BIOGEOGRAPHICAL BOUNDARIES 



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Thalassiothrix longissima Corycaeus anglicus 



JFMAMJJASOND JFMAMJJASOND 



7948 

 /949 

 1950 

 1951 

 1952 

 1953 

 195^ 

 1955 

 1956 

 1957 



Fig. 6. The seasonal abundance of a diatom (left) and a copepod (right) in 

 the North Sea, based on the Continuous Plankton Recorder Survey. The contours 

 are based on the average number per sample in each month. Small dots indicate 

 that the species was absent; open circles, large dots, and the contours are drawn 

 at abundance intervals of X2 (G. A. Robinson and J. AI. Colebrook, unpublished). 



plankton over the deep Atlantic part of the Recorder Survey and 

 a different cycle in the shallow waters of the North Sea. 



Figure 7 summarizes this difference for the most common spe- 

 cies of phytoplankton and copepods. It is based on the period of 

 nine years, 1948 to 1956; that is, the nine Januaries, etc. It is not 

 a community succession diagram in the usual sense as it does not 

 show dominance in the plankton but only the successive seasons 

 of each species. In the southern North Sea there was a marked 

 sequence of peaks of abundance of both phytoplankton and cope- 

 pods; the total effect is one of biological activity throughout most 

 of the year. In the Atlantic, on the other hand, the sequence was 

 less marked; nearly all species showed the same period of abun- 

 dance and the general impression is of a biological season shorter, 

 by two or more months, than that of the North Sea. Other meth- 

 ods confirm this result and suggest that the difference is largely 

 explained by the earlier spring outburst of diatoms in the North 

 Sea and by the relatively small autumnal phytoplankton crop in 

 the Atlantic. The seasonal cycles in both parts of the survey were 

 much more protracted than those found by Kielhorn (1952) and 

 Fish (1954) at the ocean weather station in the Labrador Sea, 

 at about the same latitude as the Recorder Survey. 



Figure 8 illustrates another aspect of long-term fluctuations in 



