i86 



NAT URL 



[August io, 191 i 



to the Commissioners to operate detrimentally in the 

 case of local authorities. 



With regard to the alleged obligation of the Crown 

 to defend the coasts from the inroad of the sea, the 

 Commissioners, with one exception, consider that the 

 evidence laid before them does not warrant this con- 

 clusion, and that there is not any settled principle of 

 the Crown or statute law to support the contention 

 that there is a responsibility for sea defence resting 

 primarily upon the nation at large; the fact that 

 there is erosion in some places does not affect the 

 nation generally, and there is not any ground for the 

 contention that sea defence is a national service. 



SEASONAL VARIATIONS OF MARINE 

 ORGANISMS. 

 r rHE present short article has been written, not for 

 J- the plankton specialist, but for readers who may 

 have had their interest aroused by some reference in 

 purely technical papers to seasonal variations. Aquatic 

 organisms (vegetable and animal) may be divided into 

 two groups, those which are fixed to the bottom or 

 cannot leave the substratum, and those which are 

 independent of it, and live swimming or floating in 

 the water. 



Of the latter, the pelagic organisms, some, such as 

 fishes, swim about actively, whilst others are passive 

 organisms, with but feeble organs of locomotion or 

 none. They float about almost like inanimate objects, 

 at the mercy of tides and currents, and they vary in 

 size from microscopic flagellates up to large medusae. 

 These more or less passive pelagic organisms (both 

 vegetable and animal) are what Hensen in 1887 char- 

 acterised under the term plankton. The study of the 

 plankton has advanced by leaps and bounds in the 

 last few years, and fresh discoveries (often the results 

 of new methods and ingeniously devised apparatus) 

 have acted as stimuli to the work. 



From being a qualitative science, planktology has 

 become quantitative, and is becoming as exact in its 

 methods as biometrics. Formerly the investigations 

 were considered sufficiently intense and accurate if a 

 net made of fine silk was pulled horizontally through 

 the water, so that the planktonic organisms were 

 filtered out and captured. Now, it is necessary to 

 use other methods, to pass the water through special 

 filters, and to centrifuge measured quantities in order 

 to catch those exceedingly small creatures which pass 

 quite easily through the finest silk cloth. These very 

 small organisms are of supreme importance, for what 

 they lack in size they make up for in numbers, and 

 some of the most keenly discussed theories in marine 

 biology of the last few years may have to be seriously 

 modified when more detailed observations have been 

 made on their occurrence. 



Serial investigations have shown that the plankton 

 varies both qualitatively and quantitatively through 

 the different seasons. It is never absent even under 

 an ice cover in fresh-water lakes, and just as the 

 opening of the buds serves as an indication of spring, 

 so the appearance of certain organisms tells of the 

 approach of the same season in the waters. Summer 

 and autumn are both equally well marked in the 

 aquatic world, and this applies to the seas as well 

 as to lakes and ponds. 



Some organisms are always present, but most 

 planktonic animals and plants appear at certain 

 seasons, and then disappear, whilst others take their 

 places. In fact, it is quite impossible to write an 

 account of the plankton of any waters from a series 

 of catches made in a period of a few weeks only. A 

 plankton investigation must extend through at least 

 ■ ear, so that catches may be examined repre- 



NO. 2 I So, VOL. 87] 



sentative of all seasons. If such a research be carried 

 out it becomes at once obvious that the plankton 

 varies according to certain external conditions, ami 

 which might be enumerated, sunlight and temperaim, 

 of the water, chemical constitution of the medium, 

 and, finally, the motion of the latter, particulai 1 

 regards vertical currents. 



The application of quantitative methods to the study 

 of the marine plankton has shown that, contra i 

 all expectation, the colder waters of the globe are more 

 productive than those of the tropics. Compared with 

 temperate and Arctic seas, the open ocean of the tropics 

 is a desert so far as the plankton is concerned, with 

 occasional oases. In our own waters the quantity of 

 plankton present varies enormously during the year. 

 Catches made with a net hauled vertically under the 

 same conditions average perhaps about 1 c.c. in 

 volume during the early months of the year. In the 

 short space of a week in March or April, this volume 

 may rise to 40 c.c, or even more, and remain con- 

 stant for a period of some weeks, falling eventuallv to 

 about 1 or 2 c.c. again during the summer. It is of 

 the utmost importance that the causes of these varia- 

 tions in the quantity of the plankton should be dis- 

 covered. What determines the productivity of any 

 particular region ? Why is there an extraordinary in- 

 crease in quantity during certain weeks of the spring 

 and autumn ? These are amongst the most funda- 

 mental questions in the biology of the sea. 



If every year was the same so far as meteorological 

 conditions were concerned, and the plankton variations 

 of successive years were absolutely identical, we should 

 never be able to do more than make speculations as to 

 the causes of such variations. Nature, however, 

 varies the meteorological and hydrographical condi- 

 tions for us, and we find that there are also corre- 

 sponding annual variations in the plankton. Thus the 

 spring and autumn maxima of the latter may be 

 earlier one year than another, or may extend over a 

 longer period. It is the work of the planktologist 

 to analyse these changes and endeavour to correlate 

 plankton variations with hydrographical and meteoro- 

 logical conditions. 



In the Irish Sea, for example, an attempt has been 

 made by Prof. Herdman and others to arrive at some 

 of the causes of the seasonal variations by taking 

 serial plankton catches through a period of several 

 years, the hydrographic conditions prevailing being 

 also observed. It ought to be possible in this way 

 to correlate certain biological and physical variations. 

 It was found in the first years of this plankton in- 

 vestigation that the spring and autumn maxima were 

 due to a large extent to an enormous increase in 

 diatoms, an increase followed by the appearance of 

 dinoflagellates. The copepods followed these spring 

 dinoflagellates, and attained their greatest develop- 

 ment in the early summer. This order of succession 

 has held good throughout all the years of this inves- 

 tigation (1907-11). Whilst, however, the highest 

 monthlv averages were in April in 1007 and iqio, thev 

 occurred in May in 1008 and 1900. The two years, 

 1907 and 1910, resembled one another so far as 

 meteorological conditions are concerned in having a 

 larger amount of sunshine during the early months 

 than was the case in tqo8 and 1909; and the question 

 arises whether this early sunshine was a determining 

 factor in the earlv appearance of the vernal maximum. 



It was expected that this year, 1911, would perhaps 

 throw some light on the question, and whatever the 

 result mav be when the hvdrographic and meteoro- 

 logical conditions are worked out, the plankton maxi- 

 mum has certninlv been very different from that of 

 anv of the previous years; 



Up to the present the cause of the vernal phyto- 



