids and be removed from the system as sludge. To reintroduce nutrients into the aquaponic system, efforts are being made to remineralize this sludge using UASB-EGSB reactors. Additionally, minerals are not released at the same rate, and their concentration in the aquaponic solution requires more complex monitoring, as it depends on the composition of the feed.
Nitrification
The primary nitrogen source in an aquaponic system is the fish feed, which contains proteins. It is preferable for the fish to consume 100% of this feed. However, it has been observed that fish only utilize around 30% of the nitrogen present in the feed. Part of the ingested feed is used for assimilation and metabolism, while the remainder is excreted through the gills or as urine and feces. The nitrogen excreted through the gills is primarily in the form of ammonia (NH3), while the urine and feces consist of organic nitrogen that is converted into ammonia by proteases and deaminases. Generally, fish excrete nitrogen as total ammonia nitrogen (TAN), which includes NH3 and NH4+. The balance between NH3 and NH4+ mainly relies on the pH and temperature. Ammonia resulting from the fish breaking down the proteins in the feed is the primary waste produced.
The process of nitrification occurs in two steps. First, the ammonia or ammonium excreted by fish is converted into nitrite and then into nitrate by specific bacteria that use aerobic chemosynthesis to produce energy. Adequate levels of dissolved oxygen are necessary for nitrification because it consumes oxygen. The first step is carried out by ammonia-oxidizing bacteria, including Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus, and Nitrosovibrio. The second step is conducted by nitrite-oxidizing bacteria, such as Nitrobacter, Nitrococcus, Nitrospira, and Nitrospina. Nitrospira is believed to be a complete nitrifier, meaning it is involved in the production of both nitrite and nitrate. These bacteria can be found in both aquaculture and aquaponic systems, primarily in biofilms attached to the media used in the biofilter. However, they can also be observed in other parts of the system.
Nitrification plays a crucial role in aquaponics because ammonia and nitrite are highly toxic to fish. Even a small amount of ammonia-nitrogen (0.02-0.07 mg/L) can cause harm to warm water fish, while nitrite-nitrogen should be kept below 1 mg/L to prevent oxygen fixation issues. Ammonia affects the fish’s central nervous system, whereas nitrite impacts their ability to fix oxygen. On the other hand, fish can tolerate nitrate-nitrogen levels of up to 150-300 mg/L.
When beginning a system, it is recommended to run it without fish so that the population of nitrifying bacteria can grow slowly and establish in the biofilters. To prevent the growth of highly competitive heterotrophic bacteria, it is important to minimize the presence of organic matter in the biofilters. Another option is to add commercially available mixes of nitrifying bacteria to the system before stocking, to speed up the colonization process. However, there are also small aquaponic systems that do not have biofilters. In these systems, nitrifying bacteria create biofilms on available surfaces, such as the walls of hydroponic compartments or inert media when the media bed technique is used.
Mass Balance: What Happens to Nutrients Once They Enter into the Aquaponic System?
Context
In order to optimize the management of aquaponic systems, it is important to understand the dynamics of nutrient cycles, which serve as the basis for their functioning. The growth of plants in hydroponic systems requires specific nutrient concentrations to be maintained throughout the various stages of growth. Therefore, close monitoring of nutrient concentrations is necessary in order to prevent deficiencies in the system water or through foliar application. Delaide et al. (2016) found that supplementing aquaponic solutions with mineral nutrients to match those found in hydroponics can result in higher yields. The first step in achieving a balanced system is to properly design and size the compartments. If the hydroponic compartment is too small compared to the fish tanks, nutrient accumulation in the water and potential toxicity issues may arise. The feed rate ratio, which is the amount of fish feed in the system relative to the plant-growing surface and type, is commonly used to initially size the system. However, Seawright et al. (1998) noted that it is not possible to achieve an optimal balance of plant needs by relying solely on fish feed as an input. Monitoring methods for maintaining system balance typically focus on the nitrogen cycle, but to ensure optimal functioning, it is necessary to closely monitor the balance of other macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Zn, B, Mn, Mo, Cu) as well. Recent studies, such as those conducted by Schmautz et al. (2015, 2016), have examined the impact of different hydroponic layouts on nutrient uptake in aquaponic tomatoes. Drip irrigation was found to produce slightly better yields, with the mineral content of the fruits (P, K, Ca, Mg) equivalent to conventional values. However, the leaves had lower levels of several nutrients compared to conventional agriculture. Delaide et al. (2016) observed that their aquaponic system lacked certain macronutrients and micronutrients, while others accumulated quickly. Graber and Junge (2009) found that their aquaponic solution had significantly lower levels of nitrogen, phosphorus, and potassium compared to hydroponics. However, they still achieved similar yields, although the quality was lower due to a potassium deficiency.
Factors Influencing the Nutrient Cycles
A plant’s nutrient uptake is influenced by factors such as light intensity, root zone temperature, air temperature, nutrient availability, growth stage, and growth rate. Research by Schmautz et al. (2016) and Lennard and Leonard (2006) has shown that the hydroponic method can also affect a plant’s nutrient uptake capacity. Therefore, it is important to choose a growing system that matches the type of vegetables being cultivated. NFT and DWC (deep water culture – raft) are appropriate for leafy greens, while drip irrigation on rockwool slabs is better suited for fruity vegetables.
Macronutrients
The function of these macronutrients within plants is outlined in the list below. There are six macronutrients that plants need in large amounts, which are Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulphur (S).
Primary and mobile nitrogen (N)
Nitrogen is a crucial nutrient that forms the foundation of all proteins and plays a vital role in various biological processes such as building structures, photosynthesis, cell growth, metabolic activities, and chlorophyll production. In the aquaponics ecosystem, nitrogen serves as an important indicator for other nutrients and is considered a key element. Generally, for optimal plant growth, nitrogen should be present in the form of nitrate, although plants can still utilize small amounts of ammonia and free amino acids. However, excessive nitrogen can lead to excessive plant growth, resulting in vulnerable, soft plants that are prone to diseases and insect damage. Additionally, excessive nitrogen can also impede the flowering and fruiting capabilities of plants.
Phosphorus (P) is both primary and mobile.
Phosphorus is a key nutrient that plays a crucial role in various processes such as photosynthesis, oil and sugar formation, and promotion of germination and root development in seedlings. Inadequate levels of phosphorus can result in reduced root development and hinder the absorption of other necessary nutrients and water.
Potassium (K) is both primary and mobile.
Potassium, which is also a primary nutrient, plays a crucial role in cell signaling by regulating ion flow through membranes to control stomatic opening. It is also involved in the development of flowers and fruits. To put it simply, potassium acts like “street light signals” that enable efficient functioning and growth. Additionally, potassium is essential for the production and transportation of sugars, water absorption, defense against diseases, and the ripening of fruits. Every plant requires potassium, and insufficient levels of this nutrient can adversely impact their growth.
To treat potassium deficiency in aquaponic plants, it is necessary due to the potential negative impacts it can have on photosynthesis, plant growth, and vulnerability to infection or infestation that may result in plant mortality.
Calcium (Ca) is both secondary and immobile.
Calcium plays a vital role in the robust development of plants and the maintenance of their firm cell walls, just as it does for our bones. Additionally, it assists in fortifying stems and aids in the generation of flowers, fruits, and vegetables. Examples of plants that are prone to lacking calcium include squash, tomatoes, and peppers.
To prevent damaging the harvestable fruits or vegetables, it is essential to address the calcium deficiency in your system. In addition, insufficient calcium levels can hinder plant growth and potentially result in plant death.
Secondary and mobile magnesium (Mg)
In aquaponics, low magnesium is frequently observed as a deficiency in plants. Magnesium significantly contributes to the internal functions of plants as it aids in the breakdown of chlorophyll processes. The treatment of magnesium deficiency is crucial for promoting the overall health and growth of plants.
Sulfur (S) is both secondary and immobile.
In the organism, sulfur plays a vital role in aiding the production of certain proteins, processing chlorophyll, and other photosynthetic enzymes. Occurrences of sulfur deficiencies are infrequent in aquaponics.
Micronutrients
Micronutrients such as iron (Fe), copper (Cu), zinc (Zn), manganese (Mn), boron (B), and molybdenum (Mo) are responsible for various deficiencies. Generally, yellowing of the leaves is a common symptom, while in the case of copper deficiency, the leaves can turn into a darker shade of green.
Iron (Fe) is not capable of moving.
Iron plays a crucial role in chloroplasts and electron transport chains, making it essential for proper photosynthesis. It can be easily determined whether there is an iron deficiency by using an Iron Checker to measure the iron levels in the water. Another way to identify iron deficiency is by observing if the plant’s leaves turn yellow while the veins remain green, a condition known as “choloris.” The addition of iron to the system should be in the form of “chelated iron,” with a suggested dosage of 5 ml per 1 m2 of grow bed. Although a large quantity of iron does not harm the system, it can lead to the tank and pipes becoming discolored.
Manganese (Mg) lacks mobility.
Manganese plays a significant role in the process of plants’ photosynthesis as it serves as a catalyst for water splitting. One can identify manganese deficiency in plants through symptoms such as decreased growth rates, a lackluster grey appearance, and yellowing between veins that remain green.
The element boron (B) is not capable of movement within an organism.
Boron plays a role in the structural polysaccharides and glycoproteins, as well as in carbohydrate transport and the regulation of certain plants’ metabolic pathways. Furthermore, boron is involved in cell reproduction and the uptake of water.
Zinc (Zn) is not able to move.
Enzymes use zinc, and zinc also plays a role in chlorophyll, impacting the overall size of plants.
Nutrient Sources for Aquaponics Ecosystems
Aquaponics relies on fish food as the primary source of plant nutrients. Nonetheless, plants require different nutrients than fish, and some nutrients from fish waste remain in a solid state that is not beneficial for plants. If the fish feed pellets do not contain all the necessary nutrients for plant growth but are sufficient for fish growth, adjustments to the system will be necessary. Fish do not require the same levels of iron, potassium, or calcium as plants in order to thrive, which may result in deficiencies of these nutrients. The following are the typical nutrient deficiencies found in Aquaponics.
Iron deficiency is a condition characterized by a lack of iron in the body.
Indications of a Lack of Iron:
- Yellowing color on plant leaves
- Spots on immature leaves
The following steps explain how to incorporate additional iron into your diet: 1. Iron supplementation:
To supplement iron in your aquaponics system, you need to add iron that the plants can absorb. This means using chelated iron. Adding chelated iron to your system will only be effective if your pH is 7.5 or lower. Your aim is 2 mg/liter, so you need to calculate your water tank’s size and add the required amount of iron every 3 – 4 weeks.
A condition of lacking potassium is referred to as potassium deficiency.
Symptoms of a lack of potassium:
- Older leaves of the plants show interveinal chlorosis and spots or scorching, which progresses to the younger leaves when the deficiency becomes more severe.
One way to enhance potassium intake:
- By Spraying – You can use potassium chloride and spray it into the plant leaves. You must repeat the process at least once a week to avoid potassium deficiency in your aquaponics plants.
- Adding a potassium supplement to your fish food through kelp meal concentrate. Other options are adding potassium sulfate or potassium hydroxide to your fish food.
Step by step, think about how to rephrase the text without changing its meaning: Insufficient levels of calcium in the body.
Symptoms indicating a lack of calcium:
- Black, dead areas of the young plant tissue, known as necrosis.
- Slight chlorosis to brown or black scorching on new leaf tips.
- Fresh leaves are distorted with hooked tips and irregular shapes.
Here are the steps to follow for supplementing calcium:
- One way to supplement calcium deficiency in aquaponics is to use hydrated (or agricultural) lime, which will also supplement calcium and magnesium besides raising the pH levels.
- Another way is by spraying calcium chloride mixed with some water on your plants. The ratio should be four teaspoons of calcium chloride per gallon of water. You can increase the dose if needed and spray once a week on your plants.
A deficiency in phosphorus.
One indicator of a lack of phosphorus is the presence of certain signs or symptoms.
- Stunted plant growth.
- Darkening of the leaves near the plants’ base.
- Purple or reddish color of the leaves.
- Spare leaf growth.
The process of adding phosphorus to fulfill a deficiency:
The prevailing approach to incorporating phosphorus into aquaponics is by utilizing rock phosphate. The additive may be directly introduced to your grow beds. It is advisable to refrain from directly introducing it into the water, and it is essential to keep your grow bed shaded from direct sunlight to prevent premature dissolution before the plant can uptake it.
To prevent nutrient deficiencies in aquaponics, it is important to maintain a water level with a pH of 6-7 and ensure that fish are fed a well-balanced diet. Another effective method is to use grow media beds, which provide the necessary nutrients for plant growth. These beds create a favorable environment for nutrient development in your system, and adding worms can further enhance this process by breaking down solids and increasing nutrient availability for the plants.
The lack of magnesium.
Signs indicating a deficiency in magnesium:
- Yellowing between the leaf veins, sometimes with reddish-brown tints and early leaf fall. Magnesium deficiency is common in tomatoes, apples, grapevines, raspberries, roses, and rhododendrons.
Here are the steps to supplement Magnesium:
- Your first solution will be Epsom salts. Epsom salts are an essential feed for high-foliage plants in the summer.
- To apply, you should dilute the salts, with 20 grams of salts per liter of water.
- Spray the Epsom salts mix fortnightly, maybe two or three times over the summer months.
- Remember that spraying in dull weather is always best, as you will want to avoid scorching the leaves.
Nutrient Losses
Aquaponics practitioners face a constant challenge of reducing nutrient loss. Nutrient loss occurs in various ways, such as through sludge settlement (37% of feces and 18% of uneaten feed), water losses, denitrification, and ammonia volatilization, among others. For instance, Rafiee and Saad (2005) found that the sludge contained 24% of iron, 86% of manganese, 47% of zinc, 22% of copper, 16% of calcium, 89% of magnesium, 6% of nitrogen, 6% of potassium, and 18% of phosphorus present in the fish feed. The sludge has the potential to retain up to 40% of the nutrients from the feed.
The loss of nitrogen through denitrification can be significant, ranging from 25 to 60 percent. Denitrification occurs under anoxic conditions with low carbon levels and involves the conversion of nitrate into nitrite, nitric oxide (NO), nitrous oxide (N2O), and eventually nitrogen gas (N2), which is released into the atmosphere. Numerous facultative heterotrophic bacteria, including Achromobacter, Aerobacter, Acinetobacter, Bacillus, Brevibacterium, Flavobacterium, Pseudomonas, Proteus, and Micrococcus sp., are responsible for denitrification. In cases where dissolved oxygen levels fall below 0.3 mg/L, certain bacteria can perform both nitrification and denitrification. Additionally, nitrogen loss can also occur through a process known as anaerobic ammonium oxidation (ANAMMOX), where ammonium is oxidized into dinitrogen gas in the presence of nitrite.
The presence of heterotrophic aerobic bacteria in aquaponic systems results in a significant loss of nitrogen that should be available for plants. This is because the nitrogen consumed by these bacteria is not accessible to nitrifying bacteria, which hinders the process of nitrification. The population of these bacteria tends to increase when the C/N ratio rises, making them more competitive and better suited for colonizing the media compared to autotrophic nitrifying bacteria.
Nutrient Balance Systems Dynamics
In an aquaponic system, the nutrient levels in the fish tanks and hydroponic solution need to be balanced for the specific needs of each subsystem. Closed aquaponic systems typically use filters to transport nutrients from the fish to the plants through nitrification. However, there is often an imbalance between the nutrient requirements of crops and the nutrients supplied by the aquaponic system. Multi-loop and decoupled systems offer an easier way to create optimal conditions for both the fish and plant sections. Modeling the system allows for the calculation of the ideal size of the hydroponic area, fish tanks, biofilters, and other equipment. This is particularly important in decoupled multi-loop systems that involve various types of equipment, such as UASB or desalination units, which need to be carefully sized. The mismatch between the nutrients provided by the fish environment and the crop’s needs must be rectified and balanced. A desalination unit proposed by Goddek and Keesman (2018) offers a solution to up-concentrate nutrients, but it only partially addresses the problem. The perfectly balanced system relies on a non-dynamic evaporation rate achievable in closed chambers with perfectly functioning plants. However, the reality is that the evapotranspiration of crops in greenhouse-based aquaponics systems depends on various factors, including climate and biological variables. The calculation of evapotranspiration (ETc) considers factors such as irradiative net fluxes, boundary layer resistance, stomata resistance, and vapor pressure deficit within the canopy using the Penman-Monteith equation. However, this equation only calculates water flux through the crop. Nutrient uptake, on the other hand, is a highly complex process that is influenced by factors such as pH and the relationships between different nutrients. The role of the microbiome in the root zone is also important but not yet included in models. Therefore, detailed modeling of nutrient balancing and system sizing in aquaponics is challenging. The simplest way to estimate nutrient uptake is to assume that nutrients are absorbed as dissolved in irrigation water and accounted for in the ETc calculation. To maintain equilibrium, all nutrients taken up by the crop should be added back to the hydroponic system.
Conclusions
Current Drawbacks of Nutrient Cycling in Aquaponics
In hydroponics, the nutrient solution is carefully determined and the nutrient input into the system is well understood and controlled. This allows for easy adjustment of the nutrient solution for different plant species and growth stages. In aquaponics, the nutrients must come from uneaten fish feed, fish solid feces, and fish soluble excretions, making it more challenging to monitor nutrient concentrations available for plant uptake. Additionally, nutrients are lost through processes such as sludge removal, water renewal, and denitrification. When sludge is removed, important nutrients like phosphorus often precipitate and become trapped in the solid sludge. Even small amounts of water renewal contribute to nutrient loss in the aquaponic system. Lastly, denitrification occurs due to the presence of denitrifying bacteria and conditions that support their metabolism.
How to Improve Nutrient Cycling?
In summary, in order to enhance plant growth in aquaponics, it is necessary to improve nutrient cycling. To prevent the loss of nutrients captured in sludge, sludge remineralization units have been created. These units aim to extract the solid form nutrients in the sludge and reintroduce them into the system in a form that plants can absorb. Another method to minimize nutrient loss is to promote plant uptake through the concentration of the aquaponic solution. This involves removing a portion of the water to maintain the same amount of nutrients in a smaller volume of water, which can be achieved by incorporating a desalination unit into the aquaponic system. Additionally, decoupled/multi-loop systems offer optimum living and growing conditions for fish, plants, and microorganisms. While some research has been done in this area, further studies should be conducted to gain a better understanding of nutrient cycling in aquaponics. Enhanced knowledge of specific macronutrient cycles, the transformation of nutrients by microorganisms, and how plants in aquaponics uptake these nutrients would greatly contribute to understanding aquaponic systems.
