SMAYDA: NET PHYTOPLANKTON IN UPWELLING WATERS 



to the question of whether Pleuroncodes' occur- 

 rence is causally linked to that of Coscinodiscus. 



Coscinodiscus and other heavily silicified 

 diatoms sink to the sea floor, as documented for 

 the Gulf of California (Round 1967, 1968). This 

 deposition contributes skeletal remains (i.e., to 

 the thanatocoenosis) and organic matter to the 

 sediments. The abundance and sinking charac- 

 teristics of this diatom population are also of in- 

 terest, since Pleuroncodes also occurs in the 

 benthos. During this benthic residence, when 

 population densities up to 250 individuals/m^ 

 have been found (Smith et al. footnote 2), it may 

 feed on detrital material (Longhurst et al. 1967). 



Various studies on phytoplankton were carried 

 out during the MESCAL II expedition of the RV 

 Thomas G. Thompson in 1973 to study upwelling 

 off Baja California, as a continuation of 1972 ac- 

 tivities in this area (Walsh et al 1974). These 

 included the routine, shipboard examination of 

 both net phytoplankton and the >20- Mm size frac- 

 tion filtered from quantitative samples. The dis- 

 covery that natural populations of a Ditylum 

 brightwelli and, possibly, Biddulphia mobiliensis 

 exhibited diel cell division has been reported 

 (Smayda in press a). 



Examination of the >20-/jm size fraction was 

 partly motivated by the need to know its composi- 

 tion and abundance, particularly that for Coscino- 

 discus. This was to evaluate the aforementioned 

 relationship possibly occurring between Pleuron- 

 codes and Coscinodiscus, and to establish the 

 latter's importance during the initial stages of 

 upwelling. This latter objective was prompted by 

 the remarkable bloom found in Magdalena Bay 

 during a later stage of the upwelling cycle. 

 Finally, such data are needed to evaluate the 

 sinking and turnover rates of the more heavily 

 silicified and dissolution-resistant components 

 of this size fraction which sink faster and repre- 

 sent an energy source for benthic secondary 

 production. 



METHODS 



Between 26 March and 6 April 1973, 22 collec- 

 tions of net phytoplankton were made at 15 sta- 

 tions (multiple sampling on different days at 

 some). A No. 20 (mesh opening of 69 /am) net 30 cm 

 in diameter sampled the upper 50 to 100 m (de- 

 pending on depth) for 30 min by repeated vertical 

 oscillations, during which the net was lowered at a 

 rate of ca. 30 m/min and retrieved at a rate of 10 

 m/min. The samples were examined microscopi- 



cally soon after collection, after placing onto a 

 slide an aliquot of the unpreserved, sedimented 

 material from an unagitated sample. 



Of the 15 stations sampled, 13 were located in a 

 sampling block measuring about 105 km long and 

 30 km wide centered approximately at Punta San 

 Hipolito off the coast of Baja California (Figure 1). 

 The coordinates of the northern- and southern- 

 most stations are lat. 27°6.7'N, long. 114°21.2'W 

 andlat. 26°28.5'N,long. 1 13°45. 5 'W, respectively. 

 The stations extended offshore from within sight 

 of land to within, or near, the California Current; 

 the inner- and outermost stations were at lat. 

 26°55.2'N, long. 114°02.2'W and lat. 26°51.2'N, 

 long. 114°10.7'W, respectively. Stations 1 and 2 

 (not shown in Figure 1) were located about 460 km 

 north of this main sampling area at lat. 30°57.8'N, 

 long. 116°32'W and lat. 28°8.2'N, long. 

 115°39.2'W, respectively. 



Quantitative samples were also collected at 13 

 stations from the surface to 50 m at 10-m inter- 

 vals, and at 75 m with 5-liter Niskin Bottles. 

 Seven stations (18 to 24) were sampled at 6-h 

 intervals while following a drogue. From samples 

 collected in the upper 30 m, 2 liters were usually 

 filtered through a 20-/im mesh net, and 3 liters 

 from greater depths. The apparatus used is illus- 

 trated in Durbin et al. (in press). The material 



Figure 1. — Location of stations ■where collections of net phyto- 

 plankton ( + ) and net and water bottle samples (•) were made 

 from 26 March to 6 April 1973 (except that net and water bottle 

 collections were made only at Stations 26 and 38 at the fre- 

 quently sampled station located off Punta San Hipolito). A rep- 

 resents stations, along with Station 27, used to illustrate the 

 occurrence of upwelling in Table 1; the outermost station is 

 Station 29. 



39 



