Coastal North Carolina (USA) has experienced 35 tropical cyclones over the past 2 decades; the frequency of these events is expected to continue in the foreseeable future. Individual storms had unique and, at times, significant hydrologic, nutrient-, and carbon (C)-loading impacts on biogeochemical cycling and phytoplankton responses in a large estuarine complex, the Pamlico Sound (PS) and Neuse River Estuary (NRE). Major storms caused up to a doubling of annual nitrogen and tripling of phosphorus loading compared to non-storm years; magnitudes of loading depended on storm tracks, forward speed, and precipitation in NRE-PS watersheds. With regard to C cycling, NRE-PS was a sink for atmospheric CO 2 during dry, storm-free years and a significant source of CO 2 in years with at least one storm, although responses were storm-specific. Hurricane Irene (2011) mobilized large amounts of previously-accumulated terrigenous C in the watershed, mainly as dissolved organic carbon, and extreme winds rapidly released CO 2 to the atmosphere. Historic flooding after Hurricanes Joaquin (2015) and Matthew (2016) provided large inputs of C from the watershed, modifying the annual C balance of NRE-PS and leading to sustained CO 2 efflux for months. Storm type affected biogeochemical responses as C-enriched

we conducted 27 continuous-flow surveys of surface water CO 2 partial pressure (pCO 2 ) along the longitudinal axis of the Neuse River Estuary (NRE), North Carolina ranging from the tidal freshwater region to the polyhaline border with the Pamlico Sound. Lateral transects were also conducted at the borders of each of three hydrologically distinct sections. The pCO 2 displayed considerable spatial-temporal variability. Likewise, net air-water CO 2 fluxes showed high spatial and temporal variability, with a maximum [release] of 271 mmol C m À2 d À1 during high river flow conditions in fall and minimum [uptake] of À38 mmol C m À2 d À1 during wind-driven, high primary productivity conditions in late spring. During high-flow conditions, pCO 2 generally decreased from the river mouth to the Pamlico Sound, similar to patterns seen in well-mixed systems. During warm, low-flow conditions, surface water pCO 2 distributions were spatially variable and dissimilar to those patterns seen in most macrotidal, well-mixed estuaries. The annual air-water CO 2 efflux from the study area was 4.7 mol C m À2 yr À1, an order of magnitude less than previously estimated for temperate estuaries. The CO 2 fluxes observed in the NRE highlight the contrasts between macrotidal and microtidal systems and suggest that global estuarine CO 2 emissions are likely overestimated by the current classification approaches. Scaling this lower efflux by the relative surface area of macrotidal and microtidal systems would reduce the global estuarine flux by 42%.

Shallow coastal waters serve an important role as long-term carbon (C) sinks because they capture terrestrial C and retain internally produced C in wetlands and sediments. We show that tropical cyclones (TCs) can lead to rapid CO 2 efflux from estuaries, driven by physical and biogeochemical perturbation of these coastal C reservoirs, and that the magnitude of TC-driven CO 2 emissions may offset C that accumulates over much longer timescales. In August 2011, Hurricane Irene passed over North Carolina's Neuse River Estuary-Pamlico Sound (NRE-PS), which is part of the second largest estuarine system in the U.S., the Albemarle-Pamlico Sound. Irene rapidly changed the NRE-PS system from a small CO 2 sink to a large CO 2 source. Irene-induced CO 2 efflux from the NRE alone was at least four times the annual riverine C input and seven times the annual atmospheric CO 2 uptake. The magnitude and duration of ecosystem disturbance from TCs vary with storm intensity and frequency but likely are qualitatively similar across many terrestrial and coastal systems. Consequently, altered TC activity under future climate scenarios may shift the balance between C accumulation in, and release from, coastal C reservoirs.

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.