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

Abstract The control of adverse effects and the possibility of removing suspended solidsfrom 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 ( δ 13 C and δ 15 N ) 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 δ 13 C and 11.9‰ for δ 15 N ), feces(−24.0‰ for δ 13 C and 6.4‰ for δ 15 N ) and biofilm (−25.1‰ for δ 13 C and 12.9‰ for δ 15 N ) 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 signaturesof suspended solidsfrom −23.8 and 7.9‰ to −24.9 and 8.3‰ for δ 13 C and δ 15 N , respectively. The fish muscle (−18.6‰ for δ 13 C and 15.4‰ for δ 15 N ) could reflect the isotopic variability of feed ingredients accumulated over time. The isotopic shiftsindicate that the contribution of three sources depends on: (1) fish metabolism; (2) water treatment devices; and (3) bacterial bio-fouling into biofilter.