478 BULLETIN OF THE BUREAU OP FISHERIES 



must, as Atkins points out, limit the abundance of the phytoplankton. He has 

 made interesting calculations of the amounts of diatoms that could be produced, 

 supposing all the phosphate in the water to be consumed. The facts so far garnered, 

 however, do not warrant the assumption that a poverty in phosphorus can safely be 

 invoked as the universal cause for the eclipse of the vernal flowerings of diatoms when 

 this event takes place. As. we have seen, a parallelism of the same sort has been 

 established with fair probability between the amount of silica in the water and the 

 abundance of diatoms. There is also good reason to believe that at times and over 

 large areas of sea the supply of available nitrogen falls below the minimum requisite 

 for their active reproduction. The strong probability that different groups of 

 planktonic plants, and even different species within the major groups, differ widely 

 in their nutritive requirement, makes the question complex. 



I have no first-hand information to offer on the richness of the Gulf of Maine 

 water in phosphoric acid, but the fact that the vernal swarmings of diatoms are 

 succeeded by peridinians and not by other diatoms over most of the area of the gulf 

 is sufficient evidence that water that is no longer fit to support rich flowerings of the 

 latter, through the exhaustion of some substance essential to their growth, still offers 

 a favorable environment for the former. 



Thus it does not seem likely that the spring diatom maxima in Massachusetts 

 Bay and in the southwestern part of the gulf generally as nearly exhaust the phos- 

 phates as Atkins found to happen in the English Channel. But if diatoms require 

 a richer supply of phosphates than peridinians do (as they certainly require a more 

 abundant supply of silica) the reduction in the available supply of this nutrient result- 

 ing from their consumption of it may terminate the flowerings of diatoms, though still 

 leaving the water rich enough in dissolved phosphates to support an abundance of 

 peridinians. It is true that Brandt (1905, p. 11) and others following him have sug- 

 gested that peridinians may need more phosphorus than diatoms, not less, but noth- 

 ing whatever is definitely known as to their requirements. 



The desirability of analyses of Gulf of Maine water for phosphoric acid at dif- 

 ferent times of year is obvious, and further speculation on the dependence of the 

 local phytoplankton on fluctuations in the supply of this nutrient is best postponed 

 until such are undertaken. 



In addition to the major foodstuffs which I have mentioned, planktonic plants, like 

 terrestrial plants, require a small but available supply of various other substances — 

 iron, sulphur, sodium, etc. Nothing whatever is definitely known as to their exact 

 requirements in this respect, but recent experiments on the cultivation of diatoms 

 have shown that some species require substances which others can do without. In 

 the case of Thalassiosira gravida, E. J. Allen (1914) was unable to obtain good cultures 

 in artificial sea water to which he added the same nutrient solution that had produced 

 abundant growth in natural sea water until a small percentage of the latter was 

 added to the artificial medium, when excellent cultures ensued. Provided this small 

 amount — 1 per cent or so — of natural sea water was added, the constituents of the 

 artificial sea water (which formed all but this trifling proportion of the culture medium) 

 could be varied within wide limits, as could its total salinity, without either hindering 

 or apparently helping the growth of the diatoms. Thus this particular genus appar- 



