Implications of Environmental Variations on Saccharina japonica Cultivation in Xiangshan Bay, China

This study took Xiangshan Bay as an example to illustrate the variation characteristics of the physicochemical environments (temperature, salinity, light, nutrients, and currents) during one kelp cultivation cycle. The study was conducted from November 2020 to May 2021 through tracking down observations. Furthermore, the environmental factors were evaluated using suitability functions of kelp growth, aiming to provide references for promoting kelp cultivation in South China. We discussed the self-limiting effect of nutrients in the culture zone. The results showed that the average temperature, salinity, and light intensity during the cruises in Xiangshan Bay kelp farm were characterized by seasonal variations. Temperature was found to be the most critical environmental factor in determining the kelp cultivation window and hence the yield in Xiangshan Bay. The dissolved inorganic nitrogen (DIN) concentrations initially decreased and then increased, while the dissolved inorganic phosphorus (DIP) concentrations remained decreasing along with the kelp cultivation. The surface tide currents were dramatically attenuated by the suspended kelp cultivation, while the quasi-steady circulations which played a key role in nutrient supplementation for kelp cultivation were not weakened by the kelp cultivation. Among the cruises, the suitability indices’ ranges for temperature, salinity, light, and nutrients were 0.02–0.94, 0.96–0.99, 0.97–1, 0.96–0.97 (DIN), and 0.92–0.95 (DIP), respectively. The results of the suitability functions demonstrated that the salinity and light conditions in Xiangshan Bay were very suitable for kelp cultivation and would not cause significant cultivation risks. Although the cultivated kelp could greatly absorb nutrients, the suitability index of nutrients remained adequate even during the late stage of cultivation, indicating the present kelp cultivation scale has not reached the carrying capacity of Xiangshan Bay and there is still much potential for development. To this end, further selective breeding of the thermal tolerance variety has become the key to improving the kelp cultivation performance in Xiangshan Bay. Meanwhile, the self-limiting effects in relation to nutrients are not significant in the Xiangshan Bay kelp farm, but it might be more significant in other kelp farms.