## 4 Discussion
For effluent, specifically, previous authors have reported that aquaculture effluents with different composition, in terms of salinity, nutrient content, presence of microorganisms and pathogens, can trigger different responses in _Ulva_ specimens [32; 50; 51]. Moreover, continuous effluent flow improves biomass production of _Ulva_ spp. [41], water flow velocity and aeration being key factors for inorganic nutrient acquisition and algae growth [46; 52]. In AQUA1, negative SGRs and yields were observed on account of _Ulva_ spp. entrapment in the aeration system, preventing the algae to properly recirculate in the tank. This led to loss of biomass and decolouration of _Ulva_ spp. Such a phenomenon has been reported before [39; 53] and can be an indication of stress triggered either by the lack of or improper aeration and consequent diminished exposure to sunlight or by other factors such as lack of nutrients, water flux or zoospore release [42; 54]. The problem was solved by increasing the pore size of the aeration system to 2 mm, which allowed the proper upward and downward movement of the algae in the tank. No entrapment of algae was latter registered in AQUA2. Throughout the experiments, _Ulva_ spp. generally showed high biofiltering efficiencies. In accordance, previous studies have shown that macroalgae from genus _Ulva_ can efficiently remediate aquaculture effluents [5; 56; 32; 46; 49; 5]. Nutrient removal rates within the same order as those reported in literature were attained in this work and, generally, NRE showed a tendency to increase in autumn for nitrogen compounds (LU2 and AQU2) and in spring for phosphorous compounds (AQUA1). Experiment LU2 indicated that nutrient load was significantly reduced after four days. This was taken into consideration in IMTA experiments. Nevertheless, medium change in AQUA2 was performed every six days due to alterations in the fish feeding regime and normal functioning of the aquaculture on account of pandemic restrictions. Several features can influence nutrient removal rates, such as, climatic conditions and light exposure. These are difficult to control but parameters, as tank design, fish stock density, algal strain, algae surface, nutrient load, aeration and hydraulic regime [32; 39], can be improved to optimise the management and operation of the system to guarantee the efficiency of the effluent treatment. Moreover, effluent nutrient composition also depends on several factors, such as fish density, feed intake, excretion rates, the presence of microorganisms, etc., which can vary seasonally. Initial nutrients levels (d0 and MC) varied within the same experiment and among experiments being in some cases, specifically NH4+, above the environmental minimum quality objectives establish in [57]. Higher removal rates were registered in effluent with elevated amounts of nutrients, especially NH4+, NO3- and NO2-. _Ulva_ spp. shows a clear preference for NH4+, but continues to take up NO3- and/or NO2- even though at lower rates. This behaviour has been also reported by other authors [58; 32] as the metabolisation of NH4+ requires less energy prior amino acid incorporation. On the other hand, the assimilation of NO3- and NO2- relies on a reduction reaction dependent on energy [55]. Therefore, NH4+ is generally assimilated faster and this uptake is more efficient [46; 59]. Generally recognised as a second limiting factor of algae growth [60], phosphorous dynamics are rarely studied in IMTA biofiltering experiments. In fish effluent, phosphorus content is often an order of magnitude less than nitrogen content and low NRE have been reported in the past [49; 61; 62]. Phosphorous concentration (PO4-) was generally low in the effluent and quantifiable concentrations were only registered in AQUA1 and AQUA2. In both IMTA assays, nutrient removal efficiencies above 40% were obtained for PO4- even at static regime. [PERSON] et al. also reported high removal efficiencies of PO4- (69-89%).