Hypoxia and biogeochemical processes concurrently impact acidification in a seasonally stratified coastal marine lake

Coastal areas experience stronger short-term pH fluctuations than the open ocean due to higher biogeochemical process rates. These fluctuations can mask or amplify the ocean acidification signal induced by increasing atmospheric pCO2. Additionally, coastal acidification can be enhanced as eutrophication-induced hypoxia develops, since the CO2 produced during respiration decreases the buffering capacity of the hypoxic bottom water.
In 2012, we examined the carbonate system in the water column of the seasonally hypoxic marine Lake Grevelingen over the full annual cycle. Monthly carbonate system, oxygen and nutrient measurements were complemented with estimates of primary production and respiration using O2 light-dark incubations. Nitrification was measured seasonally with 15N-enriched ammonium incubations.
During stratification and hypoxia, pH differed up to 0.75 units between the well-buffered oxic surface and less-buffered hypoxic bottom waters. A proton budget calculated from measured process rates indicates proton cycling was fastest in the hypoxic bottom water and buffering capacities were lower after the stratified period than before. This illustrates the importance of buffering in controlling pH dynamics and explains the increasing vulnerability of hypoxic, CO2-rich waters to any acidifying process.