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 gra zing 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 th e particulate material: chlorophyll (chl) a; chl C (C1 + C2); fucoxant hin 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 w ere 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-hexanoylox yfucoxanthin varied between 0.3 and 15 (w/w). The fucoxanthin was prob ably associated with diatoms and the 19-hexanoyloxyfucoxanthin with pr ymnesiophytes. Some chl a-derived fluorescent breakdown products were found in faecal pellets collected after grazing, but neither fucoxanth in 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 pre ferentially on diatoms, and that their overall feeding rate was depres sed when fucoxanthin: 19-hexanoyloxyfucoxanthin ratios were relatively low. By contrast, copepods 200 to 500 muM in size fed randomly or pre ferred the prymnesiophytes. When chl a concentrations were initially < 1 mug l-1, then for both size ranges of copepods, only a very small p roportion (< 10%) of the ingested chl a was converted into a-type phae opigments, the rest apparently being destroyed during digestion. In on e case, however, when copepods 200 to 500 mum were fed with water in w hich the initial chl a concentration was > 2 mug l-1, about 30 % of in gested chl a was recovered in a-type phaeopigments.