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 cont inue to excrete during the day when they are below the thermocline, th en there should be a diel variation in their mean body nitrogen conten t. Thus measurement of the body nitrogen content of migrating zooplank ton at different times of the day may provide a technique for the in s itu assessment of the rate at which they remove nitrogen from the surf ace mixed layer. We show that if the inter-individual variation in bod y nitrogen content is high and sample size is low, then detection of t he predicted diel change will tend to be obscured. However, in a simpl e laboratory experiment, a significant reduction in the mean body nitr ogen content of the copepod Calanus helgolandicus was found between fe d individuals (mean 12.91 mu g N, n=42, SD = 1.63) and those starved f or 18 h (11.89 mu g N, n = 30, SD = 1.29; I-test, t(69) = 2.92, P = 0. 0047), suggesting that the technique may be applied in the field. (C) 1997 Elsevier Science Ltd.