Assessing zooplankton advection in the Barents Sea using underway measurements and modelling

Assessing zooplankton advection in the Barents Sea usingunderway measurements and modelling

A. EDVARDSEN1,*, D. SLAGSTAD2,K. S. TANDE1 AND P. JACCARD3

1Norwegian College of Fishery Science, University of Tromsø,N-9037 Tromsø, Norway2SINTEF Civil and Environmental Engineering, Department ofCoastal and Ocean Engineering, N-7934 Trondheim, Norway3Geophysical Institute, University of Bergen, N-5020 Bergen,Norway

ABSTRACT

The coastal shelf of northern Norway and the BarentsSea is highly advective and mainly affected by theNorth Atlantic Current and the Norwegian CoastalCurrent. The oceanographic conditions are an import-ant factor for the spatial and temporal distribution ofzooplankton in the region. To quantify zooplanktonadvection over the western border of the Barents Sea, aScanfish ⁄Optical Plankton Counter (OPC), AcousticDoppler Current Profiler (ADCP), Multiple Openingand Closing Nets and Environmental Sampling System(MOCNESS), and hydrodynamic model were used.This study provides data from two time windows (Juneand July 1998) by continuous measurements betweennorthern Norway and Bear Island. The zooplanktoncommunity structure was obtained by net tows and zo-oplankton abundance fields were mapped by an OPCcounting zooplankton in the size range 0.7–14 mmequivalent spherical diameter. A simple zooplanktoncommunity structure was found with the copepod Cal-anus finmarchicus (CIII–CV) as the dominant species inthis size fraction. Ocean currents were measured by aship-mounted ADCP and the residual currents werecalculated by subtracting the tidal component obtainedfrom a hydrodynamic model. Two measurements con-ducted in June and July 1998 shows a net eastwardtransport of water of 3.5 and 1.3 Sverdrup over thesection. For the same two periods, zooplankton biomasstransport is also positive towards east but varies by twoorders of magnitude between the two measurements.

Key words: Acoustic Doppler Current Profiler,advection, Barents Sea, Calanus finmarchicus,currents, hydrodynamic model, Optical PlanktonCounter, temperature

INTRODUCTION

The coastal shelf of northern Norway and the BarentsSea is a highly productive area, with a food chain inwhich phytoplankton and zooplankton supports therecruitment of major fish stocks such as herring (Clu-pea harengus), capelin (Mallotus villosus) and cod(Gadus morhua) (Skjoldal and Rey, 1989; Gjøsæter,1998). Studies have shown that the water properties(salinity and temperature) of the Barents Sea varyconsiderably between years (Loeng, 1991). The reasonfor this has been linked to both distant and localatmospheric forcing such as wind and air pressure(Adlandsvik, 1989), but could also be caused byincidents of local bottom water drainage (Midttun,1985). This in turn alters the flow of Atlantic andNorwegian coastal water into the Barents Sea andhence the flux of salt and heat into the system. Periodsof high inflow tend to increase the ambient tempera-ture while the temperature decreases during low inflowbecause of heat loss to the atmosphere. The questionessential to the present investigation is, how do thesefluctuations in the water advection affect fluctuationsin zooplankton biomass?

Skjoldal et al. (1992) and Skjoldal and Rey (1989)have outlined in detail the possible effects of inflowingwater and transport of zooplankton to the Barents Sea.Their line of reasoning was that if periods of highinflow of water to the Barents Sea co-occur with thesummer surface aggregation of zooplankton, there willbe a potentially strong biomass transport into theBarents Sea. On the other hand, if strong inflow takesplace during the winter, when the overwinteringpopulations are found in deep waters of the NorwegianSea (>500 m depth) or in deep basins on the shelf, thenet zooplankton biomass transport will be negligible.In addition, the general contention is that intrusion ofwarm water into the Barents Sea favours an elevatedzooplankton production because of faster growth and,hence, lower mortality.

*Correspondence. e-mail: [email protected]

Received 5 January 2001

Revised version accepted 4 December 2001

FISHERIES OCEANOGRAPHY Fish. Oceanogr. 12:2, 61–74, 2003

� 2003 Blackwell Publishing Ltd. 61

There is evidence that the scheme outlined abovecannot explain the overall variation of zooplanktonbiomass in the Barents Sea. Long time series from theKola section (along 33.5�E, see Degtereva, 1979) showan inverse relationship between temperature andzooplankton biomass, which might be explained by adifferent faunal and seasonal succession in Arctic ver-sus Atlantic water masses (Tande et al., 2000). Further,Skjoldal et al. (1992) found a similar negative corre-lation between zooplankton biomass and temperaturein the northern central part of the Barents Sea.

Part of the explanation for the lack of coherencebetween data and theories may be found in thezooplankton sampling methods. Traditional net sam-ples with subsequent analysis are too time-consumingand costly to resolve the inherent spatial and temporalscale of variability in coupled physical and biologicalprocesses. A study from the California Current(Huntley et al., 1995) showed that the range of spatialvariability in zooplankton was 30–60 km (i.e. samplesobtained within this range were not independent).From the same data set, Zhou (1998) calculated adecorrelation time scale of 3 days. As long as samplingis not within the range of variability, one cannotexpect to provide confident data on distribution ofzooplankton over time and distance.

Although a substantial amount of research hasbeen conducted in these areas, there is still a needfor quantitative data on zooplankton to elucidate theinteractions between ocean climate and zooplanktonbiomass in the north-east Norwegian Sea and theBarents Sea. The objective of the present study is todemonstrate an approach for estimating zooplanktonadvection by combining instruments such as the OPC,the ADCP and a hydrodynamic model, and to test thefeasibility of this method along the western border ofthe Barents Sea.

METHODS

Study region

The transect along the western border of the BarentsSea (Fig. 1), often referred to as the Barents SeaOpening (BSO), represents the entire section throughwhich the inflow of Atlantic and Norwegian coastalwater to the Barents Sea occurs (Loeng et al., 1997). Ithas a maximum depth of �500 m, which enabled us tocover the entire water column by sampling with anOptical Plankton Counter (OPC; Focal TechnologiesInc., Nova Scotia, Canada) and a narrow band AcousticDoppler Current Profiler (ADCP; RD Instruments, SanDiego, CA, USA) at a cruise speed of 6–7 knots (kn)

(1 knot ¼ �0.5 m s)1). Unpublished data from thehydrodynamic model used in this study generallyindicates two regions, one close to the Norwegianmainland and another around 71.5�N, which haveconsistently more frequent bursts of inflow than otherregions along the transect.

Sampling techniques

The OPC was mounted on a Scanfish MK II platform(Geology and Marine Instrumentation, Snekkersten,Denmark). This is a vehicle with multiple sensors thatcan be towed at 2–10 kn at a fixed depth or along anundulating path from the surface to 500 m depth. TheScanfish also has a built-in Conductivity–Tempera-ture–Depth (CTD) sensor. At the time the fieldcampaign started, a high quality CTD was not yetinstalled in the Scanfish, and for these cruises we havegood data on sea water temperature only. In this paper,we therefore focused on temperature only to describethe physical properties of sea water along the section.Data from the mounted instruments was sent in real-time via a conducting cable to a shipboard PC, whereGlobal Positioning System (GPS) signals were incor-porated in the data stream.

The OPC detects particles in 4096 digital sizeswithin a range of 0.25–14 mm equivalent sphericaldiameter (ESD). As water flows at 0.5–4 m s)1 throughthe 50 cm2 sampling aperture, particle counts and sizesare evaluated at 0.5-s intervals on the basis of theamount of light the particles block while interceptingan LED beam focused on a photodiode receiver. Thereturned ESD is the diameter of a perfect sphere thatwould obscure the same amount of light as the particle.Sizing error averages 5% across the detection range,increasing to �15% at the lower detection limit(Herman, 1992). OPC-derived particle size distribu-tions and volume concentrations compare favourablywith data from net samples and acoustic biomass esti-mates (Herman, 1992; Herman et al., 1993). But theapplication of the instrument under different envi-ronmental settings has caused some dispute, mainlybecause of its fundamental differences compared withnet sampling of zooplankton (see, for instance: Spruleset al., 1998; Heath et al., 1999; Wood-Walker et al.,2000; Zhang et al., 2000). The digital size spectrum wasintegrated into 60 size classes of equal increments withrespect to ESD. In addition to the abundance of zo-oplankton, the biomass in terms of carbon was alsoestimated for these size classes. The zooplankton car-bon weight (W) was calculated from body length (L)given by the ESD (see Huntley et al., 1995) and theequation of Rodriguez and Mullin (1986):

62 A. Edvardsen et al.

� 2003 Blackwell Publishing Ltd, Fish. Oceanogr., 12:2, 61–74.

log W ½lg C� ¼ 2:23 log L ½lm� � 5:58

The biochemical composition of zooplankton variesdepending on environmental and ontogenetic factors(Falk-Petersen, 1981; Tande, 1982), and biomassestimates from the equation above should be inter-preted with care. However, the equation does providean efficient way to estimate biomass in terms of carbonin different size categories of zooplankton from dataobtained from the OPC.

A MOCNESS (Wiebe et al., 1985) with a netmesh size of 180 lm was used to obtained stratifiedzooplankton samples at both ends of the leg (Fig. 1).The volume filtered was typically 100–300 m3 per net.Zooplankton were sampled in the depth strata 0–20,20–50, 50–100 and 100 m bottom. Each sample wasconcentrated and preserved in 4% formaldehyde in seawater buffered with hexamine. A bactericide, 1,5-propane-diol was added (5% by volume) to the pre-servative. In the laboratory, the preserved samples

were counted in fractions not less than 1 ⁄100 of theoriginal samples. For those species recording lownumbers, a lower degree of subsampling was conduc-ted. All specimens in aliquots of approximately 1000animals were counted and identified under a Wild M3stereo microscope (Leica Heerbrugg AG, Heerbrugg,Switzerland) at 6–50·.

The ADCP was a 150-kHz narrow band, vessel-mounted sonar configured to record currents between 30and 500 m depth within 16-m thick layers. Ship’sheading was input by a GPS directly into the deck unit,while ship speed has been subtracted from relativecurrents during post-processing. Nominal ship speed formeasurements was between 6 and 7 kn. For a more de-tailed description of the ADCP, see Flagg et al. (1998).

The ADCP raw and navigation data were processedwith the GFI Common Oceanographic Data Archiv-ing System (CODAS) software package (http://gfi.uib.no/�jaccard/gficodas), an extension of the ori-ginal CODAS, providing extensive tools for processing

Figure 1. Map of the study area. The straight line is the standard cruise track for cruises referred to in this paper. This section isalso denoted as the Barents Sea Opening (BSO) elsewhere in the literature. The paths of the North Atlantic Current (NAC,open arrows) and the Norwegian Coastal Current (NCC, solid arrows) are indicated (Loeng et al., 1997). The hatched areaindicates the position of the temperature transition area between warm Atlantic water to the south and colder modified Arcticwater to the north at the end of July 1998. Isobaths are drawn for 200, 400 and 1000 m depths.


Zooplankton advection in the Barents Sea 63

vessel-mounted ADCP data. Relative currents wereaveraged over 30-min profiles, Doppler velocities werecalibrated against speed of sound determined fromCTD measurements and edited to exclude erroneousvalues. Finally, both the relative currents and the shippositions were combined to provide further calibrationfactors for correcting transducer misalignment as wellas ship velocity.

For calculation of water flux across the section, theprocessed ADCP data were regridded by depth to fitthe SINMOD (see below) vertical gridding, andaveraged for each 30 min of steaming. Hence, we have55–65 current measurements (direction and velocity)for every depth layer along the cruise track. To cal-culate the net water currents during each cruise, thetidal component obtained from SINMOD was sub-tracted from the measured flow field. For calculation ofwater discharge perpendicular to the cruise line, thecross-sectional area of each cell was multiplied by itsrespective nett water velocity. As our cruise line isclose to a north–south orientation, we used the east–west component of the net water velocity.

Circulation model (SINMOD)

The circulation model used in this study is a3-dimensional (3D), baroclinic, finite difference ‘levelmodel’ which is defined with a sequence of fixed, butpenetrable, levels. Each level has a fixed thickness,with the exception of the level at the surface and theone which is adjacent to the bottom. The primitiveNavier–Stokes equation is used to calculate the hori-zontal flow in the x and y directions, and the verticalcomponent is found from the equation of continuity.Detailed information about the model can be found inSlagstad (1987), Slagstad et al. (1989) and in Støle-Hansen and Slagstad (1991).

A regional, 3D model with 20 km grid spacing wasestablished for the Norwegian Sea and the BarentsSea (SINMOD). The model absorbs the tidal inputby harmonic forcing of each component throughthe open boundaries. Input data were taken fromSchwiderski (1980). The modelled tidal elevation andcurrents have been verified against output from othermodels especially designed for this purpose (Gjeviket al., 1990) and available data along the coast ofNorway.

To increase the resolution in the Barents Sea, anested model was established with a horizontal gridspacing of 6.67 km. Simulation runs with the hydro-dynamic model were performed for 1998 with inputfrom meteorological forcing (wind and atmosphericpressure) for the selected period and tides. The resid-ual currents depend on both the meteorological

forcing and more or less unknown open boundaries. Asmeteorological forcing is relatively well known, asignificant proportion of the discrepancy between theobserved (ADCP measurements) and simulatedtransport over the section is likely to be an effect oferror in the open boundary specifications.

To evaluate the model output, we sampled values ofcurrent in the model domain at the same spatial andtemporal location as for real ADCP measurements.The two corresponding data sets were plotted togetherand compared. In the same manner we extracted thefour tidal components M2, S2, K1 and N2 from themodel, the sum of which was averaged over the depthlayer 30–200 m and applied to the measured flow fieldfor calculation of the residual current. As our set-up ofthe ADCP did not enable current measurementsabove 30 m depth, the net current field for this layerwas adopted from the layer below.

Observations

This project was based on results from six cruisesconducted during 1998 and 1999. Because of severalreasons, there is variable coverage and quality of thedata obtained from both the OPC and the ADCP. Inthe present paper, we demonstrate our approach indetail based on data from the second cruise (July1998), extending our analysis of advection also to thefirst cruise (June 1998).

RESULTS

During the cruise in July 1998, there was a thermo-cline at approximately 40 m depth along the entiresection (Fig. 2). The gradient however, seems to bemore pronounced at the southern end of the transect.Surface temperature decreased from 11�C at thesouthern end to 9�C at the northern end of thetransect. Below the thermocline, temperature droppednorth of 73�N, which indicates the position of thePolar Front. At the slope near Bear Island the tem-perature reached a minimum of <1�C.

Flow fields

The flow fields along the section are dominated bytidal forced water movements, moving mainly in aneast–west direction at a typical speed of 20 cm s)1.The measured ADCP and the SINMOD modelledcurrents demonstrated a very good coherence both inphase and current magnitude at all depth ranges dur-ing Cruise 2 from 28 to 29 July 1998 (Fig. 3). Thenested model with a grid resolution of 6.67 km seemsto be better than the regional model (grid resolu-tion ¼ 20 km) in reproducing the measured flow. We



therefore used the nested model to estimate the tidalcomponent (M2, S2, K1 and N2) of the measured flow.By removing the tidal flow as described by SINMODfrom the measured flow, the residual east–west watervelocity was typically 0 ± 5 cm s)1 (Fig. 4).

Copepod community and biomass fields

During summer 1998, Oithona spp. and Calanus finm-archicus were the most abundant zooplankton speciesin the MOCNESS samples (Table 1). At the southernend of the section, C. finmarchicus dominated in lateJune together with Oithona spp. and copepod nauplii.In late July, Oithona spp. were dominant representing55% of total numerical abundance, and C. finmarchi-cus diminished from 41 to 19% of the total numericalabundance from late June to late July. At the northernend of the leg, the same two species were also the mostabundant where the proportion of C. finmarchicus was59% in June and 43% in July. Note that the highestnumbers of copepod nauplii were found at the south-ern end of the section.

From the OPC measurements in the size range 0.7–14 mm ESD, high concentrations of zooplanktonbiomass were found at both ends of the section withmaximum values of 10 mg C m)3 (Fig. 5, top panel).The biomass values were averaged within the definedgrid, hence local maxima along the OPC tow path canreach higher values. High values of biomass were alsofound in the 0–50 m layer for the entire section. Be-low 50 m depth, for the middle section of the leg,zooplankton biomass was lower with values rangingfrom 0 to 1 mg C m)3. At the southern end of the leg,biomass had a bimodal depth distribution. High valuesof biomass were found both in the upper layers and

close to the bottom, separated by a layer of interme-diate biomass. North of 73.25�N, values of zooplank-ton biomass increased towards Bear Island with a moreuniform depth distribution. There was, however, atendency for higher biomass at the northern slope ofthe Bear Island Trough (Fig. 5, top panel).

Advection of water and zooplankton biomass

The water transport was higher during the cruise inJune than in July. Nett transport of water was directedeastwards for both cruises with values of 3.46 and 1.32Sverdrup (Sv, 1 Sverdrup ¼ 106 m3 s)1) for the firstand second cruise, respectively (Table 2). The west-ward transport of water along the section was higherduring the second than the first cruise.

There was a stronger signal of zooplankton biomassduring the cruise in June (not shown) than the cor-responding cruise in July. Combining biomass andwater flow fields yielded a much greater biomasstransport for the first cruise compared with the second(Table 2). For both time windows, net biomasstransport was towards the east, but for the first cruise,biomass advection was two orders of magnitude higherthan on the second cruise. From June 30 to July 1, netbiomass transport to the east was estimated as�100 kg C s)1, which yields �8720 tonnes of carbonon a daily basis. For the second cruise the daily bio-mass transport towards the Barents Sea (i.e. to theeast) over the section reached �66 tonnes of carbon.

Averaging the flow of biomass along the entiresection indicated that a large amount of the biomasswas advected in the upper layers of the water column(Fig. 6). For the first cruise there were also high valuesof biomass flow at approximately 150 m. For Cruise 2,

Figure 2. Temperature (�C) along the section from Norwegian mainland (left) to Bear Island (right) for 28 July 1998(Cruise 2).



mean biomass transport changed at around 100 mdepth, with an eastwards transport above 100 m depthand a westwards transport below.

DISCUSSION

Flow fields

Currents were dominated by an east–west watermovement, consistent with the tidal wave whichpropagates in an east–west direction between the

continental shelf break and the eastern parts of theBarents Sea. Comparing our current measurementswith the SINMOD model for the two time windows,the model reproduces the measured flow fieldadequately. The nested model (6.67 km gridding) gavea better match with measured flow along the sectionthan the coarser regional model (20 km gridding). Thechoice of grid point resolution can affect the modeloutput greatly, especially when topography is steep(Kowalik and Proshutinsky, 1995). Our estimated

Figure 3. Measured and modelled total currents in different depth layers during the northbound leg of Cruise 2 (28 July 1998).Depth contours are drawn for 200, 300 and 400 m. Top panel: ADCP measurements. Lower panel: modelled values corres-ponding to the ADCP measurements.



residual currents rely on the SINMOD’s ability toprovide good estimates of the tidal component. As themeasured and modelled values of current velocities in

this study provide similar results, we believe thatthe SINMOD nested model output can be consideredreliable.

Table 1. Zooplankton abundance ateach end of the transect during Cruises 1and 2 in 1998. Numbers are given as theaverage for the entire water column,where the left hand values represents thesouthern end (70.5�N) and the righthand values represents the northern end(74.0�N) of the transect.

Species

Abundance (ind. ⁄m3) Relative abundance (%)

30 June 1998 28 July 1998 30 June 1998 28 July 1998

Calanus finmarchicus 119 ⁄ 52 96 ⁄ 264 41 ⁄ 59 19 ⁄ 43Calanus glacialis 1 ⁄ 2 0 ⁄ 4 0 ⁄ 2 0 ⁄ 1Metridia spp. 15 ⁄ 2 10 ⁄ 12 5 ⁄ 2 2 ⁄ 2Pseudocalanus spp. 14 ⁄ 0 0 ⁄ 8 5 ⁄ 0 0 ⁄ 1Microcalanus spp. 17 ⁄ 7 79 ⁄ 86 6 ⁄ 8 16 ⁄ 14Oithona spp. 54 ⁄ 18 271 ⁄ 226 18 ⁄ 20 55 ⁄ 37Onchea spp. 5 ⁄ 0 0 ⁄ 15 2 ⁄ 0 0 ⁄ 2Euphausiids 1 ⁄ 1 1 ⁄ 1 0 ⁄ 1 0 ⁄ 0Copepod nauplii 68 ⁄ 8 38 ⁄ 2 23 ⁄ 9 8 ⁄ 0

Figure 4. Eastward water velocities from Cruise 2 (28 July 1998). Dotted lines are ADCP measurements for each depth layerand the solid lines are the tidal component taken from the SINMOD model with a 6.67-km horizontal grid resolution. Themodelled tidal component is the sum of M2, S2, K1 and N2 (ordered by their amplitude) averaged within the layers from 30 to200 m depth.



The ADCP measurements were conducted twicealong the section on each cruise. Comparing thecurrent measurements of Cruise 2 (Fig. 4), a consistentpattern in net flow was found between 72.5�N andBear Island, where the time lags between the northand southbound measurements were small. However,when the modelled tidal component was removed, the

residual east–west water velocity along the sectionbetween 73.5�N and Bear Island changed from almostzero to a strong westward current between the north-bound and the southbound leg. The northern slope ofthe Bear Island Trough is steep, and as stated previ-ously, this rapid change in depth might cause hydro-dynamic models to produce an unrealistic flow field.

Table 2. Eastwards component of water and zooplankton biomass transport across the section during Cruises 1 and 2 in 1998.Zooplankton measurements for Cruise 1 were conducted mainly for the depth layer 0–200 m, thus the biomass transportestimates along the section cover only this depth stratum.

Cruise no.

Water transport (Sv) Zooplankton transport (kg C s)1)

Inflow Outflow Net flow Inflow Outflow Net flow

1 5.16 )1.69 3.46 179.21 )78.30 100.912 3.48 )2.16 1.32 4.29 )3.52 0.76

Figure 5. Values for biomass, net water and biomass movements over the section for the northbound leg of Cruise 2 (28 July1998). Upper panel: zooplankton biomass distribution for all sizes from 0.7 to 14 mm ESD. Middle panel: residual east–west water velocity calculated from ADCP measurements and the tidal component from SINMOD. Lower panel: biomasstransport across the section as obtained from the two upper panels. Note that values between ±1 lg C m2 s)1 are plotted as100 lg C m2 s)1 because of the log-transformation of directional values (negative towards west, positive towards east). Bluecoloured cells denote a biomass transport towards the west while red coloured cells are eastwards transport of biomass (into theBarents Sea). Cells with no observations are hatched.



There also seemed to be a more or less permanenteastward residual flow between 71.2 and 71.7�N,although of a variable magnitude. For the rest of thesection, there seemed to be little or no consistency forboth gross and residual current patterns between thetwo measurements. The temporal variability in theresidual current along the section seemed to operate atthe same or a shorter time interval than the time spentto measure currents along the section twice, which forthis case was less than 70 h. Hence, our samplingmight not be able to fully resolve the variability inresidual currents over the time window covered by onecruise. Identifying areas of persistent flow across thesection have to be done repeatedly to filter out thehigh frequency variability in residual currents, which

seemed to be present at a time scale less than six tidalcycles.

Based on mooring data for 2 months, flow of waterover the Norway mainland-Bear Island section hasbeen estimated as 3.1 Sv into the Barents Sea accom-panied by a flow westwards of 1.2 Sv (Blindheim,1989). Loeng et al. (1997) made similar estimates bycompiling available measurements, output from hydro-dynamic models and time series of oceanographic datacollected by Russian scientists. Our snapshots over thesame section gave a flow eastwards of 5.16 Sv and aflow westwards of 1.69 Sv, which gave a net eastwardtransport of 3.46 Sv for the first cruise. For the secondcruise, eastwards and westwards water transport wasestimated to be 3.48 and 2.16 Sv, respectively, with

Figure 6. Average biomass flow for each depth layer during the northbound leg of Cruises 1 and 2. Biomass flow is positivetowards the east (into the Barents Sea) and negative to the west.



net flow eastwards of 1.32 Sv. In a similar approach,Haugan (1999) used six ADCP transects over thesame section to calculate the residual flow. His esti-mated net flow eastwards was in a range from )9.2 to29.4 Sv with a mean of 3.8 Sv. From moored currentmeters between 71.25 and 73.75�N along the BSO, R.Ingvaldsen (Institute of Marine Research, Bergen,Norway, personal communication) estimated thewater transport for two periods of 2 days correspondingto our cruises in 1998. For 30 June and 1 July, shecalculated an eastwards water transport of 2.6 and1.8 Sv, and for 28 and 29 July the eastwards transportwas estimated 2.2 and 1.7 Sv, respectively. Wetherefore consider our estimated water transport, froma vessel mounted ADCP during a time span of lessthan 3 days, to be realistic.

The data from Haugan (1999) indicated that theflow over the section varies considerably. On a long-term basis, the eastwards water transport is expected tobe greater during autumn and winter than duringspring and summer (Loeng et al., 1997). However,recent data from moored current meters along theBSO (71.25–73.75�N) indicate a great deal of vari-ance on shorter time scales (R. Ingvaldsen, Institute ofMarine Research, Bergen, Norway, personal commu-nication). Cross-sectional flow between consecutivedays was found to vary as much as 10 Sv. Preliminarytime-series analysis of the data revealed a short-termfluctuation of water transport at time scales 55, 64 and83 h, 7 and 14 days. Over longer time scales, the samedata set showed periods of strong net westwards flowover the section, especially during the spring.

Given a highly variable flux, one either has to carryout frequent ADCP surveys or install moorings atsufficient spatial coverage to resolve the variability ofthe system in order to quantify the long-term flux ofwater. If our hydrodynamic model is able to reproducea reliable flow field over time, it will be more cost andtime efficient to use the model to calculate flux ofwater across the section over longer periods ratherthan using a vessel-mounted ADCP.

Biomass and abundance fields

Generally, high values of zooplankton biomass overthe entire water column are associated with thesouthern and the northern parts of the section (Fig. 5,upper panel). Between 71.5 and 73.2�N, zooplanktonbiomass is restricted to the upper layers of the watercolumn. For biomass measurements averaged withinthe grid (Fig. 5, upper panel), values peaked at 10 mgC m)3. Zooplankton biomass in the Barents Sea hasbeen found to vary by one order of magnitude betweenyears (Skjoldal and Rey, 1989). They reported biomass

values of typically 2–25 g dry weight m)2 over the0–200 m depth range during late spring or early sum-mer, which gives roughly 5–65 mg C m)3 using acarbon : dry weight ratio of 0.5. As our biomass dataare converted from zooplankton sizes measured by theOPC and not from direct measurements, it might bemore suitable to compare our abundance data from theOPC with other field measurements of abundance.During July in the years, 1986–88, Melle and Skjoldal(1998) reported maximum values of 1500 ind. m)3

for C. finmarchicus copepodide stages I–IV and150 ind. m)3 for stage V and adults in Atlantic watermasses. For Arctic water masses, the correspondingabundance was 30 and 100 ind. m)3, repectively, andsomewhat less for C. glacialis. For the northern Nor-wegian Sea and the south-western part of the BarentsSea in July 1989, Helle (2000) found the abundancefor each of the different copepodide stages of C. finm-archicus to vary between 10 and >3000 ind. m)3. Theseranges are similar to our abundance measurementsmapped by the OPC (Fig. 7). To evaluate our abun-dance estimates from Cruise 2, the OPC data can becompared with data from the MOCNESS tows at eachend of the section. Because of fundamental differencesin sampling methods, comparison of zooplanktonabundance between net samples and OPC is difficultto accomplish (Sprules et al., 1998; Heath et al.,1999). However, during the second cruise, the OPCgave an abundance in the range 101)104 ind. m)3 inthe smaller size fraction (Fig. 7, upper panel), andin the larger fraction the abundance range was101)103 ind. m)3 (Fig. 7, lower panel). This is con-sistent with the results from the MOCNESS hauls,which gave a total abundance for the different depthlayers in the range 102)103 ind. m)3 (Table 3).

As can be seen from the temperature profile(Fig. 2), there is a drop in temperature over the BearIsland Trough. This is a more or less permanentfeature of westward flowing modified Arctic water(R. Ingvaldsen, Institute of Marine Research, Bergen,Norway, personal communication). The shift inenvironmental conditions should also be traceable inthe zooplankton community along the section. Fromthe MOCNESS tows at each end of the section,Oithona spp. and C. finmarchicus were the overalldominating species (Table 1). At the northern end ofthe leg, C. finmarchicus is slightly more dominant thanOithona spp. for both cruises. At the southern end ofthe leg, C. finmarchicus is dominant during the firstcruise and Oithona spp. were the most abundant spe-cies during the second cruise. Calanus glacialis wasfound at both ends of the leg during the first cruise andonly in the north during the second cruise. Plotting



two size fractions from the OPC during the secondcruise shows that this difference in the zooplanktoncommunity is related to the environmental changeacross this temperature transition zone (Fig. 7). Thereis a clear shift towards smaller sized zooplankton in the

cold part of the section compared with the warmerareas south of 73�N.

Comparing the abundance of different size fractionsat the southern end of the section reveals a bimodaldepth distribution with smaller zooplankton in theupper parts of the water column and the larger onesclose to the bottom (Fig. 7). The bimodal pattern oftotal abundance is also indicated by the MOCNESSsamples taken at the southern end of the section(Table 3). This pattern was not observed in the northand the assemblage of zooplankton here seemed toconsist of smaller individuals. It has been suggestedthat vertical migration of zooplankton populations inthese waters display an ontogenetic rather than adiurnal pattern (Falkenhaug et al., 1997; Halvorsenand Tande, 1999). Hence, the differences found indepth distribution and size composition of zooplanktonbetween the northern and southern parts of thesection might be used as an indication of a shift in

Figure 7. Abundance (ind. m)3) of zooplankton plotted along the undulation path of the Scanfish ⁄OPC during the northboundleg of Cruise 2. The upper panel shows zooplankton in the size range 0.7–1.4 mm ESD, which correspond approximately to stagesCII–III to CVI of Calanus finmarchicus, and the lower panel the size range from 1.5 to 2.1 mm ESD. The latter size range willtypically reflect stage CV and adults of C. finmarchicus.

Table 3. Total abundance of all species listed in Table 1 fordifferent depth layers during Cruise 2. Data were obtainedfrom the MOCNESS.

Depth stratum

Abundance (ind. m)3)

South North

0–20 m 2312 56420–50 m 210 57050–100 m 309* 669100 m to bottom ND

ND ¼ no data.*Abundance in the depth layer 50 m to bottom.



developmental phase. Melle and Skjoldal (1998)found a delayed development of C. finmarchicus in thepolar front region. The explanation for this was thatC. finmarchicus developed too slowly in cold water forthe recruits to match the spring bloom of phyto-plankton. Further, the MOCNESS tows at the south-ern end of the section show a high abundance ofnauplii for both Cruises 1 and 2 compared with thenorthern end of the section. This might be the resultof a second generation of copepods being advectedfrom the Norwegian coastal and shelf areas into thesouthern parts of the Barents Sea. Although the OPCis capable of measuring particles down to 0.25-mmESD (Herman, 1992) we limited our size range to aminimum of 0.7-mm ESD. Thus, our OPC data do notinclude copepod nauplii, Oithona spp. and otherzooplankton smaller than CII–III of C. finmarchicus.There is, however, a generally good agreementbetween the MOCNESS and OPC derived abundancedata for the copepod component.

Biomass advection

The estimated total biomass advection for the firstcruise is 8720 tonnes of carbon per day with a cor-responding value two orders of magnitude less for thesecond cruise. If we assume that our June numbers arerepresentative for this month, the integrated biomasstransport over the section will be 0.262 · 106 tonnesof carbon per month. This falls within previous roughcalculations made by Pedersen (1995). He estimatedthe total advection from the Norwegian Sea to theAtlantic parts of the Barents Sea to be 0.272 · 106

tonnes of carbon during the month of June based ondata sampled in 1994. It is important to bear in mindthat our measurements are snapshots of a system thatshows a great deal of variance, not only for wateradvection but probably also for zooplankton abun-dance. Extrapolating values beyond the duration ofone investigation should therefore be looked uponwith great caution. Although a single survey gives acorrect estimate for a period of less than 2 days, pro-viding a time filter by averaging several measurementswill probably produce a more useful estimate of thezooplankton transport across the section.

A diurnal vertical migrating zooplankton popula-tion could cause problems when calculating the meanflux of biomass across the section, especially if themigration pattern matches the tidal phase. As diurnalvertical migration is not a pronounced feature ofzooplankton populations of Northern Norway and theBarents Sea during the summer (Falkenhaug et al.,1997; Halvorsen and Tande, 1999), in relation to thedominating semidiurnal tidal phase along the BSO,

migration will not bias the estimates of biomass flux.The zooplankton distribution along the BSO seems tocontain less variance than the flow field (results notshown here), which indicates that flow fields have tobe sampled more frequently than zooplankton in orderto resolve the variability. The logistical limitations ofour ability to measure water flux at a sufficient spatio-temporal resolution by a vessel mounted ADCP can beaided by moorings and hydrodynamic models such asSINMOD.

The period of pronounced zooplankton advectionoccurs when the population resides above 500 mdepth, mainly from March to August. In order to havea positive flux of zooplankton from the Norwegian Seaand the Atlantic Ocean into the Barents Sea, therehas to be a match between the physical and biologicalsettings. Events of large water flux during winter mightnot provide a burst of zooplankton advection, while aminor positive flux eastwards during spring and sum-mer at high zooplankton concentrations might providesubstantial zooplankton biomass transport into theBarents Sea.

The above outline provides general guidelines formonitoring zooplankton transportation over the BSO,with frequent measurements during the zooplanktongrowth season and less frequent during the autumnand winter. There is, however, an important linkbetween the long-term current regime and zooplank-ton biomass in the Atlantic part of the Barents Seathat should motivate monitoring outside the growthseason. As stated in the Introduction, the ambienttemperature in this area depends largely on the long-term advection of Atlantic water. In turn, this willaffect mortality and growth (i.e. production) ofzooplankton during the spring and summer in theBarents Sea. With respect to zooplankton, monitoringshould reflect the different developmental phases ofthe most important species, such as C. finmarchicus.The first event should occur as adults ascend to spawnin March. The next should take place as the popula-tion reaches the earliest copepodide stages (CI–III) inearly May and then another one in late May as the latecopepodide stages and adults predominate. Furthermeasurements should also take place in June and July,to track a potential second generation being advectedfrom the coastal and shelf waters off Norway. As far asthe flow field is concerned, the sampling frequencyoutlined above might not be sufficient to provide agood estimate. Taking ship time and other expensesinto account, using hydrodynamic models might pro-vide the best tool to calculate biomass flux fromzooplankton measurements over the western borderof the Barents Sea. The prerequisite for such an



approach, will be to carry out a more extensivevalidation of the model performance through com-parison with ground truth field measurements.

CONCLUSION

This study proposes a method for integrated meas-urements of zooplankton and the physical environ-ment in the Barents Sea by combining measurementsfrom different instruments. Through further develop-ment and tuning, this approach could be a stepforward in our understanding of integrated marine bio-physical processes. Here, we report the results fromtwo cruises conducted in late June and late July 1998.Net advection of zooplankton biomass across thewestern border of the Barents Sea is directed eastwardsbut varies by two orders of magnitude between the twomeasurements. The OPC and ADCP allow fast anddetailed mapping of biomass and current fields which,by repeated measurements, can provide good estimatesof zooplankton exchange rates to the Barents Sea. In abroader context, this can provide valuable informa-tion on energy flux towards higher trophic levels,which includes important commercial fish stocks inthis area.

ACKNOWLEDGEMENTS

The authors would like to thank J.T. Eilertsen andR. Buvang for valuable technical help on implemen-tation and use of the Scanfish ⁄OPC during thesecruises. We also thank Alexander Timonin andTatjana Semenova at the P.P. Shirshov InstituteMoscow for species identification and enumeration.We are also indebted to M. Zhou and Y. Zhu at theUniversity of Minnesota. Through fruitful co-opera-tion in other projects, they have brought a lot ofknow-how to the University of Tromsø. Without theirspirit and tools, this project could not have beenaccomplished. This project was supported by fundsfrom the Norwegian Research Council (project num-ber 133904 ⁄122) to Kurt S. Tande.

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Assessing zooplankton advection in the Barents Sea using underway measurements and modelling

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