LAUSR

Riverine loading of nutrients and organic matter act in concert to modulate CO2 fluxes in estuaries, yet quantitative relationships between these factors remain poorly defined. This study explored watershed-scale mechanisms responsible for the relatively low CO2 fluxes observed in two microtidal, lagoonal estuaries. Air-water CO2 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 CO2 to the atmosphere, 12.5 and 16.3 mmol C m 2 d 1 in the NeuseRE and NewRE, respectively. Large-scale pCO2 variations were driven by changes in freshwater age, which modulates nutrient and organic carbon supply and phytoplankton flushing. Greatest pCO2 undersaturation was observed at intermediate freshwater ages, between 2 and 3 weeks. Biological controls on CO2 fluxes were obscured by variable inputs of river-borne CO2, which drove CO2 degassing in the river-dominated NeuseRE. Internally produced CO2 exceeded river-borne CO2 in the marine-dominated NewRE, suggesting that net ecosystem heterotrophy, rather than riverine inputs, drove CO2 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 CO2 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. Plain Language Summary Estuaries release nearly as much CO2 to the atmosphere as is taken up over the continental shelf. However, estuarine emissions vary greatly across space and time, contributing significantly to the uncertainty of global carbon budgets. In this study, we assess spatial and temporal variability in CO2 emissions from adjacent estuaries in North Carolina, USA. These emissions varied across seasons and river discharge conditions but were relatively small when assessed as annual averages. Freshwater age (time freshwater spends in estuary before being flushed to ocean) was an important driver of CO2 dynamics in both estuaries, due to its role in regulating nutrient, DOC, and DIC supply while also affecting the rate at which phytoplankton are flushed from the system. While the relationship between freshwater age and CO2 was similar for both estuaries, we show that the various external and internal inputs of CO2 were quite different. Riverine CO2 inputs drove CO2 emissions in the river-dominated estuary, while internally produced CO2 (from community respiration) was more important in the marine-dominated estuary. We also demonstrate that poorly buffered regions in both estuaries are particularly vulnerable to acidification, with potentially negative impacts on calcifying organisms.