HORIZONTAL DISTRIBUTION OF 

 BIOLOGICAL PROPERTIES 



Various authors have drawn attention to the 

 broad correspondence of areas in which biolog- 

 ical properties have high values and areas 

 in which physical processes are known to 

 enrich the surface layer with water from below. 

 Among them are Forsbergh and Joseph (1964) 

 for surface chlorophyll a_ and productivity, 

 Brandhorst (1958) for zooplankton at Oto 300 m., 

 and Blackburn (MS., see footnote 2) for 

 micronekton at to 90 m. In this section, the 

 geometric means of all available values of 

 surface chlorophyll a, surface primary produc - 

 tivity, and standardized volume of zooplankton 

 (0 to 300 m.) are compared for 14 different 

 areas of the eastern tropical Pacific, separately 

 for the periods January to June and July to 

 December (irrespective of year). These prop- 

 erties are the only three which have been 

 measured sufficiently often to justify this kind 

 of analysis. Methodologically comparable 

 measurements of standing crop of mic ronekton, 

 and standing crop of chlorophyll a for the water 

 column, are much fewer (slightly over one 

 hundred of each are available for the eastern 

 tropical Pacific), and measurements of pri- 

 mary productivity for the water column are 

 fewer still (Blackburn, 1966; Holmes MS., 

 see footnote 1, Blackburn, MS., see foot- 

 note 2). 



Table 2 shows the numbers of observations 

 of the three properties by areas, half-years, 

 and by the operations listed (except with an 

 asterisk) in table 1. Data from operation 27 

 are not included, because they form a time 

 series, in which variation was small, in the 

 same body of water. A few unpublished values 

 of surface (0 to 3m.) chlorophyll a^ from 

 CalCOFI cruises are included (by courtesy of 

 R. Grigg and M. B. Schaefer). 



Figures 1 to 6 show the areas, recognized 

 on the basis of physical-oceanographic con- 

 ditions: 



Areas 3, 12, and 13 are regions of coastal 

 upwelling (Gunther, 1936; Reid, Roden, and 

 Wyllie, 1958; Wyrtki, 1963; Wooster and Reid, 

 1963). 



Areas 5 and 10 have mean annual depth of 

 center of permanent thermocline at < 30 m.; 

 upwelling or similar enriching phenonr.ana oc- 

 cur seasonally in some localities in area 5 

 (Forsbergh, 1963, and references; Blackburn, 

 1962); offshore equatorial upwelling possibly 

 occurs in the west of area 10 (Wooster and 

 Cromwell, 1958). 



Areas 4, 6, and 9 have mean annual depth 

 of center of permanent thermocline at 50 to 70 

 m.; equatorial upwelling occurs in area 9 

 (Wooster and Cromwell, 1958). 



Areas 2, 7, 8, cind 11 have mean annual 

 depth of center of permanent thermocline at 

 70 to 130 m.; equatorial upwelling occurs in 

 area 8 (Wooster and Cromwell, 1958). 



Areas 1 and 14 have mean annual depth of 

 center of permanent thermocline al > 1 30 m. 



The information on depth of thermocline is 

 from Wyrtki (1964, fig. 54). Some nonupwelling 

 areas with similar thermal structure (e.g., 

 areas 2, 7, and 11) are recognized separately 

 because they represent, very broadly, parts of 

 different surface current systems (Wyrtki, 

 1965). The boundaries of the areas were drawn, 

 on the basis of this physical-oceanographic 

 information, before the biological-oceano- 

 graphic data were examined. The Gulf of 

 California, north of lat. 25° N., has had 

 practically no biological-oceanographic study 

 of the kind under consideration and is there- 

 fore ignored. 



The periods, January to June and July to 

 December, were selected partly on the basis 

 of availability of biological data and partly on 

 the basis of physical conditions. It was de- 

 sirable to divide the year into periods, each 

 of which included reasonably large numbers of 

 observations for several areas and which 

 might be expected to differ in biological 

 properties in some or all areas because of 

 known differences in physical conditions. Avail- 

 ability of biological data--much scarcer than 

 physical data and very unevenly distributed 

 over the year- -dictated a limitation to two 

 periods and narrowed the choice of the periods. 



A physical basis exists for the use of two 

 6-month periods. On average, the northeast 

 trade wind is strongest and the southeast trade 

 wind weakest about March; the opposite situa- 

 tion holds about August (Bjerknes, 1961, and 

 references). Two periods can be recognized, 

 one about November to April when the north- 

 east trade wind is stronger than the southeast, 

 and the other about May to October when it is 

 weaker. These periods, which are approximate, 

 are specified from the mean monthly positions 

 of the Intertropical Convergence charted by 

 Wyrtki (1965). Thus, very broadly, in November 

 to April the amount of coastal upwelling, mean 

 depth of mixed layer, and mean velocity of 

 westerly surface currents can be expected to 

 be greater in comparable situations in the 

 northern hemisphere than in the southern; 

 in May to October they would be expected to 

 be greater in the southern hemisphere than in 

 the northern. Since these physical features 

 must affect the production and distribution of 

 organisms, some indication of seasonal changes 

 in biological properties might be given by a 

 comparison of data for the two periods. The 

 physical conditions do not vary in all areas 

 in the way and at the periods just stated, 

 however, and the numbers of observations 

 are very uneven for most properties in 

 most areas. The periods January to June and 

 July to December gave a less uneven 

 distribution of the data than any other 2 

 half-year intervals. This subdivision of the 

 year was therefore used for want of a better 

 one. 



