Carbon budget of a shallow, lagoonal estuary: Transformations and source‐sink dynamics along the river‐estuary‐ocean continuum

Abstract

A comprehensive carbon budget was constructed to quantify carbon flows through the freshwater‐marine continuum of a temperate, microtidal estuary. We performed coordinated measurements of dissolved inorganic carbon and total organic carbon fluxes to resolve spatial variability between and along the channel and shoals and diel variability across the entire estuary for 2 yr. Net ecosystem metabolism (NEM) was the most significant control on carbon flow within estuary regions. However, metabolic rates were spatially coupled such that counteracting fluxes across the channel‐shoal gradient or along the river‐ocean gradient resulted in system‐wide NEM that was closely in balance (–3.0 ± 3.3 to 1.1 ± 4.4 molC m−2 yr−1). Similarly, large diel and seasonal variability in air–water CO2 fluxes were observed during 72 spatial surveys, but these short‐term variations generally cancelled out when aggregated to annual budget terms. Although atmospheric exchanges were small (–0.2 ± 0.1 to 2.0 ± 0.4 molC m−2 yr−1), they were subject to large errors (± 4 molC m−2 yr−1) if diel variability was neglected. Internal mechanisms that maintained balanced carbon flows were strongly impacted by river discharge and were only apparent by separately quantifying channel and shoal fluxes. Notably, metabolic responses of the shoal to river forcing outweighed the responses of the channel, and the net impact was contrary to prior relationships derived from synthesis of lower‐resolution carbon budgets. Our budget demonstrates that resolution of carbon fluxes at appropriate scales, including channel‐shoal and diel variability, is critical to characterizing ecosystem function and the fate of carbon within the river‐ocean continuum.

Watershed‐Scale Drivers of Air‐Water CO2 Exchanges in Two Lagoonal North Carolina (USA) Estuaries

“…Both estuaries were divided into three morphologically distinct sections along their respective longitudinal axes (Figure ), such that each estuary's mouth is the point at which other watersheds become influential. These divisions are consistent with previous studies in the NeuseRE (Crosswell et al, ) and NewRE (Crosswell et al, ). In situ p CO 2 was measured using a combined laminar flow showerhead equilibrator and infrared detector (LI‐COR, Li‐840A).…”

“…Estuaries often receive large allochthonous organic matter loads, which support high remineralization rates and drive them toward net ecosystem heterotrophy and CO 2 degassing (Bauer et al, ; Borges & Abril, ; Cai, ; Frankignoulle et al, ). At the same time, nutrients (nitrogen and phosphorus) supplied externally or recycled internally drive high rates of autochthonous organic matter production in these ecosystems, causing some estuaries to act as CO 2 sinks (Crosswell et al, , ; Drupp et al, ; Hunt et al, ; Maher & Eyre, ). The balance between these factors varies over space and time, making it difficult to globally integrate CO 2 fluxes across these diverse ecosystems.…”

Lingering Carbon Cycle Effects of Hurricane Matthew in North Carolina's Coastal Waters

“…We believe the most appropriate value falls between these estimates. Overall, CO 2 emissions from the NRE‐PS were about an order of magnitude less than lateral C fluxes, which were dominated by DOC, a result consistent with prior events (Table ; Crosswell et al, ).…”

Parameterizing Air‐Water Gas Exchange in the Shallow, Microtidal New River Estuary

“…An EC footprint analysis (Kljun et al, ) indicates that the data screening steps were successful in limiting CO 2 flux measurements to those entirely within the aquatic domain, suggesting that adjacent forested and urban regions did not affect measured CO 2 flux (Figure S1). Averaged over the entire study period, CO 2 flux was 0.31 ± 1.4 mmol·m −2 ·hr −1 , nearly half of the 0.68 average reported in Van Dam, Crosswell, Anderson, et al () for the New River estuary and within the range reported in Crosswell et al () for 2013–2014 and 2014–2015 (−0.023 and 2.3 mmol·m −2 ·hr −1 , respectively) for the same estuary. Winds were generally moderate, not exceeding 14 m/s as a 30‐min average and were dominated by a sea breeze, land breeze oscillation (Figure d).…”