Bioindicator and biofilter function of Ulva spp. (Chlorophyta) for dissolved inorganic nitrogen discharged from a coastal fish farm — potential role in integrated multi-trophic aquaculture

In order to examine the efficiency of Ulva spp. (Chlorophyta) as a bioindicator and biofilter for dissolved inorganic nitrogen discharged from coastal fish farms, field surveys and field cultivation experiments were conducted in Gokasho Bay, central Japan. Naturally occurring green algae Ulva spp. were collected in and around a fish farm in February, May and July, and their δ15N and tissue N contents were analyzed. Values of these parameters obtained in May and July decreased with the distance from the fish cages, suggesting that Ulva spp. occurring in close vicinity to the fish farm assimilated the fish-excreted N. To evaluate this uptake, we cultured round cut pieces of thalli of Ulva ohnoi in transparent chambers beside a fish cage and at a control station for 2 weeks in May, July and September. The specific growth rate and N fixation rate of the thalli cultivated beside the fish cage showed 12.8–23.6% and 4.2–13.9 mg N g DW− 1 day− 1, respectively, which were significantly higher than those at the control station and found to be among the highest levels known for seaweed biofilters. The high growth rates during the warm season and easy cultivation indicate the suitability of Ulva ohnoi as a biofilter.