cessed for chlorophylls a, b, and c by the trichro- 

 matic method (Strickland and Parsons 1972), 

 using the equations of Parsons and Strickland 

 (1963). Unfortunately, these equations do not 

 give accurate estimates of chlorophyll b or c, and 

 despite numerous attempts to improve their ac- 

 curacy, no set of equations has been completely 

 satisfactory (Jeffrey 1968, 1981; Jeffrey and 

 Humphrey 1975). However, if the errors for gut 

 and water samples are assumed to be the same, 

 then comparisons can be made between pigment 

 concentrations in guts and water. To test this, 

 Acariia tonsa were starved for 24 h and a sample 

 of animals was examined microscopically to en- 

 sure that the guts were empty. Pure cultures of 

 the diatom Thalassiosira fluviatilis were added 

 to half of the flasks containing the copepods. 

 The animals in the remaining flasks were not 

 fed. After 4 h, the animals in all of the flasks 

 were processed for chlorophyll pigments as de- 

 scribed below. We chose to feed a diatom in this 

 experiment to find out if chlorophyll a or c might, 

 during digestion, be converted into a product 

 absorbing at the wavelengths used in measuring 

 chlorophyll b (not found in diatoms). Table 1 sug- 

 gests that this did not occur. Low levels of chloro- 

 phyll b were detected both in the culture and the 

 fed animals but not in the starved animals, indi- 

 cating that there was some contaminant in the 

 culture or a small error in the equation at high 

 chlorophylls a and c concentrations. Pigment 

 ratios in the culture and the guts of animals fed 

 from the culture were fairly stable. 



Copepods for gut contents analysis were col- 

 lected from 7 to 10 vertical tows of a CalCOFI 

 vertical tow net (335 /jm mesh). Tows were made 

 from 70 m at station 7B and from near-bottom at 

 stations N4 and N6. The tows took 2-3 min each to 

 complete and were made in rapid succession. 

 The ship was kept on station during the entire 

 sampling period. On the cruise of 12 September, 



Table 1.— Ratios of chlorophyll pig- 

 ments in a culture of Thalassiosira flu- 

 viatilis and in Acartia guts when the 

 animals were starved or fed the phyto- 

 plankton culture. 



Chlorophyll a levels per milliliter of culture 

 = 0.24 /yg; per animal = 12 ng. 



ND = a ratio that could not be computed, 

 since chlorophyll b was not detected. 



adult A. tonsa and C. pacificus were immedi- 

 ately separated from the rest of the catch. Half of 

 each sample was washed in filtered (0.5 ^m) sea- 

 water and frozen, and the other half was main- 

 tained in aerated, filtered seawater for 24 h 

 allowing them to clear their guts of food prior to 

 freezing. This empty gut group was used to de- 

 tect absorbance at wavelengths used in chloro- 

 phyll analysis that was not due to chlorophyll. On 

 26 September, collections were made in a similar 

 manner, except that half of the entire catch was 

 washed with filtered seawater and frozen and the 

 other half was maintained alive for 24 h in fil- 

 tered seawater prior to freezing. Specimens for 

 analysis were separated from the rest of the 

 catch in the laboratory. 



Chlorophyll a analysis of gut contents was con- 

 ducted by macerating 50-200 animals of each 

 species of full and empty gut groups in 90% ace- 

 tone and by reading absorbances in triplicate at 

 wavelengths of 750, 665, 645, and 630 nm on a 

 Beckman model 34 spectrophotometer. Chloro- 

 phylls a, b, and c concentrations were computed 

 by the trichromatic equations of Parsons and 

 Strickland (1963). Chlorophyll degradation prod- 

 ucts (pheopigments) in the gut contents were 

 computed as described by Strickland and Par- 

 sons (1972) and were considered as part of the 

 total chlorophyll because the processing of food 

 in the gut rapidly degrades chlorophylls. Since 

 our interest was in how much plant material was 

 present and not in the rate of food processing, we 

 include both chlorophyll and its degradation 

 products as a single indication of plant biomass 

 in the gut. 



Gut fullness was estimated in A. tonsa collected 

 on 26 September. The animals were "cleared" by 

 immersion in 85% lactic acid for 30 min and then 

 examined under 25X magnification (Hay ward 

 1980). Gut fullness was estimated independently 

 by each author and the average of the two esti- 

 mates was recorded. Attempts were made to esti- 

 mate gut fullness in C. pacificus, but there was 

 disagreement between estimates because the 

 lactic acid did not clear the animals well. 



Results 



Phytoplankton density and community struc- 

 ture were similar at station 7B on both sampling 

 dates. Cell density averaged 8-10 X 10 3 cells/1. 

 The majority of phytoplankton species found 

 were dinoflagellates (35% of the community), 

 dominated by Gymnodinium spp., and diatoms 



156 



