BIOGEOGRAPHICAL BOUNDARIES 205 



The Recorder Survey has provided an opportunity to study 

 many aspects of the plankton over a very wide area. In 1957, we 

 decided that the accumulation of material taken during the nine 

 years 1948 to 1956 was sufficient to provide an estimate of the 

 "average" distribution of plankton — an atlas which we could use as 

 the biogeographical background for further studies of annual and 

 seasonal fluctuations in the abundance and distribution of indi- 

 vidual species and of groups, or communities, of organisms. The 

 "average" distribution is the resultant of seasonal and annual fluc- 

 tuations of many diverse factors. Our objective was to discover the 

 faunal and floral patterns or shapes of distribution. Similar ob- 

 jectives have been pursued by, for example, Bieri (1959) working 

 on the chaetognaths, Brinton (1959) on the euphausiids, Rass 

 (1959) on the commercial fisheries, and by Bogorov (1958), and 

 many other Russian workers on a variety of organisms, mostly 

 in the Pacific. 



As far as possible, every Recorder route was sampled once a 

 month; for examples, see John and Brown (1958). The records 

 were divided into sections, each representing ten miles of towing 

 and bearing the plankton from about 3 m^ of water. Each ot these 

 10-mIle samples was assigned to a geographic square, and con- 

 tours of equal abundance were interpolated between the centers 

 of the squares, based on the nine-year average number per sample 

 of each organism. The distributions of more than sixty species of 

 diatoms, dinoflagellates, copepods, gastropods, tunlcates, and 

 young fish were prepared in this way to form a contribution to- 

 ward an atlas of the plankton of the northeastern Atlantic and 

 the North Sea (Colebrook et al., 1961). 



Figure 1, which is taken from this atlas, shows the distribution 

 of four species, ranging from those found only in the oceanic parts 

 of the survey to those restricted to the coastal regions and the 

 North Sea. 



A striking feature of the distribution of many species was the 

 close correspondence between the contours of abundance and the 

 position of the continental slope as defined by the 100-fathom 

 depth contour. Some organisms were abundant over deep water 

 but scarce or absent over the continental shelf (e.g., Calaniis mmor 



