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2100 0000 0300 . 0600 0900 1200 1500 1800 2100 0000 0300 0600 0900 1200 

 LOCAL STANDARD TIME 



Figure 1. — Variations in volumes of zooplankton over a 48-hour period, June 21-23, 1907, at the IGY station. 

 Sunrise during this perioid was at 0503 and simset at 18r46 hours. 



1500 1800 



COMPOSITION OF ZOOPLANKTON 



Abundance of groups of plankters expressed as 

 average numbers per 1,000 m.' of water strained 

 and as a\'erage jjercentages of total niimlters of 

 plankters is summarized in table 1. By far the most 

 numerous organisms in the samples were copepods, 

 which on the average constituted about 60 percent 

 of all organisms present. Chaetognatha and Halo- 

 sphaera viridis,- almost equal in average abun- 

 dance, were the second and third most common 

 organisms in the samples. Numerically, each aver- 

 aged less than 10 percent of the copepods. 



Distinct diel variations, with gi^ater numbers 

 during darkness, were exhibited by Ostracoda, 

 Euphausiacea, Pteropoda, and fish larva© (fig. 2). 

 Amphipoda exhibited such a die! variation only 

 during the second half of the series (fig. 2). In 

 addition to the aforementioned groups, Annelida 



(Polychaeta) and calanoid Copepoda have been 

 shown previously to exhibit distinct diel variations 

 in Hawaiian waters (Nakamura, 1967). In this 

 study, these two groups did not show the character- 

 istic peaks of abundance during darkness. Perliaps 

 if specific or generic identifications had been made 



^This phytoplnnkter was present in every sample and was 

 included in all counts. 



Table 1. — Average abundance and average percentage 

 composition of various plankters for 16 samples of the 

 48-hour series 



Plankters Average Average 

 abundance composition 



No.llflOOm? Percent 



Halosphaera^ .- 1,654 5.6 



Foraminifera 776 2.6 



Radiolaria -.. 572 1.9 



Siphonophora - 1,088 3.7 



Chaetognatha 1,626 6.6 



Annelida -- 132 .4 



Calanoid Copepoda 17,788 69.8 



Noncalanoid Copepoda.- _ 1,024 3.4 



Ostracoda 866 2.9 



Euphausiacea -- 664 2.2 



Amphipoda 1,189 4.0 



Decapod Crustacea 1,076 3.6 



Pteropoda 290 1.0 



Heteropoda - 147 .5 



Gastropod larvae 64 .2 



Pelecypoda ..- 60 .2 



Thaliacea - 61 .2 



Appendicularla .-- 422 1.4 



Fishlarvae 129 .4 



Fish eggs 36 .2 



others - 82 .3 



' Haloaphaera viridit, a pbytoplankter, numerous in all samples. 



diel fluctuations would have been revealed, for 

 Nakamura (1967) found that the following genera 

 of calanoid copepods were concentrated in the 

 upper waters during darkness: PJewrom-am.ma, 

 Neocalunus, Candacia, Undimula, and Eiwhaeta. 

 In the present study abmidance of Heteropoda 

 reached a peak during daylight as well as at night. 



VARIATIONS IN MARINE ZOOPLANKTON IN HAWAIIAN WATERS 



89 



