Abstract
The organisms that inhabit Oxygen Minimum Zones (OMZ) have specialized adaptations that allow them to survive within a very narrow range of environmental conditions. Consequently, even small environmental perturbations can result in local species distribution shifts that alter ecosystem trophodynamics. Here, we examined the effect of changing sea water temperatures and oxygen levels on the physiological performance and metabolic traits of the species forming marine demersal communities along the OMZ margins in the Costa Rican Pacific. The strong temperature and oxygen gradients along this OMZ margin provide a "natural experiment" to explore the effects of warming and hypoxia on marine demersal communities. We identified two distinct marine fauna communities separated by an environmental oxygen partial pressure threshold of 0.003-0.009 atm. The community inhabiting cooler waters with less oxygen was comprised of species with very low oxygen demands, while the second community inhabiting warmer waters with more oxygen was comprised by a higher diversity of species with higher oxygen demands. We also compared the community composition across different El Nino Southern Oscillation phases. During "neutral" and El Nino conditions, with relatively warmer temperatures and higher oxygen levels, species' average oxygen demand was higher, and species stayed at greater depths than during the cooler, low oxygen, La Nina phases. Our findings suggest that the effects of environmental temperature and oxygen levels on the structure of demersal communities within OMZs can be predicted by understanding species' oxygen demand. This study highlights the vulnerability of demersal ecosystem structures surrounding the Costa Rican OMZ to deoxygenation and warming under climate change.