Nutrient Release Coefficient: a Proposed Batch Reactor Assay to Determine the Elemental Composition of Aquaponic Nutrient Solutions

Aquaponics practitioners have empirically known for years that feeding and growing fish generates nutrient-rich water as a byproduct, which serves as fertilizer to grow plants that ultimately clean the water for the fish. Despite the efforts of a few investigators, the literature lacks a methodical scientific basis for describing the essential mechanics, relationships between element inputs and outputs within aquaponic systems. We proposed a simple high-throughput batch reactor assay for the prediction of nutrient inputs in aquaponics that can quickly determine the nutrient bioavailability and release rate of nutrients for any fish feed. We defined the term “nutrient release coefficient” as the conversion efficiency measurement of the quantity of a certain element excreted by fish in the dissolved form in the water to a unit of the same ingested element present in the feed. We proved that nutrient release coefficients are quite constant among different groups of fish and within the same group of fish sampled in different periods of time. This study confirmed that there is a major imbalance not only in the nutrient contents in fish diets, but also a major discrepancy in nutrient release dynamics since fish assimilate and excrete different elements at distinct rates (Ca < P < Mg < N < K < S). Results of nutrient release coefficient assays can be successfully replicated when fish size, feed, temperature, physiological solution, and oxygen saturation are maintained equal. This assay could be an alternative to long and cumbersome fish/plant ratio studies in aquaponics research.