VERTICAL STRUCTURE OF NEARSHORE PLANKTON 



OFF SOUTHERN CALIFORNIA: 



A STORM AND A LARVAL FISH FOOD WEB 



M. M. MuLLiN, E. R. Brooks, F. M. H. Reid, 

 J. Napp, and E. E Stewart' 



ABSTRACT 



Samples of zooplankton and phytoplankton were taken at 5 m depth intervals in the upper 50 m of 

 water off Dana Point, California, in the spring of 1980, just before and just after a local storm. Most of 

 the 43 zooplanktonic taxa, many phytoplanktonic taxa, and chlorophyll were vertically stratified. After 

 the storm, naupliar copepods, chlorophyll, and a few phytoplanktonic taxa were more abundant, and 

 several zooplanktonic taxa were more concentrated in the upper layers. The storm did not decrease the 

 vertical stratification of larval fish food, so the feeding environment after the storm was at least as 

 favorable as that before the storm, but larval fish were less abundant. 



Studies in the laboratory have provided data on 

 the kinds and abundances of food which are re- 

 quired for survival and growth of some types of 

 zooplankton found in the surface waters of the 

 Southern California Bight (e.g., Mullin and 

 Brooks 1970; Paffenhofer 1976) and of the larvae of 

 the anchovy, Engraulis mordax, (e.g.. Hunter 

 1976; Lasker et al. 1970) and jack mackerel, 

 Trachurus symmetricus , (Devonald 1983). The an- 

 chovy has overwhelmingly dominated the larval 

 fish assemblage of the area in recent years (e.g., 

 Gruber et al. 1982). Direct experimentation 

 (Lasker 1975) and indirect comparison of 

 metabolic requirements and observed concentra- 

 tions of likely food (Mullin and Brooks 1976; Cox et 

 al. 1983) have shown examples of situations where 

 only in layers or patches of anomalously high con- 

 centration of food can larval fish or copepods ob- 

 tain enough nutrition to grow. Field data on verti- 

 cal distributions indicate that extensive, sharply 

 defined layers with elevated abundances of 

 phytoplankton often exist within the euphotic 

 zone (e.g., Cullen and Eppley 1981, for chlorophyll; 

 Kiefer and Lasker 1975, for Gymnodinium splen- 

 dens; Cullen et al. 1982, for several species). 



There is also more indirect evidence of the im- 

 portance of unusually rich layers of food for the 

 survival and growth of planktonic predators: The 

 greater size of "wild" copepods relative to those 

 raised in the laboratory (Mullin and Brooks 1970), 



the limitation of egg production of a copepod popu- 

 lation (Checkley 1980b), and the failure of year 

 classes of anchovy when storms or upwelling were 

 thought to disrupt layers of food (Lasker 1981). 

 However, direct field evidence concerning starva- 

 tion or growth limitation by food of larval anchovy 

 is both limited and contradictory (Arthur 1976; 

 Methot and Kramer 1979; O'Connell 1980). 



The population of the large copepod, Calanus 

 pacificus, is sometimes concentrated in those 

 layers where autotrophic phj^oplankton is most 

 abundant (Mullin and Brooks 1972, 1976). How- 

 ever, there also are cases where no such correla- 

 tions were found (Mullin and Brooks^) or where 

 Calanus and other herbivores actively avoided a 

 layer of abundant dinoflagellates (Fiedler 1982), 

 and where feeding in such a layer was depressed 

 relative to other parts of the water column (Fiedler 

 1982; Huntley 1982). In the present study we 

 examined the vertical relations between several 

 zooplanktonic taxa and chlorophyll, a measure of 

 the autotrophic, phytoplanktonic biomass. Since 

 the nutrition of zooplankton governs growth and 

 fecundity, the vertical relations between zooplank- 

 ters and their food can affect the future supply of 

 food for fish if the zooplankton is food-limited. 



Small-bodied species of zooplankton (or larval 

 stages of larger species), some large-celled, non- 

 thecate dinoflagellates, and protozoans were of 

 particular interest as representing potential lar- 



'Institute of Marine Resources, A-018, Scripps Institution of 

 Oceanography, University of California — San Diego, La Jolla, 

 CA 92093. 



Manuscript accepted May 1984. 



FISHERY BULLETIN: VOL. 83, NO. 2, 1985. 



^Mullin, M. M., and E. R. Brooks. 1976. Unpubl.data. In- 

 stitute of Marine Resources, Scripps Institution of Oceanog- 

 raphy, University of California — San Diego, La Jolla, C A 92093. 



151 - 7fi 



