Growth and contents of C, N, and P of kelp (Saccharina japonica) cultured in Nanri Island, China and its effects on particulate and dissolved organic matter of seawater

In order to study the ecological and environmental effects of kelp aquaculture, this study investigated the growth rate, the tissue elemental contents (C, N, and P) and ratios of cultured commercial kelp (Saccharina japonica), in Nanri Island, Fujian Province, and the seasonal variations of the particulate and dissolved organic matter in the cultured area. The result showed that the wet weight of kelp varied as a power function of the length (W=0.02 L^7.57, R²=0.84). Besides, the wet weight (W=5.45 t−30.50, R²=0.96), the length (L=1.72 t+41.18, R²=0.97), and the width (W=0.30 t+5.28, R²=0.93) were linearly increased with the culture days. The tissue C/N ratio varied from 9.13 to 18.66, N/P ratio ranged from 11.32 to 18.48, and C/P ratio varied from 153.18 to 267.99. The tissue C/N ratio varied as an exponential function of N content (Y=748.30 e^−4.10X, R²=0.88), and the C/P ratio varied as an exponential function of tissue P content (Y=50.21 e^−0.60X, R²=0.92). The seawater POC, PON, and POP in the culture zone varied from 0.33 to 0.86 mg/L, from 0.07 to 0.11mg/L, and from 0.01 to 0.02 mg/L, respectively. The DOC, DON, and DOP in seawater of culture zone varied from 1.98 to 17.06 mg/L, from 0.20 to 0.55 mg/L, and from 0.01 to 0.04 mg/L, respectively. The average C, N, and P contents at harvest period were 26.17%, 1.76%, and 0.29%, respectively. The total production of cultured kelp in Fujian Province was about 768 300 tons, which indicates that the removable C, N, and P contents were about 201 300, 13 500, and 2 200 t, respectively. Our result demonstrated that the kelp aquaculture is an important source of seawater organic matter in the culture area, and is one of the important components of coastal carbon cycle.