Particulate matter dynamics and transformations in a recirculating aquaculture system: application of stable isotope tracers in seabass rearing

The control of adverse effects and the possibility of removing suspended solids from recirculating aquaculture systems (RAS) are the principal challenges facing aquaculture engineers. However, their dynamics and transformations are not yet well known. In this study, carbon and nitrogen stable isotopes values (δ¹³C and δ¹⁵N) were used as tracers of particulate matter in a seabass RAS. An isotopic mixing model was employed to estimate the contributions of particulate sources. Feed (−22.1‰ for δ¹³C and 11.9‰ for δ¹⁵N), feces (−24.0‰ for δ¹³C and 6.4‰ for δ¹⁵N) and biofilm (−25.1‰ for δ¹³C and 12.9‰ for δ¹⁵N) were identified as main sources of particulate matter. The particle traps collected a mixing of 29% of uneaten feed and 71% of feces, when drum filter eliminated all remaining uneaten feed, shifting the isotopic signatures of suspended solids from −23.8 and 7.9‰ to −24.9 and 8.3‰ for δ¹³C and δ¹⁵N, respectively. The fish muscle (−18.6‰ for δ¹³C and 15.4‰ for δ¹⁵N) could reflect the isotopic variability of feed ingredients accumulated over time. The isotopic shifts indicate that the contribution of three sources depends on: (1) fish metabolism; (2) water treatment devices; and (3) bacterial bio-fouling into biofilter.