CHLOROPHYLL-A DEGRADATION BY CALANUS-PACIFICUS - DEPENDENCE ON INGESTION RATE AND DIGESTIVE ACCLIMATION TO FOOD RESOURCES

Relative extents of Chl a degradation by Calanus pacificus were determined with ratios of a conservative tracer, Ge-68, to Chl a-derived pigment. The extent to which Chl a is degraded to colorless products can vary with food acclimation and ingestion rate. The fraction of ingested Chl a degraded by copepods acclimated to a high food concentration (4,000 cells ml-1 Thalassiosira weissflogii) for 3 d was 2-4 times greater than the fraction degraded by those acclimated to a low food concentration (1,000 cells ml-1). A fourfold decrease in ingestion rate corresponded to a twofold increase in relative extent of Chl a degradation by copepods acclimated to high food, but a similar decrease in ingestion rate had little effect on degradation by those acclimated to low food. Extent of degradation cannot be described with one overall mean value. Chl a and Chl a-derived pigments thus cannot be used as quantitative tracers of zooplankton feeding. Extension of our laboratory results to the field suggests that relative extents of degradation of pigments by copepods may increse over the course of a phytoplankton bloom. Fecal pellets produced during the early stages of a bloom may have relatively more undegraded pigment and other liable organic materials than pellets produced as the bloom declines.