SMAYDA: NET PHYTOPLANKTON IN UPWELLING WATERS 



(Longhurst 1968) in these waters. Although it is 

 omnivorous, while herbivorous it grazes on phyto- 

 plankton cells >25 jum (Longhurst et al. 1967), 

 i.e., the size class of Coscinodiscus . Indeed, these 

 authors report active grazing on this genus under 

 experimental conditions, and confirmed during 

 the present study (unpubl.). Therefore, is an 

 abundant Coscinodiscus community significant 

 causally to Pleuroncodes , whose occurrence is a 

 major biotic characteristic of the Baja California 

 upwelling system? Some calculations will be 

 made to evaluate this relationship, and to exam- 

 ine the other questions posed in the Introduction. 



The maximum observed abundance of all 

 Coscinodiscus (Brenneckella) spp. was 2,243 

 cells/liter; the mean abundance for all stations in 

 the upper 50 m was 458 cells/liter (Table 4). This 

 meager abundance contrasts with a mean of 4.3 x 

 10^ cells/liter reported for Coscinodiscus 

 eccentricus by Longhurst et al. (1967). In their 

 study, this concentration represented only 8% of 

 the total community, which was dominated by 

 several Nitzschia species. Coscinodiscus cells of 

 <20 Mm diameter were also not present in bloom 

 concentrations in the present material. Therefore, 

 unlike in Magdalena Bay, this genus was not im- 

 portant numerically, at least during the initial 

 stages of upwelling in the survey area. 



It remains obscure whether a regional patchi- 

 ness characterizes the abundance of Coscino- 

 discus during upwelling along the coast of Baja 

 California, as for Coscinodiscus asteromphalus 

 in the Gulf of California (Round 1967). Allen 

 and Cupp referred repeatedly to such patchiness 

 in other species in these waters. It is also possible 

 that the Coscinodiscus bloom reported by Long- 

 hurst et al. represents a later state in a species 

 succession. Finally, it might have represented an 

 episodic bloom in response to local, unique factors, 

 rather than reflect a general regional or suc- 

 cessional phenomenon. Nonetheless, the reported 

 summer abundance of Coscinodiscus eccentricus 

 during upwelling in 1964 remains intriguing. 

 The dynamics of Coscinodiscus populations in 

 these waters warrant further study. 



The dominant (non-setose) species numerically 

 in the >20-/jm fraction was Lauderia annulata, 

 although blooms of Schroederella delicatula and 

 Thalassiosira rotula characterized individual sta- 

 tions (Table 2). The total Coscinodiscus 

 (Brenneckella) spp. represented only about 10% of 

 the mean population numerically, but this rep- 

 resented 33% of the mean carbon; corresponding 



values (or Lauderia annulata are 47% and 28%, 

 respectively. Thus, although Coscinodiscus was 

 not as abundant as in the Longhurst et al. survey 

 it dominated the >20-jum biomass fraction during 

 MESCAL II. 



The percent of the total phytoplankton com- 

 munity represented by the >20-jum fraction can 

 be established indirectly from chlorophyll deter- 

 minations made at 10 of the stations for which 

 quantitative >20-/^m phytoplankton counts were 

 also made. The mean concentration (based on 5 

 depths) in the upper 20 m was 3.46 Mg Chi a/liter. 

 This depth is near the compensation depth; 

 chlorophyll determinations were not made at 

 depths greater than this 1% level. The significant 

 decrease in mean phytoplankton abundance be- 

 tween 20 and 30 m was pointed out previously 

 (Table 4). The mean carbon content of the non- 

 setose fraction >20 jum in the upper 20 m is 16.4 

 M g/liter. 



Longhurst et al. (1967) give a mean carbon/ 

 chlorophyll a ratio of 258:1 for their material. 

 This is exceptionally high, and contrasts with 

 a mean (n = 17) of 110:1 characterizing the com- 

 munity dominated by Gonyaulax polyedra during 

 the 1972 MESCAL I survey (Walsh et al. 1974). 

 A mean ratio of 40:1 characterized diatom- 

 dominated communities found throughout the 

 euphotic zone in the Peru Current (Lorenzen 

 1968). Applying this conversion factor yields a 

 mean carbon content of 138 m g C/liter in the upper 

 20 m in the present survey. If a similar 

 carbon/chlorophyll ratio characterizes the >20-Mm 

 fraction (it may differ with cell size), then this 

 size group (exclusive of setose species) contributes 

 at least 12% of the viable phytoplankton carbon in 

 the euphotic zone. Lauderia annulata and the 

 Coscinodiscus (Brenneckella) species each contrib- 

 ute 3.5%. The non-setose component of this size 

 grouping would appear to represent only a modest 

 portion of the phytoplankton biomass in the 

 euphotic zone. However, significant diel varia- 

 tions in this component occur, which indicate a 

 high turnover rate. The fluxes and kinetics of this 

 response are considered elsewhere (Smayda in 

 press b). 



Longhurst et al. (1967) estimated that the graz- 

 ing rate of Pleuroncodes on phytoplankton was 

 540 liters/day per animal. Its mean abundance 

 during MESCAL II was 1 animal/m^ (Whitledge, 

 pers. commun.), threefold greater than that dur- 

 ing Longhurst and coworkers' study. The total 

 phytoplankton population in the upper 20 m was 



47 



