FISHERY BULLETIN: VOL. 73, NO. 1 



retained by the net was concentrated to 30 ml, 

 preserved with hexamine + Formalin^ and 1 ml of 

 the concentrate then enumerated on board ship 

 using a Sedgwick Rafter Counting Chamber. The 

 concentrate was obtained by stopping filtration to 

 leave about 1 cm of water above the filter. 



As stated in the Introduction, cells in the size 

 class >20/umare frequently heavily silicified 

 and sink to the sea bed. Chaetoceros and 

 Bacteriastrum are usually not represented in the 

 latter (Round 1968), presumably because of rapid 

 dissolution of their silicon frustules. The various 

 objectives of the present and other, ongoing 

 studies during MESCAL II emphasized the 

 Coscinodiscus and other non-setose genera, and 

 also required real-time data for proper execution 

 of the program. Shipboard enumeration of phyto- 

 plankton was therefore necessary. Quantitative 

 shipboard enumeration of specimens belonging to 

 genera characterized by setae is difficult; their 

 setose nature makes them prone to movement 

 within the counting chamber in response to the 

 ship's vibrations and movement. For these various 

 reasons, during the numerical census repre- 

 sentatives of the genera Chaetoceros and Bacteri- 

 astrum and a species similar in general appear- 

 ance to (but probably not) Nitzschia frigida were 

 not enumerated. 



Numerical abundance was transformed into 

 carbon equivalents. From 10 to 40 cells of each 

 species (depending on abundance) were measured 

 to obtain the mean dimensions required to calcu- 

 late cell volume using appropriate mensuration 

 formulae. The carbon content was then calculated 

 from Strathmann's (1967) equation 



log C = 0.758 (log V) - 0.352 



where V is the cell volume in jum^. From this 

 cellular estimate (pg per cell), the population car- 

 bon was computed. The constant 0.352 differs 

 slightly from Strathmann's given value, and is 

 based on additional diatom analyses (Eppley, 

 pers. commun.). 



The mean population per liter (C) in the upper 

 50 m was calculated from: 



C=^ [(Co+Ci)(Z, -Zo) + (Ci +C2)(Z3-Z2) + 



. . . + (C4 + C5) (Z5 + Z4)] 



where Co, Ci, etc. are the observed cell (carbon) 

 concentrations per liter at the surface (Zq) and 10 

 m (Zi), etc., down to 50 m (Z5). Concentrations per 

 square meter of sea surface down to 50 m were 

 obtained from (C) (5 x lO'*). Samples were 

 collected at 75 m at only 9 of the 13 quantitative 

 stations because of depth. This, together with the 

 sparse populations usually found there, accounts 

 for the emphasis on the upper 50 m. 



RESULTS 



Upwelling occurred during the field program. 

 Table 1 presents some representative physical and 

 chemical parameters along a transect of three sta- 

 tions sampled on 3 and 4 April near Punta San 

 Hipolito (Figure 1). The inflow and upwelling of 

 cold, nutrient-rich water at the nearshore station 

 (27) is evident. Upwelling was usually more pro- 

 nounced near and shorewards of the 50-fathom 

 isobath. Details of this upwelling, which was in its 

 early stages, and associated biotic responses will 

 be presented elsewhere (Walsh, Kelley, Whit- 

 ledge, Huntsman, and Pillsbury in prep.). 



Net Phytoplankton 



The species identified in the net material are 

 listed in Appendix Table 1. Throughout the ship's 

 track of ca. 700 km the No. 20 net phytoplankton 

 was characterized by the genus Chaetoceros 



Table 1. — Hydrographic conditions along a transect off Punta 

 San Hipolito showing the occurrence of upwelling during 3 and 4 

 April 1973 (Stations shown in Figure 1). 



I 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



40 



