Features of the hydrobiocenosis of the biofilter of a recirculating aquaculture system

The qualitative and quantitative composition of the hydrobiocenosis in a recirculating aquaculture system's biofilter is examined. The spatial distribution of various periphyton species is studied, including their distribution across the biofilter's depth and in the surface and bottom directions. Significant taxonomic diversity of groups and species within the periphyton of a fish-breeding facility's biofilter was demonstrated. Thirty-eight species belonging to nine taxonomic groups were identified. Ciliates were the most widely represented group, comprising 52.6% of the total diversity. It was shown that different communities of periphyton species are formed at different depths (at the surface, in the middle layer and at the bottom of the biofilter): the Jaccard index was 0.59 for communities at a depth of 10 and 40 cm, 0.57 for communities at a depth of 10 and 80 cm, and 0.67 for communities at a depth of 40 and 80 cm. The substrate surface orientation also proved to be a significant factor: communities on the “upward” and “downward” facing plates differed significantly in species composition. On the upper surfaces, motile ciliates (genus Euplotes) and amoebae dominated. Sessile ciliates (Vorticella), rotifers, and oligochaetes predominated on the lower surfaces. The total number of periphyton varied from 29 443 thousand specimens/m2 in the middle layer of the biofilter to 31 336 thousand specimens/m2 at the surface and up to 31 630 thousand specimens/m2 at a biofilter depth of 80 cm. Rotifers and ciliates contributed the most to the overall abundance. Some species showed an increase in abundance in the biofilter with increasing depth, while others showed the opposite trend. Unlike the hydrobiocenosis of the biofilter for municipal wastewater treatment, the hydrobiocenosis of the biofilter of the year-round fish farm had the highest abundance of β-mesosaprobes.