Abstract
Abstract. Macrophytes growing in shallow coastal zones characterised by intense metabolic activity have the capacity to modify pH within their canopy and beyond. We observed diel pH changes in shallow (5–12 m) seagrass (Posidonia oceanica) meadows spanning 0.06 pH units in September to 0.24 units in June. The carbonate system (pH, DIC, and aragonite saturation state (ΩAr)) and O2 within the meadows displayed strong diel variability driven by primary productivity, and changes in chemistry were related to structural parameters of the meadow, in particular, the leaf surface area available for photosynthesis (LAI). LAI was positively correlated to mean, max and range pHNBS and max and range ΩAr. In June, vertical mixing (as Turbulent Kinetic Energy) influenced max and min ΩAr, while in September there was no effect of hydrodynamics on the carbonate system within the canopy. Max and range ΩAr within the meadow showed a positive trend with the calcium carbonate load of the leaves, pointing to a possible link between structural parameters, ΩAr and carbonate deposition. Calcifying organisms, e.g. epiphytes with carbonate skeletons, may benefit from the modification of the carbonate system by the meadow. There is, however, concern for the ability of seagrasses to provide modifications of similar importance in the future. The predicted decline of seagrass meadows may alter the scope for alteration of pH within a seagrass meadow and in the water column above the meadow, particularly if shoot density and biomass decline, on which LAI is based. Organisms associated with seagrass communities may therefore suffer from the loss of pH buffering capacity in degraded meadows.
Highlights
The carbonate system (pH, dissolved inorganic carbon (DIC), and aragonite saturation state ( Ar)) and O2 within the meadows displayed strong diel variability driven by primary productivity, and changes in chemistry were related to structural parameters of the meadow, in particular, the leaf surface area available for photosynthesis (LAI)
Forecasts predict that ocean acidification will become a significant threat to calcifying organisms in the near future (Orr et al, 2005; Gazeau et al, 2007; Talmage and Gobler, 2010; Waldbusser et al, 2011), many of which live in coastal habitats
The island of Mallorca lacks rivers and surface runoff, CO2 from soil respiration can be delivered to coastal waters through groundwater inputs (Basterretxea et al, 2010)
Summary
Forecasts predict that ocean acidification will become a significant threat to calcifying organisms in the near future (Orr et al, 2005; Gazeau et al, 2007; Talmage and Gobler, 2010; Waldbusser et al, 2011), many of which live in coastal habitats. Whereas watershed effects can be a significant source of pH regulation and variability in coastal, estuarine waters, these are restricted in islands, where vulnerability to ocean acidification can only be offset by metabolic-intense ecosystems able to remove CO2 (Duarte et al, 2013). This is the case of Mediterranean islands, which have small watersheds and little or no runoff to the coast, but where seagrass, Posidonia. Hendriks et al.: Photosynthetic activity buffers ocean acidification in seagrass meadows oceanica, meadows support intense metabolism (Duarte and Chiscano, 1999; Duarte et al, 2010), possibly contributing to alleviate the expected impacts of ocean acidification
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