## 4 Discussion  
In European countries with a long coastline, such as Portugal, aquaculture is generally an important economic sector, with a significant contribution to a country's Gross Domestic Product (GDP). Coastal aquaculture are settled mainly in estuarine areas and coastal lagoons, and the majority of these productions ill-treat their effluents and do not support the costs of current waste treatment solutions [38]. IMTA can provide a possible solution and promising benefits, either economical or environmental, in the long term. Moving aquaculture towards a more sustainable path aligned with Water Framework Directive, Marine Strategy Framework Directive and Sustainable Development Goals requires the involvement of both scientific community and aquaculture producers, as well as governmental entities. This conjoint work will allow developing more realistic and feasible solutions, that are efficient, low cost and low maintenance, for aquaculture productions and arise awareness and acceptance inside the sector. In this view, an IMTA-pilot system, practical and of low maintenance, was designed. Its performance and suitability as a sustainable solution of effluent-treatment for coastal aquaculture was first validated with _Ulva_ spp., as this macroalgae naturally grows in the aquaculture tanks. Our results showed that _Ulva_ spp. can successfully grow and remediate in fish effluent, demonstrating the suitability of the IMTA pilot-system as an effluent treatment unit and of _Ulva_ spp. for that purpose. Previous works have demonstrated that _Ulva_ spp. can be successfully cultivated in aquaculture effluent [39; 40; 41]. In this work, _Ulva_ spp. thrived in effluent from _Sparus aurata_ and _Dicentrarchus labrax_ production tanks, either under controlled conditions in terms of temperature and light (LU1 and LU2) or under natural climatic conditions (AQUA1 and AQUA2). The growth was generally more pronounced in the first two/three periods of medium change (until \(\approx\) 12-14 days of growth), SGRs and yields slightly decreasing until the end of the experiments. Similarly, other authors have observed that _Ulva_ spp. can generally present fluctuations on its growth, showing periods of increased growth rates followed by periods of decreased growth or reaching a stationary phase [41; 42; 43]. The obtained SGRs and biomass yield are comparable with data from [PERSON] and [PERSON] [41], which cultivated _Ulva_ spp. disks in effluent from _Mugil cephalus_, for 10 days, in a lab-scale static system as herein, but without medium change. In IMTA systems, [PERSON] et al. and [PERSON] et al. reported similar SGR values [40; 44; 45], although lower biomass yields were achieved in one's study. On the other hand, higher SGR values (>18% day\({}^{-1}\)) have been reported in previous works [5; 39; 46] which used continuous effluent flow. Several factors are known to influence the cultivation of _Ulva_ spp., so comparison among works is somewhat difficult. For instance, the different nature and chemical composition of the effluents used, different usages of _Ulva_ spp. (e. g., small thalli of collected algae or cut thalli in different shapes [41; 47; 48], cultivation of free algae specimens [32; 44] or fixed to a net [42]), different experimental conditions used (e.g., dissimilar volume and shape of the tanks, aeration systems), natural environmental conditions of the geographic area [41] can induce fluctuations in _Ulva_ spp. development. The production of _Ulva_ spp. biomass is also dependent upon seasonality [43; 49], which was denoted in this work by the higher SGRs and yields in late spring in comparison to those of late autumn experiments. In fact, experiment LU2 highlights the importance of temperature and light exposure in _Ulva_ spp. development. The optimal conditions allowed _Ulva_ spp. to thrive, showing SGR and yield within the range of those of LU1 (latespring) and higher than those from AQUA2 (lower temperatures and sunlight exposure conditions).