Influence of mariculure on tempo-spatial distribution of nitrogen and phosphorus in subtropical zone: a case study of Shen'ao Bay

We investigated the tempo-spatial variation of disovled inorganic nitrogen (DIN) and phosphate (PO₄-P) in Shen'ao Bay that is a typical subtropical aquaculture bay. Besides, we studied the effects of large-scale aquaculture activities such as fish cage and shellfish raft on the tempo-spatial distribution of nutrients, and evaluated the potential limiting factors of the nutrients. The concentration and distribution of DIN and PO₄-P showed obvious seasonal variation (DIN and PO₄-P reached their maximum values in autumn and winter, respectively, and both reached their minimum values in summer). The DIN nutrients concentrations and N/P ratios in fish cage area in spring were lower than those in oyster-macroalgae culture area and the control area. However, in the other three seasons, the DIN and PO₄-P concentrations and N/P ratios in fish cage area were higher than those in the other areas. There was no significant difference in the nitrogen and phosphorus concentrations as well as N/P ratios between oyster-macroalgae culture area and the control area in each season. Both DIN and PO₄-P were higher than the theoretical threshold for phytoplankton growth, indicating that there was no absolute nutrient limitation. The N/P ratios in summer and winter were 13.6 and 13.1, respectively, which were lower than the Redfield ratio, indicating that there was potential N limitation during these seasons. The N/P ratios in spring and autumn were 16.6 and 19.0, respectively, indicating potential P limitation. The average seasonal ratio was 14.3, indicating that N was probably the most limiting nutrient in Shen'ao Bay. DIN was dominated by nitrate nitrogen (NO₃-N) except for summer with percentages of 51.7%−92.7%, followed by ammonia nitrogen (NH₄-N) with percentages of 5.2%−43.8%, and nitrite nitrogen (NO₂-N) was lowest with percentages of 2.1%−27.2%, indicating that the thermodynamic equilibrium of N was reached. Compared with 2001, the concentrations of both DIN and PO₄-P decreased, with the trophic level changing into poor from moderate. It is shown that the productivity level of Shen'ao Bay is still controlled by nitrogen, and the tempo-spatial distribution of nutrients reflects the impact of large-scale shellfish culture to a certain extent.