||Before January 2003, the Dapeng Bay lagoon was occupied by oyster culture racks and fish farming cages. Along with the development of the Dapeng Bay National Scenic Area Administration, the government has started taking actions on removing oyster culture racks, and has kept improving the quality of lagoon water. Nowadays, the government is implementing sediment dredging plan. As to discuss the change of biogeochemical processes, this study is divided by three parts, including the first stage, before the removal of oyster culture rafts; second stage, after the removal of oyster culture rafts, and the third stage, after implementation of sediment dredging.|
At the first stage, the annual mean of water exchange time at the Dapeng Bay was approximately 10 days. At the second and third stage, the annual mean of water exchange time were 6.2 days and 8.3 days, respectively. The difference is not significant between the second stage and third stage. The trend of water exchange time is similar to the seawater exchange rate. Distributions of chlorophyll a were controlled by temperature and solar radiation, rather than by nutrient concentration throughout three-stage periods. Although the change tendency between chlorophyll a, DIN, and DIP at the third stage is similar, chlorophyll a correlated positively with DIN and DIP only in fall. The net ecosystem production (NEP) was positive (p－r > 0) at all three stages, so the Dapeng Bay was always an autotrophic system throughout the study period. Before the removal of the oyster culture racks, the NEP was 5.64 mole C m-2 yr-1, after that, it increase to 11.64 mole C m-2 yr-1. During the sediment dredging period, the NEP was 14.31 mole C m-2 yr-1. The NEP increases 106 % from the first stage to the second stage, and increases 23% from the second stage to the third stage. The environmental remediation appears to produce significant influence on NEP.
The concentration of DIN、DSi、DIP decreases by removing the oyster culture of racks. But the concentrations of particulate and dissolved organic carbon and nitrogen increase sharply after removing the oyster culture racks. Nevertheless, the concentration of dissolved organic carbon, nitrogen and the phosphorus decreases during the third stage, resulted mainly from the improvement of water quality. The system changed from the condition of phosphorus surplus (Si/N=1.8±1.2 and N/P=7.4±5.2) during the first and second stage to the condition of phosphorus limitation (Si/N=1.0±1.2 and N/P=22.2±18.7) during the third stage. The ratios of particulate organic carbon and nitrogen (POC/PN) are 7.7±1.1, 8.0±1.0, 6.5±1.3, respectively. The ratio at the third stage is very close to the Redfield ratio (C/N=6.6), which may result from the improvement of water quality.
In terms of temporal and spatial variation of various parameters, DO variability was strong in the time scale than in the spatial scale, but nutrients and POC show a decrease of spatial variability from the first stage to the third stage. The removal of oyster culture racks, and the implementation of sediment dredging plan at the Dapeng Bay have significant influence on the improvement of lagoon environment for the past ten years.