Eucalanoid copepod metabolic rates in the oxygen minimum zone of the eastern tropical north Pacific: Effects of oxygen and temperature

Abstract

The eastern tropical north Pacific Ocean (ETNP) contains one of the world’s most severe oxygen minimum zones (OMZs), where oxygen concentrations are less than 2µmolkg−1. OMZs cause habitat compression, whereby species intolerant of low oxygen are restricted to near-surface oxygenated waters. Copepods belonging to the family Eucalanidae are dominant zooplankters in this region and inhabit a variety of vertical habitats within the OMZ. The purpose of this study was to compare the metabolic responses of three species of eucalanoid copepods, Eucalanus inermis, Rhincalanus rostrifrons, and Subeucalanus subtenuis, to changes in temperature and environmental oxygen concentrations. Oxygen consumption and urea, ammonium, and phosphate excretion rates were measured via end-point experiments at three temperatures (10, 17, and 23°C) and two oxygen concentrations (100% and 15% air saturation). S. subtenuis, which occurred primarily in the upper 50m of the water column at our study site, inhabiting well-oxygenated to upper oxycline conditions, had the highest metabolic rates per unit weight, while E. inermis, which was found throughout the water column to about 600m depth in low oxygen waters, typically had the lowest metabolic rates. Rates for R. rostrifrons (found primarily between 200 and 300m depth) were intermediate between the other two species and more variable. Metabolic ratios suggested that R. rostrifrons relied more heavily on lipids to fuel metabolism than the other two species. S. subtenuis was the only species that demonstrated a decrease in oxygen consumption rates (at intermediate 17°C temperature treatment) when environmental oxygen concentrations were lowered. The percentage of total measured nitrogen excreted as urea (% urea-N), as well as overall urea excretion rates, responded in a complex manner to changes in temperature and oxygen concentration. R. rostrifrons and E. inermis excreted a significantly higher % of urea-N in low oxygen treatments at 10°C. At 17°C, the opposite trend was observed as E. inermis and S. subtenuis excreted a higher % of urea-N in the high oxygen treatment. This unique relationship has not been documented previously for crustacean zooplankton, and warrants additional research into regulation of metabolic pathways to better understand nitrogen cycling in marine systems. This study also compared metabolic data for E. inermis individuals captured near the surface versus those that were resident in the deeper OMZ. Deeper-dwelling individuals had significantly higher nitrogen excretion rates and O:N ratios, suggesting an increased reliance on lipids for energy while residing in the food-poor waters of the OMZ.