Wind-tide modulated net ecosystem metabolism in a Changjiang River Estuary aquaculture zone during mid-autumn

The Changjiang River Estuary (CRE) supports critical fisheries and aquaculture industries. Net Ecosystem Metabolism (NEM) serves as a key indicator for assessing aquaculture ecosystem health, providing early warning of hypoxia risks, determining environmental carrying capacity, and guiding scientific aquaculture management. However, the hydrologic regime of the CRE, influenced by monsoons and strong tides, makes the combined effects of high-frequency wind and tidal variations on NEM dynamics poorly understood. In this study, we deployed a high-resolution in-situ profiler at a mussel aquaculture site in the CRE during September-October 2022 to continuously measure parameters, including dissolved oxygen (DO), at 1-min intervals. We then applied a one-dimensional DO mass balance model to quantify NEM variations and elucidate the driving mechanisms under contrasting wind and tidal conditions. Results indicated a predominantly heterotrophic ecosystem, with a mean NEM value of -6.0 ± 27.4 mmol m^-2 h^-1. Mechanistic analysis revealed that wind and tidal forces regulate NEM by modulating the frontal dynamics between the Changjiang River Diluted Water (CDW) and the Taiwan Warm Current (TWC). Northerly winds enhanced heterotrophy by intensifying CDW dominance and terrestrial organic matter inputs. In contrast, easterly winds coupled with spring tides promoted autotrophy through the onshore advection of shelf surface waters (SSW). Tidal cycles further regulated NEM through vertical mixing-mediated benthic nutrient output, resulting in higher autotrophy during spring tides. These findings, derived from high-frequency observations, highlight the acute sensitivity of estuarine metabolism to shifts in dominant water masses. They provide essential insights for adaptive management strategies, such as implementing real-time environmental monitoring and selecting species aligned with water mass dynamics, to enhance aquaculture sustainability amidst climate variability. Nevertheless, limitations in the temporal scope (mid-autumn period) and localized geography necessitate future investigations into seasonal/interannual NEM variations across broader spatial scales. Integrating wider drivers, including river discharge and climate change, will yield more robust data support and mechanistic analysis for developing predictive models.