Zooplankton in relation to hydrography in the norwegian sea

Abstract

A study was made of the distribution of zooplankton organisms in the Norwegian Sea during May-August 1948-1954. It is based on plankton collected in vertical Nansen net hauls in the upper 200 m. Samples taken by the Norwegian weatherships at station M during 1950-1954 were used as a supplement. The hydrographical conditions in the area are discussed on the basis of earlier investigations, and on observations taken with the plankton samples. The greatest concentrations of plankton were found along the slopes of the continental shelves and near the submarine ridges, in the border areas between cold and temperate water, and in or near the centres of the cyclonic systems. During the sutnmer, water of Atlantic origin was poor in plankton. The most important whaling grounds correspond very well with the localities where the largest quantities of plankton were observed. At station M, more plakton was usually taken in the upper 25 m during the night than during the day. In hauls taken down to 200 m diurnal vertical migration does not influence the quantity of plankton to any extent, except for euphausiids which usually avoid the net during the day. One may broadly distinguish between a cold area, including the Greenland Sea, and a temperate area in the Norwegian Sea. The former is mainly characterized by Calanus hyperboreus, the latter by Calanus finmarchicus. In the temperate area the central part is dominated by Aglantha digitale and Limacina retroversa in summer. The distribution of neritic organisms is mainly correlated with the extension of the coastal water. In summer Evadne nordmanni may establish a local population in the open sea between Jan Mayen and Norway. Temperature is assumed to be the most important factor influencing growth and size of copepods. The horizontal distribution of various plankton organisms during the summer is shown. Calanus hyperboreus was dominant in the cold area, but also moderately abundant in the East Iceland current to the north of the Faroes. Single individuals were found all over the temperate area. Two popula- tions can be distinguished by size distributions. The smaller individuals were found in the temperate area and in border areas between cold and temperate water. Calanus finmarchicus was dominant in the temperate area, the distribution in general corresponding with the volumetric distribution of the plankton. Different populations have been distinguished on the basis of size distributions. The largest individuals live in coastal and bank waters from Bear Island to Spitsbergen and in the cold area, but the smaller individuals dominate in all areas. Those present in the cold area hale evidently been introduced from the temperate area with branches of the North Atlantic current. Pseudocalanus minutus is a native of the open ocean, being replaced in coastal areas by P. elongatus. The largest stocks were located in the southwestern corner of the Norwegian Sea, and in the border areas between cold and temperate water. P. minutus was scarce in the cold area. There were two or more populations. In the cold area the stock consisted of small individuals probably introduced from the temperate area the previous autumn. Some large individuals were found near Jan Mayen and on the banks from Bear Island to Spitsbergen. Microcalanus pusillus lives chiefly between 600 and 100 m, and is distributed quite uniformly over large areas of the Norwegian Sea. The samples are not representative of the distribution. In the cold area the individuals were small and were probably introduced from the temperate area. Microcalanus pygmaeus was mainly found in the deeper layers of the Norwegian Sea, at station M chiefly below 1000 m. In the cold area some specimens were taken in the upper 200 m. At station M the stock is probably supplied from other parts of the Norwegian Sea, and condusions have been drawn regarding the renewal of the bottom layers. Metridia longa was common in the intermediate and deeper layers of the Norwegian Sea. M. Lucens was most abundant in the eastern parts, partly introduced with the North Atlantic current. The size variations of the individuals are discussed for both species. Oncaeda borealis and Oithona helgolandica were both similar to Pseudocalanus minutus in horizontal distribution. The latter were also numerous in coastal areas, especially during the autnmn. Oithona spinirostris is quite common in the intermediate layers. The area of maximum abundance is generally limited by the 4° and 7°C. isotherms at 50 m. Aglantha digitale was mainly confined to the intermediate and deeper layers as adult form, and reproduced very heavily in June-July. The young were numerous in the upper layers in the central parts of the temperate area of the Norwegian sea, especially between Jan Mayen and Norway. Later in the autumn they were carried farther nothwards. Limacina retroversa was similar to Aglantha digitale in oceanic distribution, but was also numerous in coastal areas. Limacina helicina was restricted to the cold areas. Sagitta elegans was most abundant in the southwestern part of the Norwegian Sea. Eukrohnia hamata was common in all areas and more numerous than Sagitta elegans, ecpecially in the colder parts. As the two species also live in the intermediate and deeper layers, the material is not representative of the distribution. Ostracods were taken regularly, especially in the cold area. They live mainy in the intermediate and deeper layers. Of the amphipods, Themisto abyssorum was most usual. It was found all over the Norwegian Sea, mainly as young individuals. The highest numbers were observed in the cold area. Eggs and larvae of euphausiids were taken fairly regularly, especially in coastal and slope areas. Adult enphausiids are not caught quantitatively. Thysanoessa longicaudata was the species most frequently taken. Of the copelates, Oikopleura labradoriensis and O. vanhøffeni were most numerous. There were one or two main areas of distribution in the temperate area, but the copelates were scarce or absent in the cold area. O. vanhøffeni however, was taken in moderate numbers north of Jan Mayen in 1954. Fritillaria borealis was numerous in some coastal areas, e.g. northeast of the Faroes. Salps were taken in the Faroe-Shetland channel in July 1950 and off the west coast of Norway in August 1951. They were numerous at station M in September-November 1951 and 1953.