Introduction
Focusing on circular bioeconomy and solutions for managing finite and scarce resources such as phosphorus are issues that need to be addressed across sectors to obtain food security (Cordell et al., 2009). Land-based aquaculture of Atlantic salmon (Salmo salar) smolt and post-smolt in flow-through and recirculated system results in a large output of aquaculture sludge (Aas and Åsgård, 2017). These nutrient-rich side streams from aquaculture production constitute a valuable resource for cultivation of polychaetes Hediste diversicolor (Wang et al., 2019). The aim of this study was to recycle carbon (C), nitrogen (N), and phosphorus (P) contained in aquaculture sludge by production of polychaetes. Hereby, the bioremediation potential of H. diversicolor was assessed and a nutrient budget for recycling of C, N, and P was established.
Material and methods
Two different experiments, where feed levels were calculated based on estimated nitrogen content of diets and polychaetes, were carried out to investigate our research question. In a first experiment, we studied how the composition of two different diets, smolt sludge (S) and post-smolt sludge (PS) at four different feed levels (5-47% N) affected growth and C, N, and P recovery in H. diversicolor over a feeding period of 30 days. In a second setup, nutrient budgets of individual polychaetes supplied with two different quantities of smolt sludge (5% N, 40% N) were investigated to gain further knowledge on feed ingestion rates, feces production, respiration, excretion and assimilation of C, N, and P in polychaete biomass. Growth was calculated following Jørgensen (1990). Carbon and nitrogen in polychaetes, sludge, and feces were analyzed using an elemental analyzer. Phosphorus was oxidized with potassium peroxydisulfate and analyzed photometrically as phosphate in the same way as sea water samples containing nitrate, ammonium, and phosphate, using an autoanalyzer.
Results and discussion
In the 30-day trial, polychaetes fed with both types of aquaculture sludge were shown to incorporate C, N, and P from their diets into biomass. Specific growth rates increased with increased feed supply for polychaetes fed with both diets, and incorporation of C, N, and P in polychaete biomass was highest at the highest feed levels. C, N, and P contents in polychaetes were not affected by type of diet or feed level (Figure 1, left). Conversion factors of diet C, N, and P into polychaete C, N, and P differed between nutrients but were not significantly different between feed levels. On average, carbon and nitrogen conversion rates amounted to 2-10% and 2-15%, respectively, while values for phosphorus conversion rates were lower at 0.5-1.5% (Figure 1, right). When determining individual nutrient budgets, polychaetes supplied with more feed showed higher ingestion of smolt sludge, however, a smaller relative ingestion rate, suggesting overfeeding. Assimilation rates of C, N, and P were significantly higher in the high feeding treatment compared to the low. Accordingly, relative feces production was higher in the low treatment. Respiration was not affected by feed supply.
Conclusion
The presented findings suggest that H. diversicolor can successfully grow on aquaculture sludge. Nutrients C, N and P were incorporated successfully, however utilization rates varied between the three, with P being most poorly utilized. The species can be considered beneficial for bioremediation of aquaculture sludge, however, should not be regarded as the sole solution for efficient recycling of side streams from Atlantic salmon production.
References
CORDELL, D., DRANGERT, J.-O. & WHITE, S. 2009. The story of phosphorus: Global food security and food for thought. Global Environmental Change, 19, 292-305.
JØRGENSEN, C. B. 1990. Bivalve filter feeding: hydrodynamics, bioenergetics, physiology and ecology, Olsen & Olsen.
WANG, H., SEEKAMP, I., MALZAHN, A. M., HAGEMANN, A., CARVAJAL, A. K., SLIZYTE, R., STANDAL, I. B., HANDÅ, A. & REITAN, K. I. 2019. Growth and nutritional composition of the polychaete Hediste diversicolor (OF Müller, 1776) cultivated on waste from land-based salmon smolt aquaculture. Aquaculture, 502, 232-241.
AAS, T. S. & ÅSGÅRD, T. E. 2017. Estimated content of nutrients and energy in feed spill and faeces in Norwegian salmon culture. Nofima rapportserie 19 (2017), 1-8.