Abstract
AbstractSubtropical seagrass meadows play a major role in the coastal carbon cycle, but the nature of air–water CO2 exchanges 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 CO2 and water (evaporation), emphasizing the need for direct measurements. We describe the first direct measurements of evaporation and CO2 flux over a calcifying seagrass meadow near Bob Allen Keys, Florida. Over the 78‐d study, CO2 emissions were 36% greater during the day than at night, and the site was a net CO2 source to the atmosphere of 0.27 ± 0.17 μmol m−2 s−1 (x̅ ± standard deviation). A quarter (23%) of the diurnal variability in CO2 flux 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 CO2 exchange 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.
Highlights
IntroductionBenthic chamber-based dissolved oxygen (DO) studies have shown net heterotrophy at some sites (Asmala et al 2019)
We describe a set of unique drivers that causes air–water CO2 exchange in this system to differ from results in seagrass meadows elsewhere
Latent and sensible heat fluxes were positive for 99.9% and 90% of measurements, respectively, and heat fluxes were dominated by latent heat flux, which was greater than the sensible flux 99% of the time
Summary
Benthic chamber-based DO studies have shown net heterotrophy at some sites (Asmala et al 2019) This use of DO as a proxy for CO2 may be appropriate in siliciclastic and noncalcifying systems (Attard et al 2019), where the ratio of Seagrass CO2 flux. In net calcifying systems (common among tropical and subtropical seagrass), metabolic rates estimated using DO may be biased toward autotrophy, to an extent that is proportional to the ratio of net ecosystem production to calcification (Van Dam et al 2019a). There is a clear need for direct measurements of air–water CO2 exchanges in tropical and subtropical seagrasses Such measurements, in conjunction with rigorous ecosystem metabolism studies, may begin to help resolving the question of whether these important ecosystems are sources or sinks of carbon to the atmosphere. While wind is clearly an important driver of gas transfer in coastal waters (Upstill-Goddard 2006), especially when water is relatively deep (Ho et al 2018a), other factors like bottomdriven turbulence (Tokoro et al 2007; Ho et al 2016, 2018b), convective forcing (Rutgersson et al 2011; Czikowsky et al 2018; Van Dam et al 2019b), biological surfactants (McKenna and McGillis 2004; Ribas-Ribas et al 2018), and wave slope (Wanninkhof et al 2009) may cause variations in gas transfer irrespective of wind
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
