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Combined pressures from inland agricultural intensification and coastal development are dramatically altering estuaries’ structure and function. Despite the established global significance of estuarine carbon (C) cycling, the impact of growing anthropogenic stress on coastal C inputs and exports is unclear. To address this gap, we evaluated the magnitude and drivers of estuary C fluxes in eight sub-tropical estuaries at Low (n = 3), Moderate (n = 2), and High (n = 3) levels of nutrient enrichment. We measured changes in the concentration and isotopic composition (δ¹³C) of the major C pools (organic and inorganic) and gaseous product of C turnover (CO₂) over wet and dry seasons. Over both sampling periods estuaries classified Moderate and High emitted far more CO₂ (37 ± 10 mmol m⁻² d⁻¹) than those classified Low (6.3 ± 4 mmol m⁻² d⁻¹). However, estuaries with both high nutrients and …