PLANKTON POPULATIONS AND UPWELLING OFF THE COAST 



OF PERU, JUNE 1969 



John R. Beers,' Merritt R. Stevenson," Richard W. Eppley,' 

 AND Elaine R. Brooks' 



ABSTRACT 



Plankton populations associated with upwelling areas and the changes with time of upwelled patches 

 were studied off the coast of Peru near Supe in June 1969. Two patches, detected by their higher 

 nutrient levels, greater chlorophyll pigment concentrations, and lower surface temperatures than sur- 

 rounding waters, were each monitored for several days, during which time they gradually lost their 

 identity. Actively photosynthesizing phytoplankton crops (doubling time ca. 1.4 days) of predominantly 

 small monads and other flagellates were found in both patches. The zooplankton populations in the 

 patch areas were estimated to be consuming no more than about 25% of the daily phytoplankton pro- 

 duction. Direct determination of physical mechanisms affecting the patches showed a relatively high 

 level of vertical instabilit.v in Patch 1 which would allow for turbulent mixing and the carrying of phy- 

 toplankters below the compensation depth. A horizontal divergence was associated with Patch 2 and 

 would also have the effect of dissipating the patch. Approximate estimates of upwelling suggested ver- 

 tical velocities of about 2 X 10~~ cm/sec in both patches. 



Interest in biological production in Peruvian 

 coastal waters has been high in recent years. 

 The prosperous fish-meal industry developed 

 around the anchovy, Engrmdis ringens, has stim- 

 ulated numerous investigations by Peruvian sci- 

 entists of the food chain leading to this commer- 

 cially valuable fish (see, for example: Guillen 

 and Izaguirre de Rondan, 1968; Zuta and Guillen, 

 1970; and various "Informes" of the Instituto del 

 Mar del Peru). In addition, massive upwelling 

 and its associated biological activities along the 

 Peru coast has been of concern to investigators 

 from other countries and has resulted in several 

 international cruises which have added to our 

 knowledge of the pelagic ecology of the area. 



The distribution of upwelled water at the 

 surface is often in somewhat discrete "patches," 

 perhaps the result of dynamic physical forces 

 such as currents and/or eddies acting in the area. 

 In March-April 1966, Strickland, Eppley, and 

 Rojas de Mendiola (1969) observed low standing 



' Institute of Marine Resources; University of Cal- 

 ifornia, San Diego; La Jolla, Calif. 92037. 



^ Inter-American Tropical Tuna Commission, Scripps 

 Institution of Oceanography, University of California, 

 San Diego, La Jolla, Calif. 92037. 



Manuscript accepted July 1971. 



FISHERY BULLETIN: VOL. 59, NO. 4, 



1971. 



crops of phytoplankton (chlorophyll a, <2 fig/ 

 liter) in high-nutrient patches (e.g., surface 

 N0.3-N, 20 fig at./liter or higher). In other 

 similarly rich water phytoplankton abundance 

 was high (chlorophyll a, 15 /.ig/liter). Barber 

 et al. (1971) reported the surface water in an 

 area of recent upwelling showed a lack of the 

 "organic conditioning compounds" which may 

 be needed for phytoplankton growth. However, 

 in the nutrient-rich but low-chlorophyll patches 

 examined by Strickland et al. (1969) relatively 

 high growth rates (ca. 0.8 doubling/day aver- 

 aged over the euphotic zone) were found for the 

 phytoplankton. Similar rates (ca. 0.6 doubling/ 

 day) can be calculated from the '^N (nitrate and 

 ammonium) assimilation studies done in this 

 area by Dugdale and co-workers (University of 

 Washington Department of Oceanography, 

 1970). 



A high grazing pressure exerted by the pelagic 

 animal populations was considered by Strick- 

 land et al. (1969) as a possible underlying cause 

 for the low standing crops in the nutrient-rich 

 patches with active phytoplankton populations. 

 Ryther et al. (1970) also proposed grazing as 

 an important mechanism for the reduction of the 



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