Seaweeds are the major primary producers, and play a vital role in coastal ecosystems. Since 2000, large-scale cultivation of has rapidly developed along the Chinese coast. Nan'ao island is an important base for cultivation in Guangdong. In order to study the effects of large-scale seaweed cultivation on the aquatic environment and phytoplankton, monthly surveys were carried out in the cultivation area of Shen'ao Bay in Nan'ao island from March to June 2016. In this study, fish culture area (F), Gracilaria culture area (G), and control area (C) without cultivation activities were selected for sampling. There were three sampling points per area, marked as F₁, F₂, F₃, G₁, G₂, G₃, C₁, C₂, and C₃, respectively. Four surveys were undertaken on March 25, April 24, May 23, and June 22, 2016. March and April were the cultivation periods, while May was the harvest period, and in June, no was cultured. Dissolved oxygen (DO), pH, total nitrogen (TN), total phosphorus (TP), inorganic nitrogen (DIN), inorganic phosphorus (DIP), phytoplankton density, and chlorophyll a (Chl a) contents were measured. Moreover, the amount of N, P, and C removed from seawater and the amount of O₂ released to seawater by were estimated based on the statistical data from Nan'ao in 2016. The results showed that pH and DO in the culture area were significantly higher than those in other regions (>0.05) between the sampling areas after seaweed harvest (June). The contents of TN, TP, DIN, and DIP in the Gracilaria culture area were lower than those in other areas (<0.05) from March to April. Similarly, phytoplankton densities and Chl a contents were significantly lower in the <0.05). The total produce of in Nan'ao island for 2016 was 49729 t, removing 2212 t of N, 174 t of P, and 13000 t of C from seawater while releasing 34700 t of O₂ to seawater. Our results show that large-scale cultivation of could effectively remove N and P, improve pH and DO, and inhibit phytoplankton growth. This has the potential to control seawater eutrophication, mitigate the impacts of ocean acidification and hypoxia, and prevent harmful algal blooms.