Compared with air, water contains about 33 times less oxygen and it diffuses ∼300,000 times slower. Also well established is that oxygen availability in aquatic ecosystems varies with temperature and atmospheric pressure. As a consequence, it is an important ecological factor with wide-ranging influences, from animal body size to species distribution and diversity. However, expressing oxygen availability with a consistent metric has been problematic. Ecologists prefer to use oxygen availability in the context of solubility or concentration, whereas physiologists use partial pressure (PO2) as a metric. These two indicators are, however, not readily interchangeable and cause confusion. For instance, oxygen concentration correlates well with latitudinal body size in amphipods. Conversely, across a range of altitudes there are no clear size relationships, as one would expect given similar temperature changes across latitudes and altitudes. However, decreases in PO2 at increasing altitude do explain decreased invertebrate species richness. But this could also explain the absence of an altitudinal size relationship because reduced PO2 also means a reduction in the oxygen available to dissolve in water, even though lower temperatures allow for greater solubility. It is clear that when one metric is used as an indicator of ecological patterns without accounting for how it is affected by the other the potential for conflicting assessments in eco-physiology increases.
To address this issue, Wilco Verberk and co-workers from the School of Marine Science and Engineering in Plymouth, UK, set out to resolve this issue by returning to first principles. Using Fick's law of diffusion they derived an oxygen index that incorporates both concentration and PO2. They also included a third key factor – oxygen diffusivity (DO2). This gave them the oxygen supply index (OSI) in mol m–1 s–1 and incorporates solubility, PO2 and diffusivity as follows: OSI ∝ αO2 × PO2 × DO2. The OSI shows that oxygen exchange between organism and environment is driven by an interplay between solubility, pressure and diffusivity and can fully account for the prior inconsistencies observed when using these metrics in isolation. This also allows for temperature, which not only affect factors such as solubility but also determines the oxygen demands of ectothermic organisms. This enables the calculation of oxygen supply relative to demand, or relative OSI.
To test this new index, Verberk and colleagues re-analyzed previous data sets from prior published studies that demonstrated ecological patterns related to oxygen availability. In all cases, for body size and species richness indicators, OSI consistently was a better predictor. Their new index also accounted for effects of temperature and oxygen demand, and pointed to a counterintuitive assessment of latitudinal oxygen availability. Based on original assessments of temperature-dependent oxygen solubilities it was always assumed that polar waters had greater oxygen availability. The OSI shows that equatorial water actually has greater oxygen availability. Here, the lower solubility is off-set by increased diffusivity. But increased demand, due to warmer temperatures, results in a limit in oxygen exchange that affects maximum body size. Conversely, higher polar solubilities are off-set by reduced diffusivity, but the drastically reduced oxygen demand of animals does allow for increased maximal body size.