Biogeochemical processes and buffering capacity concurrently affect acidification in a seasonally hypoxic coastal marine basin

[摘要] Coastal areas are impacted by multiple natural and anthropogenic processesand experience stronger pH fluctuations than the open ocean. These variationscan weaken or intensify the ocean acidification signal induced by increasingatmospheric pCO₂. The development of eutrophication-induced hypoxiaintensifies coastal acidification, since the CO₂ produced duringrespiration decreases the buffering capacity in any hypoxic bottom water. Toassess the combined ecosystem impacts of acidification and hypoxia, wequantified the seasonal variation in pH and oxygen dynamics in the watercolumn of a seasonally stratified coastal basin (Lake Grevelingen, theNetherlands).

Monthly water-column chemistry measurements were complemented with estimatesof primary production and respiration using O₂ light–dark incubations,in addition to sediment–water fluxes of dissolved inorganic carbon (DIC) andtotal alkalinity (TA). The resulting data set was used to set up a protonbudget on a seasonal scale.

Temperature-induced seasonal stratification combined with a high communityrespiration was responsible for the depletion of oxygen in the bottom waterin summer. The surface water showed strong seasonal variation in processrates (primary production, CO₂ air–sea exchange), but relatively smallseasonal pH fluctuations (0.46 units on the total hydrogen ion scale). Incontrast, the bottom water showed less seasonality in biogeochemical rates(respiration, sediment–water exchange), but stronger pH fluctuations (0.60units). This marked difference in pH dynamics could be attributed to asubstantial reduction in the acid–base buffering capacity of the hypoxicbottom water in the summer period. Our results highlight the importance ofacid–base buffering in the pH dynamics of coastal systems and illustrate theincreasing vulnerability of hypoxic, CO₂-rich waters to any acidifyingprocess.

[学科分类] 地球化学与岩石