FEEDING SELECTIVITY BY COPEPODS GRAZING ON NATURAL MIXTURES OF PHYTOPLANKTON DETERMINED BY HPLC ANALYSIS OF PIGMENTS

During a period of 8 d in late September and early October 1992, 5 grazing experiments were carried out in an area off the coast of Morocco. Water was collected from the depth of the chlorophyll maximum and fed to copepods of either 500 to 1000 mum or 200 to 500 mum in size. Two chlorophylls and 2 carotenoids dominated the pigment composition of the particulate material: chlorophyll (chl) a; chl C (C1 + C2); fucoxanthin and 19-hexanoyloxyfucoxanthin. Initial concentrations of chl a in experiments were between 0.4 and 2.2 mug l-1 and those of chl c were, between 0.04 and 0.54 mug l-1. Initial concentrations of fucoxanthin were between 0.1 and 1.1 mug l-1 and those of 19-hexanoyloxyfucoxanthin between 0.07 and 0.32 mug l-1. The ratio of fucoxanthin:19-hexanoyloxyfucoxanthin varied between 0.3 and 15 (w/w). The fucoxanthin was probably associated with diatoms and the 19-hexanoyloxyfucoxanthin with prymnesiophytes. Some chl a-derived fluorescent breakdown products were found in faecal pellets collected after grazing, but neither fucoxanthin nor 19-hexanoyloxyfucoxanthin were present. Hence these carotenoids were destroyed following ingestion by copepods, so that decreases in their concentrations in the incubation medium could be used to measure grazing rates on the different types of phytoplankton independently. In this way it was shown that copepods 500 to 1000 mum in size fed preferentially on diatoms, and that their overall feeding rate was depressed when fucoxanthin: 19-hexanoyloxyfucoxanthin ratios were relatively low. By contrast, copepods 200 to 500 muM in size fed randomly or preferred the prymnesiophytes. When chl a concentrations were initially < 1 mug l-1, then for both size ranges of copepods, only a very small proportion (< 10%) of the ingested chl a was converted into a-type phaeopigments, the rest apparently being destroyed during digestion. In one case, however, when copepods 200 to 500 mum were fed with water in which the initial chl a concentration was > 2 mug l-1, about 30 % of ingested chl a was recovered in a-type phaeopigments.