NET PHYTOPLANKTON AND THE GREATER THAN 20-MICRON 



PHYTOPLANKTON SIZE FRACTION IN 



UPWELLING WATERS OFF BAJA CALIFORNIA 



Theodore J. Smayda^ 



ABSTRACT 



Between 26 March and 6 April 1973 various phytoplankton studies were carried out during the 

 MESCAL II survey in an area measuring circa 105 km x 30 km, and centered approximately at Punta 

 San Hipolito, Baja California. Upwelling was then in its early stages. The composition of 22 collections 

 of net phjftoplankton (No. 20 net), and the composition and abundance of the non-setose (i.e., excluding 

 Chaetoceros, Bacteriastrum) size fraction >20 nxn collected at various depths at 13 stations are 

 reported here. The mean carbon content in the upper 50 m contained in the >20 pm non-setose size 

 fraction was 533.5 mg C/m^ for all stations, and ranged from 306 to 1,022 mg C/m* at individual 

 stations. Based on a C/Chl a ratio of 40: 1, the mean concentration in the euphotic zone represents circa 

 12% of the total phytoplankton carbon present. Lauderia annulata (28%) and several Coscinodiscus 

 species (33%) accounted for most of the carbon found in the >20-Mni size fraction, even though the 

 latter comprised only about 10% of the mean population expressed as cell number. The mass occurrence 

 of Coscinodiscus reported previously for Magdalena Bay during summer upwelling was not observed. 

 The Coscinodiscus population and the non-setose component of the >20- /jm size fraction contributed 

 only 1.2% and 4%, respectively, of the daily caloric ingestion estimated for the crab, Pleuroncodes 

 planipes, previously reported to graze heavily on Coscinodiscus. Sinking rates (61 to 144 m/h) of 

 Pleuroncodes fecal material exceeded by onefold to fourfold those rates estimated for the various sizes 

 of Coscinodiscus and zooplankton fecal pellets sampled during the survey. The abundant crab 

 population is, thus, important in causing an exceptionally rapid deposition of unassimilated 

 phytoplankton frustules and organic material onto the sea floor. Floristic changes accompanying 

 upwelling were detectable. The occurrence of the unique diatoms Coscinodiscus (Brenneckella) 

 eccentricus and Planktoniella muriformis in these waters is apparently reported for the first time. The 

 present data together with earlier observations suggest that the net diatom community is similar in 

 the coastal waters extending from San Diego, Calif, to the Gulf of Panama. The data do not support the 

 idea that the abundance of Pleuroncodes in this upwelling system is causally linked to that of 

 Coscinodiscus. 



There is little information on the composition and 

 abundance of the phytoplankton community in 

 the upwelling waters off Baja California. The 

 available data are mostly qualitative (Allen 1924, 

 1934, 1938; Balech 1960; Cupp 1930, 1934), 

 aside from recent, cursory observations on phyto- 

 plankton cells >25 A( m which are grazed by the red 

 crab, Pleuroncodes planipes (Longhurst et al. 

 1967). 



Unique mass blooms of Coscinodiscus have 

 been observed by Longhurst et al. in the upwelled 

 waters of Magdalena Bay. This phenomenon and 

 the great abundance of Pleuroncodes, which 

 grazes on Coscinodiscus cells, may be distinctive 

 characteristics of this upwelling system. Smith et 

 al.2 have concluded that this crab is an impor- 



'Graduate School of Oceanography, University of Rhode Is- 

 land, Kingston, RI 02881. 



^Smith, K. L., Jr., G. R. Harbison, G. T. Rowe, and C. H. 



Manuscript accepted May 1974. 



FISHERY BULLETIN: VOL. 73, No. 1, 1975. 



tant herbivore in the California Current upwell- 

 ing system, where its role is comparable to that 

 of the anchovy, Engraulis ringens, in the Peru 

 Current. Longhurst (1968) has evaluated the 

 potential fishery for this galatheid crab, which 

 occurs in both the benthic and pelagic zones; some 

 crabs exhibit diurnal migrations (Longhurst et al. 

 1967). 



Pleuroncodes is generally distributed through- 

 out this region (Blackburn 1969), while informa- 

 tion on the regional distribution and abundance of 

 Coscinodiscus is lacking. It is therefore unknown 

 whether Coscinodiscus indeed generally charac- 

 terizes the phytoplankton community in these 

 upwelled waters. Clarification of this is relevant 



Clifford. Respiration and chemical composition of Pleuroncodes 

 planipes (Decapoda: Galatheidae): Energetic significance in an 

 upwelling system. Manuscr., 22 p. Woods Hole Oceanogr. Inst., 

 Woods Hole, Mass. 



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