Subtropical seagrass meadows play a major role in the coastal carbon cycle, but the nature of air–water CO2exchanges over these ecosystems is still poorly understood. The complex physical forcing of air–water exchange in coastal waters challenges our ability to quantify bulk exchanges of CO2and water (evaporation), emphasizing the need for direct measurements. We describe the first direct measurements of evaporation and CO2flux over a calcifying seagrass meadow near Bob Allen Keys, Florida. Over the 78‐d study, CO2emissions were 36% greater during the day than at night, and the site was a net CO2source to the atmosphere of 0.27 ± 0.17
Seagrass meadows play an important role in “blue carbon” sequestration and storage, but their dynamic metabolism is not fully understood. In a dense
- NSF-PAR ID:
- 10459195
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography
- Volume:
- 64
- Issue:
- 6
- ISSN:
- 0024-3590
- Page Range / eLocation ID:
- p. 2586-2604
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
Abstract μ mol m−2s−1(x̅ ± standard deviation). A quarter (23%) of the diurnal variability in CO2flux was caused by the effect of changing water temperature on gas solubility. Furthermore, evaporation rates were ~ 10 times greater than precipitation, causing a 14% increase in salinity, a potential precursor of seagrass die‐offs. Evaporation rates were not correlated with solar radiation, but instead with air–water temperature gradient and wind shear. We also confirm the role of convective forcing on night‐time enhancement and day‐time suppression of gas transfer. At this site, temperature trends are regulated by solar heating, combined with shallow water depth and relatively consistent air temperature. Our findings indicate that evaporation and air–water CO2exchange over shallow, tropical, and subtropical seagrass ecosystems may be fundamentally different than in submerged vegetated environments elsewhere, in part due to the complex physical forcing of coastal air–sea gas transfer. -
Abstract Seagrass meadows are valued for their ecosystem services, including their role in mitigating anthropogenic CO2emissions through ‘blue carbon’ sequestration and storage. This study quantifies the dynamics of whole ecosystem metabolism on daily to interannual timescales for an eelgrass (
) meadow using in situ benthic O2flux measurements by aquatic eddy covariance over a period of 11 yr. The measurements were part of the Virginia Coast Reserve Long‐Term Ecological Research study, and covered a relatively stable period of seagrass ecosystem metabolism 6–13 yr after restoration by seeding (2007–2014), a die‐off event likely related to persistently high temperatures during peak growing season in 2015, and a partial recovery from 2016 to 2018. This unique sequence provides an unprecedented opportunity to study seagrass resilience to temperature stress. With this extensive data set covering 115 full diel cycles, we constructed an average annual oxygen budget that indicated the meadow was in metabolic balance when averaged over the entire period, with gross primary production and respiration equal to 95 and −94 mmol O2m−2d−1, respectively. On an interannual scale, there was a shift in trophic status from balanced to net heterotrophy during the die‐off event in 2015, then to net autotrophy as the meadow recovered. The highly dynamic and variable nature of seagrass metabolism captured by our aquatic eddy covariance data emphasizes the importance of using frequent measurements throughout the year to correctly estimate trophic status of seagrass meadows.Zostera marina -
Abstract The prevailing hypothesis to explain pCO2rise at the last glacial termination calls upon enhanced ventilation of excess respired carbon that accumulated in the deep sea during the glacial. Recent studies argue lower [O2] in the glacial ocean is indicative of increased carbon respiration. The magnitude of [O2] depletion was 100–140 µ mol/kg at the glacial maximum. Because respiration is coupled to
δ 13C of dissolved inorganic carbon (DIC), [O2] depletion of 100–140 µ mol/kg from carbon respiration would lower deep waterδ 13CDICby ∼1‰ relative to surface water. Prolonged sequestration of respired carbon would also lower the amount of14C in the deep sea. We show that Pacific Deep Waterδ 13CDICdid not decrease relative to the surface ocean and Δ14C was only ∼50‰ lower during the late glacial. Model simulations of the hypothesized ventilation change during deglaciation lead to large increases inδ 13CDIC, Δ14C, andε 14C that are not recorded in observations. -
Abstract Streams and rivers are major sources of greenhouse gases (GHGs) to the atmosphere, as carbon and nitrogen are converted and outgassed during transport. Although our understanding of drivers of individual GHG fluxes has improved with numerous site‐specific studies and global‐scale compilations, our ability to parse out interrelated physical and biogeochemical drivers of gas concentrations is limited by a lack of consistently collected, temporally continuous samples of GHGs and their associated drivers. We present a first analysis of such a dataset collected by the National Ecological Observatory Network across 27 streams and rivers across ecoclimatic domains of the United States. Average concentrations of CO2ranged from 36.9 ± 0.88 to 404 ± 33
μ mol L−1, CH4from 0.003 ± 0.0003 to 4.99 ± 0.72μ mol L−1, and N2O from 0.015 to 0.04μ mol L−1and spanned ranges of previous global compilations. Both CO2and CH4were strongly affected by physical drivers including mean air temperature and stream slope, as well as by dissolved oxygen and total nitrogen concentrations. N2O was exclusively correlated with total nitrogen concentrations. Results suggested that potential for gas exchange dominated patterns in gas concentrations at the site level, but contributions of in‐stream aerobic and anaerobic metabolism, and groundwater also likely varied across sites. The highest gas concentrations as well as highest variability occurred in low‐gradient, warmer, and nonperennial systems. These results are a first step in providing unprecedented, continuous estimates of GHG flux constrained by temporally variable physical and biogeochemical drivers of GHG production. -
Abstract The Arctic Ocean is more susceptible to ocean acidification than other marine environments due to its weaker buffering capacity, while its cold surface water with relatively low salinity promotes atmospheric CO2uptake. We studied how sea‐ice microbial communities in the central Arctic Ocean may be affected by changes in the carbonate system expected as a consequence of ocean acidification. In a series of four experiments during late summer 2018 aboard the icebreaker
Oden , we addressed microbial growth, production of dissolved organic carbon (DOC) and extracellular polymeric substances (EPS), photosynthetic activity, and bacterial assemblage structure as sea‐ice microbial communities were exposed to elevated partial pressures of CO2(pCO2). We incubated intact, bottom ice‐core sections and dislodged, under‐ice algal aggregates (dominated byMelosira arctica ) in separate experiments under approximately 400, 650, 1000, and 2000μ atm pCO2for 10 d under different nutrient regimes. The results indicate that the growth of sea‐ice algae and bacteria was unaffected by these higher pCO2levels, and concentrations of DOC and EPS were unaffected by a shifted inorganic C/N balance, resulting from the CO2enrichment. These central Arctic sea‐ice microbial communities thus appear to be largely insensitive to short‐term pCO2perturbations. Given the natural, seasonally driven fluctuations in the carbonate system of sea ice, its resident microorganisms may be sufficiently tolerant of large variations in pCO2and thus less vulnerable than pelagic communities to the impacts of ocean acidification, increasing the ecological importance of sea‐ice microorganisms even as the loss of Arctic sea ice continues.