FISHERY BULLETIN: VOL. 73, NO. 1 



276 mg/m^, assuming a dry weight : carbon ratio 

 of 2. Smith et al. (footnote 2) report a mean caloric 

 content of 1,699 cal for Pleuroncodes during 

 MESCAL II, and cite a caloric value of 3,814 cal/g 

 dry wt for diatoms. From these data, a daily caloric 

 ingestion of phytoplankton of 568 cal/m^ within 

 the euphotic zone is calculated, which represents 

 33% of the total caloric content of the crab. 

 Coscinodiscus would contribute only 1.2% of this 

 daily caloric ingestion and the non-setose com- 

 ponent of the >20-jum size fraction 49c, based on 

 their contributions of 3.5 and 12% , respectively, to 

 the phytoplankton standing stock in the upper 20 

 m. Even at the maximum growth rates of 3 

 divisions/day for Coscinodiscus observed during 

 the survey (Smayda in press b) this genus would 

 provide a negligible fraction of the daily caloric 

 intake estimated (or Pleuroncodes. This suggests 

 that the Coscinodiscus population could not then 

 support the Pleuroncodes population; other food 

 sources were necessary. 



Smith et al. (footnote 2) demonstrated that the 

 respiration rate (as oxycaloric equivalents) of 

 Pleuroncodes is only 3% of the ingestion rates cal- 

 culated using the grazing rate proposed by Long- 

 hurst et al. (1967). Other calculations made 

 by them support their notion that the grazing rate 

 of 540 liters/day is too high, and partly accounts 

 for the discrepancy between rates. Other factors 

 which might contribute to the apparent feeding 

 inefficiency of Pleuroncodes would be high energy 

 losses as fecal material. Longhurst et al. observed 

 the copious production of fecal material packed 

 with Coscinodiscus. While the magnitude of this 

 waste production during MESCAL II can not yet 

 be evaluated, the relative rates of deposition of 

 frustules and organic matter to the sediments 

 when contained in fecal pellets and as free cells 

 can be put into perspective. 



The sinking rates (n = 24) of fecal pellets pro- 

 duced by freshly collected crabs, and determined 

 on board ship (unpubl.), ranged from 61 to 144 

 m/h. These rates exceed by 1 to 4 orders of mag- 

 nitude those calculated (Smayda 1970) for the dif- 

 ferent sizes of Coscinodiscus encountered, and 

 that (5.2 m/hr) estimated (Smayda 1969) for the 

 mean zooplankton fecal pellet size (320,000 lum^) 

 collected routinely in the >20-jum fraction. Thus, 

 while Coscinodiscus apparently contributed only 

 a negligible fraction of the daily caloric ingestion 

 of Pleuroncodes, the latter's ingestion and void- 

 ance in fecal material of this genus and other 

 heavily silicified diatoms >20 nm represent a 



means of exceptionally rapid deposition onto the 

 sea floor. 



The mean carbon content of 138 /jg/liter during 

 the initial stages of upwelling compares with a 

 mean standing stock of 566 /ug C/liter at 20 sta- 

 tions reported for this region during the 

 Gonyaulax polyedra bloom in March 1972 (from 

 Table 1 in Walsh et al. 1974). The mean carbon 

 content ranged from 23 to 100 /Ug/liter at three 

 stations sampled over a 5-mo period off La Jolla, 

 Calif. (Eppley et al. 1970). The mean concentra- 

 tion during upwelling south of the survey region 

 during June 1964 ranged from 48 yug C/liter (from 

 C/Chl a of 40: 1) to 308 y.g C/liter using data given 

 by Longhurst et al. (1967). However, the data are 

 too limited as yet for any meaningful comparison 

 of regional or seasonal variations in apparent pro- 

 ductivity in these coastal waters. They also indi- 

 cated that the net plankton was usually more 

 abundant in April (upwelling) between Punta Ab- 

 reojos and Punta Eugenia, i.e., in the present sur- 

 vey area (Figure 1). However, quantitative data 

 are needed to confirm this. 



ACKNOWLEDGMENTS 



This research was supported by National Sci- 

 ence Foundation Grant GX 33502 as part of the 

 IDOE Coastal Upwelling Ecosystem Analysis 

 program. I wish to express my thanks to Terry f 

 Whitlege, Cruise Leader during this portion of 

 the investigation, and to other members of the 

 scientific party on board then, including James 

 Kelley and John Walsh for helping to make this an 

 informative cruise. Blanche Coyne typed the 

 manuscript and drafted the figures. 



LITERATURE CITED 



Allen, W. E. 



1924. Observations on surface distribution of marine 



diatoms of lower California in 1922. Ecology 5:389-392. 

 1934. Marine plankton diatoms of lower California in 



1931. Bot. Gaz. 95:485-492. 

 1938. The Templeton Crocker Expedition to the Gulf of 



California in 1935 — the phytoplankton. Trans. Am. 



Microsc. Soc. 57:328-335. 

 1945. Vernal distribution of marine plankton diatoms 



offshore in southern California in 1940. Bull. Scripps 



Inst. Oceanogr., Univ. Calif 5:335-369. 

 Balech, E. 



1960. The changes in the phytoplankton population off 



the California coast. Calif Coop. Oceanic Fish. Invest. 



Rep. 7:127-132. 

 Blackburn, M. 



1969. Conditions related to upwelling which determine 



48 



