APPENDIX D: BRANDT _ IQ — 



August 1902 (4) 1-037 November 1902 (3) 1'26 



February 1903 (2) 1-45 May 1903 (1) 0-65 



August 1903 (1) 0-93 November 1903 (6) 1-084 



February 1904 (6) 1-015 May 1904 (2) 0-655 



August 1904 (2) 0-926 mgr. per Liter. 



These figures are of interest in several ways. From August 1902 on to November and 

 then to February, the amount of the dissolved silicic acid increased. In May 1903, the 

 amount was very small; it was greater in August and still greater in November, fn 

 February 1904, the amount was not so high as in the preceding year; but in May 1904, 

 there was again a very small quantity which again increased towards August, if we exclude 

 for the moment the abnormal value of February 1904, we obtain a curve which agrees 

 with the annual changes in a group of plankton-plants, viz. the diatoms. This parallel 

 would be still clearer, if investigations had been made in the intervals between the seasonal 

 cruises. As shown first of all for Kiel Bay and then for other North-European waters 

 also, the minimum of the plankton occurs in our waters in February and March (according 

 to the year). Immediately afterwards follows the spring-maximum, which is caused by the 

 luxuriant growth of the diatoms (chiefly Chaetoceros). In the period from May to July or 

 August, the total quantity of plankton organisms is relatively small. In late summer and 

 autumn a second maximum occurs, which is smaller in volume than the spring-maximum 

 and comes in part from the copious development of certain diatoms (especially Rhizosolenia), 

 but chiefly from the fact that the large Peridineae (Ceratium) have the main period of their 

 development in October. As the number of the Ceratium gradually decreases towards 

 February, the quantity of the plankton-organisms diminishes to a minimum. 



The spring maximum is to be explained from the circumstance, that the plant food- 

 stuffs are greatly stored up in the winter — dissolved silicic acid amongst others — and 

 then the rise of temperature and increased intensity of the light, render an immense 

 production possible of quite definite diatoms (Chaetoceros). These diatoms require a very 

 large amount of silicic acid; the dried substance of Chaetoceros consists of silicic acid 

 to about one half, that of Rhizosolenia to about one third. Before the period of propagation 

 in 1903, the water was richer in dissolved silicic acid than at any time previously. In conse- 

 quence of the very great demand for this food-stuff — occurring still only in small 

 quantity — the conditions of nourishment became so bad , that the propagation ceased 

 and the resting-stages were formed. In fact, after the diatom-maximum in May of both 

 years, the amount of silicic acid was very greatly reduced. That the silicic acid occurs 

 at a minimum at this time, and that the diatom-propagation is thus suddenly brought to 

 an end, appears very probable from the fact, that about 1 part of diatom silicic acid occurs 

 in a million parts of water at the period of the strongest increase of the diatoms. At this 

 period the relation between dissolved and diatom silicic acid was about 1 : 1 ; it is thus 

 many times more unfavourable that the relation between the inorganic and organic forms 

 of the nitrogen compounds. 



In consequence of the small demand during the summer months, a similar enrichment 

 of the water as regards silicic acid, again takes place in the period from May to August 

 or September, and this in most years brings on a late summer maximum of diatoms, which 

 soon disappears and is almost always much smaller than the spring maximum. It consists 



