2.2. Management and operating conditions
Prior to sampling, the five experimental flow-through systemswere operated as described in Section 2.1 for more than 4 months to establish steady state conditions in the biofilters each receiv-ing different nutrient loads/concentrations. The fish in each systemwere fed different fixed rations (Table 1) of a commercial troutfeed (4.5 mm EFICO Enviro 920, BioMar, Denmark) via belt feed-ers from 09:00 h to 15:00 h, corresponding to 1.2% of the initialbiomass/d (no feed spill was observed during the study). Photope-riod lasted from 07:30 to 22:00 each day. Swirl separators wereflushed once a day before feeding. Temperature during the trialsaveraged 11.5 ± 0.6◦C (n = 30) in the biofilters with only minor day-to-day variations. pH ranged between 7.40 and 7.90. Alkalinity inthe inlet freshwater was around 180 ppm CaCO3and no substancewas added to compensate for alkalinity loss induced by nitrifica-tion.