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 ( KEYWORDS: 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 TEMPERATURE( 'C) Table 1 Intervals of MOCNESS sampling in Malangen 1992 . --- a or-ioo - p--- >>g , 40 S , 8, POo"P l 'ooO---y Depth strata Stations - 80 Oo \ g ,20 ------. \ . .-. (m) 2 4 8 12 14 19 23 26 -- - OoO l 0-20 XXXXXXXX 20-50 xxxxxxxx 50-100 xxxxxxxx 100-150 xxxxxxx 150-200 xxxxxxx J FMAMJJASOND 200-250 xxxxxx SALINITY (960) b 0 250-bottom X X X :-.-- 33 50 4 0 i 33 50 -- -3 5.c;n fragments of ctenophore body tissue were also k 200 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 FMAMJJASOND Motoda splitter device (Motoda 1959). One half of the 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). No dif- ferences in vertical distribution of mesozooplankton The seasonal variation of hydrographic conditions at biomass between day and night could be detected at Stn 8 illustrates the influence of the coastal waters on Stns 8 and 19 (data not shown). However, the vertical this fjord (Fig. 2). Salinities were always high in the distributions of the biomass varied with season. High Mar Ecol Prog Ser 140: 59-70, 1996 Table 2. Mesozooplankton biomass (g AFDW m-') in Malangen from February to December 1992. The proporton of blomass (%) located above 50 m at each station is glvcn m parentheses Month Station 2 4 8 14 19 23 Feb 0.02 (0.0) 0.04 (0.0) 2.6 (15.1) 0.9 (13.4) 3.4 (14.3) - Mar 0.4 (9.9) 0.8 (10.7) 0.7 (18.2) 0.05 (29.8) 0.4 (20.0) 0.4 (42.7) AP~ 1.5 (26.0) 1.5 (42.6) 0.9 (32.4) 1.4 (51.3) 0.8 (64.0) 1.4 (64.0) May - - 10.5 (22.2) - - - Jun 28.1 (5.4) 7.5 (15.9) 9.9 (29.7) 4.9 (20.0) 13.3 (7.3) 21.4 (11.1) J ul 61.8 (4.0) 47.2 (14.7) 25.8 (7.9) 19.6 (2.3) 29.1 (10.2) 12.0 (5.1) *W 19.4 (5.8) 15.9 (3.4) 15.7 (4.1) 32.1 (1.4) 20.5 (1.0) 32.1 (2.5) S~P 4.5 (10.5) 21.6 (5.2) 15.7 (4.8) 12.5 (0.9) 15.2 (1.3) 17.6 (1.2) Oct 6.6 (10.6) 15.7 (4.0) 20.3 (1.1) 8.4 (2.6) 32.2 (0.4) 26.4 (0.9) Dec 0.4 (3.4) 4.8 (0.6) 5.8 (2.0) 1.6 (5.8) 6.0 (1.0) 2.9 (3.2) Distance in l000 m Distance in 1000 m Distance In l000 m 0 TO 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 B0 Mav 0 - 100 E 200 a 2 300 400 11) AU~JUS~I I , * ,, 2 4 8 14 19 23 Slat~onnumber Station number c0 05 rnl m-3 0.05-05 rnl m.3 0.5-1 rnl rn.3 1-3rnl rn.3 3-4m1 rn.3 Flg 3. Bolinops~sinfundibulum. Vertical distribution (m1 wet >4 m1 m.3 volume m-') along the sampling axls in Malangen from May 2 4 8 14 19 23 to December 1992 Stat~onnumber proportions of the biomass were located in the upper was sampled only once with the WP3, and due to the 50 m in spring and early summer (20 to 60% in April to patchy distribution of ctenophores, catches in the WP3 June). During fall and winter, 90 to 100% of the bio- were variable and could not be treated statistically. For ma.ss was found below 50 m. this reason, net tow samples were only used for ana- lyzing size distribut~onsof ctenophores. Much of the ctenophore biomass was lost from the MOCNESS sam- Ctenophores ples due to fragmentation of animals. This was espe- cially the case for Bolinopsis infundibulum,which is Ctenophores caught in the WP3 were in good condi- more fragile than Beroe cucumis. On average, the esti- tion. However, the volume of water filtered at each mated biovolumes of B. infundibulumper m2ba.sed on vertical tow was low (20 to 50 m3),and the vertical res- WP3 samples were 4 times larger than the estimates olution was poor. The water column at each station from MOCNESS samples (WP3:MOCNESS ratio 4 -+ Falkenhaug: Distribution of ctenophores in a fjord D~stancein 1000 m 0 10 20 30 40 50 60 70 1 0 - loo E 200 ra E 300 400 , S, I0 2 4 14 19 23 2 4 8 14 19 23 Stat~onnumber Stat~onnumber Fig. 4. Beroe cucumis.Vertical distribution (m1 wet volume m-3) along the sampling axis in Malangen from February to l I1 2 4 8 14 19 23 December l992 Stallon number 2.9). The ratio was however not significantly different as the season proceeded (Fig. 3). The biovolumes of B. from 1 (p > 0.05),due to the variability in WP3 catches. infundibulum reached maximum values in June, and The losses are considered to be variable with depth, from August on, they decreased, reaching very low due to differences between nets in tow lengths and levels in December B. infundibulum was almost ab- clogging. However, th.e distribution of B. infundibulum sent during the winter, while large volumes of Beroe was restricted to the upper 50 m (which was confirmed cucumis (>4m1 m-" Fig. 4) were present in the deeper by the WP3 net sampling) and the duration of the sur- basins of the fjord (below 200 m) during winter and fall face sampling (50 to 0 m) was less than 5 min. Thus, the (February to Apr~land August to December). During biovolumes of B. infundibulumare underestimates, but the summer period (May to July), volumes of B. are assumed to mirror variations in the seasonal, hori- cucumis were lower, and the major part of its popula- zontal and vertical distributions for this species. tion was situated above 50 m, usually within or ju.st Very few Bolinopsis infundibulum occurred in the below the layer of its prey, B. infundibulum.No differ- MOCNESS samples from February to April (less than ences were found in the vertical distribution of 0.004 m1 m-3, not shown in figures). An increase in bio- ctenophores between day and night. volumes of B. infundibulum occurred from April to Three patterns for seasonal variations of ctenophore May, when high concentrations (>4 m1 m-3) were biovolume and zooplankton biomass are shown in found in the upper 20 m layer (Fig. 3).The ctenophores Fig. 5: for Stn 2 (outside the sill), Stn 8 (inside the sill) appeared earlier in the inner part of the fjord, and and Stn 23 (fjord head). At all 3 stations, Bolinopsis maximum volumes were found further out in the fjord infundibulum appeared during the period when bio- 64 Mar Ecol Prog Ser 140: 59-70, 1996 Station 2 400 WINTER &4inopsis mlunbrtulwn Volume (rnl rn4) m Beme cucumrs mg AFWD m-3 0 50 100 150 0 2 4 0 I I - Mesozooplankton N Station 8 800 2 E 400 , rm 0a, 300 - - 400 28 30 32 34 Station 23 Salinity (Xo) 400 I 6 2 SUMMER mg AFWD m-3 Volume (rnl m-3) 100 TIME (Month) Fig. 5. Seasonal variations in Bolinops~s infundibulum (m1 m-'], Beroe cucumis (m1 m-'), and mesozooplankton biomass (mg AFDW m-' in 0-50 m) outside the s11l (Stn 21, inside the sill (Stn 8), and fjord head (Stn 23) 0 Bolinopsis masses of mesozooplan.kton above 50 m were high t salinity (May to August). The peak in B. infundlbulurn appeared somewhat earlier at the inner station (in 400- June at Stn 8) compared to the outer station (in July at Salinity (%,) Stn 2). At the outer station (Stn 2), Beroe cucumis was absent during the winter, but appeared in June, and a Fig. 6. Vertlcal distributions of zooplankton and ctenophores in Malangen during (a] winter and (b) summer. Left panel: peak was reached after the decrease of B. infundibu- Mesozooplankton biomass (mg AFDW m-3).Right panel: Bo- lurn, in August. Within the fjord, B. cucumis was pre- linopsis infundibulum, Beroe cucumis,- and density (a,) sent most of the year At the deeper Stn 8, volumes of B. cucumis were high during the winter (>790m1 m-2) but low in summer (<45m1 m-2) when B. infundibulum was found below 150 m. Bolinopsis infundibulum was was abundant. At the fjord head (Stn 23) B. cucumis present in very low numbers (not visible in the figure), was present all yea.1, at fairly constant volumes (45 to and high volumes of Beroe cucurnis (6 m1 m-" were 150 m1 m-2), and volumes of B. infundibulum were found in deeper waters, below 250 m (Fig. 6a). The always low (145 m1 m-2). summer situation (Fig 6b) was characterized by a The vertical distributions of ctenophores and their halocline at 20 meters, and a bimodal vertical distribu- prey was found to have 2 basic patterns, illustrated by tion of mesozooplankton biomass. Large volumes of B. the fall/winter period (Fig. 6a) and the late spring/early infundibulum (6.5 m1 m-3) occurred above 20 m. B. summer period (Fig. 6b) at Stn 8. In the fall/winter cucumis was found within or just below the layer of its period, the stability of water masses was low, and the prey, increasing in volume from May ( LENGTH (mm) LENGTH(mm) LENGTH(mm) Fig. 7 Bolinopsis infundibulum.Length frequency distributions of specimens captured in WP-3 net, May to October 1992. n = number of specimens measured Length frequency distributions the seed population of ctenophores is an unresolved question in many areas, but the species is often as- The coarse mesh size used in this study excluded the sumed to overwinter in the water column in small smallest and youngest ctenophore offspring from the numbers (Bigelow 1926, Fraser 1970). In the present analysis ( :L;E;m1 0 n=87 n46 -8 l 6 0 I X X3 M 6S 80 85 110 12. l* iU '70 ~~~~~~~c n=115 0 0 0 5 35 65 95 125 155 5 35 65 95 125 1% 20 50 80 If0 1Ul 170 20 50 80 110 140 170 Decembe LENGTH (mm) LENGTH (mm) n=lo Fig. 8. Beroe cucumis.Length frequency distributions of cap- 5 20 35 50 65 80 95 110125 140 1% 170 tured in WP-3net, February to December 1992. n = number of specimens measured ;mLENGTH (mm) lower during the winter (Stn 2, Figs. 4 & 5). The most tion of Bolinopsisinfundibulum in May (Fig. 7). Strati- dense samples Iof B. cucumis during Mrlnter were fication of water masses started earlier, and was most obtained when the MOCNESS incidentally touched pronounced in the inner part of the fjord (Fig. 2) where the bottom. The fate of Beroe populations during the the ctenophores start to reproduce in the spring. The winter is scarcely described in the literature, since sampling frequency in this study was too low to resolve populations located close to the bottom may be difficult the dynamics of the increase in biomass of B. infundi- to catch with traditional plankton sampling gear. Over- bulum in spring However, the biovolume increased wintering in deeper water layers has been reported for from zero to 90 m.1 m-' in 7 wk (Apnl and May). During B. ovata and the cydippid Mertensia ovum in the the next 2 wk (May 27 to June 10) the biomass of R. Canadian Arctic (Siferd & Conover 1992), while B. infundibulumincreased by more than 200% at some cucumisis thought to immigrate into the Bay of Fundy, stations. Its self-fertilization potential, extremely high Canada (Milne & Corey 1986). In the Canadian Arctic, fecundity, and rapid growth potential explain how the peaks in the abundance of B. ovata and M. ovum coin- population can increase at such high rates within a few cided, which is unusual for predator-prey relation- weeks. ships. Similarly, a closer match between predator and Overwintering adu.lts of Beroe cucumis probably prey was observed at the innermost station in Malan- spawned when their prey Bolinopsis infundibulum gen (Stn 23, Fig. 5). At this station, the biomass of B. occurred in April and May, as was indicated by the cucumiswas constant most of the year, and no distinct change in size frequency distributions (Fig. 8). Large peak in the biomass of B. infundibulumwas observed. overwintering individuals disappeared from the fjord A large overwintering population will ensure early in May, as illustrated by a decrease in biomass of B. spawning and a close match between predator and cucumisat Stn 8 (Fig. 5) and a decrease in mean length prey. in May (Fig. 8).The biomass of B. cucumisincreased in In early summer, rapid changes in biovolumes of Malangen in July, and its abundance peaked after that ctenophores were observed, which is characteristic of of B, infundibulum.This typical time lag between gelatinous zooplankton populations (Kremer & N~xon Beroe and its prey also has been described by Greve & 1976, van der Veer & Sadee 1984, Larson 1986). Stabi- Reiners (1988). lization of water masses, increased water temperatures The zooplankton in Malangen is highly influenced and abundant food supplies triggered rapid reproduc- by advective processes (Falkenhaug et al. 1995). In Falkenhaug: Distribution of ctenophores in a fjord 67 Table 3. Daily advective transport of ctenophores and zooplankton biomass Vertical distribution of ctenophores and in May and October 1992, expressed as "h of content ~nsidethe fjord A neg- copepods ative 'Net' transport value indicates that net transport is directed out of the fjord Calculations are based on current data in Falkenhaug et a1 (1995) In the Malangen area, the sun stays be- low the horizon from 28 November until May October 14 January, and above the horizon from In Out Net In Out Net 19 May until 26 July. Bolinopsis infund- Mesozooplankton 5.1 4.2 0.9 0.4 1.4 -1.0 ibulumoccurred during the midnight sun Ctenophora period with little diel variation. During Bolinopsisinfundibulurn 0.3 58 -58 000 this period, ctenophores and a large pro- Beroe crlcun~is <0.1 55 -55 0 <0.1 The calculated metabolic demand of B. infundibulum metabolic demand of B. cucumisin Malangen was cal- is equivalent to 0 4 to 0.9% of the standing stock of culated (Table 4) on the assumption that the carbon- zooplankton carbon in the upper 20 m in the penod specific respiration was 8% per day (Kremer et al. June to August. Applying a clearance rate for the 1986a).The minimum impact of B. cucumison Bolinop- closely related Bolinopsis vitrea (14.5 1 mg-' C d-l; Kre- sis infundibulumwas low at the beginning of the sea- mer et al. 1986b), the ingestion by B. infundibulumin son (5.9%),but increased to 50% in July. Predation by June to August equals 2 to 7 % of the zooplankton car- B. cucmis thus contributed to the decrease in B. bon in the upper 20 m, and 0.4 to 1 "/, of the zooplank- infundibulum in late summer. Amorphous material, ton carbon in the total fjord. These values are low com- presumably ctenophores, was found in cod stomachs pared with other studies in which ctenophores were during the winter in Malangen and during the summer found to affect their prey populations (Bishop 1967, in other northern Norwegian fjords (T. Pedersen pers. Deason & Smayda 1982, W~lliams & Collins 1985). comm.). Cod, which has been reported to feed on However, due to loss during net sampling, the calcu- ctenophores in arctic waters (Boldovsky 1944, Hansen lated ingestion by B. infundibulum in this study is 1949), may thus be a significant predator on both B. underestimated. Miller & Daan (1989) found no signif- infundibulumand B. cucumis in the study area. The icant impact of Pleurobrachia pileus on copepods off role of fish as predators on gelatinous zooplankton has the Dutch coast. Nevertheless, models have shown that probably been underestimated, and more quantitative predatory control can occur even at moderate con- work is needed on this interaction (Arai 1988). The sumption rates (Davis 1984). However, in an advective scyphozodn Cyanea cdpillatd, which is known to feed system, planktonic predators are not a part of the usual on ctenophores (Purcell 1991, Siferd & Conover 19921, predator-prey relationship as described by the Lotka- peaked In the fall (September to October, pers, obs.) Volterra equations (Aksnes et al. 1989). No significant after the decrease in biomass of B. infundibulum,and correlation between volumes of B. infundibulumand might not be an important ctenophore predator in this mesozooplankton biomass (p > 0.05) was found. The area. The fragile B. infilndibulumis sensitive to strong advective exchange of mesozooplankton biomass in water movements. Currents and winds were stronger Malangen was found to be 9% of the mesozooplankton in the fall (Falkenhaug et al. 1995),and the decrease in standing stock per day in May and June (Table 3). water stability from July onward (Fig. 3) coincided which exceeded the calculated predatory impact by B. with the decrease in biovolumes of B. infundibulum infundibulum(1.3 %, Table 2) in this period. The sam- (Fig. 6). The fall decline in the populat~on of B. pling volumes used here were large in order to over- infundjbulumwas probably caused by the combina- come the patchy nature of the ctenophores. On such a tion of decreased growth, due to low prey abundance, scale however, biological factors such as predation and increased mortality, due to predation and water may be overriden by physical factors. turbulence. The factor most often cited as affecting the seasonal The fall and winter biomass of Beroe cucumis was abundance of gelatinous predators is prey abundance high, in spite of the absence of Bolinopsis infundibu- (Greve 1971, Kremer 1976, Deason & Smayda 1982, lum (Figs. 4 & 5). My data suggest that B. cucumismay Larson 1986). The decrease in the population of not be totally dependant on the availability of B. Bolinopsis infundibulumcoincided with the seasonal infundibulum.The population of B. cucumis would decrease in mesozooplankton biomass above 50 m lack adequate amounts of prey during most of the year (e.g. 70% decrease at Stn 8 from June to September; (September to April) if the metabolic demand (8%) Table 2, Fig. 5). The total mesozooplankton biomass in remained constant and if B. infundibulumwere the the fjord continued to increase until late fall, but only a only food source. Pleurobrachia pileus has only been small proportion remained in the surface layer all year observed occasionally in this area, and could not sup- However the biomass found within the layer of B. port the large winter population of B. cucumis. There infundibulum(above 50 m) in late fall and winter, are copepods, Cyanea capillata, Aurelia aurita and a would not be sufficient to substain a large population number of hydromedusae available in Malangen on of B. infundibulum.The proportion of B. infundibulum which B. cucumis could potentially predate However, caught with empty guts increased from 63% in May I was unable to make B. cucumis feed on crustacean and June to 80% in July and August (author's unpubl. prey, A. aurita, or Rathkea octopunktatain the labora- data). In September no individuals were found with tory, and no prey items were found in guts of B. prey in their guts, which indicates food limitation in the cucumis during the winter Ctenophores are able to fall. undergo periods of food shortage or absence by utiliz- Species of Beroe are often associated with the ing their own body tissue for their metabolic require- decline of populations of other ctenophores (Greve ments and so gradually shrink in size (Kamshilov 1960, 1971, Hirota 1974, Swanberg & Bdmstedt 1991). The Kremer 19761. However, the length frequency distribu- Falkenhaug: Distribution of ctenophores in d fjord 69 - P- tions (Fig. 8) showed no decrease in size of B. cucumis Falkenhaug T, Nordby E, Svendsen H, Tande K (1995) Impact during fall and winter. On the contrary, mean size of advective processes on displacement of zooplankton increased from 29 mm in August to 70 mm in Decem- biomass in a North Norwegian fjord system. A comparison between spring and autumn. In: Skoldal HR, Hopkins ber. The nutritional source of B. cucumis during the CCE, Erikstad KE, Leinaas HP (eds) Ecology of fjords and winter remains unidentified. coastal waters. Proceedings of the 3lare Nor Symposium In summary, the seasonal dynamlcs of Bolinopsis on the Ecology of Fjord and Coastal \\'dters, Trornsa, Nor- infundibulumand Beroe cucumis were characterized way, 5-9 December 1994 Elsevier, Amsterdam, p 195-217 by alternating abundances of the 2 ctenophore species. Frank KT (1986) Ecolog~calsignif~cance of the ctenophore Large volumes of B. cucmis were present in the fjord Pleurobrachia pileus off southwestern Nova Scotia. Can J during the winter months when B. infundibulumwas Fish Aquat Sci 43:211-222 absent. A time lag in the reproductive response of B. Fraser JH (1970)The ecology of the ctenophore Pleurobrachia cucurnis on the appearance of its prey made a 'win- pileus in Scottish waters. J Cons int Explor Mer 33: 149-168 dow' in time of decreased predation risk (May to July) Gamble JC (1974) Preliminary observations on the feeding of within which the population of B. infundibulumwas Bohnopsis infundibulum. Comm Meet Int Coun Explor able to reproduce and reach high abundances. Sea ICES CM, 1974/L:14, Plankton Comm 8 p Gamble JC (1977) Populat~onstructure, feeding behav~or, Acknowledgements. I am grateful to the captain and crew of and fecundity of the lobate ctenophore, Bolinopsis ~nf~in- RV 'Johan Ruud' for their cooperation and assistance. I thank dlbulurn. Comm Meet Int Coun Explor Sea ICES CM J. T. Eilertsen. K. Lydersen and U. Normann for technical sup- 1977/L:16 port during the cruises, and A. Arentsen. G. A. Hansen, E. Greve W (1970) Cultivation experiments on North Sea Nordby, S. Jsrgensen, G. Ottesen and U. Zeller for help dur- ctenophores. Helgolander Wiss Meeresunters 20:304-317 ing sampling. Figures were drawn by Frsydis Strand. 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