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) (6.32 µg 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, NO2−-N, N/P ratio, SiO32−-Si, and salinity were determined as the key factors underlying Chl-a and phytoplankton diversity. Additionally, the results of redundancy analysis further indicated that the phytoplankton assemblage in the bivalve zone is positively related with nutrients such as NO3−-N and NH4+-N as well as water depth, while the phytoplankton assemblages in the kelp, control, and IMTA zones are associated with NO2−-N, SiO32−-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 SiO32−-Si.