A persistent mid-water column hypoxic zone with low pH and CaCO3 saturation state in Toba Inlet

• Main Author: Hare, Dr. Alex
• Format: Text
• Language: English
• Published: Western CEDAR 2022
• Subjects: Ocean acidification
• Online Access: https://cedar.wwu.edu/ssec/2022ssec/allsessions/366; https://cedar.wwu.edu/context/ssec/article/3528/viewcontent/424_d1ab0ad4a3f2409fa3580ce4d6f8efd2.pdf

Oxygen concentrations in many coastal areas have declined over the last several decades, increasing the prevalence of low oxygen zones and hypoxia ([O2] < 63 µmol kg-1). This trend is driven by warming temperatures that decrease oxygen solubility, biological metabolism, and ocean stratification, as well as by anthropogenic nutrient loading. Coastal waters, however, also contain environments naturally predisposed to low-oxygen concentrations, particularly in fjords where sills restrict sub-surface water exchange and associated re-oxygenation, and geography constrains freshwater runoff and nutrient loadings. Low-oxygen waters are also commonly associated with low pH and conditions corrosive to CaCO3 through mechanistic links between processes related to both deoxygenation and the inorganic carbon system. Such settings are therefore sensitive to further oxygen decline and additional carbon inputs, making them vulnerable to both hypoxia and ocean acidification. Here, we identify and characterize a persistent mid-water column hypoxic zone with regionally low pH in Toba Inlet in the northern Salish Sea, and evaluate the processes controlling this feature. The hypoxic zone is present year-round and is most intense near the fjord head, but extends seaward across the entire fjord (~ 40 km) at its greatest extent, often encompassing > 200 m of the water column. However, in contrast with typical fjord systems, Toba Inlet does not contain a sill, but joins several deep channels that connect to the Strait of Georgia at roughly twice the oxygen minimum depth, commonly observed around 100 m deep. Together, these characteristics imply additional processes to sub-surface water exchange maintain the extent and intensity of hypoxia and low seawater pH conditions in Toba Inlet. We investigate a suite of processes including sub-surface water exchange, riverine input, and biological production and respiration, to develop a simple model to explain the oxygen and inorganic carbon characteristics in Toba Inlet.