## 1 Introduction  
Aquaculture is an important economic sector growing worldwide to meet the current demand of fish consumption [1]. Water quality and fish health are some of the main concerns of semi-intensive and intensive productions [2]. The gross investment on this area has been mostly on infrastructures to effectively increase production, that is to grow more fish per area [3, 4], while ensuring product quality. Meanwhile, aquaculture productions worldwide produce effluents enriched with nutrients and suspended solids derived from the remains of uneaten feed and waste from fish metabolisms [5]. The discharge of these nutrient-rich effluents into the aquatic environment can cause harmful effects in these ecosystems, such as eutrophication [1, 6, 7]. Although European directives promote greener and sustainable processes [1, 8, 9], aiming at protecting aquatic ecosystems, the treatment of fish effluent is still quite disregarded. Integrated multitrophic aquaculture (IMTA) is a versatile opportunity that transforms waste resultant from aquaculture into profit by upbringing economic attractive species, such as macroalgae, mussels, oysters, capable of extracting nutrients in particulate and diluted phases in aquaculture effluent, cleaning it [10]. Macroalgae-based IMTA systems can be very effective effluent treatment units for coastal aquacultures, particularly when using macroalgae with high ability for removing nutrients [11]. In Europe, IMTA systems have been mainly applied to offshore aquacultures. For instance, macroalgae, such as kelps (class Phaeophyceae), are produced by using nutrients released from off-shore fish facilities [12; 13; 14]. In the coastal aquaculture industry, IMTA is currently poorly applied despite the advantages of these systems. Many factors can contribute for this, for instance, aquaculture producers, especially small- and medium-sized enterprises, are mainly focused in improving their monoculture activity and generally sceptical regarding IMTA systems, tending to perceive these culture systems as secondary, unproductive and time-consuming. Moreover, the application of these solutions represents a significative initial investment and requires learning new skills and more complex lines of production [13; 14]. Such perception is generally incited by the lack of knowledge and inexperience on markets and production methods [13], but also by the lack of effective, low-investment and low-maintenance solutions. With that in mind, a simple and low maintenance IMTA-pilot system was settled in a semi-intensive coastal aquaculture and its performance was validated for the green macroalgae _Ulta_ spp. (Chlorophyta) culture. Our aim was to develop a solution for coastal aquacultures, with the least impact possible to the aquaculture's normal functioning, that enables the treatment of their effluents before discharge, reducing the anthropogenic pressure on estuarine and coastal ecosystems, and ensuring compliance with the current laws and regulations. Concomitantly, a new sub-product with economical value will be produced. The increase of profit associated to the production and trade of macroalgae can be, in the future, an incentive for the implementation of such solutions. Herein, the IMTA-pilot system was applied to an aquaculture production unit of fishsponds adjacent to an estuarine area, which produces sea bream (_Sparus aurata_) and sea bass (_Dicentrarchus labrax_). Such units are commonly found in Portugal and south of Europe and typically settled near estuaries [15]. These productions units do not generally present water treatment facilities and rely on water flow provided during favourable moon cycles and currents to proceed with water changes in the facilities. Produced effluents are generally discharged untreated into the estuary after a brief residence in an effluent channel.