Photosynthetic activity buffers ocean acidification in seagrass meadows

[摘要] Macrophytes growing in shallow coastal zones characterised by intensemetabolic activity have the capacity to modify pH within their canopy andbeyond. We observed diel pH changes in shallow (5–12 m) seagrass(Posidonia oceanica) meadows spanning 0.06 pH units in September to0.24 units in June. The carbonate system (pH, DIC, and aragonite saturationstate (Ω_Ar)) and O₂ within the meadows displayed strongdiel variability driven by primary productivity, and changes in chemistrywere related to structural parameters of the meadow, in particular, the leafsurface area available for photosynthesis (LAI). LAI was positivelycorrelated to mean, max and range pH_NBS and max and rangeΩ_Ar. In June, vertical mixing (as Turbulent Kinetic Energy)influenced max and min Ω_Ar, while in September there was noeffect of hydrodynamics on the carbonate system within the canopy. Max andrange Ω_Ar within the meadow showed a positive trend with thecalcium carbonate load of the leaves, pointing to a possible link betweenstructural parameters, Ω_Ar and carbonate deposition.

Calcifying organisms, e.g. epiphytes with carbonate skeletons, may benefitfrom the modification of the carbonate system by the meadow. There is,however, concern for the ability of seagrasses to provide modifications ofsimilar importance in the future. The predicted decline of seagrass meadowsmay alter the scope for alteration of pH within a seagrass meadow and in thewater column above the meadow, particularly if shoot density and biomassdecline, on which LAI is based. Organisms associated with seagrasscommunities may therefore suffer from the loss of pH buffering capacity indegraded meadows.

[学科分类] 地球化学与岩石