Macroalgal metabolism and lateral carbon flows can create significant carbon sinks
2020
Abstract. Macroalgal beds have drawn attention as one of the
vegetated coastal ecosystems that act as atmospheric CO2 sinks.
Although macroalgal metabolism as well as inorganic and organic carbon flows
are important pathways for CO2 uptake by macroalgal beds, the
relationships between macroalgal metabolism and associated carbon flows are
still poorly understood. In the present study, we investigated carbon flows,
including air–water CO2 exchange and budgets of dissolved inorganic
carbon, total alkalinity, and dissolved organic carbon (DOC), in a temperate
macroalgal bed during the productive months of the year. To assess the key
mechanisms responsible for atmospheric CO2 uptake by the macroalgal
bed, we estimated macroalgal metabolism and lateral carbon flows (i.e.,
carbon exchanges between the macroalgal bed and the offshore area) by using field
measurements of carbon species, a field-bag method, a degradation
experiment, and mass-balance modeling in a temperate Sargassum bed over a diurnal
cycle. Our results showed that macroalgal metabolism and lateral carbon
flows driven by water exchange affected air–water CO2 exchange in the
macroalgal bed and the surrounding waters. Macroalgal metabolism caused
overlying waters to contain low concentrations of CO2 and high
concentrations of DOC that were efficiently exported offshore from the
macroalgal bed. These results indicate that the exported water can
potentially lower CO2 concentrations in the offshore surface water and
enhance atmospheric CO2 uptake. Furthermore, the Sargassum bed exported 6 %–35 % of the macroalgal net community production (NCP; 302–1378 mmol C m −2 d −1 ) as DOC to the offshore area. The results of degradation
experiments showed that 56 %–78 % of macroalgal DOC was refractory DOC
(RDOC) that persisted for 150 d; thus, the Sargassum bed exported 5 %–20 % of
the macroalgal NCP as RDOC. Our findings suggest that macroalgal beds in
habitats associated with high water exchange rates can create significant
CO2 sinks around them and export a substantial amount of DOC to
offshore areas.
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