‘Blue Carbon’, which is carbon captured by marine living organisms, has recently been highlighted as a new option for climate change mitigation initiatives. In particular, coastal ecosystems have been recognized as significant carbon stocks because of their high burial rates and long-term sequestration of carbon. However, the direct contribution of Blue Carbon to the uptake of atmospheric CO2 through air-sea gas exchange remains unclear. We performed in situ measurements of carbon flows, including air-sea CO2 fluxes, dissolved inorganic carbon changes, net ecosystem production, and carbon burial rates in the boreal (Furen), temperate (Kurihama), and subtropical (Fukido) seagrass meadows of Japan from 2010 to 2013. In particular, the air-sea CO2 flux was measured using three methods: the bulk formula method, the floating chamber method, and the eddy covariance method. Our empirical results show that submerged autotrophic vegetation in shallow coastal waters can be functionally a sink for atmospheric CO2. This finding is contrary to the conventional perception that most near-shore ecosystems are sources of atmospheric CO2. The key factor determining whether or not coastal ecosystems directly decrease the concentration of atmospheric CO2 may be net ecosystem production. This study thus identifies a new ecosystem function of coastal vegetated systems; they are direct sinks of atmospheric CO2.

No abstract

No abstract

Aquatic models used for both freshwater and marine systems frequently need to account for submerged aquatic vegetation (SAV) due to its influence on flow and water quality. Despite its importance, parameterizations are generally adopted that simplify feedbacks from SAV, such as canopy properties (e.g., considering the deflected vegetation height) and the bulk friction coefficient. This study reports the development of a fine‐scale non‐hydrostatic model that demonstrates the two‐way effects of SAV motion interaction with the flow. An object‐oriented approach is applied to capture the multiphase phenomena, whereby a leaf‐scale SAV model based on a discrete element method is combined with a flow dynamics model to resolve stresses from currents and waves. The model is verified through application to a laboratory‐scale seagrass bed. A force balance analysis revealed that leaf elasticity and buoyancy are the most significant components influencing the horizontal and vertical momentum equations, respectively. The sensitivity of canopy‐scale bulk friction coefficients to water depth, current speeds, and vegetation density of seagrass was explored. Deeper water was also shown to lead to a smaller decrease in vegetation height. The model approach can contribute to improving assessment of processes influencing water quality, sediment stabilization, carbon sequestration, and SAV restoration, thereby supporting an understanding of how waterways and coasts will respond to changes brought about by development and a changing climate.

Small lakes (lake area < 0.1 km 2 ) occupy 25%-35% of the total area of all lakes in the world (Downing et al., 2006;Verpoorter et al., 2014). However, C emissions from small lakes are usually ignored when evaluating C fluxes (Cole et al., 2007;Raymond et al., 2013), resulting in an underestimation of CO 2 emissions on a global scale (Downing et al., 2006;Verpoorter et al., 2014). Lake size has also been revealed to be one of the most critical factors controlling CO 2 emissions, with small lakes having more considerable contributions from convection to gas transfer velocities (Read et al., 2012;Vachon & del Giorgio, 2014). Heavy rainfall, wind shear, and convective mixing are essential meteorological forces that cause vertical mixing in small lakes; in shallow lakes, stratification can cause internal waves, which in turn can cause mass transport (Kimura et al., 2012(Kimura et al., , 2014(Kimura et al., , 2017. Therefore, to accurately estimate C fluxes from small lakes, it is necessary to clarify the mechanisms of vertical mixing and the collapse of stratification.There are many mountainous shallow lakes in subtropical East Asia. For example, in Taiwan, previous studies have shown that seasonal and annual C fluxes are frequently influenced by typhoon and monsoon events, but C fluxes from subtropical lakes above an altitude of 1,500 m are not well understood (