i3^ 



NATURE 



[September i6, 1909 



be unifonnlty ol conditions, but if ouglit to be recognised 

 that such legions are not tliose in whicli the planlvton is 

 of most importance to men. The great fisheries of the 

 world, rui_h as those of the North Sea, the cod fishery in 

 Norw .11 , and those on the Newfoundland Banks, are not 

 in miil-ucean, but are in areas round the continents, where 

 the planliton is irregular in its distribution. 



.•\s an example of a locality of the second type, showing 

 seasonal, horizontal, and vertical differences in the distribu- 

 tion of the plankton, we may take the centre of the Irish 

 Sea, off the south end of the Isle of Man. Here, as in 

 other localities which have been investigated, the Phyto- 

 Plankton is found to increase greatly about the time of 

 the vernal equinox, so as to cause a maximum, largely 

 composed of Diatoms, at a period ranging from the end of 

 March to some time in May — this year to May 28, in the 

 Irish Sea. Towards the end of this period the eggs of 

 most of the edible fishes are hatching as larvae. 



This Diatom maximum is followed by an increase in 

 the Copepoda (minute Crustacea), which lasts for a con- 

 siderable time during the early summer, and as the fish 

 larvre and the Copepoda increase there is a rapid falling 

 off in Diatoms. Less marked ma.\ima of both Diatoms 

 and Copepoda may occur again about the time of the 

 autumnal equinox. These two groups — the Diatoms and 

 the Copepoda — are the most important economic con- 

 stituents in the plankton. \ few examples showing their 

 importance to man may be given : — Man eats the oyster 

 and tile .American clam, and these shell-fish feed upon 

 Diatoms. .Man feeds upon the cod, which in its turn may 

 feed on the whiting, and that on the sprat, and the sprat 

 on Copepoda, while the Copepoda feed upon Peridinians 

 and Diatoms ; or the cod may feed upon crabs, which in 

 turn eat '* worms," and these feed upon smaller forms 

 which are nourished by the Diatoms. Or, again, man 

 eats the mackerel, which may feed upon young herring, 

 and these upon Copepoda, and the Copepoda again upon 

 Diatoms. All such chains of food matters from the sea 

 seem to bring one through the Copepoda to the Diatoms, 

 which may be regarded as the ultimate " producers " of 

 food in the ocean. Thus our living food from the waters 

 of the globe may be said to be the Diatoms and other 

 microscopic organisms as much as the fishes. 



Two years ago, at the Leicester meeting of the British 

 Association, I showed that if an intensive study of a small 

 area be made, hauls being taken, not once a quarter or 

 once a month, but at the rate of ten or twelve a day, 

 abundant evidence will be obtained as to (i) variations in 

 the distribution of the organisms, and (2) irregularities 

 in the action of the nets. Great care is necessary in order 

 to ensure that hauls intended for comparison are really 

 comparable. Two years' additional work since in the same 

 locality, off the south end of the Isle of Man, has only 

 confirmed these results, viz. that the plankton is liable to 

 be very unequally distributed over the depths, the localities, 

 and the dates. One net may encounter a swarm of 

 organisms which a neighbouring net escapes, and a sample 

 taken on one day may be very different in quantity from 

 a sample taken under the same conditions next day. If 

 an observer were to take quarterly, or even monthlv, 

 samples of the plankton, he might obtain very different 

 results according to the date of his visit. For example, on 

 three successive weeks about the end of September he 

 might find evidence for as many different far-reaching 

 views as to the composition of the plankton in that part 

 of the Irish Sea. Consequently, hauls taken many miles 

 apart and repeated only at intervals of months can scarcely 

 give any sure foundation for calculations as to the popula- 

 tion of wide sea areas. It seems, from our present know- 

 ledge, that uniform hydrographic conditions do not deter- 

 mine a uniform distribution of plankton. 



These conclusions need not lead us to be discouraged as 

 to the ultimate success of scientific methods in solving 

 world-wide plankton and fisheries problems, but thev 

 suggest that it might be wise to secure by detailed local 

 work a firm foundation upon which to build, and to ascer- 

 tain more accurately the representative value of our samples 

 before we base conclusions upon them. 



I do not doubt that in limited, circumscribed areas of 

 water, in the case of organisms that reproduce with great 

 NO. 2081, VOL. 81] 



rapidity, the plankton becomes more uniformly distributed, 

 and a comparatively small number of samples may then 

 be fairly representative of the whole. That is probably 

 more or less the case with fresh-water lakes, and I have 

 noticed it in Port Erin Bay in the case of Diatoms. In 

 spring, and again in autumn, when suitable weather 

 occurs, as it did two years ago at the end of September, 

 the Diatoms may increase enormously, and in such circum- 

 stances they seem to be verj' evenly spread over all parts 

 and to pervade the water to some depth ; but that is 

 emphatically nol the case with the Copepoda and other 

 constituents of the plankton, and it was not the case even 

 with the Diatoms during the succeeding year. 



I have published elsewhere an observation that showed 

 very definite limitation of a large swarm of crab Zoeas, so 

 that none were present in one net while in another adjacent 

 haul they multiplied several times the bulk of the catch 

 and introduced a new animal in enormous numbers. Had 

 two expeditions taken samples that evening at what might 

 well be considered as the same station, but a few hundred 

 yards apart, they might have arrived at very different con- 

 clusions as to the constitution of the plankton in that part 

 of the ocean. 



It is possible to obtain a great deal of interesting in- 

 formation in regard to the " hylokinesis " of the sea with- 

 out attempting a numerical accuracy which is not yet 

 attainable. The details of measurement of catches and of 

 computations of organisms become useless, and the exact 

 figures are non-significant, if the hauls from which they 

 are derived are not really comparable with one another and 

 the samples obtained are not adequately representative of 

 nature. If the stations are so far apart and the dates are 

 so distant that the samples represent little more than 

 themselves, if the observations are liable to be affected by 

 any incidental factor which does not apply to the entire 

 area, then the results may be so erroneous as to be useless, 

 or worse than useless, since they may lead to deceptive 

 conclusions. It is obvious that we must make an intensive 

 study of small areas before we draw conclusions in regard 

 to relatively large regions, such as the North Sea or the 

 Atlantic Ocean. Our plankton methods are not yet 

 accurate enough to permit of conclusions being drawn as 

 to the number of any species in the sea. 



The factors causing the seasonal and other variations in 

 the plankton already pointed out may be grouped under 

 three heads, as follows : — 



(i) The sequence of the stages in the normal life-history 

 of the different organisms. 



(2) Irregularities introduced by the interactions of the 

 dififerent organisms. 



(3) More or less periodic abnormalities in either time or 

 abundance caused by the physical changes in the sea, which 

 may be grouped together as " weather." 



These are all obvious factors in the problem, and the 

 constitution of the plankton from time to time throughout 

 the year must be due to their interaction. The difficulty 

 is to disengage them from one another, so as to determine 

 the action of each separately. 



Amongst the physical conditions coming under the third 

 heading, the temperature of the sea is usually given a very 

 prominent place. There is only time to allude here to one 

 aspect of this matter. 



It is often said that trop'cal and sub-tropical seas are 

 relatively poor in plankton, while the colder Polar regions 

 are rich. In fishing plankton continuously across the 

 Atlantic it is easy from the collections alone to tell when 

 the ship passes from the warmer Gulf Stream area into 

 the colder Labrador current. This is the reverse of what 

 we find on land, whf-re luxuriant vegetation and abundance 

 of animal life are characteristic of the tropics in contrast 

 to the bare and comparatively lifeless condition of the 

 Arctic regions. Brandt has made the ingenious suggestion 

 that the explanation of this phenomenon is that the higher 

 temperature in tropical seas favours the action of denitrify- 

 ing bacteria, which therefore flourish to such an extent in 

 tropical waters as seriously to diminish the supply of 

 nitrogen food and so limit the production of plankton. 

 Loeb.' on the other hand, has recently revived the view 

 of Murray, that the low temperature in Arctic waters so 

 1 " Darwin and Modern Science" (Cambr'dge, 1909), p. 247. 



