Ocean acidification has a wide-ranging potential for impacting the physiology and metabolism of zooplankton. Sufficiently elevated CO(2) concentrations can alter internal acid-base balance, compromising homeostatic regulation and disrupting internal systems ranging from oxygen transport to ion balance. We assessed feeding and nutrient excretion rates in natural populations of the keystone species Euphausia superba (Antarctic krill) by conducting a CO(2) perturbation experiment at ambient and elevated atmospheric CO(2) levels in January 2011 along the West Antarctic Peninsula (WAP). Under elevated CO(2) conditions (∼672 ppm), ingestion rates of krill averaged 78 µg C individual(-1) d(-1) and were 3.5 times higher than krill ingestion rates at ambient, present day CO(2) concentrations. Additionally, rates of ammonium, phosphate, and dissolved organic carbon (DOC) excretion by krill were 1.5, 1.5, and 3.0 times higher, respectively, in the high CO(2) treatment than at ambient CO(2) concentrations. Excretion of urea, however, was ∼17% lower in the high CO(2) treatment, suggesting differences in catabolic processes of krill between treatments. Activities of key metabolic enzymes, malate dehydrogenase (MDH) and lactate dehydrogenase (LDH), were consistently higher in the high CO(2) treatment. The observed shifts in metabolism are consistent with increased physiological costs associated with regulating internal acid-base equilibria. This represents an additional stress that may hamper growth and reproduction, which would negatively impact an already declining krill population along the WAP.