Photosynthetic activity buffers ocean acidification in seagrass meadows

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 (omega 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). LAI was positively correlated to mean, max and range pHNBS and max and range omega Ar. In June, vertical mixing (as Turbulent Kinetic Energy) influenced max and min omega Ar, while in September there was no effect of hydrodynamics on the carbonate system within the canopy. Max and range omega Ar within the meadow showed a positive trend with the calcium carbonate load of the leaves, pointing to a possible link between structural parameters, omega Ar and carbonate deposition.

Composition of bentonite-, smectite-rich- and ash beds from DSDP Leg 19 holes

Late Cenozoic ash deposits cored in Deep Sea Drilling Project Leg 19 in the far northwest Pacific and in the Bering Sea have altered to bentonite beds. Some bentonite layers were subsequently replaced by carbonate beds. A significant part of the Neogene volcanic history of land areas adjacent to the far north Pacific is represented by these diagenetic deposits. Bentonite beds are composed of authigenic smectite and minor amounts of clinoptilolite. Authigenic smectite has fewer illite layers than detrital smectite. Opal-A and opal-CT, abundant in Bering Sea sediment, are not found in ash or bentonite layers. The percentage of smectite in the total clay-mineral assemblage of ash beds is greater than that for adjacent terrigenous sediment, but the total amount of clay minerals in ash sequences is less than in surrounding deposits. Morphology of the 17-Å peak of smectite found in ash may represent newly formed, poorly crystalline smectite. Smectite becomes better crystallized as bentonite layers form. The percentage of smectite of the total clay-mineral assemblage in bentonite beds is greater than that in surrounding sediment, and, in contrast to ash beds, the total amount of clay minerals (mostly smectite) in bentonite layers is greater than in adjacent terrigenous sediment. Apparently, silica is not mobilized when volcanic ash layers transform to bentonite beds. Saponite-nontronite varieties of smectite and high Fe/Al and Ti/Al ratios distinguish bentonite beds derived from basaltic parent material from those beds formed from more silicic volcanic ash. These silicic ash beds produce bentonite composed mostly of montmorillonite. The basal sediment section at site 192 is rich with bentonite beds. Smectite in the upper part of this section (Eocene) was formed by low-temperature diagenesis of volcanic debris of intermediate or more silicic composition derived from arc or Pacific volcanoes. In contrast, smectite from the lowest 10 to 20 m of the sedimentary section (Cretaceous) is formed from either low-temperature or hydrothermal alteration of the underlying basaltic basement and associated pyroclastic debris. This near-basement smectite contains Mg and K acquired from sea water and Si, Al, Fe, Ti, and Mn released from the volcanic material.

Seagrass ecosystem response to long-term high CO2 in a Mediterranean volcanic vent

We examined the long-term effect of naturally acidified water on a Cymodocea nodosa meadow growing at a shallow volcanic CO2 vent in Vulcano Island (Italy). Seagrass and adjacent unvegetated habitats growing at a low pH station (pH = 7.65 ± 0.02) were compared with corresponding habitats at a control station (pH = 8.01 ± 0.01). Density and biomass showed a clear decreasing trend at the low pH station and the below- to above-ground biomass ratio was more than 10 times lower compared to the control. C content and delta 13C of leaves and epiphytes were significantly lower at the low pH station. Photosynthetic activity of C. nodosa was stimulated by low pH as seen by the significant increase in Chla content of leaves, maximum electron transport rate and compensation irradiance. Seagrass community metabolism was intense at the low pH station, with significantly higher net community production, respiration and gross primary production than the control community, whereas metabolism of the unvegetated community did not differ between stations. Productivity was promoted by the low pH, but this was not translated into biomass, probably due to nutrient limitation, grazing or poor environmental conditions. The results indicate that seagrass response in naturally acidified conditions is dependable upon species and geochemical characteristics of the site and highlight the need for a better understanding of complex interactions in these environments.