328 



Fishery Bulletin 88(2), 1990 



50 



100 



50 



X 



I— 



a. 



LU 



Q 



100 



50 



100 



SUMMER WINTER 



DISTANCE OFFSHORE (n. miles) 



20 10 50 40 30 



1 1 1 



20 



— 1 — 



10 



ISOTHERMAL 

 15 °C 



Temperature ( C) 



Microplankton (particles litre } 



Chlorophyll a (mg-m-') 



Figure 5 



Summer (January 1978) and winter (August 1977) sections of temperature, microplankton, and chlorophyll-a values along line 7G 

 over ttie Agulhas Bank, southern Benguela Current region. 



Current origin forming an upper mixed layer and cold 

 water that moves onto the Bank along the bottom in 

 summer. The warm layer had very low plankton den- 

 sities, but enhanced microplankton density and phyto- 

 plankton-a concentration were associated with the 

 thermocline. 



With the onset of winter, the influence of the warm 

 Agulhas Current water over the Agulhas Bank be- 

 comes less and heat loss to the atmosphere increases, 

 causing a cooling of the surface layer. Surface cooling 

 coinbined with the movement off the shelf of the cold 

 bottom layer makes the water column less stable and 

 susceptible to mixing by winter storms (Shannon et al. 



