limits phytoplankton growth in this region. 

 Thomas (MS."") established the limiting con- 

 centration of nitrate for growth of a tropical 

 oceanic dinoflagellate and showed that surface 

 concentrations of nitrate in the eastern tropical 

 Pacific are generally lower than this figure. 

 The total of nitrogen compounds available in 

 surface sea water, including ammonia, might 

 not be limiting, however. 



Blackburn (1966) demonstrated significant 

 positive correlations between the standing 

 crop of chlorophyll a (water-column values) 

 and each of the following at about the same 

 times and places in the eastern tropical Paci- 

 fic: primary productivity (in the same water- 

 column), standing crop of certain herbivores 

 (0 to 300 m.), and standing crop of certain 

 primary carnivores (0 to 90 m.). The regres- 

 sion (slope) coefficients showed that the stand- 

 ing crop of herbivores varied as some power 

 <i.O of the standing crop of chlorophyll a. This 

 low value of the slope suggests that efficiency 

 of utilization of plants by herbivores decreases 

 with increase of the standing crop of plants. 

 On the other hand, the relation between the crop 

 of carnivores and that of herbivores was ap- 

 proximately linear. The standing-crop ratios, 

 copepods- -plants (by estimated weight of car- 

 bon) and carnivore s--zooplankt on (by displace- 

 ment volume), were both estimated roughly at 

 0.04, but it was clear for various reasons that 

 the corresponding efficiency ratios of the food 

 chain, for standing crops of all material at the 

 appropriate trophic levels, would be higher. 



Forsbergh (1963) and Holmes (MS., see foot- 

 note 1) presented values of photosynthesis - 

 chlorophyll a ratios, which were higher in 

 eutrophic than in oligotrophic regions. Holmes 

 (MS., see footnote 1 ) gave multiple regressions 

 of primary productivity, chlorophyll a, and 

 standing crop of zooplankton on various com- 

 binations of physical, chemical, and biological 

 properties, ajid discussed their predictive 

 value. Smayda (1965) found significant positive 

 regressions of primary productivity on various 

 measures of standing crop of diatoms (number, 

 volume, surface area, and plasma volume of 

 cells) in the Gulf of Panama. He also found that 

 the rate of carbon assimilation per unit of 

 standing crop was density-dependent. 



BIOLOGICAL OCEANOGRAPHY OF 

 SPECIAL AREAS 



Some detailed studies have been made of 

 spatial and temporal changes in the distribu- 

 tion of biological properties in certain areas 

 where physical processes such as upwelling or 

 vertical wind-mixing occur. 



* Thomas, William H. Surface nitrogenous nutrients 

 and phytoplankton in the northeastern tropical Pacific. 

 (Scripps Institution of Oceanography, University of Cali- 

 fornia, 1965). 



The most thorough study of this kind was by 

 Forsbergh (1963) for the Gulf of Panama, 

 where observations were made about every 2 

 weeks for about 4 1/2 years. Coastal upwelling 

 occurs seasonally in this area, which is neritic 

 and not oceanic. Standing crop of phytoplankton 

 and primary productivity were significantly 

 greater during upwelling than during nonup- 

 welling; about as much carbon per square 

 meter of sea surface was fixed by phytoplank- 

 ton during the 4-month upwelling season as 

 during the remaining 8 months. Relative ef- 

 ficiency of photosynthesis (photosynthesis cor- 

 rected to unit standing crop and unit light 

 energy) was closely related to indices of up- 

 welling such as temperature and salinity. 

 Mean northerly (offshore) winds for 3 days 

 preceding the observation day were better 

 correlated with indices of upwelling and rela- 

 tive efficiency of photosynthesis than were 

 other means. The response of photosynthesis 

 to changes in velocity and direction ofthe wind 

 seems to be rapid. The phytoplankton- zooplank- 

 ton relationship was obscure. 



Blackburn (1962, 1963) reported the physical 

 and biological results of repetitive seasonal 

 cruises to the Gulf of Tehuantepec, which is 

 more oceanic than the Gulf of Pananna. He 

 showed that the offshore wind, which is seasonal 

 as at the Gulf of Panama, produces a char- 

 acteristic surface circulation and associated 

 thermocline topography. The thermocline be- 

 comes so shoal in some areas that its nutrient- 

 rich waters lie close to the surface. The mix- 

 ing action of the wind itself then causes some 

 transfer of these nutrients into the surface 

 layer; higher standing crops of chlorophyll a 

 and zooplankton are then produced, at or a 

 little "downstream" from the site ofthe vertical 

 mixing. Crops are much lower in other parts 

 of the Gulf, and in all parts of it at seasons 

 when the wind is weak. 



Gunther (1936) noted that standing crops of 

 phytoplankton and zooplankton were higher in 

 areas of coastal upwelling than elsewhere 

 along the coast of Peru, and Sears (1954) 

 described the types of biological disasters that 

 occur when the coastal waters of Peru become 

 exceptionally warm and which have been con- 

 sidered to be consequences of the poorly under- 

 stood phenomenon called "El Nino." 



Holmes (MS., see footnote 1) drew attention 

 to the biological richness of the Costa Rica 

 Dome, an offshore upwelling area, and pre- 

 sented many measurements. 



Similar investigations have been made in 

 noneutrophic parts of the eastern tropical 

 Pacific. Bennett and Schaefer (i960), who 

 studied the so-called "island effect" at Clarion 

 Island, found that primary production and the 

 crop of chlorophyll a increased slightly towards 

 the island, but zooplankton showed no such 

 gradient in standing crop (the crop of zooplank- 

 ton was extremely low). Griffiths' (1963) in- 

 vestigation of the distribution of biological 



13 



