FISHERY BULLETIN: VOL. 74, NO, 1 



300 



Jan. Feb. Mar. Apr. May June July Aug. 



Q. 



<v 

 Q 



100 - 



200 - 



300 



Jon. Feb. Mar. Apr. May June July Aug. 



Figure 2. — Average depth of isotherms and nitrate isopleths 

 for hydrographic stations samples over Monterey Submarine 

 Canyon, January through Augxast 1972 (data from Broenkow 

 and Benz 1973). 



Standing Stocks and Primary 

 Production 



In January, at the end of the Davidson Current 

 period, standing stocks were near their lowest 

 levels and nannoplankton dominated (Table 1, 

 Figure 3). Throughout the period from February 

 through July, however, the net plankton fraction 

 exceeded the nannoplankton. In August, the 

 standing stocks were again predominantly nan- 

 noplankton. Estimated primary production fol- 

 lowed the general trend shown by the standing 

 stocks (Figure 4), but lower production per unit 

 chlorophyll for the net plankton fraction in Jan- 

 uary and July is apparent. The highest standing 

 stock was measured in April at the time the 

 isotherms and nutrient isopleths reached their 

 highest positions (see Figure 2). At this peak, the 

 stocks were 97% net plankton, and net plankton 

 concentrations in the euphotic zone ranged from 

 4.63 to 6.88 mg Chi a m"^. Concentrations of net 



Jon Feb. Mor Apr Moy June July Aug. 



FlGURE 3. — Phytoplankton standing stocks in the euphotic 

 zone, January through August 1972. Numbers over histogram 

 bars refer to stations. 



I 



3 



Moy 



July 



Aug- 



FIGURE 4. — Estimated primary production in the euphotic 

 zone, January through August 1972. Numbers over histogram 

 bars refer to stations. 



plankton as high as 9.26 mg Chi a m'^ were re- 

 corded in June. During the April peak, the cor- 

 responding total productivity was approximately 

 1.1 g C m"2 day"^ It is difficult to equate incubator 

 productivity to in situ productivity; however, 

 these values are similar to productivity estimates 

 calculated from nutrient uptake and oxygen pro- 

 duction in the water column (Smethie 1973). 



The changes in the ratios of the two fractions 

 were largely a result of changes in the biomass of 

 the net plankton fraction. The net plankton frac- 

 tion experienced large seasonal changes in con- 

 centrations, and occasionally there was sig- 

 nificant vertical stratification within the water 

 column; however, nannoplankton fluctuations fell 

 within a much narrower range (Figure 5). There 

 were significant differences in the average con- 

 centrations in the euphotic zone of the two frac- 

 tions in all three hydrographic seasons, and both 

 fractions showed significant differences between 

 seasons (Mann Whitney U test; P = 0.01). The 



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