Evaluating nutrient circularity in integrated aquaculture systems: criteria and indicators

Nutrient circularity is an application of circular economy principles that addresses issues such as nutrient accumulation or loss, and a shift away from fossil and synthetic fertilisers. The concept is increasingly being explored and is relevant for managing nutrients more sustainably in agri- and aquaculture systems, but the standards by which nutrient circularity can be evaluated require further clarification, and quantitative indicators should be established. Identifying the nutrient circularity performance of integrated aquaculture systems is particularly relevant to better understand how the combination of multiple and complementary species farmed or naturally present in these systems can upcycle nutrients from waste. The main objectives of this study were to improve understanding of nutrient circularity and clarify how to quantify it for (integrated) aquaculture systems. To this end, criteria for describing nutrient circularity were first defined based on a literature review, and quantitative indicators were then identified for each criterion to create an indicator framework. Finally, this framework was applied to three contrasting experimental integrated aquaculture systems (i.e. aquaponic, biofloc and polyculture pond) and their conventional monoculture system counterparts from previous studies to test its ability to compare nutrient circularity in aquaculture systems. Six complementary criteria (and 21 associated indicators) for describing nutrient circularity were identified: productivity, efficiency, self-sufficiency, recycling, regeneration, diversity and complementarity. These criteria, related to circularity principles, provided a clear framework for evaluation. Application of the framework indicated that the integrated systems evaluated usually outperformed conventional monoculture systems, which highlighted the potential of integrated systems to manage nutrients more sustainably. These contrasting integrated systems showed that different pathways (e.g., microbial loops, complementarity between farmed species) can be mobilised to create and (re-)cycle nutrients. Although relatively simple indicators were developed, lack of data prevented quantification of several indicators and thus a full comparison of the systems. Overall, this study helps clarify the concept of nutrient circularity and supports the development of integrated farming systems for more sustainable use of nutrients.