Biogeochemical feedbacks to ocean acidification in a cohesive photosynthetic sediment

Ecosystem feedbacks in response to ocean acidification can amplify or diminish the diel pH oscillations that characterize productive coastal waters. We report that benthic microalgae generate such oscillations in the porewater of cohesive sediment and ask how carbonation (acidification) of the overlying seawater alters these in the absence and presence of biogenic calcite. To do so, we placed a 1-mm layer of ground oyster shells (Treatment) or sand (Control) onto intact sediment cores free of large dwelling fauna, and then gradually increased the pCO2 in the seawater above half of the Treatment and Control cores from 472 to 1216 μatm (pH 8.0 to 7.6, CO2:HCO3 from 4.8 to 9.6 x 10-4). Vertical porewater [O2] and [H+] microprofiles measured 16 d later showed that this carbonation had decreased O2 penetration in all cores, indicating a metabolic response. In carbonated seawater: (1) sediment biogeochemical processes added and removed more H+ to and from the porewater in darkness and light, respectively, than in ambient seawater increasing the amplitude of the dark–light porewater [H+] oscillations, and (2) the dissolution of calcite decreased the porewater [H+] below that in overlying seawater, reversing the dark sediment–seawater H+ flux and decreasing the amplitude of diel [H+] oscillations. This dissolution did not, however, counter the negative effect of carbonation on sediment O2 penetration. We hypothesise that the latter effect and the observed enhanced acidification of the sediment porewater were caused by an ecosystem feedback: a CO2-induced increase in the microbial reoxidation of reduced solutes with O2.

Vopel K., Marshall A., Brandt S., Hartland A., Lee C. K., Graig Cary S. & Pilditch C. A., in review. Biogeochemical feedbacks to ocean acidification in a cohesive photosynthetic sediment. Research Square. Article.

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