Nutrient Budget and Biogeochemical Dynamics in Sansha Bay, China: A Coastal Bay Affected by Intensive Mariculture

This study examines the spatiotemporal distribution of dissolved and particulate nutrients in Sansha Bay, a coastal bay characterized by complex hydrodynamics (monsoon-driven river inputs and alongshore coastal waters), and intensive mariculture. However, nutrient stoichiometry and underlying biological responses in this ecosystem remain unclear. High nutrient (39.6-87.5 mu mol L-1 dissolved inorganic nitrogen (DIN), 1.79-3.77 mu mol L-1 dissolved inorganic phosphorus (DIP)) in winter suggested entrainment of the nutrient-enriched China Coastal Current superimposed with mariculture inputs; lower nutrient concentrations (9.6-47.1 mu mol L-1 DIN, 0.31-1.57 mu mol L-1 DIP) in summer, and high Chlorophyll a (0.8-17.0 mg m(-3)) and dissolved oxygen (4.48-8.78 mg L-1), were driven by the exchange of nutrient-poor coastal waters and enhanced biological consumption. Using the Land-Ocean Interaction Coastal Zone (LOICZ) mass balance model, field observations of nutrient sources/sinks allowed construction of a robust nutrient budget, in which Delta indicated non-conservative dissolved nutrient fluxes. In both winter and summer, the external DIN and DIP additions ratio Delta DIN:Delta DIP was similar to 21.8-20.4:1, while Si(OH)(4) addition/removal occurred seasonally. Potential N biogeochemical processes were also explored. Using a three end-member mixing model, nutrient biogeochemistry (regeneration/uptake, denoted by delta) within the aquaculture ecosystem was estimated separately from complex physical mixing effects. In winter, nutrient regeneration dominated (delta DIN:delta DIP similar to 20:1; delta Si(OH)(4):delta DIN <1:2), likely due to remineralization of particles (phytoplankton with a 16:1 Redfield ratio and/or fish feed/feces with a non-Redfield ratio). In contrast, nutrient consumption dominated in summer (delta DIN:delta DIP similar to 13.5:1; delta Si(OH)(4):delta DIN similar to 2.4:1). Results of this study are relevant to other coastal bays affected by intensive aquaculture, riverine inputs and coastal waters.

Automated summary

This study examines the spatiotemporal distribution of dissolved and particulate nutrients in Sansha Bay, highlighting seasonal variations and factors influencing nutrient dynamics, with implications for other coastal bays facing similar pressures from intensive aquaculture, riverine inputs, and coastal waters.