Investigations on Zooplankton in Coastal and Offshore Waters of Western and Northwestern Norway - With Special Reference to the Copepods

Abstract

1. Zooplankton samples, taken in vertical Nansen net hauls at permanent stations on the west coast of Norway between 61°-63° N and in the Lofoten area from October 1948 to the end of December 1951 have been used for a complete analysis of the variation in volume, plankton composition and biology of the species. A study has also been made of the variations in plankton volume of selected samples collected in the same area during 1927-39 and 1946-48 with the same gear, and during 1949-51 with Clarke- Bumpus plankton samplers. Plankton material from station "M" in the Norwegian Sea, taken in vertical Nansen net hauls in steps from 2 000 m to the surface during 1950-52 has been used for studies of variations in plankton volume and of variations in length of the copepods. 2. Volume measurements were made by draining or displacement, subdivisions of the samples by the improved Lea's plankton divider. All plankton has been preserved in formalin. 3. The hydrographical conditions of the investigated areas are discussed on the basis of previous investigations, and of observations taken simultaneously with the sampling of plankton. At the coastal localities more or less continuous changes take place in the body of water due to the influence of the Baltic current and the North-Atlantic current. At Skrova in the Vestfjord the conditions seem to be more stable. A survey of the more important changes during 1949-51 is given in table 1, pp. 20-21. 4. a. On the west coast of Norway between 61°-63° N the maxima in the volume of plankton usually occur in April and June-July, in the Lofoten area in May-June and July-August. The main quantity of plankton is found in the upper 50 m from April to July, with maximum in June. Generally, the plankton was most abundant in the Lofoten area, and in all localities there was also an increase in the yearly mean volumes from 1919 to 1951. b. Plankton samples taken in earlier years show volume variations siislilar to those recorded in the present investigation. c. The quantitative distribution of plankton in the Lofoten and Vesterålen area in April-June of the years 1949-51 based on samples taken with Clarke-Bumpus plankton samplers is shown. Surface currents seem to be of great importance for the distribution, while diurnal vertical migrations of the plankton organisms have no influence, the plankton in spring being concentrated in the upper 75 m. There is, on the average, quite good agreement between the quantities of plankton calculated per m² of sea surface from the hauls with the C. B. sampler and with the Nansen net. d. At station "M" in the Norwegian Sea there was a maximum of plankton volume in the upper 100 m in June, minimum in the period September-March. The plankton was more abundant in 1950 than in 1951, the temperature of the surface layers being on an average somewhat lower in 1951. - The volumes mere generally larger than in the upper 50 m on the west coast of Norway between 61°-63° N, but smaller than in the same layer in the Lofoten area. e. A comparison is made between the quantities of plankton in various European and American waters. During spring and summer the bank water off Eggum is comparable in richness to the waters of Georges Bank off the east coast of U.S.A., and to those in the Barents Sea. The greatest concentrations of plankton hitherto recorded in Northern waters, are found in summer ancl autumn in the Barents Sea. 5. There are two periods of abundance in the total number of organisms, at Sognesjøen and Ona in March-July and August-September, in the Lofoten area in April-May and August-October. Numbers are generally higher in the Lofoten area, and were on the whole greater in 1950 than in 1949. The copepods always dominate, but cladocera, eggs and larvae of euphausiids, bottom invertebrate larvae, copelata, and some other groups may at times be of some importance. 6. The variations in the relative composition of the copepod stock are shown both as to number and volume. More than 38 species have been identified, of which only 4 are of any importance in number all the year, viz. Calanus finmarchicus, Pseudocalanus elongatus, Microcalanus pusillus and Oithona similis. Sometimes, especially in the autunm, Metridia lucens, Paracalanus parvus, Temora longicornis and Acartia clausi may also play a certain part. - In volume C. finmarchicus is always dominant but at Skrova C. hyperboreus and Pareuchaeta norvegica may also be of some importance. 7. The plankton organisms have been divided into different groups according to their origin and appearance in the plankton, and the renewal of the stocks is discussed. Organisms indicating influx of Atlantic water are listed. 8. Length distributions of copepods are shown to be useful for the determination of the origin of the populations and also for the determination of the origin and degree of mixing of different water masses. 9. The production of zooplankton in Norwegian coast waters is discussed in relation to the production of phytoplankton, likewise the greater abundance of zooplankton in spring near the edge of the continental shelf and the possible origin of the plankton populations there. The importance of the statement by MARSHALL and ORR (1952), that individual females of Calanus finmarchicus may survive and produce eggs for more than two months, is stressed. 10. The biology of the different species. a. Calanus finmarchicus. At Sognesjøen and Ona maxima in stock occur in March, June-July and September, at Eggum and Skrova in April-May and August. Spawning periods at Sognesjøen and Ona in February-March, Nay-June, July-August, and September- October, in the Lofoten area in April-May, June, and July-September. Copepodite stage V had a minimum length in February-March, maximum length in April-July, and was again smaller during the autumn. The females showed a similar variation. At Skrova there was a very striking decrease in mean length of stage V from January to March each year. - At station "M" the length variations of stage V in the upper 100 m during the late summer and autumn of 1940 was very similar to that found at Eggum, and the possible relationship is discussed. b. Calanus hyperboreus was scarce at all stations except Skrova, where a moderate stock occurs in the deeper layers all the year. Spawning takes place in February. Stage V and the females vary very little in length distribution during the year. c. Pseudocalanus elongatus had maxima in stock in May-June and August-September, at Eggum also in October-November. Maxima in spawning at Sognesjøen and Ona occurred in March, May-June, August and October, at Eggum in March-April, May-June and September. The females showed a minimum mean length in January- March, maximum length in April, and decreased again in length from the end of April. d. Pseudocalanus minutus (P. gracilis G.O. Sars) was shown to be distinct from P. elongatus. Scarce at all stations, being confined mainly to the deeper layers. Spawning probably occurs in March-April. Stage V and the females were larger than P. elongatus from the same samples. The dominance of P. minutus at station "M" is confirmed and discussed. e. Microcalanus pusillus occurred in moderate numbers with 3 or more maxima in stock and 4-5 spawning periods a year. The females had maximum size at Ona in May and October, minimum size in February -March and August. At Eggum and Skrova there was maximum length in May only. The length variations at station "M" were similar to those at Ona. The relation length-temperature is discussed. f. Microcalanus pygmaeus is shown to differ from M. pusillus both in size and in appearance. It is scarce at Skrova, but present all the year, with one spawning period a year, in February. The individuals found in deep water at station "M" are larger than those at Skrova. g. Pareuchaeta norvegica. Found in moderate numbers in deep water at Skrova. Spawning takes place all the year, with maximum in December-January and June-August. Females are more than twice as numerous as the males, but in the copepodite stage IV-V the proportion was 1:1. The mean length of stage V decreased slightly from the spring to the autumn. 11. Metridia lucens is an important component of the summer plankton, with maxima in stock in June-October. Four to five spawning periods occur. Nauplii are most common in the upper 50 m, but as development proceeds, the copepodites seek deeper water, and of the adults only 5-9 % are found in the upper 50 m. The mean length of the females showed minimum in January-February, maximum in May, and decreased again gradually during the autumn. i. Clausocalanus arcuicornis is new to the Norwegian fauna, occurring sparsely from October to January. k. Oithona similis was sometimes very abundant, with main maxima in August-September. The other copepod species are of less importance numerically. 11. Other organisms. a. Cladocera were present from March to October, most abundant in June-September. b. Adult euphausiids were seldom taken. Eggs and larvae of euphausiids were numerous from March to June with maxima at Sognesjøen and Ona at the end of March (both Meganyctiphanes norvegica and Thysanoessa inermis), in Lofoten in April and May (mainly T. inermis). c. Chaetognaths were most common on the cleeper stations, especially at Skrova. Sagitta elegans and Eukrohnia hamata were the most commonly found species, but single S. setosa occurred at Sognesjøen and Ona. d. Copelata. Oikopleura dioica was most abundant at Sognesjøen and Ona, O. labradoriensis in the Lofoten area. O. vanhøffeni occurred in moderate numbers at Eggum, singly at Ona and Skrova. Fritillaria borealis acuta was moderately numerous, but scarce at Ona. e. Thaliacea. Salpa fusiformis was recorded in the autumn of 1950 at Ona, Eggum and station "M" in 1951 at, Sognesjøen and Ona. f. Limacina retroversa was numerous at all stations except Skrova from June-July to October. g. Herring larvae were numerous at Sognesjøen and Ona in March, cod eggs and larvae plentiful in the Lofoten area in April-May, especially in 1949. The other groups of organisms, amphipods and isopods, ostracods, gasteropod larvae, larvae of bottom invertebrates, coelenterates and others, were usually of minor importance numerically.