54 ASTRID CEEVE-EULER, QUANTITATIVE PLANKTON RESEARCHES IN THE SKAGER RAK. 



besides the common Paralia and Guinardia, it cannot be identified with the ordinary 

 Guinard i a-for mation of the so-callcd Southern bank-water, reaching us in summer and 

 autumn. 



2. Influence of hydrographic conditions on the power of multiplying of the 



different plankton-associations. 



The hydrographic factors, affecting the growth of plankton in the first line, are 

 these three: light, salinity of the water, and its temperature. I will not undertake to 

 range them in accordance to tlieir relative importance for the biology of the waters. 

 A sufficicnt amount of light has certainly proved to be indispensable for the production 

 of rich vegetations, but also a proper salinity, within quite narrow limits, was in a great 

 many cases found to be a condilio si?ie qua non for a propagating to get on. There are further 

 cases, when a difference of a few degrees alone seems to account for the development 

 stopping or going on vividly. The temperature does not exercise a direct influence only 

 upon the growth, as is clearly seen from the fact, that some formations are stirred up 

 to multiplying by high temperatures and other by low ones. In the latter case, it can be 

 but a secondary effcct, owing to a larger quantity of dissolved gases at low temperatures, 

 especially carbonic acid, and possibly to a paralysing of destroying agents, acting upon 

 the p rotoplasma or other cell-contents. 



Nor will it be forgotten, that several other factors have to be paid attention to, 

 each of which being equally capable of stopping the growth, if present at a minimum. 

 Such are a sufficient råte of the necessary aliments and of oxygenc, dissolved into the 

 water. But we can dispense with discussing them here, since a supply as well of the ne- 

 cessary alimentary stuffs as of oxygene, sufficient in most cases for the råte of generation 

 enabled by the physical conditions, may be expected to be present as a rule, at least 

 near the coast. It cannot be doubtful, that even the fresh supply of aliment brought 

 from land by the Baltic Current, highly favours the developing of the diatomaceous maxima 

 in the cold season. For a similar reason the stations close to land generally exhibit a 

 much richer plankton than those situated further out, in perfect accordance with the 

 theory of Gran. 



Influence of light. 



As I shall endeavour to give låter on a special survey of the optimal conditions for 

 growth, needed by the different species, a few instances only may be quotcd here in order 

 to illustrate the capital influence of light upon the development of whole formations and 

 tlieir leading species. 



In the following diagrams, the frequency of a ruling winter-diatom, Thalassiosira 

 NordensMöldii, at S. Skag in Fcbruary is reproduced, once in 1912 ( 15 / 2 ), according to 

 Gran's tables, and then in 1914 ( 25 / 2 ), as found by myself. The difference is immense. 

 Now the mixed layer of towards 30 °/ 00 salinity, preferred by this species, is not found 

 before a depth of c. 30 m in 1914, whereas it lies at c. 15 m in 1912. It must, however, 

 be admitted, that at another station, Extra II, the limits of salinity do not alone explain 



