## 4 Discussion
The reason could be linked to the lack of alkalinity management and the absence of water change in week 5. In the aquaponics treatment, the pH of the system remained after the first experimental week rather steadily close to seven, and no drop in pH was observed after week five as in RAS (Figure 1d). Nitrification is an acid-forming process that can destroy the alkalinity of the water and result in a pH decrease [1]. Alkalinity of water is also affected by feed input, water exchange rates, hydraulic retention time and nitrifying activity [44; 45]. Alkalinity adjustment is needed in RAS, which is usually done by the addition of sodium bicarbonate or diluted NaOH [1; 44], but in aquaponics, compounds containing sodium should not be used [1]. Water exchanges also help maintaining the water quality for RAS by preventing the accumulation of harmful compounds in the system [45]. One reason for the absence of pH drop in our aquaponic system could be that the plants buffered the pH of the system [42]. During the process of absorbing nitrates from the surrounding water, the plants exchange H\({}^{+}\) and OH\({}^{-}\) between the medium and roots. Plants excrete the anion and uptake cation leaving the root medium alkaline [46; 47]. On the other hand, the stability of pH in aquaponics can also be related to anoxic parts in the biofilter causing denitrification and, thus, recovery of alkalinity [1]. The tap water analysis of the present study showed very low concentrations of sulfate. Similarly, the concentration of nitrate-N was also very low in tap water. In the area of the inlet water uptake, the sulfate concentrations for sulfate are typically very low [48], and for nitrate, they remain below 5 mg/L [48]. The guidelines of Norwegian authoritiesrecommend that water nitrite levels should be below 0.1 mg/L but there are no reference values for freshwater chloride concentration [49]. The chloride concentration was significantly higher in aquaponics tank water than in RAS, which can be linked to the smaller relative water exchange in aquaponics compared to RAS treatment. Chloride from the fish feed possibly accumulated in the systems during the experiment. The minor increase in phosphate likely originates from the fish metabolism as fish feed contains phosphorus [50; 51]. Both chloride and phosphate remained clearly below the recommended limit value of 3 mg/L for salmonids [51]. Additionally, the plants in the aquaponic system absorb phosphate from circulating water [42] but likely because of the low level of phosphate in both aquaponics and RAS, the difference between the two systems was not significant (Table 5). The plants can perform well at variable concentrations of nutrients in soilless systems [52]. In our study, the concentrations of macronutrients, i.e., nitrate, chloride, phosphate, and sulphate, were within the range required for the growth of plants in soilless systems [52; 53; 54; 55]. Chloride is a beneficial nutrient for plant growth which is required in small quantities, and less than 70 mg/L is generally safe for all plants [52]. Higher chloride concentration can affect the absorption of nitrogen and other nutrients. Phosphate is also needed in small quantities for the plant's growth. Phosphorus plays an important role in root development and flower quantity [54]. A study documented that the Kale plant (_Brassica oleracea_) performed well at 10-times lower concentrations (0.1 mM) of phosphate without compromising its growth in hydroponics [55]. Sulfur is an important nutrient for plants and required in small quantities. Plant absorbs sulfur in the form of sulfate. The sulfur requirement can vary for different plants depending upon the plant species. Generally the recommended concentration of sulfur in hydroponics nutrient solution range from 48 to 336 mg/L [12; 53]. In the present study the sulfate concentration ranged from 69.06 \(\pm\) 49.74 (aquonics) to 85.32 \(\pm\) 57.03 (hydroponics).