Ecological functions of the kelp Saccharina japonica in integrated multi-trophic aquaculture, Sanggou Bay, China

The growth rate, photosynthetic activities,and nitrogen nutrient uptake characteristics of cultured commercial kelp (Saccharina japonica) were studied in situ and under laboratory conditions. The lengths of marked kelps were measured every 10 to 20 days throughout their entire life cycle by collecting 5-10 individuals for simultaneous weight measurement. The in-situ seaweed was incubated in high light transmission polyethylene tubes (the perimeter of the tube was 25-50 cm; light transmittance was above 80%) suspended in the kelp farming area, and the photosynthetic oxygen production rate at different growth stages was measured in January, March, May, and July. In the laboratory, the total inorganic nitrogen (TIN) uptake rate was measured for discs taken from different parts of the kelp under two temperature treatments of 4°C and 10°C. The NH4-N and NO3-N selective uptake characteristics of the discs were also measured. The results showed that the wet weights are power functions that are related to both culture days (W = 1.3886 t1.362, R2= 0.9611) and kelp length (W = 0.0071 L2.0882, R2= 0.9392) during the culture period. There was a clear positive linear correlation (R2 ranged from 0.950 to 0.981) between the oxygen production rate (O2 mg/h) and the wet weight (g), and the slope (related to the photosynthetic oxygen production rates of the kelp by unit time and unit fresh wet) varied from 0.096 to 0.195 with an average of 0. 191. The oxygen production rate in unit fresh weight was lower at the first growth stage (January), but gradually increased and became stable after March. The TIN uptake rate varied between different parts of the kelp. The uptake rates of the upper part of the middle band (60-110 cm) and the base of the plant (20-50 cm) were faster than the lower part of the middle band (150-200 cm) and the marginal part of the plant. The highest TIN uptake rate was observed between 0.5 and 1 hour after nitrogen starvation, and about 64.2% to 97.1% of the TIN in the culture medium (initial concentration was 24.2μmol/L and the kelp density was 4 g/L) was removed within 24 hours. The TIN uptake and removal rate of the tested kelp incubated at 10℃ was higher than that at 4°C. The NO3-N uptake rate of the kelp discs was higher than for NH4-N, and became stable after 24 hours. The results demonstrated that the kelp has a relatively high growth rate, nutrient uptake rate, and active photosynthetic activities, which means it has valuable ecological functions