Biofilter performance, water quality, and microbial dynamics in RAS operated under high or low suspended solids load

Accumulation of total suspended solids (TSS) in recirculating aquaculture systems (RAS) may be harmful to fish and biofilter functionality, and there is a link between particulate organic matter and heterotrophic bacteria proliferation in RAS. In this study, the chronic effects of high total suspended solids (HTSS, n = 3) in RAS were assessed on water quality, biofilter performance, and microbiota, compared to RAS with low levels of suspended solids (LTSS, n = 3), during a 128 day-trial following smoltification procedures on Atlantic salmon. Higher TSS concentration (1 vs 7 mg/L) was accompanied by increased organic matter, particle concentration, surface area, and volume, and bacterial concentration and activity in the water and the biofilter. Furthermore, high TSS systems exhibited higher mean levels of ammonia and nitrite, and lower levels of nitrate. Fish biomass increase after 128 days was 14 % lower in HTSS systems. The microbiota composition in the water and biofilter-biofilm was altered by TSS load, and competition from heterotrophic bacteria may have reduced the nitrifying capacity of the biofilter. A combined nutritional-hydrobiological model was developed to assess the effect of commercially-relevant C/N ratios on biofilter performance. Nitrification was not affected by calculated C/N ratios between 4.0 and 5.4, but the dynamics of nitrification were influenced by nitrogen load and source (fish or bacteria). This model is a promising tool for biofilter assessment studies in commercially-relevant scenarios but requires further development and validation. The results indicate the potential impacts of accumulating organic matter on RAS water quality, microbiota, and fish performance. • Poor removal of faecal material in RAS increased particle load and bacterial activity. • High TSS load negatively impacted nitrification and fish biomass gain over 128 days. • TSS affected the composition of the water and biofilter microbiota. • A new model to assess C/N ratio impact on biofilter performance was developed. • Carbon-based fish waste impacted nitrogen loading and biofilter performance in RAS.