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Samples of adult female Calanus helgolandicus were collected at midday and midnight from the upper 20 m of the stratified water column in the western English Channel over 3 d in July 1983. There we]-e pronounced diel changes in population density in these samples consistent with vertical migration through the thermocline; such migration was detected at the same station in September 1979 and August 1982. Ammonium excretion rates of the surface populations at midday and midnight were compared in short-term incubation experiments and additional samples taken for estimation of gut pigment, and the activity of the digestive enzymes amylase, trypsin and laminarinase Weight-specific ammonium excretion showed highly significant diel differences, night-time ammonium excretion being 1.7 times that observed at midday. When die1 changes in population density are allowed for, the regenerated ammonium flux from female C. helgolandicus in the surface 2 0 m at midnight is estimated to be 28 times higher than at midday. Gut pigment levels and laminarinase and trypsin activity observed in the n~ght-tlme surface population were consistent with active feeding. Amylase was an exception in showing significantly lower activity at night. Observations at stations characteristic of tidally mixed and frontal conditions showed less pronounced diel changes in metabolism. The implications of these results for competitive interactions between phytoplankton species adapted to pulsed nitrogen supply in nutrient-limited surface waters are considered.

Diel patterns of ammonium excretion and grazing rhythms in Calanus helgolandicus in surface stratified waters