## 2 Materials and Methods  
### Experimental Setup  
The experiment was carried out at the Laboratory of Aquaculture-aquaponics, Department of Ichthyology and Aquatic Environment, University of Thessaly, Greece. Gilthead sea bream (_S. aurata_) and rock samphire (C. _martimum_ originated from Mount Athos; GR-1-BBGK-16,5961) were provided by a local nursery facility and the Institute of Plant Breeding and Genetic Resources, Hellenic Agricultural Organization Demeter, respectively. They were divided into three treatments under three different salinities: 8 ppt, 14 ppt, and 20 ppt, respectively. Salinity 8 ppt was chosen as a low salinity for fish and plants aiming to grow well [7]; 20 ppt as the maximum salinity to investigate the growth of both gilthead sea bream and rock samphire; and 14 ppt as an intermediate salinity where sea bream is known to show the optimum growth, beneficial somoregulatory capacity, and metabolic activity [37]. Nine autonomous small-scale aquaponic systems (135 L) were used consisting of nine fish tanks (54 L), nine hydroponic tanks (54 L), and nine sump biofilters (27 L) used as described in previous own studies [7]. Each aquaponics system consisted of three rectangular fish-rearing tanks (40 \(\times\) 35 \(\times\) 26 cm) with a total volume of 36.4 L, supported by an upflow and downflow sump filter with a volume of 53 L. The experiment lasted for 45 days. Later, fish were gradually adapted to the experimental salinities for 30 days as described previously in our own studies [7]. Upon successful adaptation of the fish to 20 ppt, 14 ppt, and 8 ppt, a small number of fish (20 individuals/aquarium) were placed in each aquaponics fish tank for 15 days until the conditioning of the aquaponic system and the filter bed took place. After the successful acclimatization of gilthead sea bream and rock samphire in the experimental salinities, 234 gilthead sea bream individuals were divided into 26 individuals per aquarium with an initial average weight of 3.90 \(\pm\) 0.05 g, and an initial average length of 6.75 \(\pm\) 0.03 cm. In addition, 54 individuals of rock samphire were divided into six individuals per system with an average initial height of 7.90 \(\pm\) 0.05 cm. Gilthead sea bream were fed by hand with a commercial pellet (1.5 mm, BIOMAR, SA-GREECE) containing 55% crude protein, 15% crude lipids, ash 9%, and gross energy 21.3 MJ/Kg of D.M., in three meals six days a week, except the seventh day where the fish remained with an empty stomach to let the system discharge from fish wastes. Fish were weighed every 15 days of the experiment, remeasuring their weight and length to redefine the amount of daily food intake. Fish tanks were cleaned, and uneaten food was removed every day by siphoning. At the end of the experiment, fish were anaest with an 0.20 mg/L MSS 222, and final fish body weights and lengths were measured. Water effluent from the fish tank and hydroponic grow bed was dropped through gravity into the mechanical filter (consisting of a porous sponge to capture the faeces and uneaten food), then the water through downflow and upflow into the biofilter containing 20 L bio balls media (\(\Phi\)31.8-38.1 mm) creating a specific surface area (SSA) of 600 cm\({}^{2}\)/cm\({}^{3}\) and 20 L ceramic ring media (\(\Phi\)25 mm), with SSA 1000 cm\({}^{2}\)/cm\({}^{3}\). The SSA of filter media was affected by feeding rate and protein content in fish feed, influencing the carrying capacity of the filter bed, ammonia removal efficiency, and the condition of the filter. The hydraulic load (HLR = 2.51 cm/min), the hydraulic retention time (HRT = 4.59 min), the recirculation rate (r = 0.90), and the water flow (Q = 3.31 m\({}^{3}\)/h) were adjusted to be similar in both treatments. The measurement of the morphometric characteristics of fish and plants was performed every 15 days during the experiment. Every 15 days, the amount of food offered was re-adjusted due to the change in weight obtained from the measurements, while the nutrition ration level (5% of b.w.)