— 11 — APPENDIX D: BRANDT 



mostly of Rhizosolenia, which require less silio.ic acid, so that a further increase in the 

 dissolved silicic acid may go on in to November. Ft is not yet explained how the cessation 

 of this second diatom maximum occurs, nor how it is replaced by the luxurious growth of 

 Ceratium. 



The spring maximum of the diatoms is very different in different years, as I have 

 shown from volume-curves in an earlier publication ^ In 1891, the quite unusual case 

 occurred, that the spring maximum of the diatoms was smaller than that of the autumn. 

 The relatively small amount of dissolved silicic acid detected in February 1904, as shown 

 above, leads to the conclusion that the spring maximum will have been unusually small 

 in this year also. Such a conclusion stands in agreement with the great abnormality, 

 that the Ceratium forms were already occurring in great quantities in the middle of June. 

 If the diatoms have not increased so greatly in the spring of 1904, the silicic acid must 

 have been used up so much, that only the minimum is present for Chaetoceros, conse- 

 quently, the other food-stuffs have been used in less quantities than usual and the Ceratium 

 forms (constantly occurring in Kiel Bay) have been able to propagate very strongly about 

 two months earlier, in consequence of the greater nourishment. 



Though I have hitherto considered it almost impossible, that phosphoric acid could 

 occur reduced to a minimum, basing this conclusion on the older information in literature 

 concerning the amount of this important food- stuff in sea-water, yet, from the new analyses 

 of Raben, which have given very much smaller values than the earlier investigations, I 

 must recognise the probability that, just as the silicic acid is at a minimum at the end of 

 the spring maximum, so the Ceratium propagation in autumn is dependent on the amount 

 of dissolved phosphates. The fact that the autumn maximum is caused by the propagation 

 of plant-species different from those of the spring maximum, shows that plants, with 

 different food-requirements, find the most favourable conditions of life and above all of 

 nourishment at the different periods of the year. According to the as yet unpublished 

 investigations of Dr. Raben, concerning the best method of exactly determining the amount 

 of phosphoric acid in sea-water, this amount seems to vary a great deal. In February and 

 May, only a small quantity of phosphoric acid occurred (0-14 — 0'25mgr. P^ 0^ in the liter), 

 but in autumn, a great deal usually (to 1-46 mgr. in a liter of Baltic water), so that much 

 phosphoric acid is present at the beginning of the propagation-period of Ceratium. 



Rearing experiments on the commonest plankton-plants under the influence of different 

 food- stuffs, offer a means of determining their food-requirements, and the most favourable 

 composition of sea-water as a nourishment-solution for plants. From such experiments, 

 with which I am at present engaged, it appears that the addition of calcium phosphate 

 alone, in autumn, causes a very distinct increase of microscopic plankton-plants, whilst 

 without this the propagation is extremely small. From similar experiments, Knauthe (I.e.) 

 has already shown it to be in high degree probable, that the poverty of plankton, which 

 has been remarked in ponds and freshwater lakes during the summer, is due to the lack 

 of phosphates. As the method of quantitative determination of phosphoric acid in seawater 

 has already been so far tested by the above-mentioned preliminary investigations, that 

 accurate results may be expected, a large number of investigations on water-samples 

 taken from the North Sea and Baltic at different times should now be made. These will 

 show whether the phosphates occur at a minimum according to the period , e. g. in 



' Die Fauna der Ostsee, insbesondere die der Kieler Buclit (Verhdl. Deutsch. Zool. üesellscli. 1897). 



2' 



