FISHERY BULLETIN: VOL. 71, NO. 1 



cies were found at 125 m, and of these 64, 22 

 occurred only in samples from that depth. Thus, 

 the chlorophyll maximum layer, which has a 

 higher species diversity (as measured by the 

 number of diatom species) and which may con- 

 tain species not found at shallower depths ap- 

 pears to offer unique features as a biological 

 habitat. 



THE CENTRAL PACIFIC 



Studies conducted on Expeditions CLIMAX I, 

 CLIMAX II, and ARIES III near the axes of the 

 North and South Central Pacific Gyres have pro- 

 vided us with a large amount of data concerning 

 the vertical distribution of chlorophyll and pro- 

 ductivity in the water column and their rela- 

 tionship with other physical, chemical, and 

 biological parameters. Comparison with data 

 collected over much wider areas on other expe- 

 ditions leads us to believe that the relations ob- 

 served in the Central Gyres may be pertinent 

 to much of the oceanic Pacic. 



The vertical distribution of chlorophyll and 

 net production observed during these four stud- 

 ies have been summarized in Table 2. All studies 

 show a well-defined subsurface accumulation of 

 chlorophyll which varies in width from 50 to 

 75 m and contains maximum concentrations of 

 chlorophyll in excess of 0.10 mg/m^. The core 

 of the layer always occurred below the depth 

 penetrated by 1% of the surface radiation. In 

 fact, more than half of the total chlorophyll 

 within the water column was observed below 

 this depth. 



The rate of production per unit chlorophyll 



decreases with depth from a maximum at about 

 20 m, but this is partially offset by the increase 

 in the amount of chlorophyJl, and production 

 rates as high as 0.13 mg C/m^/hr have been 

 observed in the maximum layer in the South Pa- 

 cific. Our in situ experiments indicate that 7% 

 to 20 % of the total production in the water col- 

 umn occurs below the 1 % light level. These are 

 minimum values since our in situ studies did 

 not reach the level of no productivity. The total 

 rate of production throughout the water column 

 is variable on rather small spatial and temporal 

 scales, but appears to be considerably greater 

 than maximum estimate of 100 mg C/mVday 

 (8.3 mg C/m^/hr) estimated by Koblentz- 

 Mishke et al. (1970). 



The vertical distribution of chlorophyll and 

 several relevant properties are illustrated in 

 Figure 4. Data points are mean values of ob- 

 servations made in replicate on six 24-hr stations 

 in the South Central Pacific (CLIMAX II Ex- 

 pedition) and represent an area of 60 square 

 miles and a time span of 6 days. Above 200 m 

 there was an average of 12.35 mg chlorophyll 

 per square meter sea surface. Of this, over half 

 occurred below the estimated depth of 1 % light. 

 We estimated the light intensity at the core of 

 the maximum layer to lie between 0.10% and 

 0.26% of incident radiation. The vertical dis- 

 tribution of phaeophj^in is similar to that of 

 chlorophyll. 



The accumulation of both chlorophyll and 

 phaeophytin occurs within the pycnocline. On 

 a local scale, these layers may move up and down 

 with the pycnocline, for instance in response to 



Table 2. — Mean value and 95% confidence limits of the mean for data relative to the vertical distribution of light, 

 chlorophyll a, and productivity at two stations in the North and South Central Pacific Ocean. 



48 



