VENRICK: SYSTEMATIC SAMPLINC IN KCOSYSTKM 



face maximum layer (0.10-0.20 mgm-M centered 

 between 90 and 120 m. 



The present study was conducted in two parts. 

 In part A, a computer was used to sample nine 

 semiartificial populations derived from continu- 

 ous vertical profiles of chlorophyll fluorescence. 

 Changes in the fluorescence per extractable 

 chlorophyll unit with depth iKiefer 1973) and 

 smoothing of small-scale features during the 

 pumping procedure result in a profile which repre- 

 sents only the grosser features of the true distribu- 

 tion. From the vertical profiles, the total popula- 

 tion along the vertical axis was calculated, 

 allowing the accuracy of various sampling strate- 

 gies to be determined directly. Study B was con- 

 ducted in the field where restricted systematic and 

 stratified random samples were collected simul- 

 taneously from the population. In this study, a real 

 population was studied but the total population 

 could only be approximated. 



METHODS 



Analytical Procedures 



offset the increase in fluorescence per unit of ex- 

 tractable chlorophyll with depth, one conversion 

 equation was used down to and including the 

 chlorophyll maximum and another below the 

 maximum. The conversion factors were deter- 

 mined by analysis of chlorophyll extracted from 

 discrete water samples collected periodically dur- 

 ing the cruise. The surface value of each continu- 

 ous profile was set to 0.03 mg/m-^ and the minimum 

 value below the maximum to 0.01 mg/m-^ these 

 were the mean values of extracted chlorophyll ob- 

 served at the surface and at 200 m, respectively. 

 The horizontal scale was adjusted to bring the 

 mean maximum value of all profiles to 0.156 

 mg/m'\ the average maximum of the discrete sam- 

 ples. A typical adjusted profile is presented in Fig- 

 ure 2. 



These semi-artificial populations were sampled 

 with four stratified random designs ( Table 1 ). The 

 success of SR depends upon the extent to which the 

 strata can be made internally homogeneous. In an 

 attempt to achieve this, the stratum boundaries of 

 SR-1 and SR-2 were determined as much as possi- 

 ble by the hydrographic, biological, and chemical 



Chlorophyll a was determined fluorometrically 

 according to the procedure of Yentsch and Menzel 

 (1963) as modified by Holm-Hansen et al. (1965). 

 Water for discrete, extracted chlorophyll samples 

 was obtained with Nansen bottles. Water for con- 

 tinuous vertical profiles was obtained with the 

 seawater pumping system described by Beers et 

 al. (1967) and was passed through a fluorometer 

 equipped with a flowthrough door. 



Stud) A 



The chlorophyll fluorescence profiles were taken 

 during September 1968, on 9 consecutive days 

 during which time the ship followed two drogues 

 which were set at 10 m depth to follow the mixed 

 layer. These were launched at lat. 27°00'N, long. 

 155°18'W and moved in a northwesterly direction 

 at speeds between 0.5 and 1.5 kn covering 345 km 

 in 9 days. The profiles were not made at the same 

 time of day. The closest two profiles were sepa- 

 rated by 13 h, the most distant by 40 h. Additional 

 aspects of these profiles and accessory data have 

 been published (Scripps Institution of Oceanog- 

 raphy 1974). 



The fluorescence profiles were read at 1-m in- 

 tervals and translated into units of approximate 

 chlorophyll down to a depth of 180 m. In order to 



50 



a^ioo 



Q 



15 



TEMPERATURE (°C) 



20 25 



1 1 1 \ I I I I 1 I 1 1 1 T" 



CHLOROPHYLL (mg/m3) 

 05 iO 15 .20 .25 .30 



"TT 



- A 



- A 



- A 



A 

 A 

 A 

 A 



I M I I [ I M I I [ I I I 



CHLOROPHYLL 



M M M M I M I 

 TEMPERATURE " 



150 



L- hAh  



Figure 2.— a typical population of chlorophyll values derived 

 from a continuous profile of fluorescence (27 September 1968) 

 and sampled in study A, together with the temperature values 

 from the associated hydrocast. Triangles indicate the location of 

 samples in restricted systematic design 3; bars represent the 

 boundaries of strata used in stratified random design 1. 



619 



