SMAYDA: NET PHYTOPLANKTON IN UPWELLING WATERS 



{affinis, curvisetus, debilis, didymus, socialis) in 

 species and abundance. The genus Coscinodiscus 

 was a conspicuous co-dominant, but varied in rela- 

 tive abundance from station to station. The re- 

 markable colonial diatom Planktoniella murifor- 

 mis (Loeblich et al. 1968; Round 1972) was also 

 prominent throughout this region. Nonetheless, 

 some apparent regional differences are notewor- 

 thy. 



At Station 1 located near Punta Kolnett a very 

 rich, diverse net plankton community occurred on 

 26 March dominated by Chaetoceros and 

 Nitzschia spp. and Thalassiothrix frauenfeldii . 

 Asterionella japonica, Eucampia cornuta, and 

 Lithodesmium undulatum were other abundant 

 diatoms. This community stands out from others 

 in the importance of Asterionella (many small 

 pennate diatoms were attached to the colonies), 

 which was not found in subsequent net tows. Also 

 unlike subsequent stations, Phaeocystis cf. 

 pouchetii was common while Coscinodiscus spp. 

 were not. Allen (1945) has reported extensive 

 blooms of Phaeocystis off southern California. 

 This colonial haptophycean is well known for its 

 apparent adverse effects on certain fisheries in the 

 North Sea during mass blooms. 



The lack of nutrient data at Stations 1 and 2 

 prevents assessment of possible upwelling. How- 

 ever, when sampled on 27 March the upper 50 m of 

 the latter station was considerably warmer 

 (15.29°C at m, 14.94°C at 50 m) than at Station 1 

 (14.53°C at m, 11.42°C at 40 m). The net phyto- 

 plankton community was considerably poorer and 

 dominated by Ceratium spp.; peridinians were 

 frequent, and the diatoms Biddulphia mobil- 

 iensis, Coscinodiscus spp., and Planktoniella sol 

 were common. This community suggests that up- 

 welling was weak, if occurring. 



The principal features of the net collections {n = 

 20) made in the intensive survey area (Figure 1) 

 are: 1) the community at the deepwater stations 

 (16, 31, 32) located seaward of the 50-fathom 

 isobath was less abundant and differed somewhat 

 relative to the shallower stations; 2) the composi- 

 tion at the latter stations was generally similar; 

 and 3) a slight change in apparent species domi- 

 nance occurred by the end of the 12-day sampling 

 period. 



At the outer, deepwater Station 16 (30 March) 

 Chaetoceros affinis and curvisetus dominated; 

 Ceratium and Peridinium spp. were also common. 

 At Stations 31 and 32 (4 April), Bacteriastrum 

 dominated together with the above Chaetoceros 



species and decipiens and socialis. Coscinodiscus 

 spp. were subordinate; Asterolampra marylandica 

 and cf Pyrocystis lunula were frequent. The lower 

 relative abundance and the difference in domi- 

 nant species at these outer stations are also 

 reflected in the quantitative samples (Table 2). 

 The lowest mean concentration occurred at Sta- 

 tion 32 (quantitative samples were not collected at 

 Stations 16 and 31). The physical-chemical data 

 indicate that upwelling was not occurring at Sta- 

 tion 16 and was insignificant, if taking place, at 

 Stations 31 and 32. 



At the nearshore Station 34 (4 April), where the 

 hydrographic conditions were similar to Station 

 27 (Table 1), the Bacteriastrum component 

 important at Stations 31 and 32 was absent and 

 Thalassiosira rotula dominated along with the 

 Chaetoceros spp. This increased importance of 

 Thalassiosira rotula relative to samples collected 

 a week earlier is also noted in the series collected 

 near Punta San Hipolito (Stations 3 to 38) (Figure 

 1). The nearshore communities were otherwise 

 dominated by different proportions of Chaetoceros 

 and Coscinodiscus spp. The Coscinodiscus compo- 

 nent was especially prominent at Stations 10, 17, 

 and 19, for example. (The net tows frequently 

 contained pennate diatoms which might have 

 been scoured from bottom sediments during 

 upwelling.) 



The apparent differences in net community 

 composition, abundance, and species succession 

 during the 10-day sampling period in the inten- 

 sive survey area probably reflect variations in in- 

 tensity of upwelling, which was just beginning 

 based on aerial reconnaissance of sea-surface 

 temperatures prior to the ship's arrival in the sur- 

 vey area. Between 28 March (Station 3) and 30 

 March (Station 13) cold water ascended 10 m at 

 the fixed station near Punta San Hipolito (Figure 

 1; Pillsbury, pers. commun.). 



Quantitative Samples 



Numerical Abundance 



The results of the quantitative census of the 

 non-setose species in the >20-/jm size fraction are 

 presented in Table 2. The mean population level in 

 the upper 50 m ranged from about 2,110 to 9,800 

 cells/liter. Lauderia annulata dominated numeri- 

 cally (from 35 to 60% of total abundance) through- 

 out the area, except at the last station (38) sam- 

 pled (3%) where Thalassiosira rotula dominated 



41 



