Impact of biogeochemical processes on pH dynamics in marine systems

Abstract

Uptake of anthropogenic carbon dioxide (CO2) from the atmosphere has resulted in a range of changes in ocean chemistry, including the lowering of pH, collectively referred to as ocean acidification. Rates of coastal-zone acidification exceed those of the open ocean since coastal-ocean pH is influenced by many other processes than absorption of CO­2 alone. These processes do not only play a role in long-term acidification but also impact pH on seasonal timescales. Examples are enhanced atmospheric sulphur and nitrogen deposition, as well as eutrophication, the latter which can additionally result in the development of low-oxygen waters. The degree to which these processes induce a change in pH depends both on their rates and the extent to which the water can buffer acid production or consumption. This acid-base buffering capacity has been shown to decrease substantially in hypoxic waters, suggesting that low-oxygen conditions exacerbate ocean acidification. In this dissertation the key factors controlling the seasonal pH variability and longer-term pH changes in both the coastal and open ocean were examined, showing that buffering mechanisms play a crucial role in the impact of any biogeochemical or physical process on pH. Monthly or seasonal water-column chemistry and process-rate measurements in a transiently hypoxic coastal marine basin indicate that, despite generally higher process rates in the surface water of the basin, the amplitude of pH variability as mainly governed by the balance between primary production and respiration is greater in the seasonally-hypoxic bottom water, due to a considerable reduction of its acid-base buffering capacity in summer. A proton budget, based on these measurements and set up for each season, shows that the net change in pH is much smaller than the flux of protons induced by each of the individual processes. The interplay between absorption of atmospheric CO2 and atmospheric sulphur and nitrogen deposition in the coastal ocean was found to depend on the water-column concentration of CO2 relative to the atmosphere. If the atmospheric concentration surpasses that of the surface water, then this part of the coastal ocean is most sensitive to CO2-induced acidification, but least affected by additional acidification resulting from atmospheric acid deposition. Although coastal seas will become up to a factor 4 more sensitive to atmospheric deposition-induced acidification between the present-day and 2100, the annual change in proton concentration will only increase by 28% at most. Finally, a set of general expressions describing the sensitivity of pH to a change in ocean chemistry was derived. These expressions, which can include as many acid-base systems as relevant and are thus generally applicable, were tested on several long-term open ocean data sets. For each of these sites, pH can be properly predicted if seasonal cycles of temperature, salinity, total alkalinity (TA) and the total concentrations of acid-base species are known. By the end of the 21st century a change in most acid-base parameters will induce a comparably greater pH excursion. This increased vulnerability is driven by enhanced CO2 concentrations and slightly moderated by the projected global warming.