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Distributional and Seasonal Patterns of Ctenophores in Malangen, Northern Norway

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Distributional and Seasonal Patterns of Ctenophores in Malangen, Northern Norway MARINE ECOLOGY PROGRESS SERIES Vol. 140: 59-70.1996 Published September 12 Mar Ecol Prog Ser Distributional and seasonal patterns of ctenophores in Malangen, northern Norway Tone Falkenhaug* Norwegian College of Fishery Science, University of Tromse, N-9037 Tromse, Norway ABSTRACT: Annual cycles of abundance of the ctenophore species Bolinopsis ~nfundibulumand Beroe cucumls wel-e studied in the Malangen fjord, northern Norway, during 1992. The seasonal dynamics were characterized by alternating abundances of the 2 species A biomass increase of B infundibulum occurl-ed in April and May, I-eaching a peak of 2 to 7 m1 m-.' wet volume in the surface layers in sum- mer. The specles started to reproduce in the inner part of the fjord, and then propagated throughout the entire fjord system. Length frequency distributions indicated that B. infundibulum were reproducing continuously dunng the summer months. Biovolumes of B. infundibulum decreased in August, leaving a small number of individuals (<l~nd. per 5700 m" to overwinter in the water column. Small individu- als of B. cucurnls occurred in the surface layers in spring B~ovolumesof this species were low during summer, but increased in the fall, and a large overwinterlng population (2 to 6 m1 m') was present in the deep bas~nsof the fjord (below 200 m) from September to April. The calculated predatory impact of B. cucunlls on B. ~nfundibulunlIncreased during summer (from 6 to 501%)),and contr~butedto the autumn decrease in volume of R infundibulun~.The tvintel- population of B, cucurnls would be short of food ~f ctenophores were the only prey Poss~blemechanisms contnbutlng to the observed patterns are discussed. KEY WORDS: Ctenophora . Seasonality Zooplankton Prcdat~on Fjord Northern Norway INTRODUCTION winter (Mountford 1980). In order to understand the role of ctenophores in the pelagic food web, more Ctenophores have been observed to occur in quantitative information is needed on population strongly seasonal pulses in many locations (Nelson dynamics and seasonal distributions. 1925, Greve 1970, Baker 1973, Hirota 1974, Larson The lobate ctenophore Bolinopsis infundibulum is a 1986). High feeding and growth rates combined with common macroplanktonic organism of northern and high fecundity enable ctenophores to react quickly to Arctic seas (Bigelow 1926, Zelickman 1972, Lenz changes in the environment, and the annual biomass 1973). Because this species is an extremely delicate extremes often span several orders of magnitude. planktonic animal, information regarding its biology Ctenophores are considered to be important regulators and distribution is scarce. Like other lobates, the feed- of mesozooplankton in coastal marine waters (Kremer ing rate of B. infundibulum is high (Gamble 1974, 1979, Greve & Reiners 1980, 1988). However. in spite 1977), and closely related lobates are known to control of many reports on mass occurrences of ctenophores their prey populations (Reeve & Walter 1978). Species in northeastern Atlantic waters during summer, the of the genus Beroe feed primarily on other ctenophores amount of quantitative data on the vertical distribution (Swanberg 1974, Harbison et al. 1978) and may signif- and seasonal abundance is 1.imited and little is known icantly affect the population structure of B. infundibu- about the fate of ctenophore populations during the lum, and thus indirectly modify the population dynarn- ics of other zooplankton at lower trophic levels (Kamishlov et al. 1958, Kamishlov 1960). The interac- tion between Beroe cucumis and its ctenophore prey 0 Inter-Research 1996 Resale of full artlcle not pennitfed Mar Ecol Prog Ser 140- 59-70, 1996 Fig. 1 The Malangen fjord system with sampling stations. Stn 2 is situated 15 km NW of Stn 4 (indicated by arrow) was described as an 'ecological feedback system' by with shallow sills (10 and 30 m). A more detailed Greve (1971, 1981). description of the fjord is given in Falkenhaug et al. The ecology of ctenophores in northern Norwegian (1995). waters has not been investigated previously, and the Data on ctenophores, zooplankton and hydrography distribution of ctenophores is largely unknown within were collected once a month from February to Decem- this region. However, Bolinopsis infundibulum and ber in 1992. Stations (Fig. 1) were situated along an Beroe cucumis have been observed to occur in high axis extending from waters outside the fjord mouth to abundances in coastal waters off northern Norway the fjord head (Stns 2 to 23). Two more stations were during summer. The purpose of the present study was situated in the outlets of adjacent fjords (Stns 12 and to obtain quantitative information on the seasonal 26). Temperature and salinity were measured at each abundance and spatial distribution of ctenophores in a station with a Neil Brown CTD prof~ler.Methods and boreal marine ecosystem, and investigate the interac- results with respect to physical oceanography in the tion between 3 trophic levels. Here I present seasonal, fjord are presented in Falkenhaug et al. (1995) Sam- horizontal and vertical variations in occurrences of ples of ctenophores and other zooplankton were mesozooplankton, B. infundibulum and B, cucumis obtained from 5 to 7 depth strata at each station by a during 1992 in a North Norwegian fjord system, and 1 m2 MOCNESS with 180 pm mesh size nets (Wiebe et suggest possible mechanjsms contributing to the al. 1985) towed at 1 knot (Table 1). The duration of observed patterns. each tow was 30 to 40 min and volume of water filtered was 560 to 1250 m3 per tow. Stns 8 and 19 were sam- pled by both day (10:OO to 14:OO h local tune) and night MATERIALS AND METHODS (22:OO to 02:OO h local time) in order to detect changes in the vertical distributions of ctenophores and meso- Malangen (Fig. 1) is located 30 km to the south of zooplankton. Tromser, in northern Norway (69" 30' N, 3.8" 21' E).The Ctenophores were immediately removed from the fjord is 50 km long with an average width of 5 km and samples, and placed in a 1000 pm sieve for the removal a maximum depth of 400 m. It is separated from the of excess water. The total wet volume of both species in ocean to the northwest by a 200 m deep sill and is con- the sample was assessed when the animals had been nected to adjacent fjords through 2 narrow sounds poured into a measuring cylinder. Torn specimens and Falkenhaug: Distribut~onof ctenophores in a fjord 6 1 Table 1 Intervals of MOCNESS sampling in Malangen 1992 TEMPERATURE ( 'C)--- p--- . a or-ioo - "P >>g , 40 S , 8, POo l 'ooO---y Depth strata Stations - 80 Oo \ . g 20 ------ \ .-. , . -- - OoO (m) 2 4 8 12 14 19 23 26 l 0-20 XXXXXXXX 20-50 xxxxxxxx 50-100 xxxxxxxx 100-150 xxxxxxx 150-200 xxxxxxx J FMAMJJASOND 200-250 xxxxxx SALINITY (960) b 0 :-.-- i -- 250-bottom X X X 33 50 4 0 33 50 -3 5.c;n fragments of ctenophore body tissue were also 200 k l included in these volume measurements. Following 240 the removal of all ctenophores from the sample, the 280 , 320-rL l remaining zooplankton sample was split with a 1 I,7-1 J Motoda splitter device (Motoda 1959).One half of the FMAMJJASOND sample was frozen in liquid nitrogen for determina- tions of ash free dry weight (AFDW) and the other was preserved in 4% formalin solution for later species identification and enumeration. In this paper, I have only presented the biomass data. Frozen samples were thawed in filtered seawater at room temperature and shrimps, fish larvae, euphausilds and chaetognaths were sorted out (data not presented here).The remain- 0 2 I I I l I I I ing zooplankton (mainly copepods) were dried to con- FMAMJJASOND stant weight at 60°C and ashed at 500°C to obtain dry TIME (Months) weight and AFDW. Fig. 2. Seasonal variation in (a)temperature ("C)and (b)salin- For a more careful sampling of ctenophores, vertical ity at Stn 8 in 1992. (c) Stability in the upper 20 m at dif- hauls with a WP3-net (1 m opening diameter, 1000 pm ferent stations in Malangen in 1992 mesh size; UNESCO 1968) fitted with a large cod-end (20 1) were made at each station. The nets were hauled at a speed of 0.5 m S-' in 2 or 3 depth strata with the use deeper layers, and temperatures increased towards the of a closing mechanism. Body lengths and the wet vol- bottom, with a small annual range (5 to 7°C) below ume of undamaged ctenophores in the net were mea- 120 m. Heating of the surface layer occurred during sured immediately after collection. Body sizes, defined the summer months, but temperatures never exceeded as gut length of Bolinopsis infundlbulum and total 10°C. A halocline at a depth of 10 m occurred during body length of Beroe cucumis, were measured to the summer (May to August) due to freshwater run-off nearest mm with a 1 mm grid under a petri dish. Gut from the river Mdlselv at the fjord head. The stratifica- contents (as present or non-present) of undamaged tion started earlier, disappeared later, and was more individuals were noted immediately after collection. pronounced in the inner part of the fjord system Due to cod-end feeding and losses of gut content when (Fig. 2c). disturbed (pers. obs.), the gut content analyses were only used as indications of seasonal variations in the feeding activity. Non-gelatinous zooplankton Biomasses of mesozooplankton were low at all sta- RESULTS tions during the winter, increased during the summer season and reached maximum values (25 to 30 g Hydrography AFDW m-') in late summer and fall (Table 2).
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