Short-term effects of abrupt salinity changes on aquaculture biofilter performance and microbial communities

Authors: COSTIGAN, ELIZA - University Of Maine; BOUCHARD, DEBORAH - University Of Maine; ISHAQ, SUZANNE - University Of Maine; MACRAE, JEAN - University Of Maine

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2024
Publication Date: 10/13/2024
Citation: Costigan, E., Bouchard, D.A., Ishaq, S.L., MacRae, J.D. 2024. Short-term effects of abrupt salinity changes on aquaculture biofilter performance and microbial communities. Water. 16. Article 2911.

Interpretive Summary: Recirculating aquaculture systems (RASs) have become an increasingly popular source of high-quality protein production, with strong potential to supplement traditional, water intensive aquaculture methods. In RASs, optimal conditions allow for low water exchange, targeted at around 300 L per kg feed. Thus, to maintain the water quality required for healthy animal growth, the recirculated water must be treated to remove or transform fish waste products. When fish consume proteins in food, some of the nitrogen is excreted through the gills in the form of ammonium. Most aquatic animal species cannot tolerate more than 1.0 mg L-1 of total ammonia nitrogen (TAN) in water; ideally, the concentration for long-term exposure should be kept under 0.05 mg L-1. It is imperative that the ammonia-nitrogen be converted to its less toxic form, nitrate, before the RAS water is recirculated or discharged. This conversion, nitrification, is a two-step process where ammonium (NH4 + ) is converted to nitrite (NO2 -), and then to nitrate (NO3 -), in an aerobic environment. The microorganisms that perform these conversions in biofilters grow in biofilms on plastic beads or other filter media. These microorganisms, also called nitrifiers, are sensitive to changes in their environment, such as changes in pH, temperature, and salinity. Changes in salinity of RASs can be convenient when raising anadromous species such as Atlantic salmon, as this eliminates the need for multiple biofilters operated at different salinities. This could be particularly beneficial for smaller-scale RASs, though shifting salinities abruptly has proven to be a hindrance to biofilter performance.

Technical Abstract: In recirculating aquaculture systems (RASs), ammonia excreted by fish must be converted to the less toxic nitrate before recirculation. Nitrifying microorganisms in biofilters used for this transformation can be sensitive to changes in salinity, which can present issues for systems that raise anadromous fish such as Atlantic salmon. Freshwater biofilters maintained at a low level of salinity (such as biofilters operated in coastal areas) may be better equipped to handle more drastic salinity shifts; therefore, experiments were performed on freshwater and low-salinity (3 ppt) biofilters to assess their ability to recover nitrification activity after an abrupt change in salinity (3, 20, and 33 ppt). Two-week tests showed full nitrification recovery in freshwater biofilters after a shift to 3 ppt but no ammonia oxidation in 20 or 33 ppt. Low-salinity-adapted filters (transitioned from 0 to 3 ppt) showed a small recovery (about 11%) after a shift to 20 ppt, and no activity when shifted to 33 ppt. Illumina sequencing revealed that, while nitrification was slowed or stopped with shifting salinities, the nitrifiers survived the salinity increases; conversely, the heterotrophic communities were more greatly affected and were reduced in proportion with increasing salinity. This work indicates that biofilters operated at low salinity may recover more quickly after large salinity changes, though this slight benefit may not outweigh the cost of low-level salinity maintenance. Further research into halotolerant heterotrophs in biofilms may increase the effectiveness of nitrifying biofilters under variable salinities.