BEERS ET AL,, PLANKTON AND UPWELLING OFF PERU 



average, be a very small addition to the total. 

 If the ciliate populations found off Peru with 

 their dominance of oligotrichous forms i-eceive 

 much of their nutritional requirements through 

 functional chloroplasts in their endoplasm, their 

 direct consumption of phytoplankton would prob- 

 ably be lower than assumed. The gymnostome 

 ciliate, Mesodhuum rubnim, for which good evi- 

 dence of endocellular chloroplasts exists (Taylor, 

 Blackbourn, and Blackbourn, 1969) was not in- 

 cluded in this calculation. 



The Cahinus standing stocks at the four sta- 

 tions associated with Patch 1 (0-100 m) were 

 estimated to be consuming an average of 22 mg 

 C m-/day. These estimates were derived using 

 the data of Mullin and Brooks (1970) on inges- 

 tion by the various developmental stages of Cal- 

 anus helgolandicus. The average net primary 

 production over the euphotic zone at these four 

 stations was found to be 1035 mg C/m2/day. 

 Thus the Cakinus population, which was an aver- 

 age of 21 Vf of the total 103 /x net biomass was 

 consuming only a little more than 2''/> of the 

 plant production. Even if the remaining 73% 

 of the zooplankton population were migrating 

 to the euphotic zone and consuming phytoplank- 

 ton at the same rate as Calanus the total con- 

 sumption estimate would still be less than 10 "Jr 

 of the daily production. Of course, a significant 

 number of the zooplankters may not be herbi- 

 vores and also many are much larger forms than 

 Calamis and it is probable that their daily in- 

 gestion as a percentage of their bodily carbon 

 would be lower than that of Calanus. The zoo- 

 plankton populations below 100 m which might 

 migrate vertically to feed have not been con- 

 sidered here. The majority of tows taken on 

 this cruise were during daylight hours but no 

 significantly greater abundance was evident in 

 the few tows taken during the hours of darkness. 

 The level of dissolved oxygen at 100 m and below 

 in Peruvian coastal waters is low (usually 

 <1 ml/liter). However, Mullin (1966)' found 



' Mullin, M. M. 1966. Vertical distribution of zoo- 

 plankton occurring in the oxygen minimum layer off 

 Peru. In University of California, Institute of Marine 

 Resources, Research on the marine food chain, Progress 

 report, January 1966 - December 1966, p. 359-369. (Un- 

 published manuscript.) 



numerous zooplankton species inhabiting the 

 o.xygen-poor waters off Peru, and some species 

 even showed their greatest abundance at these 

 depths. Nevertheless, in terms of total zoo- 

 plankton biomass the upper 100 m would probab- 

 ly be of much greater importance than lower 

 depths. 



In summary, our estimates call for no greater 

 consumption by the zooplankton than about 25% 

 of the daily primary production. Coupling this 

 with the fact there was no indication that the 

 actively photosynthesizing phytoplankton crop 

 in either patch was increasing with time but, in 

 fact, was actually disappearing, indicates some 

 mechanism other than grazing must be at least 

 partly responsible. Likewise, the fact that there 

 was no significant increase in the phaeophytin 

 level or in the chlorophyll/phaeophytin ratio as 

 the patch was monitored with time argues 

 against zooplankton grazing as a principal cause. 

 Dugdale and Goering (1970) in their study of 

 biological production in the Peru Current during 

 a period of high diatom levels indicated grazing 

 was not the principal source of "loss" of phyto- 

 plankton and that the combined anchovy and 

 zooplankton grazing was at a daily level of about 

 20 "^r of the standing crop. It was further sug- 

 gested that, of these, the anchovy were a quan- 

 titatively more important grazer than the zoo- 

 plankters. 



Strickland et al. (1969) suggested three al- 

 ternate hypotheses to grazing which implicated 

 physical factors as mechanisms for patch dis- 

 appearance. In the present study, estimates of 

 vertical shear and stability indicated that turbu- 

 lent mixing was occurring in the upper 50 m in 

 Patch 1. 



Although a lack of current measurements lim- 

 its our ability to accurately determine local mo- 

 tion within Patch 1, an order of magnitude 

 estimate for the rate of upwelling in the patch 

 is possible from a consideration of the size of the 

 patch and associated biological productivity. 

 From Figure 4 the patch size was found to be 

 10 km by 5 km by 50 m, in the east-west, north- 

 south and vertical dimensions, respectively. The 

 corresponding volume of the patch is 25 X lO'"" 

 cm^. The patch is assumed to be 50 m thick, 

 below which a subsurface poleward flow is 



873 



