ZOOPLANKTON IN METABOLISM OF LAKES 



89 



ferent oi-.uaiiisms behave differently under 

 the same conditions. Also, in many cases 

 animals in the laboratory behave differently 

 I'rom those in their natural liabitat. This 

 fact makes it extremely difficult to correlate 

 laboratory and field work. "It seems prol)- 

 able, however, that the nltimate picture of 

 plankton ic behavioi- will come from such a 

 correlation." 



Collection Methods 



The accuracy of the results of quantita- 

 tive estimations of plankton has been ques- 

 tioned by many workers. Plankton catches 

 are subject to certain errors Avhich may be 

 enumerated as follows (Ricker 1938) : (1) 

 lack of uniformity of horizontal distribu- 

 tion, (2) ability of plankters to avoid cap- 

 ture, (3) collection errors, (4) errors in 

 enumeration. Ricker (1937) has worked 

 out certain statistical limits, but as Lan^- 

 ford (1938) points out these limits apply 

 only when the distribution of plankters is 

 knoAvn to be random. Langford believes 

 that the effects of horizontal variation can 

 be overcome by taking' a number of dupli- 

 cating series and that greater accuracy can 

 be obtained by making counts of a smaller 

 number of organisms from each or a frac- 

 tion of mixed samples from one depth than 

 by counts on single samples. Tn lakes with 

 even bottom contours, a single central sta- 

 tion will give a fairly good representation 

 of the whole lake, but in lakes with rough 

 and uneven bottom contours a considerable 

 difference in the quantity of the pelagic 

 plankton has been shown to exist (Southern 

 and Gardiner 1926 and others). Southern 

 and C4ardiner (1926) and others observed 

 that the net plankton was less in daytime 

 than at night, and they believed that this 

 was due to the ability of the plankters to 

 see and avoid the net. This difference oc- 

 curred most markedly in the upper lighted 

 areas. They found an extreme ratio of 2.4 

 between night aiul day catches in Lough 

 Derg in the case of DapJniid longispiud. 

 Ricker, however, found no difference in the 

 day and night abundance of Episcluira, 

 which lives in the upper waters. The abil- 

 ity of plankters to see and avoid the net 



was furtlier studied by Ullyott (1936) who 

 used black nets. He found that black nets 

 reduced the difference in some eases but had 

 no effect for most species. It is believed 

 that individual species exhibit differences in 

 ability to see and swim away from the net 

 or trap, and that tliese individual differ- 

 ences in l)ehavior must be known before a 

 general explanation of the differences in 

 day and night catches can be given. Worth- 

 ington and Ricardo (1936) found this to be 

 ti'ue in Lake Rudolph where Diaptomus 

 crowded near the surface showed no differ- 

 ence in day and night catches, while the 

 Cladocera in deep portions of the lake 

 showed marked differences in quantity. 



Collection errors are largely due to the 

 type of collecting apparatus used even when 

 such apparatus is handled in the most care- 

 ful manner. The old vertical haul closing 

 net had the advantage of covering all the 

 water from the bottom to the surface, it was 

 easy to handle and transport but had the 

 very serious drawback of being very un- 

 reliable and variable in the amount of water 

 strained. Ricker (1938) found that even 

 Avhen handled with the utmost care a new 

 No. 20 net after five hauls had its efficiency 

 decreased as much as one-third or even one- 

 half. A Xo. 10 net gave good and fairh' 

 consistent results for the larger forms, but 

 failed to capture most of the smaller zoo- 

 plankton (one-half of the Sy)ichuefa and 

 one-fourth of the nauplii) . It has also been 

 shown that there is a great difference be- 

 tween vertical hauls and stage hauls, ver- 

 tical hauls giving more plankton. Lang- 

 ford in comparing the closing net with the 

 plankton trap found wide ranges in effi- 

 ciency at different depths and for different 

 distances of vertical haul. In a haul from 

 2.5 meters to the surface the efficiency for 

 various organisms varied between 55-84 per 

 cent with a mean of 65 per cent, while in 

 the least efficient haul from 10 to 5 meters 

 the net caught only 10 per cent of the 

 plankters captured by the trap. It has been 

 suggested that some of the material may be 

 spilled when the net closes, and that such 

 losses account for the greater efficiency when 

 the net is hauled from shallow depths to 



