Nutrient Dynamics in Aquaponic Systems

Aquaponics

The integration of aquaculture and the hydroponic cultivation of plants has been examined repeatedly over the past five decades with a wide variety of system designs, plant and aquatic animal species, and experimental protocols. Closed, recirculating systems have generally been identified as the most appropriate aquaculture system types for integration with hydroponics because nutritional and environmental conditions can be maintained at levels enough for hydroponic plant culture. Aquaponics yields comparable plant growth as compared with conventional hydroponics despite the lower concentrations of most nutrients in the aquaculture water. Increased CO2 concentrations in the above ground environment and changes in the biomes of the root zone are thought to be the main reasons for this.

The use of aquaculture effluents increases the challenge of monitoring the nutrients within the solution. It is indeed harder to control the composition of a solution where the nutrients originate from a biological degradation of organic matter than to follow the evolution of the nutrients’ concentration in a precisely dosed hydroponic solution based on mineral compounds. Moreover, a plant’s nutritional needs vary during the growth period in accordance with physiological stages, and it is necessary to meet these needs to maximize yields. This means that the aquaponics system operator needs to be a lot more hands-on compared to the typical hydroponics operator where a protocol or systematic steps in a user manual are followed.

In order to recycle aquaculture effluents to produce plant biomass, it is necessary to optimize the recycling rates of phosphorus and nitrogen. Several factors can influence this, such as the fish species, fish density, quality of the fish feed, water temperature, the type of plants and the microbial community. Therefore, it is of prime importance to understand the functioning of the nutrient cycles in aquaponics.

Origin of nutrients:

The major sources of nutrients in an aquaponics system are the fish feed and the water added into the system. With reference to fish feed, there are two main types: fishmeal based, and plant protein based feed. Fishmeal is the classic type of feed used in aquaculture where lipids and proteins are supplied from the fishmeal and fish oil. However, for some time now, concerns regarding the sustainability of such feed have been raised and attention drawn towards plant based diets. A meta-analysis conducted in 2012 revealed that the use of plant proteins in fish feed can influence the growth of fish if incorporated in high proportions. Indeed, plant proteins can have an impact on the digestibility and levels of anti-nutritional factors of the feed. Phosphorus originating from plants and thus in the form of phytates does not benefit, for example, salmon, trout and several other fish species. Very little is known of the impact of varying fish feed composition on crop yields.

Classical fish feed is composed of 6-8 macro ingredients and contains 6-8% organic nitrogen, 1.2% organic phosphorus and 40-45% organic carbon with around 25% protein for herbivorous or omnivorous fish and around 55% protein for carnivorous fish. Lipids can be fish or plant based as well. Once fish feed is added into the system, a substantial part of it is eaten by the fish and either used for growth and metabolism or excreted as soluble and solid faeces, while the rest of the given feed decays in the tanks. In this case, the feed leftovers and metabolic products are partly dissolved in the aquaponics water, thus enabling the plants to uptake nutrients directly from the aquaponics solution.The full article is for subscribed members only. To view the full article please subscribe. It’s FREE!Log In Register

Author

  • Manager, Aquaculture, Department of Animal Sciences, Stellenbosch University

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