STA 5B3 



361 CL /TON 



THERMOSTERIC ANOMALY (CL /TOfJ) 



200 250 300 350 400 



WARM SIDE — •- 

 MIDDLE --•- 

 COOL SI DE *- 



; 7 KM — »| 



Figure 21. — Thermosteric anomaly ( 5 -[.) profile and 5 -p- depth curves for the first triplet (B) of hydrocasts at front 

 5 (22 April 1961). c = cool side; m = middle; w = warm side; • = Nansen bottle depths In profile. 



The nitrate profile of the E series (fig. 29) 

 shows no marked features in the upper waters, 

 but nitrate concentration in the middle is less 

 than on either side below 30 m. Oxygen iso- 

 pleths, on the other hand, generally are deeper 

 in the middle below 30 m. (fig. 27), indi- 

 cating the relationship between the oxygen and 

 nitrate concentration. Oxygen is used in the 

 oxidative decay of dead matter, which accounts 

 for the relationship observed. 



Chlorophyll a distribution. -- The amount of 

 phytoplankton is estimated by the chlorophyll 

 a content of the water. Figure 30 shows the 

 profile of chlorophyll a determined from the 

 data of a triplet of Van Dorn plastic sampler 

 casts at the time of the B series of hydro- 

 casts. The profile maximum (>0.6 mg. chl. 

 a m.-^) occurs at about 25 m. at the "middle" 

 station. The station maxima correspond 

 roughly to the pycnocline (fig, 21). The depths 

 of the station maxima correspond to relatively 

 high values of dissolved oxygen but there is no 

 obvious correlation between chlorophyll a. and 

 dissolved oxygen. Although the location of the 

 profile maximum in the middle suggests that 

 phytoplankton has been aggregated by (or pro- 

 duced at) the front, if should be borne in mind 



that the samples were at only four depths: 

 surface, 25, 40, and 100m. Maxima as great as 

 that found at 25 m. in the middle may actually 

 have existed to either side but at depths not 

 sampled. The reasons for sampling at only four 

 depths per cast were (1) to obviate a marked 

 time lapse between samplings during which the 

 front might shift, and (2) to free the water 

 filters of earlier samples in time for later 

 ones . 



Productivity .-- The great importance of min- 

 imizing the difference in starting times of 

 productivity experiments confined our obser- 

 vations of productivity to the surface water. 

 Incubation of samples should start as close to 

 local apparent noon as possible. It took at 

 least half an hour to cross the front (at 9 knots); 

 hence the unavoidably few observations. 



Surface productivity increased on passing 

 from the warm side to the cool; the values 

 were 6.87, 11.88, and 16.38 mg. C/m.^/day. 

 These observations were made a day later 

 than those of chlorophyll a. 



Zooplankton and micronektondistribution .- - 

 In the introduction I stated that there is a 

 widely held opinion that a front (a convergent 



