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

Paulo Mira Fernandes; Ludvik Wolfgang Forbord Fiksdal; Paula Rojas-Tirado; Stian Ringbakken Stenhaug; Sujan Khadka; Sara Calabrese; Mark Darryn Powell; Håkon Dahle; Ingrid Bakke; Ole-Kristian Hess-Erga; Simen Fredriksen

doi:10.1016/j.aquaculture.2025.743559

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

Paulo Mira Fernandes^*, Ludvik Wolfgang Forbord Fiksdal, Paula Rojas-Tirado, Stian Ringbakken Stenhaug, Sujan Khadka, Sara Calabrese, Mark Darryn Powell, Håkon Dahle, Ingrid Bakke, Ole-Kristian Hess-Erga, Simen Fredriksen

^*Corresponding author for this work

School of Biology

Research output: Contribution to journal › Article › peer-review

Abstract

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.

Original language	English
Article number	743559
Pages (from-to)	1-15
Number of pages	15
Journal	Aquaculture
Volume	614
Early online date	16 Dec 2025
DOIs	https://doi.org/10.1016/j.aquaculture.2025.743559
Publication status	E-pub ahead of print - 16 Dec 2025

Keywords

C/N ratio
Heterotrophs
Microbiota
Nitrification
Recirculating aquaculture systems
Tss

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Fernandes-et-al-2025-Biofilter-performance-Aquaculture-614-743559-CCBY
Copyright © 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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Fernandes, P. M., Fiksdal, L. W. F., Rojas-Tirado, P., Stenhaug, S. R., Khadka, S., Calabrese, S., Powell, M. D., Dahle, H., Bakke, I., Hess-Erga, O.-K., & Fredriksen, S. (2026). Biofilter performance, water quality, and microbial dynamics in RAS operated under high or low suspended solids load. Aquaculture, 614, 1-15. Article 743559. Advance online publication. https://doi.org/10.1016/j.aquaculture.2025.743559

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

abstract = "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.",

keywords = "C/N ratio, Heterotrophs, Microbiota, Nitrification, Recirculating aquaculture systems, Tss",

author = "Fernandes, \{Paulo Mira\} and Fiksdal, \{Ludvik Wolfgang Forbord\} and Paula Rojas-Tirado and Stenhaug, \{Stian Ringbakken\} and Sujan Khadka and Sara Calabrese and Powell, \{Mark Darryn\} and H{\aa}kon Dahle and Ingrid Bakke and Ole-Kristian Hess-Erga and Simen Fredriksen",

note = "Funding: This research was sponsored by the Norwegian Seafood Research Fund (Fiskeri- og havbruksn{\ae}ringens forskningsfinansiering, FHF, fhf.no) project nr. 901735 «Mikrobiota assosiert med atlantisk laks i overgangen fra ferskvann til sj{\o}vann i RAS: effect av. h{\o}y/lav partikkelbelasting og konsekvenser for fiskevelferd og -helse (MikroRAS)» (Microbiota associated with Atlantic salmon in the transition from fresh- to seawater in RAS: effects of high/low particle load and consequences to fish welfare and health).",

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doi = "10.1016/j.aquaculture.2025.743559",

language = "English",

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journal = "Aquaculture",

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T1 - Biofilter performance, water quality, and microbial dynamics in RAS operated under high or low suspended solids load

AU - Fernandes, Paulo Mira

AU - Fiksdal, Ludvik Wolfgang Forbord

AU - Rojas-Tirado, Paula

AU - Stenhaug, Stian Ringbakken

AU - Khadka, Sujan

AU - Calabrese, Sara

AU - Powell, Mark Darryn

AU - Dahle, Håkon

AU - Bakke, Ingrid

AU - Hess-Erga, Ole-Kristian

AU - Fredriksen, Simen

N1 - Funding: This research was sponsored by the Norwegian Seafood Research Fund (Fiskeri- og havbruksnæringens forskningsfinansiering, FHF, fhf.no) project nr. 901735 «Mikrobiota assosiert med atlantisk laks i overgangen fra ferskvann til sjøvann i RAS: effect av. høy/lav partikkelbelasting og konsekvenser for fiskevelferd og -helse (MikroRAS)» (Microbiota associated with Atlantic salmon in the transition from fresh- to seawater in RAS: effects of high/low particle load and consequences to fish welfare and health).

PY - 2025/12/16

Y1 - 2025/12/16

N2 - 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.

AB - 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.

KW - C/N ratio

KW - Heterotrophs

KW - Microbiota

KW - Nitrification

KW - Recirculating aquaculture systems

KW - Tss

U2 - 10.1016/j.aquaculture.2025.743559

DO - 10.1016/j.aquaculture.2025.743559

M3 - Article

SN - 0044-8486

VL - 614

SP - 1

EP - 15

JO - Aquaculture

JF - Aquaculture

M1 - 743559

ER -

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

Abstract

Keywords

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