The Ecological Response Of Diatom Community To The Gracilaria Lemaneiformis Cultivation In Shen'ao Bay,Nan'ao Island,guangdong Province

Rapid development of marine aquaculture has caused a severe degradation in the coastal environments of China in recent years,and various remediation methods,including physical,chemical and biological approaches,have emerged to remediate the coastal waters.Among such methods,cultivation of large seaweed Gracilaria lemaneiformis has been accepted as a highly cost-effective approach.However,most studies on the bioremediation capacity of Gracilaria lemaneiformis focused on laboratory control tests or short time,in-situ field experiments.In this study,a long-term field monitoring strategy was carried out in the aquaculture area of Shen'ao Bay,Nan'ao Island,Guangdong Province from April 2014 to July 2017,aiming to demonstrate the bioremediation capacity of Gracilaria lemaneiformis in an in-situ mariculture ecosystem through physicochemical measurements and diatom community analyses.The main findings include the following aspects:1.Water quality analysis.The temporal?from February 2017 to January 2018?seawater physicochemistry showed that total nitrogen?TN?,total phosphorus?TP?,SiO₃-Si,and chemical oxygen demand(COD_Mn)were generally lower during the Gracilaria cultivation period?GCP?than that during the non-cultivation period?NGCP?.On the contrary,salinity,dissolved oxygen?DO?,pH,water temperature and transparency during the GCP were higher than that during the NGCP.The results suggest that water quality during the GCP was better than that during the NGCP.2.Nutrient concentrations of settling particulate sediments?from April 2014 to July 2017;67 samples in total?.Two sediment traps were installed in fish-cage area?Trap A?and Gracilaria cultivation area?Trap B?.TN and TP of the settling particulate sediments from Trap A were higher than that from Trap B,and their values from both traps were typically lower during GCP than during NGCP.These results indicate that large-scale cultivation of Gracilaria lemaneiformis can reduce the nutrient concentrations of in-situ environment as well as the surrounding areas.3.A total of 230 diatom species?including varieties?belonging to 54 genera,14families and 6 orders were observed in the settling particulate sediments from the two sediment traps,and Thalassionema nitzschioides,Thalassiosira binata,Cocconeis scutellum var.parva and Paralia sulcata were the dominant species.4.Diatom community analysis,including species richness,absolute abundance,planktonic/?benthic+attached?ratio,percentage of dominant species and species composition.The diatom communities in Trap A seem not to respond to Gracilaria cultivation,while the diatom dynamics in Trap B respond sensitively to Gracilaria cultivation.In Trap B,significantly higher diatom diversity was found during the GCP,while higher values in absolute abundance,planktonic/?benthic+attached?ratio and percentage of Thalassionema nitzschioides were observed during NGCP,indicating high nutrient levels.All these data imply that large-scale cultivation of Gracilaria lemaneiformis strongly influenced the diatom assemblage dynamics through increasing the diversity and reducing the cell density,and may help control the microalgal blooms.