Skate blood-oxygen affinity under different temperatures and pCO2 levels

Dissolved oxygen is not evenly distributed throughout the water column nor across geographic regions, but is rather concentrated in near-surface areas where it can diffuse into the water or in areas where it is produced via photosynthesis (Keeling et al., 2010). When water masses are isolated away from photosynthesizing organisms and away from the surface, low oxygen content, or hypoxic, environments arise. Directional climate change will likely result in increasing severity and frequency of hypoxic events in the nearshore environment (Breitberg et al., 2015).

Understanding species-specific responses to hypoxia is critical to understanding how populations are impacted, and thus how management can be most effective. Unfortunately, calculating species-specific hypoxia tolerance (Pcrit) is a labor and time intensive procedure (see methods for Chapter 1). It has recently been proposed that an alternative physiological measure can correlate with Pcrit measurements. Mandic et al. (2009) found a strong statistical relationship between Pcrit and a measure of oxygen partial pressure in the blood at 50% oxygenation (P50) in twelve species of sculpin. The percent oxygenation of whole blood changes as blood cells pass through the gills (Hochachka and Somero, 2002). A low P50 therefore means blood will equilibrate to 100% oxygenation at a lower oxygen pressure, signifying higher hypoxia tolerance.

Blood oxygen affinity, and thus arterial and venous blood content, is both temperature and pH specific. Under directional climate change predictions, increasing temperatures will reduce the ability of hemoglobin to bind oxygen, as will a decrease in pH. In teleost fishes, a shift in pH can also result in a decrease in the maximum amount of oxygen able to be carried (Root effect), thus reducing maximum blood oxygen content (Rummer et al., 2013, Verde et al., 2007). Although it is generally thought elasmobranch blood does not exhibit a Root effect (Cox et al., 2017), few species have been explicitly studied.

By mechanistically determining which species are physiologically equipped the handle the changing environmental conditions, we may be able to better predict which species will be “winners” and which will be “losers” under directional climate change.