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Journal of Marine Systems 50 (2004) 61–77

Diurnal vertical motions over a seamount of the

southern Gulf of California

Arnoldo Valle-Levinsona,*, Armando Trasvina Castrob,Guillermo Gutierrez de Velascob, Rogelio Gonzalez Armasc

aCenter for Coastal Physical Oceanography, Ocean, Earth and Atmospheric Sciences Department,

Old Dominion University, Norfolk, VA 23529, USAbCentro de Investigacion Cientıfica y Educacion Superior de Ensenada en Baja California Sur,

La Paz, Baja California Sur 23050, MexicocCentro Interdisciplinario de Ciencias Marinas, La Paz, Baja California Sur 23000, Mexico

Received 13 December 2002; accepted 30 September 2003

Available online 4 June 2004

Abstract

A 6-month time series of velocity and scattered sound intensity profiles is used to document diurnal vertical motions over a

seamount, El Bajo Espıritu Santo, in the southern Gulf of California, Mexico. The vertical motions persisted throughout the

period of measurements and is a noteworthy finding in this study. Data were collected with a 153.6-kHz acoustic Doppler

current profiler (ADCP) deployed in water f 300 m deep between June 19 and December 8, 1999. Vertical velocities and

scattered sound intensity anomalies recorded by the ADCP showed a well-defined diurnal periodicity throughout the period of

observation. Peaks of both variables coincided with the timing of sunrises and sunsets and featured phase shifts consistent with

upward or downward motions. It was likely that the diurnal peaks in vertical velocities and scattered sound intensity anomalies

were associated with motions of the deep scattering layer that depicted vertical migrations of euphausiids and other

zooplanktonic groups. This was suggested by plankton samples collected over the seamount early in the deployment and also

given the frequency of the emitted sound and the documented dominant taxa of El Bajo Espıritu Santo. Typical upward

velocities, at sunset, were 3 cm/s while downward velocities, at sunrise, were 4 cm/s over vertical spans of 100–150 m. The

persistence of diurnal vertical motions throughout the 6 months of measurements was attributed to injections of zooplankton

swarms from other parts of the Gulf of California by means of near-surface advective fluxes. Once in the area of the seamount,

continued vertical migrations effected by the zooplankton groups probably reduced their chances of being swept away from this

area as subtidal flows at depth were considerably weaker ( < 5 cm/s) than those near the surface (>10 cm/s) and frequently in

opposite direction.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Vertical motion; Seamount; Zooplankton

0924-7963/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmarsys.2003.09.016

* Corresponding author. Tel.: +1-757-683-5578; fax: +1-757-683-5550.

E-mail address: [email protected] (A. Valle-Levinson).

A. Valle-Levinson et al. / Journal of Marine Systems 50 (2004) 61–7762

´
1. Introduction
Vertical motions of zooplankton and other organ-

isms in the ocean and in lakes have been widely

documented with acoustic methods (e.g. Pluedde-

man and Pinkel, 1989; Robinson and Gomez-

Gutierrez, 1998; Luo et al., 2000). The rate of

these motions is usually inferred by estimating

slopes of the acoustic signal strength during the

periods of rapid changes in depth and time. Fewer

studies have been able to document directly mea-

sured values of these vertical migration rates (e.g.

Rippeth and Simpson, 1998; Winant et al., 1994;

Thomson and Allen, 2000; Pinot and Jansa, 2001;

Liljebladh and Thomasson, 2001). The availability

of acoustic Doppler current profilers with four

transducer heads distributed in a Janus configura-

tion (two orthogonal pairs) has allowed redundant

measurements of vertical velocities. In addition to

measuring each component of the horizontal veloc-

ity, each pair of transducer heads records a value of

vertical velocity that allows for quality control of

the data. The purpose of this study is to document

diurnal vertical motions over a seamount in the

southern Gulf of California throughout a 6-month

period that extends over the latter half of 1999.

This is the first time that such migrations are

recorded in the Gulf of California with such tem-

poral resolution and over such an extended period,

and also constitutes one of the few extended

records of diurnal vertical migrations anywhere in

the ocean. This purpose was achieved with data of

vertical velocities and acoustic strength anomalies

recorded by an acoustic Doppler current profiler

deployed in the ‘El Bajo Espıritu Santo’ seamount.

1.1. Study area

The El Bajo Espıritu Santo seamount is located in

the southern Gulf of California, about 9 nautical

miles off the Bay of La Paz and off the Espıritu

Santo Island (24j42VN and 110j18VW, Fig. 1). The

seamount is surrounded by a deep channel to the east

that reaches 1000 m, and typical depths elsewhere

reaching f 400 m. The seamount summit consists of

two shallow (depth of f 20 m) peaks, 100 m apart

joined at a common depth of about 100 m (Klimley

and Nelson, 1984).

The El Bajo Espıritu Santo is recognized by fish-

ermen and oceanographers alike as an area where

large quantities of fish congregate. Fishermen and

divers are attracted to the area by tuna, dolphin fish,

marlin, manta ray, whale shark and the large schools

of hammerhead shark that assemble here (Klimley and

Butler, 1988; Muhlia-Melo et al., 2003). In the vicin-

ity of the seamount, high concentrations of zooplank-

ton and fish larvae are also found. Fish larvae

diversity is high, 104 different species have been

identified here. This represents nearly 30% of all

species present in the Gulf of California (Gonzalez-

Armas, 2002) concentrated in an area that is only

f 0.05% of the entire gulf.

Seasonal temperature changes are large in the

southern Gulf of California. Tropical conditions occur

during the months of May to November, sea surface

temperatures (SSTs) vary between 25 and 29 jC.During the rest of the year, SSTs are more typical of

subtropical latitudes, ranging from 19 to 24 jC (un-

published data from the authors). The main tidal

components in the gulf are the M2 (semidiurnal) and

K1 (diurnal). In this context the seamount is a subtrop-

ical, shallow seamount. The frequency of the tidal

forcing (K1 +M2) is superinertial since the local iner-

tial period is 28.6 h.

A number of physical processes should contribute

to the local enhancement of biological productivity at

different time scales (Trasvina et al., 2003). Increased

vertical shear of the currents generates mixing over

the seamount’s summit that promotes efficient redis-

tribution of nutrients to the photic zone as reflected by

high chlorophyll-a concentrations (Santamarıa-del-

Angel et al., 1994; Klimley and Butler, 1988).

Three-dimensional tidal advection is important during

spring tides. Surface hydrographic fields change

quickly due to frequent, short-lived wind bursts that

force current jets out of the neighboring Bay of La

Paz. Impinging large-scale flows perturb the vertical

structure along the flanks of the seamount. Low

frequency (1–3 weeks) cool and warm filaments,

consequence of the mesoscale structure in the Gulf

of California, also reach the seamount (Soto-Mar-

dones et al., 1999).

In addition to biomass, planktonic diversity is also

high in the Bajo Espıritu Santo region, as 104 different

species of fish larvae have been identified. This is

thought to be due to the specific geographical location

Fig. 1. Site of ADCP deployment (star) showing the bathymetry (contoured at 100 m intervals) around El Bajo Espıritu Santo seamount in the

context of the Gulf of California. The island appearing on the lower left portion of the figure is Espıritu Santo.

A. Valle-Levinson et al. / Journal of Marine Systems 50 (2004) 61–77 63

of the seamount. Outflows from the Bay of La Paz

promote higher diversity of species by carrying zoo-

plankton from a region that is biologically distinct

from the seamount. Also, complex mesoscale struc-

tures (eddies, jets) in the southern Gulf of California

are likely to transport eggs and fish larvae across the

gulf (Hamman et al., 1988, 1998; Green-Ruiz and

Hinojosa-Corona, 1997). Amador-Buenrostro et al.

(2003) report a mesoscale eddy just east of the sea-

mount that affects local hydrographic fields. Their

infrared imagery is consistent with the circulation

around the eddy being capable of transporting proper-

ties from an upwelling region off the eastern coast of

the gulf to the area around the seamount.

The reason for the documented aggregation of

fish and larvae around the seamount area, however,

is not completely clear. In higher latitude seamounts,

the interaction between bathymetry and energetic


subinertial components of the tide produces reso-

nance, resulting in closed circulations on the sea-

mount or trapped baroclinic waves around it

(Chapman, 1989; Rhines, 1969; Huthnance, 1974).

In particular, the El Bajo Espıritu Santo seamount is

located below the critical latitude (30j N) and

therefore the main tidal components are superinertial.

This study presents observational evidence of the

presence of vertical migrating particles, believed to

be zooplankton, around the seamount for a period of

at least 6 months.

2. Data collection and processing

A 6-month time series of velocity and sound

intensity profiles is used to document diurnal vertical

motions in the water column over a seamount of the

southern Gulf of California, Mexico. The profiles

were obtained with a RD Instruments 153.6 kHz

Broad Band acoustic Doppler current profiler (ADCP)

during the period between 23:00 (local time) on June

19, 1999 and 09:00 on December 8, 1999 (days 170–

342). The instrument was deployed at 24j42.656VN,110j17.727VW on a taut-wire buoy over a depth of

f 300 m. The instrument’s transducers faced upward

at f 50 m from the sea bed and were oriented at 20jfrom the vertical. The mooring was located at a

distance of f 1 km to the northeast of the crest of

El Bajo Espıritu Santo (Fig. 1). The ADCP recorded

30 pings distributed over 40-min intervals and 40 bins

with a length of 6 m. The first bin was centered at

around 63.5 m from the sea bed, or 13.5 m from the

transducers, and the last usable bin was bin 37, at

f 279 m from the bottom. Additionally, a thermistor

chain was deployed near the ADCP to examine the

temporal changes in the vertical structure of the

thermal field. Unfortunately, the thermistor chain

was not successfully recovered. The variables

recorded by the ADCP that receive most attention

here are the vertical component of the flow w (in cm/

s) and the intensity of sound scattering in the water

column I (in arbitrary units).

The vertical component of the flow was slightly

trimmed from values where the error velocity, the

difference between the redundant vertical velocities

measured independently by each pair of ADCP trans-

ducers, fell outside F 3 cm/s. This threshold was

determined empirically through examination of the

time series of error velocity that showed randomly

distributed (in time) sporadic values that exceeded

such threshold throughout the water column.

The intensity of the sound scattered I was normal-

ized as in Plueddeman and Pinkel (1989) and Rippeth

and Simpson (1998). The aim of this normalization is

to derive a measure of the scattering strength in order

to describe temporal changes (i.e., the normalization

consisted in determination of intensity anomalies

IVrelative to a mean intensity hI i). Following Rippeth

and Simpson (1998):

IVðz; tÞ ¼ Iðz; tÞ � hIiðzÞ ð1Þ

where I(z,t) = 10log10[RSSI(z,t)], and RSSI is the

received signal strength indicator as recorded directly

by the ADCP. The mean intensity hI i(z) is depth-

dependent and calculated over an arbitrary period.

3. Data description

Both w and IV(z,t) showed clear diurnal periodici-

ties throughout a large portion of the water column.

This is first observed in the w contours for each full

month of deployment (Fig. 2). Pulses of negative

w appeared in the early part of each day, while

positive w developed in the late portion of each day.

The regularity of these pulses throughout the 6 months

of observation is remarkable and a first indication of

vertical migrations throughout the period. The timing

and extent of these pulses are better illustrated in Fig.

3, where shorter periods, selected arbitrarily, have

been portrayed. The vertical extent of the pulses

reached between 120 and 150 m, occasionally even

exceeding 150 m. It is clear that negative w (descend-

ing motions) appeared toward the first third of the day

and positive w (ascending) occurred at the beginning

of the second third of the day. A slight shift in these

motions is noticeable at times in Fig. 3 in such a way

that descending motions occurred initially near the

surface and later at depth, whereas ascending motions

appeared initially at depth. The sampling interval (40

min) hindered a better elucidation of these phase

shifts.

Closer examination of all w velocities as a function

of the hour of the day indicated that descents occur at

Fig. 2. Color contours of vertical velocity w versus depth (distance from ADCP transducers in meters) and time for the months covered completely by the deployment. Positive (yellow

and red) values represent ascending velocities. Note the persistent alternation between descending velocities early in the day and ascending velocities late in the day.

A.Valle-L

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s50(2004)61–77

65

Fig. 3. Same as Fig. 2 but for three 10-day periods, selected arbitrarily.

A.Valle-L

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s50(2004)61–77

66


sunrise, between 05:30 and 07:00, and ascents devel-

op at sunset, between 18:00 and 20:00 (Fig. 4). Also,

the phase shift from surface to bottom on descents and

vice versa on ascents is more noticeable in this

representation. The average descent velocity for the

Fig. 4. Vertical velocities w at selected distances from the ADCP transducer

continuous, thick line denotes the average w of all observations (asterisks)

light shaded regions denote times of sunrises and sunsets throughout the 6-

compared in the lowermost panel with the darkest line being the closest t

entire 6 months of observations is a little more than 2

cm/s, while the average ascent velocity is a little less

than 1 cm/s. These w velocities were uncorrelated to

the horizontal velocities, which represents, together

with the timing of the w peaks, another indication of

s, for the entire deployment, as a function of the hour of the day. The

at each time. Dark shaded regions indicate periods of darkness while

month deployment. Vertical velocity averages at different depths are

o the transducers.


migrating particles or organisms. At speeds of 2 cm/s,

the particles would descend 48 m in the 40 min

sampling interval. Likewise, at 1 cm/s, particles would

ascend 24 m in 40 min. Figs. 2 and 3 show that the

w pulses span only two 40-min intervals. Therefore, in

order to traverse 100–150 m as observed, the particles

would need to move at larger w than the average 2 cm/

s on descent and 1 cm/s on ascent.

Indeed, it was not surprising to note that the

w velocities appeared larger upon examination of

Fig. 5. Vertical velocities w at 127 m from the ADCP transducers, for the e

shown separately. Thick lines and shaded regions are the same as in Fig.

their distribution as a function of the hour of the day

for each month, separately (Fig. 5). Average descent

velocities for June, August, September and Decem-

ber were f 4 cm/s, while average ascent velocities

were close to 3 cm/s. These are likely underestimates

of the actual vertical velocities because of the 40 min

sampling interval. In July, October and November,

monthly w averages were smaller than in the other

months. This may be the result of having fewer (or

different types of) scatterers in the study area during

ntire deployment, as a function of the hour of the day. Each month is

4.

Fig. 6. Profiles of the logarithm of the monthly mean intensity

anomaly (arbitrary units).


those months. Another noteworthy feature is the time

progression in the occurrence of the descent peak,

from June to December, in agreement with the delay

in sunrise, and also the advancement of the ascent

peak in conjunction with earlier and earlier sunsets.

Thus, the w velocities portrayed in Figs. 2–5 strong-

ly suggest that vertical motions observed over El

Bajo Espıritu Santo are related to diurnal motions of

the deep scattering layer and represent vertical

migrations of zooplanktonic organisms. It is inferred

that the migrations are carried out by zooplankton

because the frequency of the ADCP (153 kHz)

detects relatively large (1–4 cm long) organisms

and because similarly long and fast vertical excur-

sions have previously been documented for such

organisms (Youngbluth, 1976; Lavaniegos, 1996;

Robinson and Gomez-Gutierrez, 1998; Pinot and

Jansa, 2001). These suggestions are further supported

by the logarithm of the intensity anomaly IV.In the determination of IV, according to Eq. (1), the

average interval has been taken as each month,

separately. Therefore, we refer to anomalies relative

to the monthly means. The monthly means of the log

of the intensity (Fig. 6) indicated that there were less

scatterers (or organisms) at the study site in June and

July and that the monthly mean abundance changed

little from August to December. The distributions of

the anomalies for five full months of observation

illustrated clearly the diurnal vertical excursions of

the organisms (Fig. 7). Marked anomalies were ap-

preciable as vertical streaks that appeared at the same

times as w peaks. These streaks occasionally were

very thin because they portrayed rapid migrations.

Other times, the streaks were well defined and joined

to patches of high anomalies centered around mid-

night, like those around day 200 or 250 where the

diurnal cycle was nicely illustrated. The thickness of

the patches reached 60–70 m, in agreement to those

observed by Robinson et al. (1995). The periods of

highest anomalies, from the monthly mean profile,

were related to energetic near-surface flows, not

necessarily to monthly tidal or moon phase, as pre-

sented in the discussion.

The synchronization between w peaks and large I Vcan be confirmed in the distributions of IV as a

function of hour of the day for different depths in

the water column (Fig. 8). This representation shows

that peaks of IV, denoting increased organism con-

centrations, occurred at sunrise and sunset. In addi-

tion, at sufficiently large distances away from the

transducers (e.g. >50 m), daytime IVwere lower than

nighttime IV. In contrast, close to the transducers

daytime IVwere larger than nighttime IV. These shiftsof IVfrom day to night reinforce the concept of vertical

migrations of organisms consisting of daytime

descents and nighttime ascents.

4. Discussion

Diurnal vertical migrations of organisms respond-

ing to day/night cycles have been documented in

many other studies (e.g. Longhurst and Williams,

1979; Roe, 1984; Hays, 1995, 1996; Robinson and

Gomez-Gutierrez, 1998). The studies have shown that

copepods and euphausiids are capable of carrying out

migrations of hundreds of meters. Copepods and

euphausiids are also the most likely to be detected

by a 150-kHz ADCP (Rippeth and Simpson, 1998;

Pinot and Jansa, 2001). Even though there was no

frequent biological sampling in the El Bajo Espıritu

Fig. 8. Same as Fig. 4 but for the logarithm of the intensity anomaly IV(or lIa as the ordinate label).


Santo during the 6-month ADCP deployment, it is

highly likely that the ADCP sampled euphausiids,

copepods, pteropods and/or mesopelagic fish larvae

(Aceves-Medina et al., submitted for publication). In

fact, net sampling during the first few days of ADCP

Fig. 7. Contours of the logarithm of the intensity anomaly IV(arbitrary units)time for the months covered completely by the deployment. These are anoma

full and new moon are indicated by yellow and dark circles, respectively.

deployment in June of 1999 (Gonzalez-Armas 2002;

Gonzalez-Armas et al., 2002) and in previous years

has shown that the zooplankton communities in the

area are dominated by juveniles and adults of euphau-

siids of transitional and equatorial affinity (Brinton et

as a function of depth (distance fromADCP transducers inmeters) and

lies relative to the monthly average profile depicted in Fig. 6. Days of


al., 1986; De Silva Davila and Palomares Garcıa,

2002). The species that are most likely found in the

El Bajo Espıritu Santo during the second half of the

year (June–December) are Nyctiphanes simplex,

Nematoscelis difficilis and Euphausia distinguenda,

the first one being the most abundant (Elorduy and

Caraveo, 1994; Robinson et al., 1995; De Silva Davila

and Palomares Garcıa, 2002). Even though all the

above aspects of the present study have been

addressed to a certain extent in previous investiga-

tions, this is the first time that diurnal vertical migra-

tions of zooplankton, as well as their migration

speeds, are described with an ADCP in the Gulf of

California. Most importantly, the unique and innova-

tive aspect of this study is the documentation of an

extended period, almost 6 months long, of vertical

excursions of zooplankton at one point. In an attempt

to explain the persistence of vertical excursions during

such a long period, this study also discusses the

probable circumstances that allow retention of zoo-

plankton in El Bajo Espıritu Santo, a seamount that

provides food and shelter to a great number of species

in the Gulf of California.

The pulses of vertical motion illustrated in this

investigation were observable throughout the 6

months of observation (Figs. 2 and 3). This is in

contrast to events lasting a few days, as described in

the Hebridean Shelf (Rippeth and Simpson, 1998) or

the Bullmarsfjord (Liljebladh and Thomasson, 2001),

where advection seemed to transport the zooplankton

patch away from the measuring location. Even

though the vertical motions in El Bajo Espıritu Santo

appeared similar throughout the period, the anomalies

of the sound scatter intensity IVsuggested that the

zooplankton composition and concentrations fluctu-

ated. Different zooplanktonic composition was con-

jectured from the extent of the excursions portrayed

in Fig. 7. Some upward excursions reached near the

surface (e.g. days 186–188, or 263–265, or 305–

325), while others extended to f 150 m from the

transducers (e.g. days 198–202, or 290–291, or

328–334), still some 100 m from the surface. Such

variations in the extent of the excursions have been

attributed to sun irradiance (or cloudiness) and moon

phases (brightness of the moon) (e.g. Pinot and Jansa,

2001), which were not clearly influential in this data

set (Fig. 7). However, the suggestion that variability

in species composition was responsible for variations

in excursion lengths observed here still requires field

validation. Fluctuations in zooplankton concentra-

tions were seen in monthly averages of I (Fig. 6),

which indicated an increase from July to August and

relatively small changes from June to July and from

August to December. Shorter term fluctuations in

zooplankton concentrations were also inferred from

variations in IVthat were related to hydrodynamic

agents.

The periods of most intense IVwere indeed related

to energetic surface flows (Fig. 9). A total of nine

episodes of large positive anomalies with different

duration were identified and centered on days 200,

210, 219, 232, 247, 260, 271, 290 and 330 (see also

Fig. 7). These episodes coincided with periods of

subtidal (passed through a Lanczos filter with half-

power of 40 h) surface currents that typically

exceeded 10 cm/s. Most (six out of nine episodes)

were associated with southeastward flow and seem to

have produced a longer lasting effect than the north-

westward episodes. The most energetic part of these

pulses was concentrated near the surface, which leads

to the speculation that during these nine episodes

zooplankton patches were transported near the surface

toward El Bajo Espıritu Santo. It is further hypothe-

sized that once in the area of the seamount, organisms

benefited by migrating vertically not only to find

plentiful food toward surface waters at night and to

avoid predation during the day, but also to avoid being

advected away and increase their chances to stay in

the area of the seamount where there was abundant

food for them. In doing so, they in turn represent an

ample food source to upper trophic levels in areas

where their daytime descents were limited by shallow

seabed ( < 200 m). This easy availability (shallow, in

daylight, and with prolonged residence times) of food

to upper trophic levels could have caused zooplankton

patchiness through daily gap formations over rugged

bathymetry (Genin et al., 1988, 1994). The restricted

vertical migrations by zooplankton over the seamount

and consequent vulnerability to predators may help

explain the congregation of large fish and mammals in

this area.

The issue of longer residence time in the sea-

mount area by zooplankton may be explored further

through examination of passive particle and verti-

cally migrating particle displacements at different

depths as derived from progressive vector diagrams.

Fig. 9. Low-passed currents at different distances from the ADCP transducers. Vertical shaded bars represent periods with intense positive IVderived from Fig. 7.


An easier visualization of various progressive vec-

tors diagrams, corresponding to different depths, is

achieved through representation of the separate

displacement contributed by each of the two com-

ponents, i.e., eastward and northward displacements

of a passive particle. This allows identification of

the most advective episodes during the observation

period (Fig. 10). Periods of greatest IV (shaded bars

in Fig. 10a) corresponded to rapid displacements in

the E–W direction, but only in near-surface

regions. This correspondence reiterates the idea that

zooplankton patches were advected near the surface

to El Bajo Espıritu Santo. Displacements in both

E–W and N–S directions were large near the

surface but short near the bottom. Water tempera-

ture recorded at the ADCP transducer head (i.e., at

a depth of 250 m) showed that few of the

energetic episodes were also related to advection

of different water properties even near the bottom.

Comparatively, water temperature from a nearby

location at a depth of 30 m showed temperature

changes corresponding to near-surface advective

forcing. Therefore, migrations toward the bottom

would generally minimize transport of organisms

away from the seamount but migrations toward the

surface would increase the chances for them to be

swept away. Perhaps, this is why during most

energetic events, maximum values of IV are ob-

served at 150 m from the transducers, rather than

further up in the water column. The reduction of

horizontal advection can be illustrated with the

horizontal displacements of a vertically migrating

particle.

The diurnal vertical migration pattern used to

simulate horizontal displacements is a simplification

of the observed behavior: a particle stays near the

surface (20 m) at night; descends at dawn at

speeds similar to those found in the ADCP record

Fig. 10. Components of the displacement of a passive particle at different distances from the transducers (upper two panels) and low-passed

water temperatures at two locations in the water column (lower panel). The darkest line, scaled by the left ordinate, represents water temperature

at the ADCP transducers and the other line, scaled by the right ordinate, denotes 30 m depth temperature at a nearby location. Vertical shaded

bars represent periods with intense positive IVderived from Fig. 7.


(4 cm/s), stays at depth (250 m) during daylight

and ascends again at dusk. Progressive vector dia-

grams shown in Fig. 11 result from computing the

horizontal displacements of such a vertically mi-

grating particle, using observed horizontal currents

from the ADCP record. Near-surface and bottom

passive displacements are also included in these

figures to provide additional information on the

horizontal flow. Fig. 11 shows near-surface and

near-bottom displacements for two periods: between

days 274 and 278 and between days 335 and 339.

These periods follow large IVevents in the last week

of September and November, respectively (see also

Fig 7). Large differences in horizontal displace-

ments between near-surface and near-bottom loca-

tions arise from the vertical shear of the flow.

Relatively strong surface currents produce the long

displacement observed in the surface particle. In

contrast, the horizontal path of a vertically migrat-

ing particle shows the tendency to remain close to

its original position in a vertically sheared flow.

Despite the strong surface flows the displacement is

Fig. 11. Five-day progressive vector diagrams of a surface (20 m) passive particle (dark gray line), a bottom (250 m) passive particle (light gray

line) and a particle migrating vertically in a sheared flow (dark line). (a) Period from October 1 to 5 (days 274–278), and (b) from December 1

to 5 (days 335–339).


significantly smaller, during the 5-day periods illus-

trated, than the one experienced by a particle

remaining near the surface. These simple experi-

ments illustrate two instances in which a particle

migrating vertically can remain closer to its original

position than one that remains at a fixed depth, in


agreement with those of Rippeth and Simpson

(1998).

The energetic surface flows during the period of

study were likely caused by wind forcing. Even

though there were no nearby wind velocity meas-

urements available, this was inferred from the near-

surface manifestation of the events and the frequent

bidirectional character of the response with increas-

ing depth. They could have also been caused by

baroclinic cyclonic eddies that can be observed in

the Gulf of California (Amador-Buenrostro et al.,

2003). Distinct forcing was associated with the

pulse around day 330, which was produced by a

cyclonic gyre that covered most of the entrance to

the Gulf of California as observed in sea surface

temperature imagery. In this case, although the

subtidal flows were most intense near the surface,

the flows at depth were in the same direction as

those at the surface. The irregular periodicity of the

pulses indicated that fortnightly forcing by the tides

had little influence on the flow modulation and on

the anomalous zooplankton concentrations. Al-

though the data presented here do not show that

the seamount area is particularly different from its

surroundings, the work of Amador-Buenrostro et al.

(in press) suggests that the flow structure is greatly

modified by it, and the study by Gonzalez-Armas et

al. (2002) show significant congregations of zoo-

plankton over the seamount. The details on how the

flow is modified and why zooplankton is aggregat-

ed in this area remain to be elucidated.

5. Conclusion

A 6-month deployment of a 153.6-kHz ADCP

illustrated vertical diurnal migrations of zooplankton

organisms over a seamount in the southwestern Gulf

of California throughout nearly 6 months from June to

December of 1999. The persistence of zooplankton

migrations over such an extended period is a novel

aspect of this study and is attributed to near-surface

advective episodes that inject organisms into the

seamount region. Diel vertical migrations carried out

by these organisms reduce their chances of being

swept away from the food-rich seamount area. In

turn, their permanence and/or their seabed-limited

vertical descents in the seamount region should con-

stitute a sizable food source for the ecologically rich

and diverse El Bajo Espıritu Santo.

Acknowledgements

This work was completed while AVL was on study

leave at the Centro de Investigacion Cientıfica y

Educacion Superior de Ensenada (CICESE), campus

La Paz. He was supported by U.S. NSF Project

9876565. Data collection was supported by CON-

ACYT, Mexico under contract 2600-N9509, by

CICESE and by the Centro de Investigaciones

Biologicas del Noroeste under project RP3. Instru-

ment deployment was carried out from CICESE’s RV

F. Ulloa and retrieval from the Leo of the Port

Authority in La Paz. We thank their crew for their help

during deployment and retrieval operations. ATC is a

Mexican SNI grant recipient. RGA is a COFAA-I.P.N.

grant recipient. The comments and suggestions to this

manuscript of two anonymous reviewers, J. Dower

and R. de Silva Davila are greatly appreciated.

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