zoo DISCOVERY REPORTS 



SEASONAL VARIATION IN THE VERTICAL DISTRIBUTION 

 OF THE STANDING CROP OF ZOOPLANKTON 

 DESCRIPTION OF THE OBSERVATIONS 

 It has been shown by Mackintosh (1937) that one of the most marked features of the plankton cycle 

 in Antarctic waters is that some of the most common species, including Rhincalanus gigas, Calanus 

 acutus and Eukrohnia hamata, accomplish marked seasonal vertical migrations by which they maintain 

 themselves within the limits of their environment. They drift northwards during the summer when 

 they are concentrated both day and night in the Antarctic surface layer and southward during the 

 winter when the greater number have moved into the warm deep current. 



Although a knowledge of the relative numbers of animals caught at each depth illustrates this 

 seasonal migration it gives little indication of its effect on the vertical distribution of the standing crop 

 of plankton. As the volume of a sample is a function of the size of the organisms just as much as it is 

 of the number present, it does not necessarily follow that because there has been a numerical increase 

 there will be a rise in volume (i.e. the total mass) of the plankton. In this section, however, it will be 

 shown that in the Antarctic and to a lesser extent sub-Antarctic zones there is a winter increase in the 

 concentration of standing crop of zooplankton in the deeper waters comparable to the numerical 

 increase described by Mackintosh. 



Probably the best way of representing the results is to use block diagrams in which the area of a 

 block at a particular depth (or, more precisely, in the limited column of water through which the net 

 has been fished) is proportional to the volume of the sample taken from that depth. The results from 

 the repeated lines in o°, 20 E and 8o° W, together with the observations in 20 W, no E, 160 E 

 and 90 E, have been plotted in this fashion in Figs. 1-6. The vertical scale represents depth in metres 

 and the horizontal scale latitude, with south to the right of the page. Stations are plotted according to 

 their latitude except where large volumes or the proximity of stations makes this impossible. The 

 approximate latitudes of the Antarctic convergence (A.C.) and, where possible, the sub-tropical 

 convergence (S.T.C.) as determined from the continuous thermograph records of surface temperature, 

 are also shown. Stations which occurred during daylight hours are indicated by a ' O '• Gaps occur in 

 the observations where samples were not available, or where the presence of phytoplankton made it 

 impossible to measure the volume of the sample (see p. 198). 



In plotting the volumes allowance has been made for the different duration of hauls at certain 

 depths by making the width of each block equivalent to the volume of plankton for each 50 m. of the 

 haul. For example, the width of a 50-0 m. haul is equivalent to the total volume of the sample, while 

 the width of a 500-250 m. block is equivalent to one-fifth the volume of the sample since the haul was 

 five times longer than the shallower haul. Width thus represents concentration per 50 m. haul (i.e. 

 local density of plankton), and area the volume of the catch from that depth interval. 



In drawing conclusions from these diagrams it should be remembered that the disposition of the 

 water layers in the Antarctic is such that the 50-0 and 100-50 m. hauls sample the Antarctic surface 

 layer, the 250-100 m. hauls sample partly the Antarctic surface layer and partly the warm deep layer, 

 while the deeper hauls sample the warm deep layer exclusively. In the sub-Antarctic conditions are 

 more complicated and no such simple distinction can be drawn. Only constant or large-scale variations 

 are being considered, and the minor irregularities, which are usually evident from station to station, 

 must be ignored. 



