478 
BULLETIN OF THE BUEEAU OF FISHEKIES 
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- 
