Aggregation of the Arctic Copepod Calanus Hyperboreus Over the Ocean ﬂoor of the Greenland Sea

ARTICLE IN PRESS

Deep-Sea Research I 53 (2006) 310–320 www.elsevier.com/locate/dsr

Aggregation of the Arctic copepod Calanus hyperboreus over the ocean ﬂoor of the Greenland Sea

H.J. Hirchea,Ã, S. Muyakshinb, M. Klagesa, H. Auelc

aAlfred Wegener Institute for Polar and Marine Research, Kolumbusstr. 1, D-27568 Bremerhaven, Germany bInstitute for Applied Physics of the Russian Academy of Sciences, Uljanov Str. 46, 603950 Nizhny Novgorod, Russia cMarine Zoology, University of Bremen, D-28334 Bremen, Germany

Received 4 January 2005; received in revised form 26 July 2005; accepted 12 August 2005 Available online 20 December 2005

Abstract

According to combined observations from vertical plankton tows, dredging with epibenthic nets 1 m above the ocean floor, video recordings and acoustic data from a scanning sonar obtained during descent and during deployment on the ocean floor, the calanoid copepod Calanus hyperboreus was aggregated in high concentrations near the ocean floor of the Greenland Sea between 2300 and 2500 m during late July and August. Concentrations were highest very close to the ocean floor and decreased rapidly further upward. These nearly mono-specific aggregations were apparently drifting in cloud-like formations with a horizontal extension of ca. 270 m with the near-bottom currents. Maximum abundances observed were up to 2 orders of magnitude higher than in the water column. The biomass in the bottom 20 m layer was around 18% of the biomass in the rest of the water column. Stage composition, reduced metabolic rates and insensibility to mechanical stimuli indicate that these C. hyperboreus were representing the resting population. The fact that high concentrations were observed during deployments lasting 41 d and in 3 years suggests that aggregation near the ocean floor is a regular, rather than an extraordinary, pattern in the life history of C. hyperboreus in the Greenland Sea, but there is need for comparison with other seas and eventually other Calanus species. r 2005 Elsevier Ltd. All rights reserved.

Keywords: Greenland Sea; Aggregations; Diapause; Calanus hyperboreus

1. Introduction the benthic boundary layer (BBL) is often biologi- cally rich; in comparison with the water above, The zooplankton of the near-bottom zone of the zooplankton biomass exhibits an increase in the oceans is difficult to study as it is time-consuming BBL (Wishner, 1980). Copepods were the dominant and requires modifications to the sampling gear in taxa in the BBL in the Panama Basin as well as in order to quantify the distance to the ocean floor other BBL studies of deep sea plankton from a wide (e.g. Christiansen et al., 1999). Therefore, few variety of areas (Smith, 1982; Wishner, 1980). The studies have been conducted. They showed that inhabitants of the near bottom layer were divided into two groups by Vereshchaka (1995), those living ÃCorresponding author. accidentally in the vicinity of the ocean floor and E-mail address: [email protected] (H.J. Hirche). those constantly living there. According to Wishner

(1980) benthopelagic plankton biomasses decrease et al., 1991; Wishner et al., 1988), and in the deep exponentially with depth. At 1000 m, the biomass is waters of shelf basins (Sameoto and Herman, 1990; 1% that of the surface zooplankton, at 5000 m Herman et al., 1991). However, large concentrations about 0.1%. of the genus Calanus have to our knowledge never Video and still cameras mounted on remotely been reported close to the floor of the deep ocean. operated vehicles (ROV) have brought unexpected While Auel et al. (2003) used material from the encounters of organisms usually found in the upper EBS to conduct physiological measurements, in this water layers. Thus Gutt and Siegel (1994) found study we describe the abundance of C. hyperboreus swarms of the Antarctic krill Euphausia superba near the ocean floor and compare it with the water near the ocean floor at 480 m depth. During a study column using a combination of plankton net of deep-sea scavengers in the Greenland Sea samples and acoustic data from a scanning sonar. (Premke et al., 2003) using the ROV ‘‘VICTOR Special attention was paid to assessment of the near 6000’’ and benthic lander systems equipped with a bottom profile of the copepod distribution and the scanning sonar, strongly increased optical and temporal variability of the occurrence of these acoustic signals were observed close to the ocean aggregations during the period of the lander floor, indicating dense aggregations of organisms at deployment. ca. 2500 m depth. Samples from an epibenthic sledge (EBS, Brandt and Barthel, 1995) that collects material 1 m above the ocean floor revealed the 2. Material and methods copepod Calanus hyperboreus as the major inhabi- tant of the aggregations (Auel et al., 2003). The samples and data were collected during RV Aggregations of Calanus species have been observed ‘‘Polarstern’’ cruises ARK XVI/2 (August 2000) and before in the surface (Marshall and Orr, 1955), in ARK XVII/1 (August 2001) to the Greenland Sea intermediate depths (Alldredge et al., 1984; Miller (Fig. 1). The sample protocol is shown in Table 1.

Fig. 1. Sampling area for multinet, epibenthic sledge, and lander sta. F (shaded circle) and lander sts B and E (black circles) in the Greenland Sea. Arrows indicate the main bottom current direction during 1997–2002 (modiﬁed from Premke et al., 2003). ARTICLE IN PRESS 312 H.J. Hirche et al. / Deep-Sea Research I 53 (2006) 310–320

Table 1 Sampling protocol for epibenthic sledge (EBS), multinet (MN) and benthic lander (BL) in the Greenland Sea. Sonar in the water column (W) and/or on the bottom (B)

Station Date Depth (m) Position Equipment Trawling distance (m) Volume (m3) Lat/long

151 01/08/2000 2427 791180N EBS 990 326.7 41260E 152 01/08/2000 2298 791170N EBS 660 217.8 41390E 177 05/08/2000 2329 791050N EBS 720 237.6 41260E 255 17/08/2000 2295 791040N EBS 660 217.8 41340E 278 20/08/2000 2271 791040N EBS 600 198 41400E 259 17/08/2000 2250 791040NMN 041200E B 17/07/2001 2504 781500NBL 021420EW/B E 13/07/2001 2524 781500NBL 051520EW/B F 21/07/2001 2250 791060NBL 01340EW

2.1. Zooplankton sampling 100 cm width by 33 cm height and a mesh width of 300 mm was mounted on the sledge at 1 m height. Vertical distribution of C. hyperboreus was The haul distances were calculated on the basis of studied at sta. 259 from stratified vertical hauls the GPS derived positions of the ship at the start with a multinet (Hydrobios, Kiel; 0.25 m2 mouth and end of the haul; during this study it varied from opening, 200 mm mesh). The water column was 600 to 990 m (Table 1). On deck the samples were sampled in 8 depth intervals (2250–2000–1500– washed through a 300 mm sieve and preserved in 4% 1000–500–200–100–50–0 m). The samples were pre- buffered formalin. For this study, all developmental served in 4% buffered formalin. All C. hyperboreus stages of the copepod C. hyperboreus were sorted were counted and stages determined. out and counted. In some cases, the sample was split down to 1/32 with a plankton splitter. 2.2. Dry mass 2.4. Acoustic measurements Dry mass of the C. hyperboreus population was calculated by using 150 mg for CIII (Hirche, 1997) Acoustic measurements were conducted on the and 510 mg for CIV (Auel et al., 2003). As Auel et al. ocean floor and during the descent in the water found different dry mass in CV and adult females column. On the ocean floor two lander experiments (AF) from the upper 100 m and near the sea floor, were carried out as described earlier by Premke et respectively, we applied the surface values (CV al. (2003). A tripod lander was equipped with a 1900 mg; AF 4350 mg) to specimens collected in the ‘‘Simrad’’ Mesotech MS1000 Scanning Sonar Sys- water column, and the sea floor values (CV 2160 mg; tem (SSS) and an acoustic Doppler current meter AF 3540 mg) to specimens collected in the EBS. (Aanderaa Instruments RCM-11).The SSS, contain- ing a sonar head (‘‘Simrad’’ 1071 series, working 2.3. Epibenthic sampling frequency 675 KHz), microcomputer and hard disk, was adapted to autonomous operation and config- For sampling of hyperbenthic zooplankton an ured to detect scattering objects in the plane of EBS (detailed description in Brandt and Barthel, the acoustical beam rotation at distances of 50 m 1995) was trawled over the ground at a mean with range sampling resolution of 0.1 m during a 5- velocity of 1 knot. A net with a mouth opening of min rotation period at 0.91 angle steps. To achieve ARTICLE IN PRESS H.J. Hirche et al. / Deep-Sea Research I 53 (2006) 310–320 313 high-angle resolution and to decrease bottom tion was made over the ring between 15 and 30 m reverberation, a transducer with a narrow (2.71) from the lander for each 5-min rotation of the sonar conical beam was used. Sonar data were recorded head. Additional averaging was applied by includ- during the lander descent and on the ocean floor. ing 4 rotations of the sonar head.

2.5. Post-processing of sonar data 2.5.3. Descent in the water column To estimate the vertical distribution of zooplank- 2.5.1. Vertical profile in the near-bottom layer ton biomass in the water column we have used the To estimate the vertical distribution of the data set recorded during the descent of the lander at plankton near the bottom, we used sonar data stations B, E and F. In this case the integration was obtained on sta. B. The lander stood on the ocean made over the ring between 15 and 30 m from the floor with a small inclination (Fig. 2). Thus during lander for each rotation of the sonar head. The one rotation the sonar beam was transmitted resulting time sequences were smoothed to reduce towards the bottom then in the opposite direction fluctuations of the echo signals caused by the flow into the water column above the lander. The disturbances of the descending lander. distribution of the intensity of echo signals along the beam in these positions represents a vertical 2.6. Post-processing of acoustic Doppler current profile of the plankton abundance. The horizontal meter distribution of the intensity obtained from the directions orthogonal to the extremes was used for Direction and speed of the current recorded with normalization. All these distributions with spatial an RCM-11 every 2 min were recalculated in north resolution 0.1 m were calculated as averages on and east components. Then they were smoothed in narrow (101) sectors for 10 rotations of the sonar order to obtain representative curves. head. 3. Results 2.5.2. Time series on the ocean floor Temporal variability of the plankton concentra- 3.1. Topography and hydrography tion on the ocean floor was measured for 18 h at sta. B and for 26 h at sta. E. The echo integration The sampling stations were located in Fram procedure used to obtain a description of the spatio- Strait, the only deep connection between the North temporal distribution of scattering objects in the Atlantic and the Arctic Ocean (Fig. 1). Bottom vicinity of the lander was described in detail by topography leads to a splitting of the warm West Premke et al. (2003). A similar averaging procedure Spitsbergen Current, carrying Atlantic water north- was applied to gather information about temporal ward, into at least three branches. One enters the variability of the abundance of zooplankton while Arctic Ocean north of Svalbard, a second flows the lander was deployed on the ocean floor. In order northward along the northwestern slope of the to avoid confusion with amphipods attracted by the Yermak Plateau, and the third recirculates immedi- fish baits on the lander (see Premke et al., 2003), the ately into the Fram Strait at about 791N(Rudels et sectors where approaching amphipods were ob- al., 2000). This region is characterized by strong served were excluded from averaging. The integra- annual fluctuations in ice coverage, whereas the

Sonar head Width of the sonar beam Spatial resolution

50 m

30 m

7.2 m 2.7 m 2.7° Sea floor 0.23 m

Fig. 2. Scheme of the sonar operation on the ocean floor (vertical plane). ARTICLE IN PRESS 314 H.J. Hirche et al. / Deep-Sea Research I 53 (2006) 310–320 eastern part of Fram Strait is generally ice free 3.3. Epibenthic samples during the summer months (Rudels et al., 2000). Furthermore, the complex bottom topography Samples with the EBS were taken on four strongly influences the mesoscale current regime different days during a period of 20 d. Note that on the surface, where a number of eddies have been the vertical profile with the multinet at sta. 259 was observed (Gascard et al., 1988; Schauer et al., 2004), taken at the same location just 4 h after recovery of and near the bottom (see Fig. 1; Premke et al., the EBS at sta. 255. The abundance and stage 2003). distribution of C. hyperboreus in the EBS is presented in Table 2. C. hyperboreus made up the 3.2. Vertical distribution largest portion of the samples (91.2–99% of abundances). On the ocean floor stage distribution The vertical profile of the abundance of C. was shifted towards the older stages as compared to hyperboreus was described by Auel et al. (2003). the water column. CIII was almost absent, and CV Vertical distribution of abundance was bimodal, was the dominant stage. Total abundance varied 3 with peaks in the upper 100 m and between 1000 from 25 to 355 ind m (mean 181) and was on and 1500 m; a minimum was found between 100 and average 57 times higher on the ocean floor than in 500 m. Only stages CIII to adults were present, with the deepest multinet (2000–2250 m); at sta. 152, with stage IV dominating all but the lowest layer, the highest concentration of C. hyperboreus in the followed by CV, CIII and AF. No males were EBS, the factor was 115. Biomass on the bottom observed. In Table 2, we present the biomass of four (EBS) was 18 (sta. 278) to 238 (sta. 152) times higher developmental stages in each depth layer. Biomass than in the lowest multinet (mean 56). distribution was also bimodal, but because of the higher portion of older stages the biomass concen- 3.4. Video observations tration in the 50–100 m layer (12.7 mg m3) was more than twice that of the 0–50 m layer The characteristic body shape and swimming (5.4 mg m3). The minima were between 200 and behavior of C. hyperboreus can be easily identified in 1000 m (1.5 mg m3) and below 2250 m the video recordings during the descent of the ROV (3.0 mg m3). ‘‘VICTOR 6000’’ (for link to supplementary video- clips see Appendix A). This allows a qualitative estimate of the relative distribution of C. hyperbor- Table 2 Drymass (dm, mg m3) and stage composition (% in each depth eus in the water column. In this recording we layer) of three copepodite stages (CIII–CV) and adult females focussed on the water column approximately 100 m (AF) of Calanus hyperboreus from the multinet at sta. 259 and over the ocean floor at sta. PS64/458. Increased epibenthic sledge trawls (EBS) from five stations in the Greenland concentrations were found between the ocean floor Sea and 50 m. Gear CIII CIV CV AF Total dm 3.5. Acoustic backscattering and zooplankton Depth dm % dm % dm % dm % biomass in the water column (m)

Multinet 0–50 0.2 16.7 2.2 65.2 1.8 13.6 1.3 4.5 5.4 The smoothed profiles of the acoustic back- 100 0.3 18.2 2.1 42.4 5.5 28.3 4.8 11.1 12.7 scattering obtained during the descents at sts B, E 200 0.0 0 0.3 38.5 0.8 30.8 1.7 30.8 2.8 and F are shown in Fig. 3. In order to compare 500 0.0 11.1 0.2 44.4 0.4 22.2 0.9 22.2 1.5 acoustic backscattering intensity to zooplankton 1000 0.1 26.7 0.3 40 0.6 20 0.9 13.3 1.8 1500 0.2 23.2 1.0 33.9 3.5 32.1 2.6 10.7 7.3 biomass, the acoustic data were scaled to the same 2000 0.0 2.9 0.8 44.1 2.7 41.2 1.7 11.8 5.3 magnitude as the C. hyperboreus data from the 2250 0.0 0.7 0.5 40.9 2.1 0.4 6.6 3.0 multinet samples. Thus they are of good use for EBS Stations comparing depth distributions and stations. Below 151 0 0 5.5 15.9 92.5 62.6 52.0 21.5 150.1 the surface layer the three acoustic profiles show a 152 0 0 40.5 22.4 473.2 61.7 199.7 15.9 713.4 similar pattern with a smaller maximum above 177 0 0 24.3 23.4 270.1 61.3 110.1 15.3 404.6 500 m and a second, larger, peak between 1000 and 255 0.1 0.1 34.7 30.3 284.7 58.7 86.6 10.9 406.1 1500 m (Fig. 3). Especially in the deeper layers the 278 0.0 0.1 2.2 17.2 35.0 65.0 15.6 17.6 52.8 acoustic profiles at sts E and F agree well with the ARTICLE IN PRESS H.J. Hirche et al. / Deep-Sea Research I 53 (2006) 310–320 315

0 h gives the abundance of plankton per 1 m2 in the MN EBS layer with the thickness h. E min The normalized integral distribution MðhÞ= E max ðC =aÞ as a function of h reaches unity at inﬁnity 1000 E B min 0 B max (Fig. 6). Using this integral distribution one can estimate the effective thickness of the epibenthic

B layer enriched by zooplankton (Table 3) The Depth (m) F 2000 concentration of zooplankton biomass over the ocean ﬂoor was estimated from the curve ﬁtting

CðhÞ¼Cðh1Þ expðaðh h1Þ, 3000 0.1 1 10 100 1000 where CðhÞ is Plankton concentration; Cðh1Þ is -3 Biomass (mg m ) concentration at distance h1 from ocean floor; a ¼ 0:25. Fig. 3. Vertical distribution of Calanus hyperboreus biomass Concentrations were calculated for minimal and from multinet (MN) samples together with smoothed vertical maximal concentrations from EBS samples (Table profiles of the acoustic backscattering during descents of landers at sts B, E, and F. On ocean floor values from epibenthic sledge 3). Integrated biomass over the ocean floor was (EBS) and acoustic backscattering from sts E and B converted to calculated according to the equation biomass (see text, ‘‘min’’ and ‘‘max’’ values correspond to minimum and maximum lines in Fig. 5). IntðhÞ¼ðCðh1Þ=aÞðexpðah1Þexpðaðh h1Þ. The concentration decreased rapidly with distance from the ocean floor indicating increased multinet counts. It is also noteworthy that the concentrations only up to 20–40 m above the ocean backscatter at sta. B is one order lower than at sts E floor. The integrated biomass in the bottom 20 m 2 and F. Interestingly at sts B and E the acoustic (mean 1780 mgm , max. 3674, min. 272) represents backscattering increases sharply below 2000 m, 18% (max. 37, min. 3) of the biomass of the C. while at sta. E no such increase is seen. hyperboreus population in the water column at sta. 259. 3.6. Vertical distribution near the ocean floor from sonar data 3.7. Temporal variability of backscattering during deployment The two panoramas presented in Fig. 4 show the horizontal distribution of the sound scattering Fig. 5A shows the evolution of the averaged objects in the vicinity of the lander on sta. B. The backscattering intensity around the lander at sts B distance between the white rings is 10 m. On panel and E during the deployment in the range from 15 A, representing the beginning of the observation to 30 m in 6 sectors that were free of amphipods. At (see also Fig. 5), only rare spots are seen which both stations the acoustic signal shows a large probably correspond to marine snow or individual variability in intensity during the deployment. At copepods. Panel B shows the panorama at the sta. B the signals were stronger than at E during the highest concentration of plankton near the ocean whole period of the deployment. The recording floor corresponding to the peak in Fig. 5. started at a relatively low intensity and reached a A normalized vertical profile of the distribution large peak after 7 h (Fig. 5). Thereafter 3 more of zooplankton near the ocean floor is shown in peaks, but at lower intensity, passed by the Fig. 6. For normalization the value of concentration deployment. The range of highest to lowest intensity at the height of the sonar head (2.7 m) above the covered 2.5 orders of magnitude. The distribution of ocean floor was used. The shape of this curve is the particle clouds passing by the sonar was rather similar to typical profiles of the concentration of homogeneous for ca 1.5 h. At current speeds of ca. particles transported passively by turbulent flow 5cms1 (Fig. 5B) this corresponds to a diameter of near a rough surface (Kosyan and Pykhov, 1991). the aggregations of ca. 270 m. Intensity at sta. E The exponential fit CðhÞ¼C0 expðahÞ for the started at a very low level and increased after 4 h to normalized concentration is also shown in Fig. 6. a higher level, that was maintained for the rest of The integral MðhÞ of this fit expanded to the height the observations, except for pronounced peak after ARTICLE IN PRESS 316 H.J. Hirche et al. / Deep-Sea Research I 53 (2006) 310–320

Fig. 4. Panorama with minimum (A) and maximum backscattering (B) obtained with scanning sonar system at sta. B. Distance between circles is 10 m. 14 h. The range of intensity covered only 1.5 orders 4. Discussion of magnitude; the repetition rate of the peaks was much lower at the low intensity level, but the peaks According to our combined observations from had a similar duration as those at sta. B. vertical plankton tows, dredging with epibenthic ARTICLE IN PRESS H.J. Hirche et al. / Deep-Sea Research I 53 (2006) 310–320 317

Table 3 3 Vertical distribution of zooplankton concentration Ch (mg m ) 2 and integrated biomass Int Ch (mg m ) over the sea ﬂoor estimated from acoustic measurements, using maximum (Ch max) and minimum (Chmin) values from EBS samples (Table 2)

Distance from Ch max Int Ch max Chmin Int Chmin sea ﬂoor (m)

0 67.74 0 915.20 0 1 52.80 60 713.40 810 2 41.16 106 556.10 1442 3 32.08 143 433.50 1934 4 25.01 171 337.90 2318 5 19.49 193 263.40 2617 6 15.20 210 205.30 2850 7 11.85 224 160.00 3032 8 9.23 234 124.00 3173 9 7.20 243 97.25 3284 10 5.61 249 75.81 3370 20 0.46 270 9.28 3649 40 0.003 272 0.04 3674

nets, video recordings and acoustic data, C. hyperboreus was aggregated in high concentrations in the proximity of the ocean floor of the Greenland Sea in late July and August. Video and acoustic data indicate increasing concentrations from 50 m above towards the ocean floor, with highest Fig. 5. Temporal variations of acoustic backscattering converted concentrations very close to the floor. These to biomass during the lander deployment on the ocean floor at sts aggregations were apparently drifting with the B and E (A); module of current speed measured with acoustic near-bottom currents in cloud-like formations, with current meter at the same stations (B). Lines correspond to a horizontal extension of ca. 270 m. Maximum minimum and maximum biomass values used in Fig. 3. abundances observed were up to 2 orders of magnitude higher than in the water column. The biomass of the bottom 20 m layer was around 18% of the biomass in the rest of the water column. The good agreement between the relative differences of biomass in the water column and the near-bottom as measured by net samples and sonar supports the validity of the acoustic data. The fact that high concentrations were observed during deployments lasting 41 d and in consecutive years (2000 and 2001; this study, 2003; Klages unpublished) indicates that aggregation near the ocean floor is a regular rather than an extraordinary pattern in the life history of C. hyperboreus in the Greenland Sea. According to respiration measurements by Auel et al. (2003), CV and female C. hyperboreus from the EBS had approximately half the respiration rate of those collected in surface waters. This, together with the insensibility to Fig. 6. Normalized vertical profile of zooplankton biomass from mechanical stimuli, indicates that C. hyperboreus sonar operation on the ocean floor. For details see text. found during our study were most likely representing ARTICLE IN PRESS 318 H.J. Hirche et al. / Deep-Sea Research I 53 (2006) 310–320 the resting population. Stage composition was very (1984) from a submersible revealed an extremely similar to observations by Hirche (1997) in the dense, persistent and widespread aggregation of Greenland Sea at the same time of the year. The non-migrating CV Calanus pacificus californicus in a presence of a smaller part of the population in the band 2073 m thick at a depth of 450 m, about upper 100 m indicates that the population had 100 m above the bottom. They suggest that the partly descended to overwintering depth, while the aggregation occurred as a continuous layer, occupy- other part was still accumulating lipids. From lipid ing the entire basin above an oxycline at the sill contents and respiration rates Auel et al. (2003) depth. Quiescent behavior, low laminarinase activ- estimated a survival of the bottom dwelling speci- ity, low protein content, high lipid content and mens of only 76–110 d. Consequently, the deep evidence of low excretion rate all suggest that the population would be expected to die before emer- copepods were in a state of diapause. The authors ging from resting, as the ascent of C. hyperboreus in assumed these aggregations of diapausing copepods the Greenland Sea usually takes place in April/May to be associated with seasonal upwelling cycles. (Hirche, 1997). On the other hand, metabolism may Depth of diapause may vary from stage to stage have been overestimated because of sampling stress at any one location and geographically between on the copepods. Furthermore, the copepods may stages. In the central Greenland Sea C. hyperboreus not yet have reached the deepest diapause phase and occupies the greatest depth of the herbivorous hence metabolism may have decreased further. copepods. Overwintering stages were found only Thus, Hirche (1983) found respiration rates in below 500 m, except for females, and below 1000 m diapausing CV Calanus finmarchicus to amount to in the West Spitsbergen Current and in the only 20% of that of CV in surface waters. Norwegian Sea (Hirche, 1991, 1997). In the The observations from the Greenland Sea repre- Eurasian Basin of the Arctic Ocean the overwinter- sent the first record of high concentrations of ing population was centred in the upper 900 m dormant copepods in vicinity to the deep ocean (Dawson, 1978), with copepodite stages between floor. Sømme (1934) reported on the collection of 300 and 500 m, females in the upper 150 m, and large numbers of C. hyperboreus in the Oslofjord at males between 400 and 700 m (Brodskii and Nikitin, 225–200 m by using a sledge that keeps the net at a 1955; Kosobokova, 1982; Pavshtiks, 1983). In the distance of 1 or 2 m from the bottom, very similar to Canadian Basin, females were found at 200 m and our EBS. He points also to other samples that younger stages (CIII–CV) deeper (500–1000 m) indicated a stronger concentration of animals near during November–February (Ashjian et al., 2003). the bottom in places where the depth is less than the The factors controlling diapausing depth and the maximum depth of distribution. In the deep Scotian mechanisms of adjustment to a certain depth are Shelf basins large populations consisting mainly of still unclear. From the low levels of swimming CIV and V C. finmarchicus, C. hyperboreus, and C. activity observed in freshly collected specimens of C. glacialis were observed by Herman et al. (1991). The finmarchicus by Hirche (1983) and Ingvarsdo´ ttir, aggregations occurred between 200 m and the sea 1998, Heath et al. (2004) concluded that these stages floor, reaching concentrations of up to 20,000 m3 have little active control over their vertical location, (Sameoto and Herman, 1990) from May to late fall. and their vertical distribution to be a product of Aggregations of Calanus species in the water their buoyancy and vertical mixing. They refer to column have been observed before in various seas. Visser and Jonasdottir (1999), who suggested that In the Black Sea maximum concentrations of wax esters in addition to energy storage also serve as passive and non-feeding CV and CVI Calanus an important regulator of buoyancy in overwinter- euxinus were observed in summer within a narrow ing C. finmarchicus, as they have a thermal density band underlying the pycnocline. Oxygen expansion and compressibility higher than that of content in this layer varied between 0.8 and 0.6 ml/l sea water. However, their assumption that the and was apparently not limiting (Vinogradov et al., density of water within the copepod is the same as 1990). In the Great South Channel, Wishner et al. that outside ignores the fact that most biological (1988) located a dense, nearly monospecific con- membranes are semi-permeable and hence well able centration of C. finmarchicus forming an extensive, to regulate the flux of ions and thus affect overall nearly continuous surface layer that may have density. In experiments covering a temperature occupied an area of over 2500 km2. In the Santa range between 1.5 and 10 1C (representing a Barbara Basin, California, in fall Alldredge et al. density gradient 41.1 sigma kg1 m3)CVand ARTICLE IN PRESS H.J. Hirche et al. / Deep-Sea Research I 53 (2006) 310–320 319 female C. hyperboreus from the same sample to the ocean floor exposes the copepod population remained motionless, suspended in the water for to a different predator community. Bottom bound several hours, indicating rapid adjustment of their predators like Boreomysis relicta, cumaceans, natant density (Hirche, 1987). decapods, crinoida, anthozoans like Cerioantharia The discrete layers of copepods observed by sp., hydrozoa like Candelabrium sp., and especially several authors rather seem to point to active depth a relatively high concentration of ctenophora control. Interestingly, discrete layers were observed (Klages unpubl. observations) may exert a larger in association with strong physical or chemical pressure than the pelagic predators higher up in the gradients such as the oxycline in the Black Sea, but water column. also in locations that could not be correlated with any sharp discontinuities of temperature, salinity or Acknowledgements dissolved oxygen (Alldredge et al., 1984). Miller et al. (1991) suggested that maintenance of narrow We thank U. Babst for counting the EBS layering by CV C. finmarchicus as observed by these samples. authors in Slope Water off southern New England implies that the resting CV stock has an environ- mental cue to depth, and that they adjust their Appendix A. Supplementary materials positions at least intermittently, using a specific daytime isolume. In the deep waters of the central Supplementary data associated with this article Greenland Sea, however, the majority of the over- can be found in the online version at doi:10.1016/ wintering stages lives much deeper than the 600 m j.dsr.2005.08.005. inhabited by C. finmarchicus off New England, making the perception of light cues rather unlikely. References Density gradients are around 0.025 sigma kg1 m3 between 1000 and 2500 m (e.g. Bude´ us et al., 1998), Alldredge, A.L., Robinson, B.H., Fleminger, A., Torres, J.J., and chemical discontinuities have not been observed King, J.M., Hamner, W.M., 1984. Direct sampling and in situ as yet. Hence, the deep C. hyperboreus population observation of a persistent copepod aggregation in the may just have lost their cues for depth orientation mesopelagic zone of the the Santa Barbara Basin. Marine and thus have got stuck at the ocean floor. Biology 80, 75–81. Ashjian, C.J., Campbell, R.G., Welch, H.E., Butler, M., Van Alternatively, the common encounters of this Keuren, D., 2003. Annual cycle in abundance, distribution, species near the ocean floor (see above) may indicate and size in relation to hydrography of important copepod a normal behavioral pattern, which has just escaped species in the western Arctic Ocean. Deep-Sea Research I 50, our attention so far. 1235–1261. There is certainly need to study samples from Auel, H., Klages, M., Werner, I., 2003. Respiration and lipid content of the Arctic copepod Calanus hyperboreus over- close to the ocean floor from other regions of the wintering 1 m above the seafloor at 2300 m water depth in the range of this species. 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