Spatio-Temporal Distributions And Characteristics Of Particulate Organic Matter And Settling Particulate Matter In The Mariculture Area Of Shen’ao Bay,Guangdong

Shen’ao Bay located in the northeast part of Nan’ao Island, Guangdong province, is the major cultivation base for large seaweed Gracilaria lemaneiformis in China, which houses a typical integrated multi-trophic aquaculture(IMTA) system. To reveal the role of G. lemaneiformis cultivation in eutrophic coastal seawaters, the matter cycling and the key biogeochemical processes in IMTA, seawater particulate organic matter(POM) and settling particulate matter(SPM) were collected from different culture sites of Shen’ao Bay roughly monthly from April, 2014 to December 2015. Based on the analyses of their mass concentrations and geochemical compositions, the main conclusions were reached as follows:1. Based on the G. lemaneiformis cultivation period and seasonal variation in Shen’ao Bay, the temporal changes of POM in water column can be divided into 3 periods. A) Spring(G. lemaneiformis cultivation period). In this period, the POM consisted of microalgae. As the large-scale cultivation of G. lemaneiformis can remove excess nutrients in coastal seawater and has allelopathy inhibitory effects on the growth of marine microalgae, the biomass of microalgae was low. Consequently, the concentration of POM in this stage was low and showed a little variation. B) Summer. After G. lemaneiformis culture and water temperature increased in summer, the growth of microalgae accelerated gradually. As a result, the concentration of water POM was increased and fluctuated significantly. C) Autumn and winter. The increasing current velocity caused by strong north-eastern winds, led to stronger shear forces and resulted in the resuspension of sediments and elevation of nutrients in water column. The concentration of POM in this period was high and fluctuated dramatically.2. The average values of total nitrogen(TN) and phosphorus(TP) of SPM in Shen’ao Bay were 2185.4mg/kg and 497.3mg/kg, respectively. During G. lemaneiformis cultivation period, the contents of TN and TP were relatively low. It has been shown that G. lemaneiformis can reduce N, P from SPM effectively. Compared to other similar bays, the TN content in Shen’ao Bay was higher than those in the Dapeng Bay, Jiaozhou Bay and Luoyuan Bay. However, the TN content in Shen’ao Bay was lower than these bays.3. Pearson correlation and cluster analyses were used to assess the origin of major and minor elements. The results showed that SiO2, Al2O3, Fe2O3, Co, Pb, Cr, Rb, Sr, Mn, Zr and Ti originate from terrestrial source and K2 O, MgO and CaO were sourced mainly from marine organic matter. Meanwhile, Zn, Cu and Ni had multiple sources and mostly came from Huanggang River, which is located to the north of Shen’ao Bay and through the regions of copper and zinc.4. Single factor appraisal method demonstrated that, the content of the TP was relatively low except at the fish-cage culturing site. However, the nitrogen pollution was relatively serious. The level of Cu from the SPM was higher than the Class II marine sediment standard(GB 18668-2002), and the content of Zn higher than the Class I marine sediment standard. Only Pb in the SPM was below the standard level of Class I.5. The C/N, δ13C and δ15N values in the SPM from Shen’ao Bay fell within range of marine planktonic microalgae, indicating that the organic matters was mainly originated from marine organisms. A cross-site comparison revealed that the marine matter had greater influence on the G. lemaneiformis and fish-cage culture sites than the control site.