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Abstract Seagrass meadows play an important role in “blue carbon” sequestration and storage, but their dynamic metabolism is not fully understood. In a denseZostera marinameadow, we measured benthic O2fluxes by aquatic eddy covariance, water column concentrations of O2, and partial pressures of CO2(pCO2) over 21 full days during peak growing season in April and June. Seagrass metabolism, derived from the O2flux, varied markedly between the 2 months as biomass accumulated and water temperature increased from 16°C to 28°C, triggering a twofold increase in respiration and a trophic shift of the seagrass meadow from being a carbon sink to a carbon source. Seagrass metabolism was the major driver of diurnal fluctuations in water column O2concentration and pCO2, ranging from 173 to 377 μmol L−1and 193 to 859 ppmv, respectively. This 4.5‐fold variation in pCO2was observed despite buffering by the carbonate system. Hysteresis in diurnal water column pCO2vs. O2concentration was attributed to storage of O2and CO2in seagrass tissue, air–water exchange of O2and CO2, and CO2storage in surface sediment. There was a ~ 1:1 mol‐to‐mol stoichiometric relationship between diurnal fluctuations in concentrations of O2and dissolved inorganic carbon. Our measurements showed no stimulation of photosynthesis at high CO2and low O2concentrations, even though CO2reached levels used in IPCC ocean acidification scenarios. This field study does not support the notion that seagrass meadows may be “winners” in future oceans with elevated CO2concentrations and more frequent temperature extremes.
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Variability and Controls of p CO 2 and Air‐Water CO 2 Fluxes in a Temperate River
Abstract Measurements of riverine dissolved inorganic carbon, total alkalinity (AT), pH, and the partial pressure of carbon dioxide (pCO2) can provide insights into the biogeochemical function of rivers, including the processes that control biological production, chemical speciation, and air‐water CO2fluxes. The complexity created by these combined processes dictates that studies of inorganic carbon be made over broad spatial and temporal scales. Time‐series data like these are relatively rare, however, because sampling and measurements are labor intensive and, for some variables, good measurement quality is difficult to achieve (e.g., pH). In this study, spectrophotometric pH and ATwere quantified with high precision and accuracy at biweekly to monthly intervals over a four‐year period (2018–2021) along 216 km of the Upper Clark Fork River (UCFR) in the northern Rocky Mountains, USA. We use these and other time‐series data to provide insights into the processes that control river inorganic carbon, with a focus onpCO2and air‐water CO2fluxes. We found that seasonal snowmelt runoff increasedpCO2and that expected increase and decrease ofpCO2due to seasonal heating and cooling were likely offset by an increase and loss of algal biomass, respectively. Overall, the UCFR was a small net source (0.08 ± 0.14 mol m−2 d−1) of CO2to the atmosphere over the four‐year study period with highly variable annual averages (0.0–0.10 mol m−2 d−1). The seasonally correlated, offsetting mechanisms highlight the challenges in predictingpCO2and air‐water CO2fluxes in rivers.
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- PAR ID:
- 10571691
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 130
- Issue:
- 2
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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