DIEL CHANGES IN THE PARTICULATE ENVIRONMENT, FEEDING-ACTIVITY AND DIGESTIVE ENZYME CONCENTRATION IN NERITIC ZOOPLANKTON

Diel changes in the particulate environment and nutritional metabolism of neritic zooplankton were studied in the Bay of Villefranche-sur-mer (France) for 30 h during February. Although the survey was carried out over a period of negligible wind speed, the changes in the particle size spectra and species composition of the Zooplankton sampled suggested an abrupt and temporary oscillation in the water mass at the second sampling time. The rest of the 30-h period was characterized by gradual changes which usually returned to conditions close to, but different from, the starting ones. A clear diel rhythm was recorded for the biochemical composition of the phytoplankton, probably related to the periodicity of the photosynthetic metabolism. The feeding rate of Clausocalanus arcuicornis Giesbrecht, 1888, measured by gut fluorescence, also showed diel rhythmicity with maximum rate at night. Digestive enzyme activities were measured on the total Zooplankton population composed of five major species: Clausocalanus arcuicornis, Paracalanus sp., Calanus helgolandicus Claus, 1863, Acartia clausi Giesbrecht, 1889, and Oikopleura dioica Fol, 1859. The activities of the carbohydrases showed varying patterns of periodicity with one or two night maxima. The proteases showed a different pattern with acidic proteases peaking just before dawn and decreasing gradually during the day and no periodicity for trypsin-like activity. No association could be established between the enzyme changes and those of either the food supply or the species composition of the Zooplankton sampled, suggesting that both feeding and digestion diel rhythms are controlled by an internal clock. Influences of internal, external and experimental factors on the digestive enzyme activity rhythm are discussed. The resulting scheme of short term regulation was compared with that proposed earlier for medium term changes.