Tidal effects on variations in organic and inorganic biogeochemical components in Changjiang (Yangtze River) Estuary and the adjacent East China Sea

Nutrients and dissolved organic carbon (DOC) including carbohydrates are essential components that drive the carbon cycle in estuarine and coastal ecosystems. The flood-ebb and spring-neap tidal variations in DOC, carbohydrate and nutrient concentrations were investigated in the Changjiang Estuary and the adjacent East China Sea. The DOC, monosaccharide (MCHO), SiO₃²⁻, PO₄³⁻, NO₃⁻, and NO₂⁻ concentrations fluctuated notably with variations in tidal phases and tidal cycles. The SiO₃²⁻, NO₃⁻, PO₄³⁻, and NO₂⁻ concentrations were significantly correlated with salinity at flood/ebb tides over a tidal cycle, which indicates that physical mixing might be a major factor affecting tidal variations in nutrient elements. However, there were no significant correlations between DOC, carbohydrate concentrations and salinity. Asymmetrical flood-ebb tidal variations in DOC, carbohydrate and nutrient concentrations were observed, which might result from variations in Changjiang freshwater input due to asymmetrical flood-ebb tidal duration time and velocities in the Changjiang Estuary. There were statistically significant flood-ebb tidal differences in SiO₃²⁻, NO₃⁻, and NO₂⁻ concentrations during spring tide in the Changjiang Estuary and neap tide in the East China Sea. DOC and carbohydrate concentrations showed more complex tidal behaviors due to their specific physical and biological characteristics. Principal component analysis indicated that physical mixing driven by tidal forces was an important factor influencing tidal variations in these organic and inorganic components. Other physical and biological processes, such as absorption and microbial degradation, might have also contributed to the tidal variations in these components. Tidal forcing plays a considerable role in controlling temporal variations in organic and inorganic nutrients and should be considered when sampling in estuarine systems and estimate the fluxes of these components.