90 NATURE 
[| NovEMBER 26, 1896 
respects, and consequently that the different strata of water 
were characterised by different amounts of plankton, and by 
different species. Samples of plankton were afterwards col- 
lected by the Swedish hydrographical expeditions, at the same 
time as samples of water for physical and chemical research. 
The examination of the plankton was carried out by Dr. 
Aurivillius, who took charge of the animal plankton, and by 
myself, who undertook the vegetable. 
Having examined a large number of samples, I have lately 
found that the plankton of the Skagerack and Kattegat can be 
classed according to the prevailing species, and in this way I 
distinguished four types, namely: (1) 7Z7¢fos-plankton, (2) 
Didymus-plankton, (3) Trécho-plankton, and (4) Sira-plankton. 
(1) The 7Z7zpos-plankion is characterised by its scarcity in 
diatoms and its abundance in cilioflagellates and entomostraca, 
which give to the spirit, in which the samples are preserved, an 
orange or yellow colour, all the other kinds of plankton colour- 
ing it more or less deep green. Among the entomostraca, 
according to the publications of Dr, Aurivillius, Paracalanus 
parvus, Pseudocalanus elongatus, and Evadne spinifera are the 
most abundant. Among the cilioflagellates Ceratzuze tripos, 
with the variety #acroceros, is the most common. C. dzvergens, 
C. furca, and C. fesus occur in less numbers. Diatoms are, as 
I have said, scarce, the most abundant being Coscénodzscus 
conctnnus and Rhézosolenia gractlima. In winter and early 
spring the unicellular alga, Halosphera viridis is found ir. 
abundance. This kind of plankton characterises the water of 
the Baltic current, and prevails in the summer in the Kattegat 
and Skagerack. The organisms consist chiefly of euryheline 
and eurythismic species, which can withstand the dilution of the 
salter North Sea water by the slightly saline Baltic water. 
It seems very probable that this first type of plankton may by 
future researches be split up into different kinds. _We may 
thus, perhaps, distinguish one kind, characterised by Halosphera 
veridts, and occurring in the winter, another by A’hzzosolenta 
gractllima, occurring in the summer, one with Paracalanus 
parvus, and another with Psewdocalanus elongatus, and so on. 
In all cases it seems to be certain that the water containing 
this first type is derived from the North Sea as well as from the 
Baltic. 
(2) The Didymus-plankton consists principally of diatoms, 
among which the most characteristic species are Ch@efoceros 
curcisetus, Ch. didymus, Ditylum Brightwellit, Rhizosolenta 
alata, and gract/iima (the latter probably a residuum of Type 1), 
Skeletonema costatum and Thalasstothrix Frauenfeldiz (the latter 
probably common to Type 3). A silicoflagellate, Déctyocha 
speculum, occurs constantly, but not abundantly. The cilio- 
flagellates, as well as the entomostraca, are scarce. 
This kind of plankton was predominant in the Skagerack and 
Kattegat in November 1893, filling the fjords from the bottom 
to the surface. With the water, containing this kind of plank- 
ton, the herring arrived on the shores of Scandinavia. It seems 
to have been a very large bulk of water that at this time set in 
to the coast, as it drove away the whole of the summer water 
from bottom to surface. 
The diatoms of this type are not known from the Arctic 
Ocean or from the Northern Atlantic, but are well known from 
the coasts of France and Belgium and the English Channel. It 
seems thus to be beyond doubt that the water came from the 
southern North Sea, along the western coast of Denmark. The 
temperature, as well as the salinity, were found to be variable, 
but the plankton constant. In the Gullmarsfjord the water at 
the surface had a temperature of 7° C., at a depth of 30 m. nearly 
12°, and at the bottom only 4°to 5. The salinity amounted 
respectively to about 26-27, 32 and 33 to 37 per thousand. This 
variation may be explained by the mixture of the water of the 
second type with the water previously present in the Kattegat. 
Probably the warmest water was the most pure water of Type 2, 
and corresponds to one of the kinds of water called by the 
Swedish hydrographers the fark-water. 
(3) The third type of plankton, the Zyécho-plankton, is dis- 
tinguished by its diatoms, especially the following species :— 
Thalassiosira longissima, Rhizosolenta styliformis, Chetoceros 
atlanticus (in a less degree also by Ch. 4orealis and its variety 
brightwellit), and Beddulphia mobilensis. The first-named 
species occur abundantly and almost pure in the Northern 
Atlantic, south of Iceland; the last-named I observed at 
Plymouth, West Scotland, and in the North Sea. This plank- 
ton may thus be considered a Northern Atlantic plankton. At | 
NO. 1413, VOL. 55] 
the Scandinavian coast it seems to occur very rarely in a pure 
state; in fact I have seen it only once, in February of this 
year, gathered at the bottom of the Christiana fjord (100 m.), 
where the temperature amounted to 7°5° C., and the salinity to 
34-76 per thousand, the highest figures obtained by the hydro- 
graphical examinations of all the samples gathered in February 
1896. 
On the other hand, this plankton was frequently found 
mixed with the next type in samples collected at the time 
named. 
(4) The fourth type, the Szva-plankton, consists also mainly of 
diatoms, but of different species, the most characteristic being 
Thalasstosira Nordensktoldi,and Th. gravida, Chetoceros gren- 
landicus, Ch. soctalis, Ch. scolopendra, Ch. teres, Niutschta 
sertata, many of which belong to the Arctic seas, and some of 
which are new to science. Among the cilioflagellates the most 
abundant is a variety arclzca of Ceratium trépos, distinguished 
by Dr. Aurivillius as a constituent of the plankton of Baffin’s 
Bay. 
There can be no doubt about the Arctic origin of this type. 
It occurred in the Skagerack and Kattegat this year in February 
and March, always more or less mixed with(3)and(1). In the 
Skagerack the water with Types (3) and (4) was covered by a 
shallow layer of water with Type(1); but in the Kattegat it 
reached the surface. The admixture of Type (3) shows that the 
water on or before its arrival at the coast of Sweden was mixed 
with Atlantic water. The temperature and the salinity were 
found to vary greatly, owing to the admixture of the slightly 
saline Baltic water, at this time of the year very cold. 
I have observed the same type of water in some slides col- 
lected on the west coast of Scotland by Mr. George Murray, 
and sent to me by Mr. Grove. These samples had been 
gathered in the spring of 1888—a year remarkable in England 
as an unusually cold one. 
As far as the plankton researches are advanced at present, 
we may conclude that the surface water around the Swedish 
coast consists in the summer of water from the North Sea mixed 
with Baltic water; that in the autumn its place is taken by 
water from the southern part of the North Sea. and in the 
winter by water from the Northern .\tlantic and the Arctic 
Ocean. Whether these changes occur regularly every year, or 
in certain years only, cannot be answered for the moment. 
Probably the last change is in correspondence, as Prof. Petters- 
son has recently suggested, with variations in the amount of 
water which the Gulf Stream carries past Iceland, westwards to 
Davis Strait and eastwards to the Arctic Ocean. 
I think I have proved by the above that the examination of 
plankton is a matter of the greatest interest, not only in relation 
to hydrography, but also to meteorology and to fishery questions. 
There can be no doubt about the close connection between the 
state of the sea and the movements of the air, and the still 
obscure causes of the migration of fishes may be found to be 
intimately connected with the change of water containing 
different kinds of plankton. 
It is thus an important matter that the plankton of the North 
Sea should be thoroughly and systematically examined ; but for 
this, international co-operation of all the nations around the 
North Sea is required. I imagine that a central station, under 
the direction of competent persons, and provided with adequate 
accommodation, might be erected. Samples could be collected 
at certain intervals, and by the same kind of apparatus at 
different stations, and sent to the central one for examination. 
The details should be published every month, and the general 
results formulated in a way that would be useful to hydro- 
graphers, meteorologists, &c. The marine biological stations 
already in existence will probably be found willing for co-opera- 
tion in such an undertaking, but they will be able to collect 
plankton only near the shores or at short distances from them. 
For getting samples from the open seas, the officers of the 
steamers crossing the North Sea and the Northern Atlantic 
might be found willing to assist, as the plankton may, as Dr. 
John Murray hinted to me, be procured by pumping water into 
asilk net. I recently tried this method whilst crossing from 
Edinburgh to Géttenburg. I fastened the net to the pump 
when the deck was being washed, and in this way I obtained 
sufficient plankton to prove that in the last days of July the 
North Sea was almost free from diatoms, and its plankton 
consisted mainly of cilioflagellates and entomostraca. 
P. T. CLEVE. 
