Dissolved Inorganic Nutrient Biogeochemistry in an Urbanized Coastal Region: A Study of Dapeng Cove, Shenzhen

Dissolved inorganic nutrients are pivotal in maintaining the material and energy balance of marine ecosystems, impacting the survival and dynamic succession of marine organisms. To gain a deeper understanding of the source and sink characteristics of dissolved inorganic nutrients in bays affected by human activities and to elucidate the processes involving filter-feeding shellfish in relation to these nutrients, this study investigated the source and sink dynamics of dissolved inorganic nutrients in the Dapeng Cove sea area of Shenzhen. Over the past decade, a significant change in the N/P ratio within the survey area has been observed, suggesting a shift in nutrient limitation from nitrogen to phosphorus or phosphorus-silicon limitation. This induced change in the N/P ratio, along with Si/N and Si/P ratios, may facilitate the growth of cyanobacteria and, subsequently, alter the proportions of diatoms, dinoflagellates, and cyanobacteria. Seasonal fluctuations in human disturbance intensity and precipitation determine the seasonal and spatial distribution of nutrients in the bay, thereby influencing the bay ecosystem metabolism. The Land-Ocean Interactions in the Coastal Zone (LOICZ) model analysis revealed that the bay represents a major source of inorganic nitrogen and a source of phosphate in spring, summer, and autumn, while acting as a sink for phosphate in winter. Furthermore, rivers and groundwater represent the primary sources of phosphate and inorganic nitrogen in the bay. The bay exhibits an annual net ecosystem metabolic rate of 7.06 mmol C/m2/d, with denitrification dominating the nitrogen cycle at 12.65 mmol C/m2/d. Overall, the Dapeng Cove ecosystem displays net production exceeding respiration, classifying it as an autotrophic system. Additionally, the nitrogen cycle in the sea area is predominantly driven by denitrification. The analysis also revealed that the impact of oyster proliferation on the physical and chemical factors in the surveyed area is relatively weaker than that of surface runoff and groundwater inputs.