Watershed-Scale Drivers of Air-Water CO_2 Exchanges in Two Lagoonal North Carolina (USA) Estuaries

摘要

Riverine loading of nutrients and organic matter act in concert to modulate CO_2 fluxes in estuaries, yet quantitative relationships between these factors remain poorly defined. This study explored watershed-scale mechanisms responsible for the relatively low CO_2 fluxes observed in two microtidal, lagoonal estuaries. Air-water CO_2 fluxes were quantified with 74 high-resolution spatial surveys in the neighboring New River Estuary (NewRE) and Neuse River Estuary (NeuseRE), North Carolina, which experience a common climatology but differ in marine versus riverine influence. Annually, both estuaries were relatively small sources of CO_2 to the atmosphere, 12.5 and 16.3 mmol C m~(-2) d~(-1 )in the NeuseRE and NewRE, respectively. Large-scale pCO_2 variations were driven by changes in freshwater age, which modulates nutrient and organic carbon supply and phytoplankton flushing. Greatest pCO_2 undersaturation was observed at intermediate freshwater ages, between 2 and 3 weeks. Biological controls on CO_2 fluxes were obscured by variable inputs of river-borne CO_2, which drove CO_2 degassing in the river-dominated NeuseRE. Internally produced CO_2 exceeded river-borne CO_2 in the marine-dominated NewRE, suggesting that net ecosystem heterotrophy, rather than riverine inputs, drove CO_2 fluxes in this system. Variations in riverine alkalinity and inorganic carbon loading caused zones of minimum buffering capacity to occur at different locations in each estuary, enhancing the sensitivity of estuarine inorganic C chemistry to acidification. Although annual CO_2 fluxes were similar between systems, watershed-specific hydrologic factors led to disparate controls on internal carbonate chemistry, which can influence ecosystem biogeochemical cycling, trophic state, and response to future perturbations.