| Summary: | Increased atmospheric carbon dioxide leads to ocean acidification and carbon dioxide (CO₂) enrichment of seawater. Given the important ecological functions of seagrass meadows, understanding their responses to CO₂ will be critical for the management of coastal ecosystems. This study examined the physiological responses of three tropical seagrasses to a range of seawater pCO₂ levels in a laboratory. Cymodocea serrulata, Halodule uninervis and Thalassia hemprichii were exposed to four different pCO₂ treatments (442–1204 μatm) for 2 weeks, approximating the range of end-of-century emission scenarios. Photosynthetic responses were quantified using optode-based oxygen flux measurements. Across all three species, net productivity and energetic surplus (P G:R) significantly increased with a rise in pCO₂ (linear models, P < 0.05). Photosynthesis–irradiance curve-derived photosynthetic parameters—maximum photosynthetic rates (P max) and efficiency (α)—also increased as pCO₂ increased (linear models, P < 0.05). The response for productivity measures was similar across species, i.e. similar slopes in linear models. A decrease in compensation light requirement (E c) with increasing pCO₂ was evident in C. serrulata and H. uninervis, but not in T. hemprichii. Despite higher productivity with pCO₂ enrichment, leaf growth rates in C. serrulata did not increase, while those in H. uninervis and T. hemprichii significantly increased with increasing pCO₂ levels. While seagrasses can be carbon-limited and productivity can respond positively to CO₂ enrichment, varying carbon allocation strategies amongst species suggest differential growth response between species. Thus, future increase in seawater CO₂ concentration may lead to an overall increase in seagrass biomass and productivity, as well as community changes in seagrass meadows.
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