Changes in estuarine carbon and nitrogen budgets along a gradient of land-use intensity

Estuaries play an important role in global carbon and nitrogen cycles. However, land-use change can have a significant, yet poorly understood, effect on benthic and whole-estuarine carbon and nitrogen cycles. To better understand how land-use change influences estuarine carbon and nitrogen cycles, this study measured benthic metabolism and denitrification over four seasons in three sub-tropical estuaries along a gradient of land-use change. Carbon and nitrogen budgets were also constructed for the three estuarine systems. The highest annual benthic respiration rates were found in the high land-use intensity estuary. The gross primary production/respiration ratio showed that on average the estuaries shifted from being net autotrophic to net heterotrophic as land-use intensity increased. Denitrification rates increased from an annual average of 5.1± 3 to 23 ± 18 umol m^-2 d^-1, but the percentage of the atmospheric, river and wastewater nitrogen load removed via denitrification decreased (>100% to 50%), over the land-use gradient. As such, the estuaries become net exporters of carbon and nitrogen to the ocean as land-use intensifies. Our findings show that estuaries can switch from assimilating carbon and nitrogen to directly exporting carbon and nitrogen to the ocean due to human modification.