Impacts of Integrated Multi-Trophic Aquaculture on Phytoplankton in Sanggou Bay

Integrated multi-trophic aquaculture(IMTA)has been considered as an ecofriendly culture system providing a potential solution to environmental risks caused by intensive monoculture system.However,the impact of IMTA on phytoplankton remains unclear.In this study,the spatial and temporal variations of phytoplankton in Sanggou Bay were investigated seasonally based on 21 sampling sites covering three cultivation zones(bivalve zone,IMTA zone,and kelp zone)and one control zone(without aquatic cultivation).In total,128 phytoplankton species,with diatoms and dinoflagellates as the dominant groups,were obtained across the whole year,and the mean Shannon diversity index(H')and species richness(SR)were determined as 1.39 and 9.39,respectively.The maximum chlorophyll a(Chl-a)(6.32μg L^(-1))and plankton diversity(H'of 1.97)occurred in summer and autumn,respectively.Compared to other zones,the bivalve zone displayed significantly higher Chl-a and lower H'in majority of time.Pairwise PERMANOVA analysis indicated that the phytoplankton assemblage in the bivalve zone was significantly different with the control and kelp zones,while the IMTA zone maintained close to other three zones.Based on generalized additive models,temperature,NO_(2)^(-)-N,N/P ratio,SiO_(3)^(2-)-Si,and salinity were determined as the key factors underlying Chl-a and phytoplankton diversity.Addi-tionally,the results of redundancy analysis further indicated that the phytoplankton assemblage in the bivalve zone is positively re-lated with nutrients such as NO_(3)^(-)-N and NH_(4)^(+)-N as well as water depth,while the phytoplankton assemblages in the kelp,control,and IMTA zones are associated with NO_(2)^(-)-N,SiO_(3)^(2-)-Si,and salinity.Taken all observations into consideration together,it can be inferred that IMTA can effectively reduce Chl-a level compared to bivalve monoculture by reducing the nutrients.However,the SR,H’,and species composition of phytoplankton are primarily determined by local environment factors such as temperature,water depth,salinity and SiO_(3)^(2-)-Si.

The phosphorus cycle in the Sanggou Bay

The phosphorus cycle is studied during 2013–2014 in the Sanggou Bay（SGB）, which is a typical aquaculture area in northern China. The forms of measured phosphorus include dissolved inorganic phosphorus（DIP）, dissolved organic phosphorus（DOP）, particulate inorganic phosphorus（PIP）, and particulate organic phosphorus（POP）.DIP and PIP are the major forms of total dissolved phosphorus（TDP） and total particulate phosphorus（TPP）,representing 51%–75% and 53%–80%, respectively. The concentrations and distributions of phosphorus forms vary among seasons relative to aquaculture cycles, fluvial input, and hydrodynamic conditions. In autumn the concentration of DIP is significantly higher than in other seasons（P〈0.01）, and higher concentrations are found in the west of the bay. In winter and spring the phosphorus concentrations are higher in the east of the bay than in the west. In summer, the distributions of phosphorus forms are uniform. A preliminary phosphorus budget is developed, and shows that SGB is a net sink of phosphorus. A total of 1.80×10^7 mol/a phosphorus is transported into the bay. The Yellow Sea is the major source of net input of phosphorus（61%）, followed by submarine groundwater discharge（SGD）（27%）, river input（11%）, and atmospheric deposition（1%）. The main phosphorus sink is the harvest of seaweeds（Saccharina japonica and Gracilaria lemaneiformis）, bivalves（Chlamys farreri）,and oysters（Crassostrea gigas）, accounting for a total of 1.12×10^7 mol/a. Burial of phosphorus in sediment is another important sink, accounting for 7.00×10^6 mol/a. Biodeposition by bivalves is the major source of phosphorus in sediment, accounting for 54% of the total.