The Production of Zooplankton in a Landlocked Fjord - The Nordåsvatn near Bergen, in 1941-42 (With special reference to the copepods)
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1. In the Nordåsvatn, a land-locked water near Bergen, weekly observations were carried out from May to November 1941 and from April to July 1942 for the main purpose of studying the production of zooplankton. Investigations were made at 3 main stations, 2 in the inner part, 1 in the outer part of the Nordåsvatn, comprising hydrographic-chemical observations, water samples for phytoplankton and samples of zooplankton, collected in vertical hauls with a closing net. In 1941 the hauls in the inner fjord were mostly made in 2 steps from the border of the H2S layer to the surface, in the outer fjord in 3 steps from bottom to surface. In 1942 the hauls were made in 3-4 steps from bottom to surface in all parts of the fjord. 2. The plankton material was immediately fixed in formalin. Definite fractions of the samples were taken out by Lea's Plankton Divider and counted. 3. A. The topography of the Nordåsvatn and its hydrographic conditions 1941-42 are dealt with. Investigations by Gaarder (1916) have shown that the Nordåsvatn is a landlocked water and can be divided into an outer and an inner fjord, separated by a threshold in 12 m depth at Bønes. In the inner fjord the water layer below 15 m may be stagnant for years, and, as a rule, contains H2S. The renewal of the bottom water takes place irregularly, chiefly in winter and early spring, in years with little precipitation. In the outer fjord the water is renewed regularly once a year. B. The surface layer is well aerated, with great changes in temperature and salinity during the year. The upper 1 m generally contains brackish water. Below 1 m the salinity commonly is higher than 27 ‰. C. The hydrographic-chemical investigations in 1941-1942 include observations on temperature, salinity, and of oxygen content. D. The picture given by Gaarder (1916) of the hydrographic conditions, has not been essentially altered. A renewal of the bottom water of the outer fjord took place in the early spring of 1941 and could to a minor degree be perceived in the inner fjord. In the early spring of 1942 a very extensive renewal of the bottom water took place in all parts of the Nordåsvatn. E. The oxygen in the deeper water layers of the outer fjord was consumed more rapidly in the summer of 1942 than that of 1941. 4. The population of zooplankton is at first treated as a whole. The number of organisms was low in May-June 1941, increased in July, and had maxima in July-August and in October in the inner fjord, at the end of August in the outer fjord, numbers there being twice as high as in the inner part. In 1942 numbers were low in April, increased in May, and had maxima in May, June, July, in the outer fjord probably in September. 5. The distribution of plankton organisms in relation to the hydrographic conditions is discussed. 6. The copepods form the greater part of the zooplankton, with the exception of June-August and October 1941, when the cladocera and benthonic evertebrate larvæ were very numerous in the inner fjord. In 1941 the percentage of copepods was higher in the outer fjord than in the inner part. In 1942 the case was the other way. 7. 16 species of Calanoida, 5 of Cyclopoida, and 14 of Harpacticoida have been found. The bulk of the copepod stock is composed of 5 calanoids and 2 cyclopoids, namely, Calanus finmarchicus (mean percentage 1941: 0.4, 1942: 8.4 %), Pseudocalanus minutus (13.3-7.6 %), Paracalanus parvus (May -September 1941 3.1 %, May-December 1941: 11.3 %, May- September 1942: 1.8 %), Temora longicornis (4.5-14.2 %), Acartia spp. (6.7-4.5 %), Oithona spp. (60.1-56.2 %), Oncæa spp. (1.8-6.1 %). 8. The copepod fauna of the Nordåsvatn is very poor as compared with that of the fjords outside. 9. Calanus finmarchicus was scarce in 1941, with maxima in August and October. In April 1942 it was numerous, numbers decreasing steadily to September. In 1941 some spawning occurred between July and October, mainly in August-September. A new stock introduced from the fjord outside the Nordåsvatn in April 1942 mainly consisted of nauplii and early copepodite stages. A new spawning took place in the outer fjord in September. In 1941 the bulk of the Calanus in the inner fjord frequented the water layer between 12 and 15 m, in the outer fjord the upper 30 m, copepodites of stage V kept below 30 m. In 1942 the vertical distribution varied a little, but stage V, as a rule, frequented the deeper water layers. 10. Paracalanus parvus had always its greatest stock at Paradisholmen. In 1941 there were maximum numbers in August and October. In 1942 numbers were comparatively high in April, decreased to July and were again higher in September. Propagation takes place most part of the year. Maximum occurrence in the surface and intermediate water layers, some also in deeper waters. 11. Pseudocalanus minutus. Maximum numbers in the inner fjord: August and October 1941, May and August 1942. In the outer fjord: September 1941, June and September 1942. Propagation takes place most part of the year, maximum spawning in April, June-July and September-October. Vertical distribution as that of paracalanus, on an average however, a higher percentage in the deeper water layers. 12. Centropages hamatus was always scarce. In 1941 maximum stock in July-August, in 1942 in June. Maximum spawning in May, July and September. 13. Temora longicornis was moderately numerous in July-September 1941, maximum number in August. In 1942 it was more abundant, with maximum in June. Maximum of spawning in May, July-August and September-October 1941, and May 1942. With some exceptions, most frequent between 15 and 5 m in the inner fjord, 20 and 10 m in the outer part. 14. Acartia clausi. Maximum numbers in August and October 1941, very scarce April-July 1942. Vertical distribution as for Temora. 15. Acartia longiremis. More common than the previous species. Maximum numbers in May and July both years, the May maximum lacking in the inner fjord in 1942. More restricted to the surface layers than Temora. In 1942 maximum occurrence in the upper 5 m in the inner fjord. 16. Oithona helgolandica is the most common copepod of the Nordåsvatn. The inner fjord had maximum numbers in July-August 1941 and May and July 1942, the outer fjord maxima in September 1941, June and probably September 1942. Maximum of propagation in August. Vertical distribution in 1941 mainly in the intermediate and deeper water layers, in 1942 in all depths, but maximum occurrence in the upper 15 m. 17. Oithona spinirostris. Scarce, in the deeper water layers, in 1941 confined to the outer fjord. 18. Oithona nana. Moderately numerous, maximum numbers in July-August and October both years. Keeps in the upper 10-15 m, in the inner fjord in the upper 5 m. 19. Oncæa borealis. Scarce, in 1941 only caught in the outer fjord, below 30 m. 20. Oncæa subtilis. New to the Norwegian fauna. In 1941 maximum numbers in the inner fjord in August and October, in the outer fjord a small stock and irregular variation in numbers. In 1942 the stocks were larger in all parts of the Nordåsvatn, with maxima in April, June-July, and September or later. Propagation the greater part of the year, in 1941 most intensively from July to October, in 1942 maximum of propagation in March and July in the inner fjord, in April and July-August in the outer part. Vertical distribution: In summer in the upper 5-10 m, in August-September a more even distribution in all water layers, from October to April in deeper water. 21. Common to all copepods: Spawning and maximum of numbers as a rule occur earlier in the inner fjord than in the outer part. 22. Evadne nordmanni. Scarce November-April, numerous May-October. In 1941 maxima in July-August and October, in 1942 in June. Numbers were always highest in the inner fjord. 23. Podon spp. a) Podon polyphemoides. In 1941 present May-October, numerous June-August. In 1942 scarce June-September. b) P. intermedius. Scarce July-October 1941, lacking in 1942. c) P. leuckarti. In moderate numbers June-October 1941 and April-September 1942. Maxima in May and June both years. 24. Larvæ of different decapods occurred in summer. Of great interest is the capture of larvæ of Porcellana platycheles Pennant from August 15 to October 10, 1941. 25. Euphausiids. Adults of Thysanoessa inermis were caught singly in the outer fjord from July 18. to September 12. 1941, larvæ were caught during all the time of investigation, in 1942 also in the inner fjord. Nauplii occurred June 6. 1941 and April 10.-21. 1942. One single calyptopis larva of Meganyctiphanes norvegica was taken in the outer fjord May 7. 1942. 26. Sagitta elegans in 1941 was confined to the outer fjord. In 1942 it was taken at all stations. Maximum of spawning occurred in August and October 1941, in May, June and possibly August 1942. Adults and eggs were mostly caught in the deeper water layers, newly-hetched larvæ and younger individuals above 15 m. - Of Sagitta setosa a few specimens were caught. 27. Oikopleura dicoica was numerous June-December 1941, with maxima in July, August, and October. In 1942 it was scarce, most common in May. 28. Fritillaria borealis occurred abundantly. In 1941 with maxima in April, July, and October, in 1942 maxima (March), April, and May. 29. Larvae of different bottom evertebrates, mussels, polychætes, cirripedians and others, were fairly numerous, especially in the autumn. 30. 20 species of coelenterata were identified with more or less certainty. In 1941 Melicertum octocostatum was the most common medusa, in 1942 Sarsia tubulosa. Of the etenophora, Pleurobrachia pileus was fairly abundant, with mass occurrences of eggs and larvæ in June, August, and September 1941. Larvæ of Cerianthus lloydi were found in 1942. 31. Of the turbellaria, Alaurina composita and larvæ of Notoplana almata, or nearly allied species, were caught. 32. Fish eggs and larvæ, chiefly of Clupea sprattus, some also of Pleuronectes flesus, P. limanda, Gadus callarias and G. merlangus, were taken in May-August 1941 and April-July 1942.
SerieFiskeridirektoratets skrifter, Serie Havundersøkelser
vol 7 no 7