Benthic buffers and boosters of ocean acidification on coral reefs

[摘要] Ocean acidification is a threat to marine ecosystems globally. Inshallow-water systems, however, ocean acidification can be masked by benthiccarbon fluxes, depending on community composition, seawater residence time,and the magnitude and balance of net community production (NCP) andcalcification (NCC). Here, we examine how six benthic groups from a coralreef environment on Heron Reef (Great Barrier Reef, Australia) contribute tochanges in the seawater aragonite saturation state (Ω_a).Results of flume studies using intact reef habitats (1.2 m by 0.4 m),showed a hierarchy of responses across groups, depending on CO₂ level,time of day and water flow. At low CO₂ (350–450 μatm),macroalgae (Chnoospora implexa), turfs and sand elevatedΩ_a of the flume water by around 0.10 to 1.20 h⁻¹ –normalised to contributions from 1 m² of benthos to a 1 m deep watercolumn. The rate of Ω_a increase in these groups was doubledunder acidification (560–700 μatm) and high flow (35 compared to8 cm s⁻¹). In contrast, branching corals (Acropora aspera)increased Ω_a by 0.25 h⁻¹ at ambient CO₂(350–450 μatm) during the day, but reduced Ω_aunder acidification and high flow. Nighttime changes in Ω_aby corals were highly negative (0.6–0.8 h⁻¹) and exacerbated byacidification. Calcifying macroalgae (Halimeda spp.) raisedΩ_a by day (by around 0.13 h⁻¹), but loweredΩ_a by a similar or higher amount at night. Analyses ofcarbon flux contributions from benthic communities with four differentcompositions to the reef water carbon chemistry across Heron Reef flat andlagoon indicated that the net lowering of Ω_a bycoral-dominated areas can to some extent be countered by long water-residencetimes in neighbouring areas dominated by turfs, macroalgae and carbonatesand.

[学科分类] 地球化学与岩石