Bioremediation efficiencies of <Emphasis Type="Italic">Gracilaria verrucosa</Emphasis> cultivated in an enclosed sea area of Hangzhou Bay,China

Due to the discharge of nutrients into the East China Sea, severe eutrophication has appeared in Hangzhou Bay. Therefore, we cultivated Gracilaria verrucosa on a large scale in the Jinshan enclosed sea with an area of 1.72 km² in the northern part of Hangzhou Bay to perform bioremediation. The Fengxian enclosed sea with an area of 2.3 km² and 50 km far from Jinshan was used as the control. The results showed that the Hangzhou Bay was severely eutrophicated before G. verrucosa cultivation. During the period of cultivation between August 2006 and July 2007, the annual growth rate of G. verrucosa was 9.42% day⁻¹, and the sea water quality was improved from worse than grade IV to grades II–III, with the concentration of dissolved inorganic nitrogen (DIN) and PO₄-P significantly lower than that in the Fengxian enclosed sea (p < 0.01). The concentration of NH₄-N, NO₃-N, NO₂-N, and PO₄-P after G. verrucosa cultivation was decreased by 54.12%, 75.54%, 49.81%, and 49.00%, respectively. The density of phytoplankton in the Jinshan enclosed sea with cultivation of G. verrucosa was 6.90 –126.53 × 10⁴ cells m⁻³, which was significantly lower than that in the Fengxian enclosed sea. In addition, species diversity, richness, and evenness was significantly increased after cultivation of G. verrucosa in the Jinshan enclosed sea compared with that in the Fengxian enclosed sea. The density of Skeletonema costatum, Prorocentrum micans, and Prorocentrum donghaiense, which were the usual species of red tides at the coastal sea of China, in the Jinshan enclosed sea with cultivation of G. verrucosa was significantly lower. Based on these results, if the water quality in the Jinshan enclosed sea were to be maintained at grade I (DIN ≤0.20 mg  L⁻¹) or II (DIN ≤0.30 mg  L⁻¹), 21.8 t or 18.0 t fresh weight of G. verrucosa need to be cultivated, respectively. These results indicated that large-scale cultivation of G. verrucosa could play a significant role in the bioremediation of Hangzhou Bay.