FISHERY BULLF.TIN: VOL. 69, NO. 4 



Samples for pigment analysis were also col- 

 lected 2 to 3 hr before local apparent noon 

 from 13 depths between the surface and 100 to 

 200 m. The upper 6 to 10 depths sampled were 

 within the photic zone, depending on its depth, 

 and were chosen on the basis of the thermal 

 structure of the water column. Sample depths 

 were evenly spaced through the mixed layer and 

 evenly but more closely spaced across the ther- 

 mocline. Samples were always taken at the base 

 of the photic zone and at two depths below to 

 at least twice the photic zone depth. Chloro- 

 phyll-a and phaeopigment concentrations were 

 determined by a fluorometric technique (Strick- 

 land and Parsons, 1968). Water samples were 

 fractionated by the same procedure described 

 for the carbon-uptake experiments except What- 

 man GF/C glass fibre filters coated with 2 ml 

 of I'yr MgCOs suspension were used in place of 

 membrane filters, and the netjilankton chloro- 

 l)hyll fraction was calculated from the difference 

 between fractionated and unfractionated values. 

 Duplicate values for each fraction were averaged 

 (mean coefficients of variation were 10 ± 2'"f for 

 the nannoplankton and 22 ± 8/r for the net- 

 plankton fraction) . The use of glass filters may 

 have led to no more than a 10 /f underestima- 

 tion of nannoplankton chlorophyll-a (Malone, 

 in press a). 



Samples for phytoplankton enumeration and 

 identification were preserved with Lugol's so- 

 lution made basic with sodium acetate in place 

 of acetic acid. Aliquots of 100 ml were placed 

 in Nessler tubes and the cells allowed to settle 

 for 72 hr. Depending on the concentration of 

 cells, from 50 to 90 ml of the supernatant was 

 then siphoned off and 2 ml aliquots were added 

 to settling chambers. After 48 hr the samples 

 were counted by the inverted microscope tech- 

 nique of Utermohl (Lund et al., 1958). All or- 

 ganisms longer than about 30 jjl were counted 

 at 100 V while smaller cells were counted in 100 

 random fields at a magnification of 400 x . For 

 lack of better criteria, phytoplankton having di- 

 mensions of 30 X 22 (U, or less were classified 

 as nannoplankton and those with larger dimen- 

 sions as netplankton. This did not present 

 much of a problem, however, because the nanno- 

 plankton fraction was dominated by cells whose 



longest dimension was in the range of 2 to 15 /n, 

 while the netplankton fraction was dominated 

 by chain-forming diatoms with cell lengths of 

 40 fjL or more, e.g., Nitzschia pacifica. Dominant 

 netplankton forms were identified to species, and 

 less numerous forms to genus. The remaining 

 phytoplankters were cla.ssified as pennate or 

 centric diatoms, thecate or nonthecate dinoflag- 

 ellates, coccolithophores, silicoflagellates, or 

 "others." Mean coefficients of variation be- 

 tween duplicate samples were 14 ± 4Sf for the 

 nannoplankton fraction and 27 ± 11 Tl for the 

 netijlankton fraction. 



Standard hydrographic and bathythermo- 

 graph casts were made 2 to 4 hr before local 

 apparent noon in conjunction with productivity 

 and standing crop measurements to estimate the 

 vertical distributions of dissolved inorganic ni- 

 trogen compounds, temperature, and density in 

 the water column. Additional hydrographic 

 casts made for the CalCOFI Program in Mon- 

 terey Bay are utilized in this paper. Nitrate con- 

 centrations were determined by the manual 

 procedure described by Strickland and Parsons 

 (1968) and ammonium by the phenolhypochlo- 

 rite method (Solorzano, 1969). A Secchi disc 

 was used to estimate photic zone depths (3.5 X 

 Secchi disc reading) . 



The ratio of phaeopigments-to-chlorophyll in 

 the water column (to 100 m for inshore stations 

 and to 200 m for offshore stations) was used 

 as a rough index of relative grazing pressure on 

 the phytoplankton standing crojj (Lorenzen, 

 1967; Beers and Stewart, 1969). In the pre- 

 sent study, a highly significant (P = 0.01) re- 

 gression of phaeopigment concentration on logio 

 transformed zooijlankton wet weights was found, 

 and it was concluded that the phaeopigment- 

 chlorophyll ratio could be used as a first order 

 index of grazing pressure. 



TEMPORAL VARIATIONS IN 

 MONTEREY BAY 



ENVIRONMENTAL FACTORS 



The hydrographic conditions observed at 

 CalCOFI 3 from October 1969 to February 1971 

 are summarized in Figure 2 and Table 1. Sur- 



802 



