Award: OCE-1737096

Kelp forests are among the world's most productive marine ecosystems. They are widespread in the coastal ocean, especially in temperate and sub-polar environments. Because they are so highly productive they support many other marine organisms, including fish, shellfish, and marine mammals. Their hyper-productivity also suggests that they may be able to locally reduce the impacts of ocean acidification (OA). Some coastal organisms are very sensitive to increasing seawater acidity caused by increasing levels of carbon dioxide. Shellfish larvae, in particular, are already known to be impacted by OA. We explored the possible that hyper-productivity by growing kelp may reduce this effect near to the kelp. We did so by deploying a large array of instruments that measure water column properties inside and outside of a kelp forest in Monterey Bay, California. These instruments recorded seawater circulation oxygen levels, and acidity, at minute-to-minute to day-to-day to week-to-week timescales during two summers. We saw clear differences in water chemistry between the arrays inside and outside of the kelp forest as well as between surface waters and near-bottom waters. Surface water pH was elevated inside the kelp compared to outside, suggesting that the kelp canopy locally decreased surface ocean acidification. But we observed the greatest acidification stress deeper in the water column where levels of carbon dioxide (partial pressure) reached levels as high as 1,300 μatm, about 3 times the partial pressure of carbon dioxide in air. In these acidic waters, aragonite undersaturation (ΩAr < 1) occurred on several occasions. Aragonite is an important mineral grown by developing shellfish larvae and undersaturation of the local seawater would cause stress. At this site, kelp canopy modification of seawater properties, and thus any ameliorating effect against acidification, is greatest in a narrow band of surface water, whereas the greatest acidification stress is located well below the surface canopy. The spatial disconnect between stress exposure at depth and reduction of acidification stress at the surface warrants further assessment of utilizing kelp forests as provisioners of local OA mitigation. Last Modified: 09/09/2022 Submitted by: Robert B Dunbar