Changes in phytoplankton as indicated by spectrophotometric chlorophyll estimations 1952-53 59 



So when, largely by his efforts, the Woods Hole Oceanographic Institution was 

 founded, they could begin with a basic knowledge of the phytoplankton which is still 

 lacking in Plymouth. But quantitative work on the production of phytoplankton in 

 the English Channel has been carried out since 1921, first by calculation from the 

 changes in hydrogen ion concentration brought about by photosynthesis (1922) and 

 since 1922 from the consumption of phosphate (1923). The arrival of a modern 

 spectrophotometer however made it possible to estimate the phytoplankton crop by 

 extracting the chlorophyll from collodion filter membranes on which even the smallest 

 green flagellates had been retained. This was done in the year 1951-52, and by a 

 culture method, similar to that of bacteriology, the organisms were grown and 

 multiplied so that even those originally very sparsely distributed were not missed 

 (Atkins and Jenkins, 1953). 



The chlorophyll method of course gives the phytoplankton content of the water 

 when sampled, whereas the phosphate calculations give the amount produced over a 

 period. The two may be very different. A beginning was thus made in obtaining a 

 better knowledge of what plants were present — and of when they flourished — also 

 such work provides a basis for the study of the movement of water masses tagged by 

 a known algal flora. 



I therefore asked my collaborator Miss P. G. Jenkins to continue this research 



and to give her results, which she has done as follows. 



W. R. G. A. 



ORIGIN OF SAMPLES AND THEIR EXAMINATION 



Water was collected with a Nansen-Pettersson water bottle at the international hydrographic 

 station England No. 1 (El), twenty miles S.W. from Plymouth, at a series of depths from m to 

 70 m bottom. Two htres of each sample were filtered through a collodion (Gradocol) membrane 

 of average pore diameter one micron. 



The phytoplankton cells and the suspended matter which remained on the disks were examined 

 under the low-power microscope. Then 10 ml of an 80 per cent aqueous acetone solution was used 

 to extract the plant pigments from each membrane. 



A " Unicam " spectrophotometer with 4 cm cuvettes served to measure the minimum percentage 

 transmission in the red between 640 and 670 my.. These values were converted into concentrations 

 of chlorophyll in mg 1 read off from a graph of the transmissions of 80% aqueous acetone solutions 

 of a dry commercial chlorophyll plotted against their concentrations. This graph and the absorption 

 spectrum of the chlorophyll may be seen in the 1953 paper. Using 10 ml of the aqueous acetone 10 

 extract the plankton from a litre of water, it is obvious that the chlorophyll has been concentrated 

 one hundred times, so 1 mg/l as read oflFis equivalent to 001 mg, 1 or to 10 mg, m^. An allowance was 

 of course made for the actual volume extracted. 



In winter, i.e. November to February, the colour of the extracts was slightly yellow and the green 

 was almost imperceptible. The colour deepened with the spring growth to a deep olive green, and 

 lightened in the summer samples. 



EXAMINATION OF THE PLANKTON ON THE DISKS 



The disks varied in their intensity from a dark grey or chocolate to a very light sandy colour. It 

 was impossible to deduce the amount of chlorophyll in the extracts from these shades. That deduction 

 could only be made when the disks were a uniform faint green. They were often covered with diatoms 

 of various species, and at times had green spots due to the presence of some species of the Chloro- 

 phyceae. The Dinoflagellate Ceratium tripos occurred at every depth on Aug. 10. 1953. Fibres were 

 frequently seen. Many copepods were found on the disks and as they were so numerous in the spring 

 of 1953 their numbers were counted and set out in Table I. The totals in the second line from the 



bottom'(S means) are based on the sum of the means for 2-5, 7-5, 12-5 67-5 m for 14 depths. 



This sum is then multiplied by 5 (for 5 m intervals) and the totals in the bottom line are expressed as 



