Ocean Acidification

Ocean alkalinity enhancement (OAE) is an emerging approach for atmospheric carbon dioxide removal (CDR). The net climatic benefit of OAE depends on how much it can increase CO₂ sequestration relative to a baseline state without OAE. This so-called “additionality” can be calculated as follows:

So far, feasibility studies on OAE have mainly focussed on enhancing alkalinity in the oceans to stimulate CO₂ sequestration (C_OAE); however, the primary focus has not been on how such anthropogenic alkalinity would modify the natural alkalinity cycle and associated baseline CO₂ sequestration (ΔC_baseline). Here, I present incubation experiments in which materials considered for OAE (sodium hydroxide, steel slag, and olivine) are exposed to beach sand to investigate the influence of anthropogenic alkalinity on natural alkalinity sources and sinks. The experiments show that anthropogenic alkalinity can strongly reduce the generation of natural alkalinity, thereby reducing additionality. This is because the anthropogenic alkalinity increases the calcium carbonate saturation state, which reduces the dissolution of calcium carbonate from sand, a natural alkalinity source. I argue that this “additionality problem” of OAE is potentially widespread and applies to many marine systems where OAE implementation is considered – far beyond the beach scenario investigated in this study. However, the problem can potentially be mitigated by dilute dosing of anthropogenic alkalinity into the ocean environment and the avoidance of OAE in natural alkalinity cycling hotspots, such as in marine sediments. Understanding a potential slowdown of the natural alkalinity cycle through the introduction of an anthropogenic alkalinity cycle will be crucial for the assessment of OAE.

Ocean acidification (OA) can impact aquaculture because reduced pH may negatively affect the calcification in bivalve species. Photosynthetic activity can naturally generate an OA buffering effect, favouring the calcification process by increasing the surrounding seawater pH. Therefore, the incorporation of macroalgae into bivalve farms may be a strategy to mitigate the impacts of acidification on the industry. In this study, we evaluated the modification of seawater chemistry by the metabolic activity of the blue mussel Mytilus chilensis and three macroalgae (Ulva sp., Chondracanthus chamissoi and Macrocystis pyrifera), in monocultures and co-cultures under ambient and acidified initial conditions in three closed-environment experiments. In all three experiments, photosynthesis and respiration modulated seawater chemistry, resulting in higher values of pH, oxygen concentrations, and aragonite saturation state (Ω_Ara) in macroalgal monocultures compared to mussel monoculture. In co-cultures, pH, oxygen concentrations and Ω_Ara were higher than in mussel monoculture but lower than in macroalgal monoculture. In co-cultures, the OA buffering effect (pH > 7.7, Ω_Ara > 1) was observed during daytime, but unfavourable conditions for calcification were observed during nighttime. These results are species-specific, with a greater capacity for pH increase for Ulva sp. and Ch. chamissoi and limited capacity for M. pyrifera in both initial pH treatments. Results of the enclosed environment experiments indicate that the presence of macroalgae in co-cultures did not guarantee favourable conditions for mussel calcification in acidified conditions.