Meeting Abstract
Diel vertical migration (DVM) of zooplankton between shallow and deep waters profoundly impacts the transport of nutrients and carbon through the water column. Despite the acknowledged importance of this so-called active flux to ocean biogeochemistry, these contributions remain poorly constrained, in part because daily variations in metabolic rates are not considered or are modeled as simple functions of temperature. This project is part of a larger study that uses transcriptomic, proteomic, physiological and biochemical methods to characterize the metabolic consequences of daily physiological rhythms and DVM for a model zooplankton species, the abundant subtropical copepod Pleuromamma xiphias . We sampled copepods at 6-hour intervals during their migratory cycle and measured rates of oxygen consumption, ammonium excretion, fecal pellet production and metabolic enzyme activity. Fecal pellet production was highest during late night, consistent with several hours of feeding near the surface. Similarly, activity of glutamate dehydrogenase (corresponds to nitrogen excretion) and citrate synthase (pace-setting enzyme for citric acid cycle of aerobic respiration) were highest during the night. Surprisingly, oxygen consumption rates were highest in copepods collected from deep water during mid-day. In addition, the activity of the electron transport system, which typically correlates to oxygen consumption rate, did not change over the daily cycle. These results show that activity of metabolic enzymes and metabolic rates both vary during diel vertical migration, but that peaks in cellular enzymatic activity and organismal physiological rates may be offset. This potential lag would have consequences for using enzymatic markers to predict physiological rates.
