Microscale Variations in the Distribution and Consumption of Oxygen at the Sediment-Water Interface in Sanggou Bay in Summer

Integrated multi-trophic aquaculture has attracted widespread attention and is considered an effective way to purify the water environment and improve the sustainability of aquaculture. However, the biodeposition produced by large-scale, high-density raft-type shellfish and seaweed aquaculture activities has been shown to be the main reason for the enrichment of organic matter in sediments. Organic matter enrichment changes the biogeochemistry of sediment, which is the main reason for hypoxia in the benthic environment. Especially in the summer, water column stratification in terms of water and salinity can lead to hypoxia in the benthic environment. Besides, the hypoxic conditions in sediment promote anaerobic metabolism and sulfate reduction, which increase the dissolved sulfide concentration in the interstitial water. It was reported that dissolved oxygen (DO) in the sediment is depleted within a range of a few millimeters to a few centimeters in surface sediments. The diffusion boundary layer (DBL), which generally varies between a tenth of a millimeter and a few millimeters above the sediment, greatly influences substance exchange across the sediment-water interface. Current studies on the environmental effect of aquaculture mainly focus on the influence of aquaculture activities on the environment, and the influence mechanism of aquaculture activities on the sediment microenvironment remains unknown. Therefore, it is necessary to study the influence of aquaculture activities on the sediment microenvironment and to explore the influence mechanism of aquaculture activities on the distribution of physical and chemical factors in the sediment microenvironment.In this study, microscale variations in the distribution and consumption of oxygen at the sediment-water interface under high temperature conditions was investigated in the summer of 2022 in Sanggou Bay, a typical integrated multi-trophic aquaculture area in northern China. A high-resolution microelectrode system was used to investigate the profile distributions of DO, hydrogen sulfide (H2S), and pH in sediments with a spatial resolution of 1 mm collected from the shellfish monoculture area (SF), shellfish and seaweed polyculture area (SF-SW), seaweed monoculture area (SW), and offshore area (OF). The thickness of the DBL of different aquaculture areas was determined according to the DO distribution at the sediment-water interface and the inflection point of the profile distribution. Sediment cores were sliced at a thickness of 1 cm and the profile distribution of sediment organic carbon, the median grain size, and other factors in the surface sediment in different aquaculture areas were measured. The diffusive oxygen flux at the water-sediment interface was calculated from the gradient of the DO concentration in the DBL, and the sediment oxygen consumption rate in different aquaculture areas was calculated from the DO profile distribution in the sediment.The results showed that the mean thickness of the DBL in the SF, SF-SW, SW, and OF areas were (1.5±0.3), (1.0±0.3), (2.0±0.8), and (1.3±0.2) mm, respectively; there was no significant difference between different aquaculture areas. The mean sediment oxygen penetration depth (OPD) was (12.49±1.59), (12.17±0.09), (15.49±0.79), and (14.87±1.27) mm, respectively. The sediment OPDs in the SF-SW and SF areas were significantly lower than those in the SW and OF areas. The maximum concentrations of H2S were (5.73±0.04), (4.80±0.08), (3.30±0.19), and (3.97±0.38) μmol/L, respectively. The mean diffusive oxygen flux in the SF, SF-SW, SW, and OF areas were (24.10±1.89), (49.53±10.24), (26.69±13.13), and (24.79±7.95) mmol/(m2·d), respectively. The diffusive oxygen flux in the shellfish and kelp polyculture area was significantly higher than that in other areas. The diffusive oxygen flux at the sediment-water interface was affected by the DO concentration of the overlying water and the sediment oxygen consumption rate. The diffusive oxygen flux in the SF area was influenced by the thickness of the DBL, the DO concentration of the overlying water, and the DO concentration at the sediment-water interface. The main factor influencing the diffusive oxygen flux in the SW and SF-SW areas was the DO concentration of the overlying water.In conclusion, the biodeposition produced by aquaculture activities significantly affects material cycling in surface sediments, which promotes sediment oxygen consumption and the production of hydrogen sulfide. The influence of biodeposition produced by shellfish farming on the sediment chemical characteristics and microprofiles may be stronger than that of kelp debris. The DO concentration of the water environment is the main reason affecting organic matter mineralization in the sediments of aquaculture areas. This study also provides scientific and technological support for an in-depth understanding of the influence of aquaculture on the sediment microenvironment and the benthic carbon cycle.