Nutrient Budget and Biogeochemical Dynamics in Sansha Bay, China: A Coastal Bay Affected by Intensive Mariculture. Issue 9 (10th September 2021)

• Record Type: Journal Article
• Authors: Han, Aiqin; Kao, Shuh‐Ji; Lin, Wenfang; Lin, Qiaoyun; Han, Lili; Zou, Wenbin; Tan, Ehui; Lai, Yao; Ding, Guangmao; Lin, Hui
• Is Part Of: Journal of geophysical research. Volume 126:Issue 9(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-09-10
• Subjects: nutrients -- stoichiometry -- biogeochemical processes -- mariculture -- coastal bay; Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2020JG006220
• Languages: English
• ISSNs: 2169-8953

Abstract

Abstract: 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 μmol L −1 dissolved inorganic nitrogen (DIN), 1.79–3.77 μmol 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 μmol L −1 DIN, 0.31–1.57 μmol 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 Δ indicated non‐conservative dissolved nutrient fluxes. In both winter and summer, the external DIN and DIP additions ratio ΔDIN:ΔDIP was ∼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 δ) within the aquaculture ecosystem was estimated separately from complex physical mixing effects. In winter, nutrient regeneration dominated (δDIN:δDIP ∼20:1; δSi(OH)4 :δDIN <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 (δDIN:δDIP ∼13.5:1; δSi(OH)4 :δDIN ∼2.4:1). Results of this study are relevant to other coastal bays affected by intensive aquaculture, riverine inputs and coastal waters. Plain Language Summary: Sansha Bay is a coastal bay that undergoes intensive mariculture activities, and is influenced by China Coastal Current in winter and by South China Sea Warm Current in summer. Nutrient biogeochemistry is therefore complex but there is a lack of information from system scale biogeochemical studies. Using the Land‐Ocean Interaction Coastal Zone (LOICZ) mass balance model, the external DIN and DIP additions ratio was estimated, ∼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 δ) within the aquaculture ecosystem was estimated separately from complex physical mixing effects. In winter, nutrient regeneration dominated (δDIN:δDIP ∼20:1; δSi(OH)4 :δDIN <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 (δDIN:δDIP ∼13.5:1; δSi(OH)4 :δDIN ∼2.4:1). This study is one of the first attempts to use a combination of field observations, the LOICZ budget model and three end‐member mixing model to characterize external nutrient addition/removal and internal nutrient uptake/regeneration in a complex shelf system subjected to intensive mariculture activities, riverine input, and the influence of adjacent coastal seawater, and is important in defining the mariculture ecosystem in terms of a river‐shelf continuum. Key Points: Bay nutrients showed spatiotemporal variation influenced by complex hydrology and aquaculture. Moreover, DIN and DIP were consistently added; silicate was added in winter, removed in summer Construction of nutrient/nitrogen budget allowed quantification of biogeochemical processes Nutrient regeneration dominated in winter; uptake and regeneration co‐occurred in summer