Land-use intensity alters both the source and fate of CO₂ within eight sub-tropical estuaries

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 high turbidity produced less CO₂, and thus exported more DIC, than expected from hydrodynamics (freshwater flushing time). Differences in estuary phytoplankton biomass (Chla concentrations) corresponded with differences in the biological CO₂ production (respiration) rates estimated from δ¹³C-DIC variations, although respiration rates were higher than predicted based on hydrodynamics (surface area/discharge) in high nutrient, low turbidity systems. Together these findings demonstrate that land-use intensification can alter both the source and the production of estuary CO₂, and suggest that the direction of this shift can depend on ancillary factors like turbidity as well as nutrient enrichment. Evidence that human alterations to coastal ecosystems can shift the balance between DIC downstream export and CO₂ emissions outside of the range predicted by hydrodynamic factors like residence time, surface area, and discharge has implications for global C models