A technique for the in situ assessment of the vertical nitrogen flux caused by the diel vertical migration of zooplankton

It is argued that if herbivorous zooplankton exhibiting diel vertical migration generally feed only above the thermocline at night, but continue to excrete during the day when they are below the thermocline, then there should be a diel variation in their mean body nitrogen content. Thus measurement of the body nitrogen content of migrating zooplankton at different times of the day may provide a technique for the in situ assessment of the rate at which they remove nitrogen from the surface mixed layer. We show that if the inter-individual variation in body nitrogen content is high and sample size is low, then detection of the predicted diel change will tend to be obscured. However, in a simple laboratory experiment, a significant reduction in the mean body nitrogen content of the copepod Calanus helgolandicus was found between fed individuals (mean 12·91 μg N, n=42, SD =1·63) and those starved for 18 h (11.8911 μg N, n=30, SD= 1·29; t-test, t69=2·92, P=00047), suggesting that the technique may be applied in the field.