FISHERY BULLETIN: VOL. 69, NO. 4 



present. If the doubling rate for plankton is 

 1.4 days, then to maintain a constant concen- 

 tration level requires that the water in the patch 

 is removed at a rate of 2.1 x 10'° cmVsec. The 

 southward transport of the undercurrent be- 

 neath the patch is estimated at 20.2 x 10>° cmV 

 sec, or about 10 times the flow required for re- 

 placement of water in the patch. Upwelled water 

 is required to replace the water being- removed 

 along the outer boundary of the patch. Since 

 we assume that most, if not all of the water used 

 to replace water lost from the patch, passes 

 through the bottom of the patch at 50 m, the 

 ascending velocity of water at 50 m is estimated 

 to be 4.1 X 10~- cm/sec for the patch. This is 

 probably an upper limit since other processes 

 also act to reduce the biomass. Unfortunately, 

 only one sub-euphotic zone sample (30-50 m) 

 from Patch 1 is available, and it shows a chloro- 

 phyll level only 20 ^r of the average in the up- 

 per layer. However, if the ciliate and other 

 zooplankton populations were grazing this ma- 

 terial in which no new organic carbon was being 

 produced, it would only be a relatively short time 

 before a marked diminution of the chlorophyll 

 level would be expected. In addition, chlorophyll 

 levels above and below the compensation depth 

 may not be a comparable index to phytoplankton 

 abundance as there is an ai)parent decrease in 

 chlorophyll level in many jihytoplankters when 

 kept in the dark for any period of time. Thus, 

 the plant cell population in the sub-euphotic 

 waters may be underestimated. The phytoplank- 

 ton cells near to the compensation depth had a 

 lower photosynthetic rate, g C/g Chi o/hour 

 (University of California, Institute of Marine 

 Resources, 1970, see footnote 3), than those 

 higher in the water column. Eppley, Holmes, 

 and Strickland (1967) showed that cells in such 

 a physiological state sink at a more rapid rate 

 than faster growing phytoplanktei's. While 

 sinking or upwelling alone may not result in 

 moving material out of the euphotic zone and 

 preventing a "bloom" from developing, this com- 

 bined with the turbulence may be a significant 

 contributing factor. Dugdale and Goering 

 (1970) following a high chlorophyll patch of 

 water over 5 days, concluded that approximately 

 85% of the phytoplankton production at that 



time v/as being lost through sinking and mixing 

 processes. At one site examined more closely, 

 IB'^r of the standing crop was lost daily. 



Continued upwelling with the consequent 

 spreading out and or sinking of the surface 

 waters is another mechanism which would re- 

 sult in masking any bloom that might have de- 

 veloped had the upwelled water mass remained 

 more localized. This was suggested as a means 

 of "preventing" blooms in regions of divergences 

 in the Antarctic (Beklemishev, 1958). Hori- 

 zontal divergence of Patch 2 with a relatively 

 greater eastward water motion of the nearshore 

 side was indicated by our current measurements. 

 Water on the nearshore side would appear to 

 "stretch" in the horizontal plane and be supple- 

 mented by ascending subsurface water. Exam- 

 ination of the graphic reconstruction of chlor- 

 ophyll distribution (Figures 1 and 2) suggests 

 a spreading out of the chlorophyll patches, be- 

 coming diluted in the surrounding area with 

 time. Since dynamic vertical mixing is not in- 

 dicated by the Richardson numbers for Patch 2, 

 the decrease in chlorophyll may be largely attrib- 

 uted to the divergence and associated upwelling. 

 Horizontal mixing, however, is undoubtedly an 

 important dispersing mechanism in both 

 Ijatches. A rough estimate of upwelling based 

 on the horizontal divergence was made by using 

 the current measurements from Patch 2. Up- 

 welling in the ])atch is assumed to be confined to 

 a surface layer 50 m thick, where the vertical 

 velocity is a maximum at 50 m (the same depth 

 as the poleward flow of the coastal undercur- 

 rent). The resulting estimate of vertical veloc- 

 ity at 50 m is 2.5 x IQ-^ cm/sec and compares 

 favorably with the upwelling rate computed with 

 greater accuracy for a nearby coastal zone 

 (Smith et al., 1971). The surface concentration 

 of nitrate declined in the patches over time as 

 did the total chlorophyll pigments concentration 

 (Patch 1, Figure 5). Nitrate consumption by 

 the phytoijlankton was calculated, assuming 1 g 

 nitrate assimilated per 6 g of carbon fixed in 

 photosynthesis, and the indicated plant consump- 

 tion was only about one-third of the observed 

 nitrate decline. This observation again suggests 

 mixing of water in the patches with surrounding 

 less rich water. 



874 



