Distribution, annual cycle, and vertical migration of acoustically derived biomass in the Arabian

26 Pages·2002·0.82 MB·English

Deep-SeaResearchII49(2002)2377–2402
Distribution, annual cycle, and vertical migration of
acoustically derived biomass in the
Arabian Sea during 1994–1995
Carin J. Ashjiana,*, Sharon L. Smithb, Charles N. Flaggc, Nasseer Idrisib
aDepartmentofBiology,WoodsHoleOceanographicInstitution,WoodsHole,MA02543,USA
bRosenstielSchoolofMarineandAtmosphericSciences,UniversityofMiami,4600RickenbackerCauseway,Miami,FL33149,USA
cDepartmentofAppliedScience,BrookhavenNationalLaboratory,Upton,NY11973,USA
Received10April2001;receivedinrevisedform1November2001;accepted2December2001
Abstract
ThedistinguishingcharacteristicthatsetstheArabianSeaapartfromotheroceanicregionsistheregularoscillation
of monsoonal atmospheric conditions that produces predictable periods of upwelling or convective mixing, with
associated biological response, during the Southwest and Northeast monsoons, respectively. This oscillation is also
evidentincycles of standingstocksofzooplankton andmicronekton. Theverticaldistribution andspatialpattern of
zooplankton and micronekton biomass were estimated using an acoustic Doppler current profiler along a 1000-km
transectextendingfromthecontinentalshelfofOmantothecentralArabianSeaduringtencruisesontheR/VThomas
G. Thompson (November 1994–December 1995). The influence of the Southwest Monsoon, and accompanying
upwellingandenhancedacousticallyderivedbiomass,wasthedominantfeatureinthespatial-temporaldistributionsof
bothzooplanktonandmicronektonneartheOmanicoast.Thedielverticalmigrationofpredators(myctophids,pelagic
crabs),andtheseasonalchangesinthestrengthofthissignal,wasthemostsignificantpatternobservedinthevertical
distribution of biomass and imparted a strong day–night signal to the integrated upper water-column biomass.
Significant differences in the magnitude of integrated upper water-column biomass, both zooplankton (day) and
migrator-zooplankton (night), were seen between inshore and offshore of the atmospheric Findlater Jet. A station
locatedinthecentralArabianSeademonstratedseasonalchangesinbiomassovertheyear,despitebeingquitefarfrom
the influence of the monsoonal oscillations. Predation pressure was greater offshore of the Findlater Jet than in the
regioninshoreoftheJetorinthecentralArabianSea.ThepelagiccommunityoftheArabianSeamayhaveevolvedlife
history strategiesto coincidewith thepredictable monsoonalcycle. r2002Elsevier Science Ltd. Allrights reserved.
1. Introduction ocean’s response to oscillating monsoon winds.
The Arabian Sea is characterized by a regular,
Seasonal cycles in abundance and distribution seasonal monsoon cycle consisting of the North-
of zooplankton and micronekton in the northern east Monsoon (NEM: December–February; pre-
Arabian Sea are influenced profoundly by the vailing winds from the NE), and the Southwest
Monsoon (SWM: June–September; strong, pre-
vailing winds from the SW, centered on the
*Correspondingauthor.
E-mailaddress:[email protected](C.J.Ashjian). troposphericFindlaterJetwhichrunsSW:NEwell
0967-0645/02/$-seefrontmatterr2002ElsevierScienceLtd.Allrightsreserved.
PII:S0967-0645(02)00041-3
2378 C.J.Ashjianetal./Deep-SeaResearchII49(2002)2377–2402
offshore of the coast of Oman), punctuated by onshore–offshore declining gradient in mesozoo-
Intermonsoon periods (e.g., Currie et al., 1973; plankton biomass (e.g., Qasim, 1977; Smith et al.,
Schott et al., 1990; Morrison et al., 1998). The 1998b; Wishner et al., 1998). The community
prevailing winds have dramatic effects on the composition of the zooplankton also may be
hydrographic and advective fields of the region, modified during the SWM; some copepod species
with attendant influences on biological processes (e.g.,Calanoidescarinatus)increasedinabundance
and distributions. During the SWM, upwelling of in greater proportions than other species (e.g.,
nutrient-rich water off Oman promotes primary, Smith, 1982; Smith et al., 1998b).
and hence secondary, production (e.g., Banse, The influence of the monsoonal oscillation on
1987; Brock et al., 1991, 1992; Brock and hydrographic and advective structure, the atten-
McClain, 1992). The offshore (100–400km) and dant changes in plankton and fish biomass,
alongshore (>1000km) extent of relatively cool production, community structure, and the utiliza-
sea surface temperatures (o261C; or approxi- tion and cycling of carbon, were addressed during
mately 41C cooler than the adjacent sea surface the multidisciplinary Arabian Sea Expedition of
not affected by upwelling) make this one of the 1994–1996underthejointauspicesoftheUSJoint
largestupwellingareasknown.Onlytheupwelling Global Ocean Flux Study (JGOFS; National
areasassociatedwiththeBenguelaCurrentSystem ScienceFoundation)andtheForcedUpperOcean
(LutjeharmsandStockton, 1987)havedimensions Dynamics Project (Office of Naval Research).
similartotheoneoffofOman;allotherupwelling During the study period, a series of ten cruises
regions (Peru, 1983; Somalia, Brown et al., 1980; were conducted on the R/V Thomas G. Thompson
northwestAfrica,Estrada,1974)arelessextensive. inthenorthernandcentralArabianSeaoverafull
The situation is different during the NEM, when year, spanning the complete monsoon cycle. The
cool, dry winds from the northeast cause con- Thompson was equipped with a 153kHz RDI
vective mixing and associated primary productiv- acoustic Doppler current profiler, which operated
ity (Marra et al., 1998; Madhupratap et al., 1998; continuously throughout all of the cruises. The
Barber et al., 2001). The two monsoonal regimes presentstudyutilizesthebackscatterintensitydata
are separated by Spring and Fall Intermonsoon collected by this instrument to analyze spatial and
periods, characterized by a reduction of wind seasonalpatternsinzooplanktonandmicronekton
stress over the ocean surface and consequential biomass across a 1000-km transect of the Arabian
diminishing of the upper mixed-layer depth Sea. The transect was situated so that the atmo-
(Gardner et al., 1999). As a result, primary spheric Findlater Jet (SWM) crossed the center
production and phytoplankton standing crop are (Fig.1) so that the transect surveyed regions both
minimal during the Spring Intermonsoon (Gun- inshoreandoffshoreofthelociofstrongestSWM
derson et al., 1998; Marra et al., 1998; Caron and winds. The abundance and distribution patterns
Dennett, 1999; Gardner et al., 1999). This annual documentedbytheADCPspananentireyearand
oscillation should be evident in cycles of standing were obtained continuously across the transect, in
stock of zooplankton and micronekton, especially contrast to the net-based sampling, which was
in the upwelling region within 600km of limited to a smaller subset of the cruises and
the Omani coast (Smith et al., 1998b; Wishner occurred at discrete locations (Smith et al., 1998b;
et al., 1998). Wishner et al., 1998). Hence, the ADCP data
The SWM, and its associated upwelling and describe the temporal and spatial patterns with
enhanced primary production, is accompanied greater temporal and spatial resolution than
by an elevation in zooplankton biomass (e.g., possible from the net-based sampling and compli-
Qasim, 1977; Smith and Codispoti, 1980; Smith, ment the results of those studies.
1982, 1984; Matthew et al., 1990; Baars and Acoustic Doppler current profilers (ADCPs)
Oo.sterhuis, 1997; Brink et al., 1998; Smith et al., have been used to describe the spatial and
1998a; Wishner et al., 1998). Another pattern temporal patterns in the distribution of zooplank-
observed during the SWM is a pronounced ton biomass in many oceanic regions and
C.J.Ashjianetal./Deep-SeaResearchII49(2002)2377–2402 2379
scattering off of the organisms collected in the net
tows. In reality, organisms that are not collected
quantitatively in zooplankton net tows, such as
fish, fast swimming euphausiids, and siphono-
phores, also may contribute substantially to the
backscatter intensity (Wiebe and Greene, 1994;
Holliday and Pieper, 1995; Stanton et al., 1993,
1994). However, analyses of patterns in biomass
derived from ADCP backscatter intensity have
demonstratedthattheinstrumentprovidesreason-
able and biologically meaningful descriptions of
the biomass field and its association with hydro-
graphic characteristics. Acoustic Doppler current
profilers are particularly adept at documenting
changes in the vertical distribution of biomass in
the water column associated with the diel vertical
migration of zooplankton and micronekton
(Plueddemann and Pinkel, 1989; Buchholz et al.,
1995; Heywood, 1996; Ashjian et al., 1998; Luo
et al., 2000).
The diel vertical migration of acoustically
derived biomass from the ADCP in the Arabian
Sea is likely to be dominated by the migration of
Fig.1. Location of transect in the northern Arabian Sea. myctophidfish,sincetheregionischaracterizedby
Distancesalongthetransectarecalculatedasdistanceoffshore. these fish which produce a strong backscatter
Standard stations (S1–S11, S15) for the US JGOFS Process
intensity signal, and, seasonally, by the vertically
studycruises(Smithetal.,1998a)areshown.Theapproximate
locationoftheatmosphericFindlaterJetrelativetothetransect migrating pelagic crab Charybdis smithii (Currie
and standard stations is demonstrated. Seasonal upwelling et al., 1973; Nafpaktitis, 1978; Gj(cid:1)saeter, 1981,
occurs inshore of the Findlater Jet (S2–S4) during the SW 1984; Ropke et al., 1993; Van Couwelaar et al.,
Monsoon. 1997; Herring et al., 1998; Luo et al., 2000). The
pelagic crab C. smithii has been observed in high
abundanceintheupperwatercolumnduringboth
environments (Flagg and Smith, 1989; Pluedde- the SW and NE monsoons. This species ranges in
mann and Pinkel, 1989; Heywood et al., 1991; length from 6–70mm and should contribute
Smithetal.,1992;FischerandVisbeck,1993;Roe substantially to the backscatter intensity when
and Griffiths, 1993; Ashjian et al., 1994; Flagg present. Comparison of backscatter intensity dis-
et al., 1994; Lyons et al., 1994; Zhou et al., 1994; tributions collected simultaneously using a
Batchelder et al., 1995; Buchholz et al., 1995; 153kHz ADCP and 12kHz sonar in the Arabian
Heywood, 1996; Ashjian et al., 1998; Luo et al., Sea suggested that the strongly migrating layers,
2000). In such studies, backscatter intensity observed by both acoustic instruments, were
routinely is converted to equivalent zooplankton composed of fish whereas the non-migrating
biomass using a biomass-intensity regression layers, seen only in the ADCP backscatter
derived from net-collected zooplankton samples intensity, were attributed to zooplankton (Luo
and spatially and temporally coincident acoustic et al., 2000). Although mesozooplankton also
backscatter intensities. Such a relationship pro- migrate on a diel basis in this region (Smith et al.,
vides only a rough estimate of the potential 1998b; Wishner et al., 1998), the larger size of the
biomass in the water column, since it assumes micronekton should produce most of the back-
that the total backscatter intensity is derived from scatter intensity signal for the vertically migrating
2380 C.J.Ashjianetal./Deep-SeaResearchII49(2002)2377–2402
layers. Biomass in the upper water column should currentprofilerwasoperatingcontinuouslyonthe
be dominated by non-migrating copepod species ten cruises of the Arabian Sea study, greater
during day but overwhelmed by the biomass of temporal (diel and seasonal) and spatial (horizon-
verticallymigratingfishandpelagiccrabsatnight. tal and vertical) resolution and coverage of the
The vertically migrating myctophids and pelagic distributionofplanktonandnektonwereachieved
crabs are important predators because they feed than was possible from the net-based sampling
primarily during night on zooplankton of the programsalone(Smithetal.,1998b;Wishneretal.,
upper water column (e.g., Gj(cid:1)saeter, 1984; 1998).
Dalpadado and Gj(cid:1)saeter, 1988; Kinzer and
Schulz, 1991; Kinzer et al., 1993; Van Couwelaar
et al., 1997). 2. Methods
The goals of the present study were to (1)
document the seasonal changes in integrated Tencruiseswereconductedin1994–1995across
biomass, particularly those associated with the a transect extending from near the coast of Oman
changing monsoonal periods, across a 1000-km (181300N, 571180E) to 1000km offshore (141270N,
transect in the northern Arabian Sea, (2) contrast 651000E) on the R/V Thomas G. Thompson during
the magnitude and cycles of biomass inshore and both the US JGOFS Arabian Sea Process Study
offshoreof the atmospheric Findlater Jet and also and the Forced Upper Ocean Dynamics Program
at a reference station located in the central (Table 1; Fig.1). Four of the cruises surveyed the
Arabian Sea removed from the influence of the transect line twice, resulting in 14 total transects.
monsoons, (3) describe seasonal and spatial Datawerecollectedcontinuouslyin5minaverages
changes in the diel vertical migration behavior of using a hull-mounted, 153kHz RD Instruments
themigratingbiomass(myctophids,pelagiccrabs), acoustic Doppler current profiler (ADCP). Most
and (4) evaluate the potential predation pressure datawerecollectedin8mdepthbins,however4m
of these vertically migrating myctophids in the depth bins were used over the shelf. The ADCP
different regions. Because the acoustic Doppler backscatter intensity data were calibrated and
Table1
Datesandcruisenumbersofthe14transectsusedinthisstudy
Cruise–Transect Dates Season CentralArabianSea
TN042-outgoing November30–December9,1994 NEMonsoon
TN042-ingoing December9–13,1994 NEMonsoon
TN043 January17–31,1995 NEMonsoon *
TN044-outgoing February10–16,1995 NEMonsoon
TN044-incoming February16–21,1995 NEMonsoon
TN045 March23–April7,1995 Intermonsoon *
TN048-outgoing June26–July4,1995 SWMonsoon
TN048-incoming July4–9,1995 SWMonsoon
TN049 July30–August11,1995 SWMonsoon *
TN050 August30–September12,1995 SWMonsoon *
TN051-outgoing September21–October2,1995 Intermonsoon
TN051-incoming October2–5,1995 Intermonsoon
TN053 November6–18,1995 NEMonsoon *
TN054 December12–26,1995 NEMonsoon *
Cruises TN043, TN045, TN049, TN050, TN053, and TN054 were conducted as part of the US JGOFS study (National Science
Foundation).CruisesTN042,TN044,TN048,andTN051wereconductedaspartoftheForcedUpperOceanDynamicsProgram
(OfficeofNavalResearch).Theseasonsofthemonsoonsampledbyeachcruisearenotedand,withtheexceptionofTN044-incoming,
areidenticaltothosedefinedbyMorrisonetal.,1998.CruisesduringwhichthecentralArabianSeastationwassampledalsoare
noted(*).
C.J.Ashjianetal./Deep-SeaResearchII49(2002)2377–2402 2381
range-correctedaccordingtothemethodsofFlagg using the functional regressions of Wiebe et al.
andSmith(1989)andRDInstruments(1990).The (1975) and Wiebe (1988). The biomass-intensity
ADCPdatawereenteredintoandaccessedfroma relationship was derived by comparing zooplank-
Common Oceanographic Data Access System ton dry weight from discrete depth ranges with
(CODAS) database, developed by E. Firing at spatially and temporally averaged coincident
the University of Hawaii. The ADCP transducer backscatter intensity data from the ADCP. The
ontheThompsonismountedonapodthatextends overall correlation between backscattered inten-
below the hull, producing a remarkably low-noise sity, in dB, and log of the dry weight, in mg/m3,
environment and one free of bubble interference was 0.68. A Model 2 regression was used to
frombubble-sweep-down.TherangeoftheADCP convert all backscatter intensities (dB) in the
excluded the upper B20m of the water column database to approximate equivalent dry weight,
because of the depth of the instrument on the hull using the following relationship: Log (dry
10
of the ship and the blanking interval of the weight)=5.996+0.0662(cid:2)intensity. Data from
instrument. The deck unit of the ADCP was both day and night were utilized in deriving this
located in a climate-controlled laboratory that relationship. It is likely that the 1m2 MOCNESS
experienced constant temperature during the undersampled the night biomass because the net
period of data collection. may not effectively capture fast swimming micro-
Duringsixofthecruises,alocationfaroffshore nekton. Unfortunately, sampling with a net
in the Arabian Sea (101000N, 651000E), removed appropriate to capture fast swimming micronek-
from the influence of the monsoons and the ton, such as a 10m2 MOCNESS, was not
oxygen minimum zone, was sampled to serve as conducted from the Thompson during these
a reference point (station S15) (Fig.1; Table 1). cruises, so estimates of micronekton abundance
The ADCP datafrom this location were extracted were not available. Hence the biomass estimates
and used to verify the consistency of the ADCP derived from the regression, particularly for the
data over the course of the study period, and to datacollectedduringthenight,mayunderestimate
establishabaselineofminimal seasonalityintotal the actual biomass present.
biomass and diel vertical migration behavior to All data between the beginning and end points
compare with those characteristics observed along of the transect were extracted from the database
the 1000-km transect line (Fig.1). Examination of without vertical or temporal averaging. The
the backscatter intensity from this location re- various cruise tracks frequently did not lie
vealed little variation in minimum intensity preciselyalongthetransect.Excursionsbothalong
(noise), but a degradation in the penetration of and off the transect occurred during stations and
theacousticsignalanddepthrangeovertheperiod during the execution of grid surveys by towed
of the study. instruments (cruises TN042, TN044, TN048,
Vertically discrete zooplankton samples were TN051) (Lee et al., 2000). Furthermore, station
collected using a 1m2 Multiple Opening/Closing keeping resulted in multiple time periods being
Net and Environmental Sensing System (MOC- surveyed at a single location. Hence, a secondary
NESS; Wiebe et al., 1976), equipped with 150mm data set consisting of data along the transect line
meshnets,duringthreeofthecruises(Smithetal., was derived from the complete data set by
1998b). These samples were used to derive an removing observations collected during station
empirical relationship between backscatter inten- keeping and then projecting the position of each
sity from the ADCP and zooplankton biomass. off-track ADCP profile orthogonally back onto
Zooplankton biomass was determined from the the transect line. The primary and secondary data
net plankton samples by first determining the sets were plotted as vertical sections as a function
displacement volume of each preserved sample of time (primary data set) or distance offshore
using methods modified from Ahlstrom and (secondary data set) for the 14 transects.
Thrailkill (1963) and then converting the displace- Integrated water-column biomass was calcu-
ment volume to equivalent dry weight (mg/m3) lated for both the 1000-km transect (edited data)
2382 C.J.Ashjianetal./Deep-SeaResearchII49(2002)2377–2402
and the offshore station over a 20–120m depth fromthedatafield)basedonfieldvariance,sensor
interval. Because the ADCP is not located at the error, and the uncertainty in the estimation of the
sea surface, but mounted on the hull of the ship, mean.
datawerecollectedonlybelow20m.Thedepthsto Diel vertical migration (DVM) in the data was
which reliable estimates of backscatter intensity identified byexamining thevertical distributionof
(% good >92; e.g., Flagg and Smith, 1989) were biomass relative to the timing of day and night
obtained shoaled dramatically during night, pre- overthesampledwatercolumn.Themediandepth
sumably because of the absence of scatterers in (depth at which 50% of the biomass is located
deeperlayersduetoupwarddielverticalmigration above and below) was selected to represent the
(FlaggandKim,1998;similarpatternobservedby verticaldistributionofbiomassinthewatercolumn
Herring et al., 1998). Hence, only those profiles and to identify periods of diel vertical migration
that extended to 120m were considered in this (e.g., Ashjian et al., 1998). This statistic best
analysis to ensure that backscatter intensity represented the observed vertical distribution of
measurements were present for all depths consid- biomass of the several statistics considered (depth
ered when integrating and comparing biomass. ofmaximumbiomass,meandepth).Useofasingle
Because of the strong diel signal, day and night statistic to represent vertical distribution could be
integrated biomasses were considered separately. misleading if multiple migrating scattering layers
Day and night integrated biomasses and chlor- were observed, such as seen in Luo et al. (2000).
ophyll a concentrations were further processed by For the present data, only single migrating layers
objectiveanalysis(OA)accordingtothemethodof were observed. The median depth was calculated
Mariano and Brown (1992) to calculate predation foralldatacollectedduringatransect(noremoval
pressureindicesandtoregressdaytimebiomasson of data points because of station keeping or
chlorophyll a concentration. Objective analysis is excursions from the transect line). Potential
useful in projecting data collected asynoptically along-track variation in migration behavior was
onto temporally and spatially uniform fields. A examinedbycomparingvertical distributions from
modified format was adopted to analyze the different along-track locations for each cruise.
averaged data over distance (1000-km along Vertical migration velocities were calculated
transect) and time (180 days) for all cruises. The from the median depth of biomass. The median
observation variable (i.e. daytime biomass, night- depth first was smoothed over time using a
time biomass, chlorophyll a concentration) was 10-point running mean (B50min) to reduce
decomposedintoameanfield,anaturalvariability small-scale variation. Vertical migration velocities
(mesoscale) field, and a subgrid-scale noise error then were calculated between each successive data
field (for equations, see Mariano and Brown, point as the change in depth (m) per time period
1992). The mean field was estimated by a two- (s). Most observations were separated by 5min
dimensional bicubic spline with adjustable intervals except when a profile was missing from
smoothness and tension parameters. The mean the data because of poor quality.
distributions were removed from the data before Periodswhendielverticalmigration(DVM)was
OAcalculations.Aninterpolationgridwasdefined strong were identified using a DVM Index. This
based on the number of points and resolution in index reflects the adherence of the vertical
space and time for the output field estimate distribution of the median depth to a sinusoid
(101(cid:2)101 grid, 0.011 resolution). At each grid curve (e.g., Pearre, 1979), which approximates
point, a weighted local average of the influential the diel changes in depth of isolumes through the
detrended data points was calculated and sub- water column. Before calculating the index, the
tractedfromeachinfluentialdetrendeddatapoint. vertical distribution of the median depth for each
The output for each variable was an OA field of day was time-standardized so that sunrise and
identical spatial and temporal extent calculated sunsetoccurredat0600and1800,respectively,and
from the sum of the OA estimate, the local depth-normalized to a range of 0–1 so that the
average,andthetrend,andatrendfield(deviation maximum median depth was set to 1 and the
C.J.Ashjianetal./Deep-SeaResearchII49(2002)2377–2402 2383
minimum to 0 (e.g., Ashjian et al., 1998). An both of along-transect distance (Fig.2a) and of
‘‘ideal’’ distribution then was calculated for each time (Fig.2b) from the March–April, 1995 cruise
day by converting the observed times of each are shown as examples of typical vertical sections
profile to equivalent radians and calculating the observedforthe14transects.Astrongdielvertical
cosine function of the sinusoid curve. The sum of migration signal was the prevalent feature of the
squares (SS) of the deviation then was calculated verticaldistributionforallofthecruises,withonly
for each day and used as the DVM index: a single migrating layer observed. Greatest bio-
mass was observed near the surface during night
1 XN
dev:SS¼ ðobserved median depth and at depth (>250m) during day. Strong and
N(cid:4)1 i
i rapidverticalredistributionsofbiomassasasingle
(cid:4)predictedmediandepthÞ2; coherent mass were associated with periods of
i
sunrise and sunset. Most profiles collected during
fori¼1toN;whereN isthenumberofprofilesin dayextendedquitefarintothewatercolumn.The
that day. Therefore, a lower value of the DVM penetration of the acoustic signal to depth was
indexindicatesgreateradherencetoanidealDVM limited during night because of the aforemen-
distribution, as simulated by a sinusoid. tioned absence of sound scatterers (zooplankton
The vertical range of the acoustic data was and micronekton). The along-transect vertical
limited during both day and night for the last two distribution (Fig.2a) demonstrates the temporal
cruises (November 1995 (TN053); December 1995 discontinuities resulting from station-keeping and
(TN054)). Because of an apparent degradation of the editing of the data.
the ADCP signal, the DVM index was not The vertical distribution of the median depth
calculated for December 1995 (TN054). Examina- clearly showed the effect of DVM on the distribu-
tion of the vertical sections of intensity from the tion of biomass, following a sinusoid pattern
reference station located in the central Arabian throughout the 24h, and was an effective means
Sea over the period of the study demonstrated of representing the vertical distribution of the
that, although the depth range of the instrument biomass (Fig.2b, upper panel). Occasionally, the
clearlywasreduced,themagnitudeoftheintensity median depth of biomass deviated from this
did not appear to be affected. Comparisons pattern when the biomass signal was reduced
involving the magnitude of biomass from Novem- (e.g., Fig.2b, day 28). Vertical velocities (Fig.2b,
berandDecember1995wereconsideredwithcare. lower panel) showed prominent peaks of upward
Statistical analyses were conducted using MA- or downward movement (migration) associated
TLAB (Mathworks, Inc.) or the Statistical Analy- with times of sunset or sunrise, respectively.
sis System (SAS; SAS Institute, Inc., 1985). For
multiple comparisons of means, analysis of var- 3.2. Spatial and temporal distribution of ADCP
iance was used to identify whether significant biomass
differences existed and the Student-Newman-
Keuls post-hoc test was used to identify which The ADCP biomass present in the upper 120m
means or groups of means were significantly during day was considered to be zooplankton,
different (Zar, 1984). while that present during night was considered to
be zooplankton plus those organisms that migrate
to the upper 120m during night, primarily
3. Results myctophidfish,swimming crabs,andeuphausiids.
Because of the different compositions of the
3.1. Vertical distribution of acoustically-derived biomass, day and night distributions and patterns
biomass of biomass were considered separately. The
influence of the SWM (June–September, 1995)
Theverticaldistributionsofacoustically-derived was clear in the along-transect distribution of
biomass(hereinafterADCPbiomass)asafunction vertically integrated (20–120m) zooplankton
2384 C.J.Ashjianetal./Deep-SeaResearchII49(2002)2377–2402
Fig.2. Vertical distributions of biomass from March–April, 1995 (TN045) as a function of along-track distance (n=585 profiles)
(upperpanel),andtime(n=3447profiles)(middlepanel).Periodsofday(white)andnight(dark)areshowninthebarsacrossthetop
ofeachpanel.Themediandepthofbiomassasafunctionoftimealsoisshown(blackline).Thelowerpanelshowstherateofvertical
displacementofthemediandepthofbiomass,calculatedafterapplyinga10-pointrunningmean.Verticalprofilesweregriddedtoa
densityof1point/20mintheverticaland1point/5kmfordistanceor1point/hfortimeinthehorizontal.Thealong-tracklocationsof
standardUSJGOFSprocessstudystations(S2–S10)areshownasreference(upperpanel).
biomass throughout the year (Fig.3a). Elevated also was observed inshore of the Findlater Jet
zooplankton biomass was observed inshore (sta- during the SWM (July); this peak corresponds
tions S2–S5) of the Findlater Jet (Jet located with the peak in zooplankton biomass.
between stationsS6 and S7; seeFig.1)duringlate When the mean, vertically integrated ADCP
June–early October, extending offshore to the biomass for regions inshore (stations S2–S4;
Findlater Jet region during a single cruise in 104–403km along-transect) and offshore (stations
August. Peak zooplankton biomass was observed S7–S11; 584–1064km along-transect) of the
inJulyatstation S3,with the greatest areal extent Findlater Jet for each sampling period are
of enhanced biomass occurring during the late compared, seasonal patterns emerge (Fig.4).
SWM(September).ElevatednightADCPbiomass Biomass was significantly greater during night
(the sum of zooplankton and migrator biomasses) (migrators plus upper-ocean zooplankton) than
was observed offshore during the NEM and day (upper-ocean zooplankton only) for each
Spring Intermonsoon (SI) periods from December sampling period within each region (ANOVA,
to April (Fig.3b). Elevated night ADCP biomass po0:0001). Similar temporal patterns were
C.J.Ashjianetal./Deep-SeaResearchII49(2002)2377–2402 2385
Fig.3. AnnualdistributionofintegratedADCPbiomassintheupperwatercolumn(20–120m)forday(upper)andnight(lower)asa
functionofalongtrackdistanceandtime.Thealong-tracklocationsofthestandardUSJGOFSprocessstudystations(S1–S11)are
shownasreference.Dataweregriddedtoadensityof1point/15daysintheverticaland1point/25kminthehorizontal.Gapsinthe
distributionresultedwhenalong-transectlocationswerenotsampledduringthatperiod(dayornight).Notethescalechangebetween
dayandnight.NocruisestookplaceinMayandmostofJune1995.
observed for both zooplankton and zooplankton intermediate biomass during the 1994 NEM, and
plus migrators within each region. Greatest lower biomass during the fall 1995 NEM (ANO-
biomass inshore was observed during the SWM, VA, po0:001; Student Newman Keuls (SNK),
2386 C.J.Ashjianetal./Deep-SeaResearchII49(2002)2377–2402
Fig.4. Average biomass in the upper water column (20–120m) for regions inshore (S2–S4; 104–403km from shore) and offshore
(S7–S11; 584–1064km from shore) of the atmospheric Findlater Jet along the transect line during day (zooplankton) and night
(zooplanktonandmigrators).Onlythoseprofilesextendingdownto120mwereused.Standarddeviationsareshownwiththeerror
bars.Eachmeanwascalculatedutilizing392–659profiles.Separatemeansarecalculatedforeachlegforcruiseswithbothanincoming
and outgoing leg. The periods of the Northeast Monsoon (NE), Spring Intermonsoon (SI), Southwest Monsoon (SW), and Fall
Intermonsoon(FI)areseparatedbydashedlines.
po0:05). The patterns of both types of biomass December 1995 for all four region/diel period
offshore were the reverse, with greatest biomass groupings (with the exception of September–
generally observed during the fall 1994 NEM and October 1995, but see large variability in mean).
Intermonsoon periods, lower biomass during the This may have resulted from interannual varia-
SWM, and lowest biomass during the early NEM bilityin biomass between the two years or may be
in 1995 (ANOVA, po0:001; SNK, po0:05). For a consequence of the degradation of the ADCP
repeat transects within a cruise, biomasses usually signal at the end of 1995 (see methods section).
were similar, suggesting that spatial and temporal Averages of integrated biomass (20–120m) at
patchiness had little effect for those periods. thecentralArabianSeaoffshorestation(S15)were
Biomasses were lowest during November and compared to averages from the regions inshore