Ocean alkalinity enhancement (OAE) is being investigated as a potential carbon dioxide removal strategy because it can increase the ocean’s capacity to store atmospheric CO2 while potentially mitigating ocean acidification. By enhancing alkalinity, seawater can take up additional CO2 and convert it into stable dissolved inorganic carbon forms, providing long-term climate benefits. However, uncertainties remain regarding its real-world effectiveness and ecological impacts, particularly in coastal environments, making field-based studies essential.
We previously conducted a small-scale study on sandy sediments investigating the feasibility and environmental effects of both limestone and olivine (link to prior study). This study gave us two key insights. Firstly, that smaller scale field studies provide key insights into the feasibility and effects of OAE. And secondly, to more accurately assess the effectiveness and biological effects of OAE we needed to increase the size of study area to better reflect real-world conditions (because key organisms such as seagrasses extend beyond the size of our first very small-scale field study). Building on these insights, we increase the size of our field plots from 1.2 m x 1.2 m to 12 m x 12 m (100 times increase in scale). For this study we have chosen to work within an organic-rich system (a seagrass bed) and focus solely on limestone in a method referred to as coastal liming.
Coastal liming is the purposeful addition of limestone, or calcium carbonate (CaCO3), onto high organic coastal sediments to increase alkalinity. Limestone is a potentially useful alkaline material for ocean alkalinity enhancement as it has very few impurities, is cheap and widely available. When added to acidic environments, such as high organic ocean sediments, the limestone dissolves and releases CO32- from CaCO3, allowing the excess H+ ions to assimilate with CO32-, thereby reducing acidity and CO2 concentration in seawater. Additionally, if calcium carbonate could stimulate ecosystem function this would provide a dual benefit of increasing alkalinity while contributing to supporting seagrass health.
Coastal liming has its roots in the terrestrial practice of agricultural liming, whereby limestone is added to crops to reduce soil acidity and enhance crop yields. Hence, the motivation behind investigating coastal liming in a seagrass bed is two-fold. Firstly, we are interested in its potential to enhance carbon dioxide removal and secondly, we are interested in whether it can benefit seagrass communities. Our goal is to provide real-world evidence on suitability of limestone as an alkalinity source in coastal environments. In austral autumn 2026, we began a field study in an estuary in southern Tasmania to identify if limestone is a suitable alkaline source and if there are effects on the marine life.
This blog will document our study and inform on what we observed.