Estimating carbon storage and flux in sea urchin barrens following kelp forest collapse

Mitigating climate change impacts due to excess carbon dioxide will require knowledge of blue carbon sources and sinks. Marine ecosystems including seagrass meadows, mangroves, salt marshes and kelp forests store organic carbon while inorganic carbon is bound as calcium carbonate (CaCO₃) in shells. Kelp forest ecosystems however can shift from carbon bound in kelps to sea urchin barrens composed of CaCO₃. We examine this ecosystem shift with respect to carbon by estimating purple, Strongylocentrotus purpuratus, and red sea urchin, Mesocentrotus franciscanus, population abundances (in 2022-2023) in northern California. In these rocky habitats (25,400 Ha), we estimate there are 5.8 billion purple and 577 million red sea urchins in northern California. This conservative estimate results in an estimated 98,000 t of CaCO₃ for a total of 11,800 t carbon (46 g C/m²) in these sea urchin barrens. Sea urchin calcification releases an estimated 30,171 t carbon dioxide (CO₂) while the eventual dissolution results in the uptake of an estimated 43,000 t CO₂ (or 11,800 t carbon). More research is needed to quantify CaCO₃ storage/flux, source/sink dynamics, carbon sequestration and the potential for CO₂ removal given ecosystem shifts from kelp forest to sea urchin barrens.