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Role of environmental seasonality in the turnover of a cetacean community in the southwestern Gulf of California

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La Paz Bay is a distinct region within the Gulf of California whose rich cetacean community exhibits an intense annual overturn. We studied the environmental conditions that could drive this change over the course of a year. Cetacean biomass was estimated from monthly surveys, with concurrent collection of water-column measurements of temperature, salinity, nutrients, chlorophyll a (chl a), and biogenic matter fluxes. The water-column structure showed 3 major conditions: deep mixing during winter, stratified isopycnal shoaling in spring and early summer, and deep stratification during late summer and autumn. Chl a and relative fluxes of biogenic silica and calcium carbonate indicated a seasonal succession of primary producers in response to the observed evolution of hydrography. During the periods of mixing and isopycnal shoaling, the bay provided suitable habitat for blue whales, bottlenose dolphins, and common dolphins, while fin whales, Bryde’s whales, and short-finned pilot whales were numerically dominant during the period of stratification. To provide a regional context to the observed seasonality, we fitted temporal least-squares to an 11 yr monthly time series of satellite-derived wind, sea surface temperature (SST), and chlorophyll concentration (CHL). Within the bay, the SST followed the annual monsoonal shift in the wind, whereas CHL showed a bi-modal pattern, with a main peak occurring under mixing conditions in winter and a second peak under isopycnal shoaling in spring/early summer. The regional fitting suggested that the latter period was driven by a localized intraseasonal phenomenon that could be responsible for the higher biological richness of the bay compared to the surrounding gulf.
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MARINE ECOLOGY PROGRESS SERIES
Mar Ecol Prog Ser
Vol. 487: 245260, 2013
doi: 10.3354/meps10217
Published July 30
INTRODUCTION
Biological hotspots in the epipelagic zone have been
described as areas where dynamic processes in the
physical environment lead to enhanced productivity
and aggregation of consumers relative to their sur-
roundings (Palacios et al. 2006). In these areas,
upwelling, mesoscale eddies, and fronts may act in
concert with the local geomorphology to generate
conditions that greatly promote the availability of
© Inter-Research 2013 · www.int-res.com*Email: m.pardo@comunidad.unam.mx
Role of environmental seasonality in the turnover
of a cetacean community in the southwestern Gulf
of California
Mario A. Pardo
1,5,
*
, Norman Silverberg
1
, Diane Gendron
1
, Emilio Beier
2
,
Daniel M. Palacios
3,4
1
Centro Interdisciplinario de Ciencias Marinas, Instituto Politécnico Nacional, La Paz, Baja California Sur 23096, Mexico
2
Centro de Investigación Científica y de Educación Superior de Ensenada - Unidad La Paz, La Paz, Baja California Sur 23050,
Mexico
3
Cooperative Institute for Marine Ecosystems and Climate, Institute of Marine Sciences,
Division of Physical and Biological Sciences, University of California, Santa Cruz, California 95060, USA
4
NOAA, NMFS, Southwest Fisheries Science Center, Environmental Research Division, Pacific Grove, California 93950-2097,
USA
5
Present address: Posgrado en Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México,
Distrito Federal 04510, Mexico
ABSTRACT: La Paz Bay is a distinct region within the Gulf of California whose rich cetacean com-
munity exhibits an intense annual overturn. We studied the environmental conditions that could
drive this change over the course of a year. Cetacean biomass was estimated from monthly sur-
veys, with concurrent collection of water-column measurements of temperature, salinity, nutri-
ents, chlorophyll a (chl a), and biogenic matter fluxes. The water-column structure showed 3 major
conditions: deep mixing during winter, stratified isopycnal shoaling in spring and early summer,
and deep stratification during late summer and autumn. Chl a and relative fluxes of biogenic silica
and calcium carbonate indicated a seasonal succession of primary producers in response to the
observed evolution of hydrography. During the periods of mixing and isopycnal shoaling, the bay
provided suitable habitat for blue whales, bottlenose dolphins, and common dolphins, while fin
whales, Bryde’s whales, and short-finned pilot whales were numerically dominant during the
period of stratification. To provide a regional context to the observed seasonality, we fitted tempo-
ral least-squares to an 11 yr monthly time series of satellite-derived wind, sea surface temperature
(SST), and chlorophyll concentration (CHL). Within the bay, the SST followed the annual mon-
soonal shift in the wind, whereas CHL showed a bi-modal pattern, with a main peak occurring
under mixing conditions in winter and a second peak under isopycnal shoaling in spring/early
summer. The regional fitting suggested that the latter period was driven by a localized intra-
seasonal phenomenon that could be responsible for the higher biological richness of the bay
compared to the surrounding gulf.
KEY WORDS: Ecological succession · Seasonal variability · Nutrient supply · Stratification ·
Biogenic matter fluxes · Trophic levels · Marine hotspots
Resale or republication not permitted without written consent of the publisher
Contribution to the Theme Section ‘Biophysical coupling of marine hotspots’
FREEREE
ACCESSCCESS
Mar Ecol Prog Ser 487: 245260, 2013
prey for large fauna (e.g. Wingfield et al. 2011). Al -
though these hotspots are often detectable through
remote sensing of the ocean’s surface, other areas
that appear oligotrophic and devoid of dynamic fea-
tures at the surface may also attract large feeding
predators and even influence their migration pat-
terns (e.g. Domeier et al. 2012). In such areas, the
biological production maxima may be in the subsur-
face in the presence of a deep/sharp thermocline,
underscoring the importance of measuring hydro-
graphic and biological parameters, both at the sur-
face and in the water column, for the characterization
of biological hotspots.
The most productive areas of the Gulf of California
(hereafter ‘the gulf’; Fig. 1) are located along its east-
ern (continental) side and in the northern region due
to winter upwelling and tidal mixing, respectively
(Lluch-Cota 2000). The southwestern gulf (peninsu-
lar side) is comparatively less produc tive, except for
La Paz Bay (hereafter ‘the bay’), whose photosyn-
thetic pigment concentrations remain high year-
round compared to its surroundings, constituting an
isolated spot of high phytoplankton biomass (Santa-
maria-del-Angel et al. 1994, Luch-Cota & Teniza-
Guillén 2000, Kahru et al. 2004). The bay sustains a
diverse megafauna that includes at least 16 ceta cean
species of temperate, tropical, and subtropical affini-
ties (Flores-Ramírez et al. 1996, Salvadeo et al. 2009).
It also hosts a growing colony of California sea lions
Zalophus californianus (Szteren et al. 2006) and is
visited by whale sharks Rhincodon typus and spine-
tail devil rays Mobula japanica, which arrive in win-
ter, spring, and early summer to feed on zooplankton
(Clark & Nelson 1997, Ketchum-Mejía 2003, Croll et
al. 2012). The rich cetacean community exhibits a
strong annual overturn, with migratory species such
as blue whales Balaenoptera musculus and hump-
back whales Mega ptera novaeangliae occurring
in winter and spring, whereas species with more
tropical affinities, like Bryde’s whales Balaenoptera
edeni, bottlenose dolphins Tursiops truncatus, and
short-finned pilot whales Globicephala macro rhyn -
chus, occur mostly during the summer and autumn.
Fin whales Balaenoptera physalus are resident in the
gulf and, together with the common dolphins Delphi-
nus spp. occur year-round (Flores-Ramírez et al. 1996,
Salvadeo et al. 2009). The hydrographic con ditions
that support this cetacean diversity and underlie the
species’ replacement have not been studied.
In the present study, we posit that seasonal forcing
of oceanographic conditions in the bay, including
surface mixing driven by northwesterly winds in
winter (Badan-Dangon et al. 1991) and cyclonic
circulation in summer (Monreal-Gómez et al. 2001,
Sánchez-Velasco et al. 2006), lead to enhanced nutri-
ent supply to the base of the food web, attracting low
trophic level prey for planktivorous and piscivorous
cetaceans. Warmer conditions in summer would, in
turn, be favorable for the aggregation of higher
trophic level prey looking for a suitable habitat to
spawn near the coast (e.g. Staaf et al. 2008), attract-
ing teutophagous cetaceans. To assess the specific
hydrographic and biological conditions underlying
species’ replacement in the cetacean community of
the bay, we make use of water-column data collected
as part of a multidisciplinary time-series investiga-
tion, aimed at examining monthly changes in physi-
cal structure, nutrient and chlorophyll a (chl a) con-
246
Fig. 1. La Paz Bay, in the southwestern Gulf of California (in-
set). The gray star in the main panel indicates the position of
the oceanographic station and the site of the sediment trap
in the deepest part of the bay (~410 m)
Pardo et al.: Role of environmental seasonality in a cetacean community
centrations, vertical fluxes of biogenic matter, and
cetacean biomasses over the course of a year. The
results are put in a regional context (i.e. the south-
western gulf) using time series of remotely sensed
(satellite) measurements of surface wind, tempera-
ture (SST), and chlorophyll concentration (CHL).
Although direct measurements of the prey field
would have probably enhanced our understanding of
the relationships between cetaceans and the dynam-
ics of their physical habitat in the bay, collection of
such data was not possible due to logistical and
financial constraints. Indeed, quantitative and com-
prehensive assessments of the intermediate trophic
levels in the southwestern gulf are lacking. However,
considering that most cetacean species must con-
stantly search for food due to their high energetic
needs (Trites et al. 1997, Barlow et al. 2008), we inter-
pret their occurrence in the bay, at least in part, as a
response to the prey availability, which is in turn
aggregated by suitable physical and biological
mechanisms. Further, the evolutionary and ecologi-
cal adaptations of cetaceans to exploit specific types
of prey, such as zooplankton, small pelagic fish, or
squid (Pauly et al. 1998), facilitate such interpreta-
tion. This argument has been widely used to charac-
terize cetacean habitats (e.g. Au & Perryman 1985,
Hamazaki 2002, Doniol-Valcroze et al. 2007, Praca et
al. 2009) as well as to model cetacean abundance as
a function of predominant environmental conditions
(e.g. Becker et al. 2010, 2012, Gerrodette & Eguchi
2011, Forney et al. 2012), even when there is no
direct measurement of the potential prey.
MATERIALS AND METHODS
Study area
La Paz Bay is the largest and deepest embayment
in the Gulf of California (Fig. 1), with an area of
~2160 km
2
and a maximum depth of ~410 m. Water
exchange with the surrounding gulf occurs mainly
through the northern channel, called Boca Grande
(Salinas-González et al. 2003, Obeso-Nieblas et al.
2004). The bay lies within a tropical-subtropical tran-
sition zone that seasonally alternates between 2 well-
defined periods as a result of the monsoonal regime
that dominates the entire gulf. The temperate season,
from November to April (winter to spring), is domi-
nated by strong northwesterly winds that enhance
evaporation and increase the surface salinity, which
induces deep vertical convection (i.e. sinking). More
moderate southwesterly winds blow during the warm
season, from May to October (summer to autumn).
During this period, the water column receives the
influence of tropical waters, the thermocline deep-
ens, and the upper layer stratifies (Badan-Dangon et
al. 1991, Adams & Comrie 1997, Bordoni et al. 2004).
Hydrographic and biological conditions
The changes in the physical structure of the water
column and their influence on the base of the food
web, through nutrient supply, help us to identify the
predominant ecological conditions that attract differ-
ent cetacean species at different times of the year.
A number of physical, chemical, and biological vari-
ables were measured at an oceanographic station
located over the deepest part of the bay (~410 m;
Fig. 1). Between 17 February 2007 and 18 February
2008, 13 CTD profiles were taken to depths ranging
from 50 to 340 m. Temperature, salinity, and density
data were standardized to 1 m depth means. From
these values, we computed the Brunt-Väisälä fre-
quency (cycles h
−1
; also known as buoyancy fre-
quency), a measure of the degree of stratification
(Wahl & Teague 1983). The depth of maximum buoy-
ancy frequency in a profile corresponds to the depth
of the pycnocline. At the same site, 13 profiles of
Niskin-bottle samples were taken at discrete depths
according to 6 levels of light penetration (0.1, 1, 10,
33, 55, and 100%) estimated from Secchi disc meas-
urements following the Beer-Bourguer-Lambert law
(Walker 1982, Bustillos-Guzmán & Lechuga-Devéze
1989). Concentrations of dissolved silica (H
2
SiO
4
),
phosphate (PO
4
−3
), and total dissolved inorganic ni -
trogen (NO
2
+NO
3
+NH
4
) were measured from these
samples (Strickland & Parsons 1972), as well as the
concentration of chl a (Ritchie 2008). Since measure-
ments were taken at different times and in some
cases different depth levels, we performed an objec-
tive interpolation of these variables using a Gaussian
weighting function (Jalickee & Hamilton 1977, Boyer
et al. 2005) with 30 d horizontal and 1 m vertical
scales to represent the temporal evolution. All data
were truncated below 100 m depth since preliminary
evaluation of the results showed that most of the
variability was concentrated above that level.
Biogenic matter fluxes
The sinking particulate matter is indicative of the
nature of biogenic components and thus the ecologi-
cal succession taking place in the upper layers
247
Mar Ecol Prog Ser 487: 245260, 2013
248
(Bishop 1988, Silver & Gowing 1991, Silverberg et al.
2006), which could trigger the incursion of different
cetacean species according to their feeding require-
ments. We analyzed samples from a Technicap
®
PPS
3/3 trap of 0.125 m
2
aperture, which was anchored
and suspended at ~310 m depth at the same site as
the oceanographic measurements (Fig. 1). The sink-
ing matter was collected in separate bottles during 7
to 15 d periods each and then fixed with a preser -
vative solution of 4% buffered formaldehyde satu-
rated with sodium tetra borate. The total mass flux, in
g m
−2
d
−1
, was estimated from 4 sub-samples, which
were centrifuged for 25 min at 3000 rpm (~1600 × g),
decanted, and washed with distilled water. The col-
lected material was weighed after a 72 h drying
period at ~50°C. The lithogenic fraction of the total
flux was subtracted since our interest was only
related to the biological processes. From the total
biogenic fractions, we analyzed the proportions of
biogenic silica, or opal (SiO
2
·nH
2
O), and calcium car-
bonate (CaCO
3
).
Local and regional seasonality
Because of their great mobility, the incursions of dif-
ferent cetacean species into the bay could be the
result of both local and/or regional conditions. There-
fore, it was important to address the larger spatial
context in which the hydrographic and biological con-
ditions within the bay occur. Also, since
the small sample sizes yielded by the
present study (14 monthly data points)
prevented us from quanti ta tively corre-
lating cetacean densities to the monthly
evolution of the water column within
the bay, it was important to compare
those conditions to longer time series of
surface variables and put them in the
spatial context of the southwestern gulf.
We therefore characterized the season-
ality of the entire region, from north of
Loreto Bay to south of La Paz Bay (Fig.
2), using an 11 yr time series of remotely
sensed SST and CHL as proxies for the
physical and biological en vironment.
The monthly CHL data came from the
Sea-viewing Wide Field-of-view Sensor
(SeaWiFS) aboard the satellite Orb-
view-2 (O’Reilly et al. 1998, 2000,
Hooker & McClain 2000), with a pixel
resolution of 1.39 km. The monthly SST
data came from the Advanced Very
High Resolution Radio meter (AVHRR)
aboard NOAA satellites (Program
Pathfinder 5.0; Walton et al. 1998, Casey
& Cornillon 1999, Kilpatrick et al. 2001),
with a spatial resolution of 4.89 km. Ad-
ditionally, we used the monthly wind
velocity data from the SeaWinds sensor
aboard the NASA satellite QuikSCAT
(Freilich 2000). Due to the coarser
spatial resolution of this product (13.9
km), only the measurement point closest
to the bay was used to compute the local
seasonality since the other available
nearby points were on land or too far
from the bay. All re motely sensed vari-
Fig. 2. Spatial representation of the 11 yr seasonal least-squares fits of sea sur-
face temperature (SST; upper panels; in a variable color scale to highlight spa-
tial gradients) and chlorophyll concentration (CHL; lower panels; same color
scale among panels) for the southwestern Gulf of California. The periods
shown were chosen following the maximum and minimum CHL values of the
sea sonal least-squares fit within La Paz Bay, which are denoted here and in
Fig. 7 as I, II, III, and IV
Pardo et al.: Role of environmental seasonality in a cetacean community
ables were obtained through the Environmental Re-
search Division’s Data Access Program of NOAA,
NMFS, Southwest Fisheries Science Center (http://
coastwatch. pfeg. noaa. gov/ erddap/ index. html). A spa-
tially explicit characterization of the seasonal cycle of
the southwestern gulf (Fig. 2) was done by fitting tem-
poral least-squares with annual and semi-annual har-
monics (Emery & Thomson 1998, Ripa 2002) to the re-
motely sensed variables. Within the polygon of the
bay (see map in Fig. 3), the temporal behavior of each
variable was calculated using the same analyses, ap-
plied to the mean of all monthly values. The periods of
maxima and minima resulting from the local (i.e.
within the bay) CHL seasonal analysis were chosen to
portray the results of the regional (i.e. southwestern
gulf) analysis of SST and CHL (Fig. 2).
Cetacean population density
We conducted monthly visual surveys within the
bay over a systematic zig-zag arrangement of tran-
sects (Fig. 3) aboard the 28 ft (8.5 m) RV ‘CICIMAR
XV’ at ~18 km h
−1
between 6 February 2007 and 23
March 2008. Two trained observers simultaneously
searched for cetaceans with the aid of 7 × 50 hand-
held binoculars (Fujinon
®
FMTRC-SX) equipped with
compass and vertical reticles, independently cover-
ing both sides of the transect line, from the front of
the vessel to an angle of 90°. A team of 4 observers
rotated every 40 min. Observations were made from
a platform at 5.09 m effective visual height. The per-
pendicular distance (x) from the transect line to the
sighting was calculated following Lerczak & Hobbs
(1998). The animals were approached to confirm
species identification only when they were within
~1.5 km of the transect line (i.e. closing mode tech-
nique; Dawson et al. 2008). Most of the large species
were easily identified beyond this distance, whereas
some dolphin schools remained unidentified as well
as some whales recorded too far from the transect
line. Search effort was suspended during the ap -
proach and the time spent with the animals as well
as when the Beaufort sea-state was higher than 3.
Monthly population densities (individuals km
−2
)
were estimated using distance sampling line-transect
techniques (Buckland et al. 2001) by modeling a
detection probability function g (x), based on the dis-
tribution of all perpendicular distances from the tran-
sect line to the groups sighted of each species. Since
the cetacean surveys have continued within and out-
side the bay after the completion of the present study,
we used all of the perpendicular distances available
through April 2012 to improve the modeling of the
detection functions (Fig. 4). We established a priori
truncation points (w) based on the frequency distri-
bution of distances. The effective half-strip width (μ)
was estimated from the detection function to convert
the linear effort into an effectively sampled area
(Thomas et al. 2002). Several mathematical functions
(uniform, half-normal, and hazard-rate) and expan-
sion series (cosine, sine, simple polynomial, and her-
mit polynomial) were tested, and Akaike’s informa-
tion criterion (Burnham & Anderson 2002) was used
to choose the best fit (Table 1). This function, evalu-
ated at zero perpendicular distance, represents the
detection probability
ˆ
f(0). Mean group sizes
ˆ
E(s)
were estimated for the odontocetes and the fin
whale, whereas for the blue and the Bryde’s whale,
the few sightings of >1 animal were split into individ-
ual detections to avoid the increase of the variance
due to the indeterminacy of the expected group size.
For all species, we assumed that all animals located
directly on the track-line were detected and counted
(i.e. g (0) = 1). Finally,
ˆ
f(0) was used, together with the
number of counted groups (n) and the total transect
249
Fig. 3. Total survey effort by month (bars) and the average length of transects in each month (dotted line with circles). Map
shows the polygon of La Paz Bay (red line) and the transect track lines (dark grey lines)
Mar Ecol Prog Ser 487: 245260, 2013
length (L), to estimate a point density value (
ˆ
D) for
each month (Thomas et al. 2002). The variance
(D) and the lower and upper limits of the 95%
confidence intervals (D/C, D · C) were estimated by
a 999 iteration bootstrap analysis of samples (i.e.
transects) at each stratum (i.e. month). For the less-
frequent species, we only calculated the encounter
rate of groups as the number of sightings recorded in
the total linear effort in each survey. Although both
the short-beaked common dolphin Delphinus delphis
and the long-beaked common dolphin Delphinus
capensis occur in the Gulf of California, we treated
them at the genus level, given the difficulty in identi-
fying them to species level in most sightings.
Cetacean biomass
The population density estimates (individuals km
−2
)
were converted into values of biomass (t km
−2
) to
make the species comparable. This was
done by multiplying the estimated density
by the mean species-specific body mass
values previously reported for the Califor-
nia Current (Barlow et al. 2008 and refer-
ences therein). These values come from
both direct measurements and regression
models of body mass as a function of the
mean body length (Table 2).
RESULTS
Hydrographic and biological conditions
Temperature dominated the density
structure in the water column (Fig. 5). Cold
water (<18°C) occurred throughout the first
100 m during the winter (February 2007
and January to February 2008). During
March 2007, the upper 75 m were above
20°C. From April to July, a doming of the
isotherms took place, and water below
17°C penetrated the surface layer up to
10 m. From June to November, the upper
25 m warmed above 25°C. December was a
transition period in which the temperature
in the upper 55 m cooled below 21°C. The
Brunt-Väisälä frequency (Fig. 6a) showed
3 major conditions over the year, defined
by the depth and degree of stratification.
High values indicate a strong stratification,
whereas low values mean strong mixing.
The low buoyancy contours in February to March
2007 and January to February 2008 indicated deep
mixing in the upper water column. During these win-
ter periods, the pycnocline (i.e. the maximum buoy-
ancy frequency along the profile) deepened to at least
100 m. Then, during the spring and early summer, the
buoyancies in the upper 25 m marked a period of
isopycnal shoaling, when the pycno cline almost
reached the surface. This doming of isopycnals lasted
4 mo, until early August, and it was followed by a
thickening of the stratified upper layer in the late
summer and autumn, marking conditions of deep
stratification, with the pycnocline around 40 m depth.
These conditions prevailed until December 2007,
when a mixed period developed again.
For nutrients, we only show the concentration (µM)
of the sum of all components (Fig. 6b) since concen-
trations of dissolved silica, phosphate, and total dis-
solved in organic nitrogen followed similar patterns
over the course of the year. Relatively high concen-
var
250
Fig. 4. Detection probability function (g(x); black line), estimated from the
distribution of per pendicular sighting distances (gray bars). The estimated
effective half-strip width (μ) is shown as a dashed vertical line. The total
number of distances used (n) is presented, specifying the number of dis-
tances from the study period and area (first value within parentheses) and
the distances taken from subsequent years and/or areas aboard the same
platform (second value within the parentheses)
Pardo et al.: Role of environmental seasonality in a cetacean community
trations of nutrients were found in the water column
during the mixing conditions in the winter, but there
was an evident depletion in the upper 50 m during
February and March 2007. The pycnocline shoaling
of the spring and early summer brought higher sub-
surface concentrations of nutrients to just below the
thermocline, reaching up to 50 μM in the top 100 m.
The surface and sub-surface concentration of nutri-
ents decreased in the late summer and autumn.
Two different types of chl a concentration peaks
occurred (Fig. 6c): during the winters of 2007 and
2008, high concentrations (~1.5 mg m
−3
) were re -
corded above the pycnocline in the upper 40 m and
upper 70 m, respectively, whereas under conditions
of shallow stratification, higher values (~2.5 mg m
−3
)
occurred as 2 sub-surface maxima in May and Au -
gust, just below the pycnocline. Low chl a concentra-
tions (<0.5 mg m
−3
) in the upper 100 m characterized
the deep stratified conditions of the late summer
(September to November). Biogenic silica contri -
buted strongly to the total biogenic flux from winter
to early summer, reaching a maximum of 60% in
April 2007, whereas carbonate (CaCO
3
) components
dominated during the late summer and autumn,
reaching 48% in October 2007 (Fig. 6d). The 2 com-
ponents showed completely opposite seasonal pat-
terns. A Pearson’s test resulted in a correlation of
−0.7032 (p = 0.002, 95% CI = [−0.8324, −0.5016], n =
40, effective degrees of freedom (N
eff
) = 17 following
Davis 1978).
Local and regional seasonality
The seasonal cycle of surface wind, SST, and CHL
showed different patterns within the bay. The wind
followed the monsoonal cycle (Fig. 7a). The annual
pattern of the temperature was unimodal, with a
251
Species Mean
ˆ
D Mean
ˆ
N %CV Group size %CV
ˆ
f(0) %CV Function Expansion
μ
(km) %CV w
(ind. km
−2
) (ind.)
ˆ
D,
ˆ
N
ˆ
E(s)
ˆ
E(s)
ˆ
f(0) series
μ
(km)
Bryde’s whale 0.0007 (0.0003, 0.0016) 1 (1, 4) 45.0 0.77 (0.48, 1.23) 22.4 Half-normal Hermite 1.30 (0.81, 2.07) 22.4
polynomial
Fin whale 0.0022 (0.0012, 0.0034) 4.7 (3, 7) 32.2 1.3 (1.1, 1.6) 9.51 0.75 (0.56, 1.05) 23.1 Hazard rate Cosine 1.42 (1.08, 1.86) 13.5 4
Blue whale 0.0034 (0.0019, 0.0058) 7 (4, 13) 38.9 0.98 (0.80, 1.19) 9.7 Uniform Cosine 1.02 (0.84, 1.24) 9.7 3
Bottlenose dolphin 0.1911 (0.0950, 0.3842) 412 (205, 828) 36.3 18 (13.6, 24) 14.52 1.76 (1.18, 2.62) 20.3 Hazard rate Hermite 0.57 (0.38, 0.84) 20.3 2
polynomial
Common dolphin 0.3943 (0.2140, 0.7263) 850 (461, 1566) 31.6 85.1 (60.4, 119.9) 17.41 1.18 (0.98, 1.41) 9.3 Half-normal Cosine 0.85 (0.71, 1.02) 9.3 2.4
Short-finned pilot 0.0154 (0.0044, 0.0454) 33.2 (9, 98) 70.3 27.7 (15.8, 48.8) 27.32 0.53 (0.35, 0.90) 30.1 Uniform Cosine 1.86 (1.32, 2.64) 16.6
whale
Table 1. Parameter results from the distance sampling analyses. Point estimates are provided, followed by the 95% confidence interval in parentheses and the percent-
age of the coefficient of variation (%CV) in a separate column to the right. From left to right: the mean population density (
ˆ
D), the mean total abundance (
ˆ
N), the esti-
mated group size (
ˆ
E(s)), the detection probability (
ˆ
f(0)), the effective half-strip width (μ), the mathematical function and the expansion series used in the model chosen,
and the a priori truncation point (w). (–) not available
Species Common name Mean body
mass (t)
Mysticetes
Balaenoptera edeni Bryde’s whale 16.477
Balaenoptera physalus Fin whale 42.150
Balaenoptera musculus Blue whale 57. 230
Odontocetes
Tursiops truncatus Bottlenose dolphin 0.188
(offshore)
Delphinus spp. Common dolphins 0.080
Globicephala Short-finned pilot 0.608
macrorhynchus whale
Table 2. Values of body mass used to standardize the density
estimates of the dominant species (after Barlow et al. 2008)
Mar Ecol Prog Ser 487: 245260, 2013
maximum in September and a minimum in January
(Fig. 7b). In contrast, the CHL pattern was bimodal,
with a maximum in January (I in Fig. 7c), a decrease
in March (II in Fig. 7c), a secondary peak at the end
of May (III in Fig. 7c), and the main minimum in Sep-
tember (IV in Fig. 7c). Note that, except for the very
warm September period when CHL values were low-
est throughout the region (IV in Fig. 2), the values
within the bay tended to be higher than in the gulf
waters offshore. These values were similar to adja-
cent coastal areas in January (I in Fig. 2), lower than
offshore and coastal areas in March (II in Fig. 2), and
significantly higher than anywhere else in June (III in
Fig. 2). The least-squares SST values within the bay
were slightly warmer than elsewhere in January and
cooler the rest of the year, considerably so in June.
Cetacean population density
Altogether, the effective search effort within the
bay during the 14 mo totaled 3937 km (mean ± SD =
281 ± 137 km; Fig. 3). Four mysticete and 6 odon -
tocete species were identified from 276 sightings. The
blue whale, fin whale, Bryde’s whale, common dol-
phin, bottlenose dolphin, and short-finned pilot whale
were the most frequent species observed (Table 1,
Fig. 8). The humpback whale, sperm whale Physeter
macrocephalus, dwarf sperm whale Kogia sima, and
killer whale Orcinus orca were only sporadically
recorded (Fig. 8). Differences in the estimated effec-
tive half-strip widths (μ in Fig. 4) between species
typically suggest interspecific variations that deter-
mine their detectability, such as their body sizes,
grouping behavior, and/or level of surface activity
(e.g. Barlow & Forney 2007, Williams & Thomas 2007).
Mysticetes in general had the widest effective half-
strip widths due to their larger body sizes and taller
blows. Among the odontocetes, the short-finned pilot
whales had the largest effective half-strip width,
probably because of the combination of large groups
and large body sizes. They were followed by the com-
mon dolphins, whose high level of surface activity
and tendency to aggregate in very large groups make
them detectable at large distances. Bottlenose dol-
phins had the shortest distance range, which could be
attributed to their tendency to approach the vessel
and to the small group sizes recorded within the bay.
252
Fig. 5. Monthly progression of hydrographic variables in La Paz Bay (from objective interpolations using a Gaussian weighting
function). Each cast is shown as a vertical gray line
Pardo et al.: Role of environmental seasonality in a cetacean community
Cetacean biomass
Overall cetacean biomass was dominated by the
mysticetes and displayed 3 major peaks (Figs. 6e & 8).
The first occurred in spring, from May to June 2007,
the second covered late summer and autumn (Sep-
tember to December), and the third and highest was
in February 2008 (Fig. 6e). The odontocetes showed
an opposite pattern from the mysticetes (Fig. 6e):
They in creased in biomass when mysticetes de -
creased, showing 2 main peaks during July to August
2007 and in Jan uary 2008. The first mysticete peak of
the spring (0.62 t km
−2
) resulted from the co-occur-
rence of the 3 most frequent species but was domi-
nated by the blue whale (Fig. 8). In contrast, the
peaks of the late summer and autumn (0.73 and 0.63 t
km
−2
, respectively) were dominated by the fin whale
in the absence of the blue whale and the occurrence
of the Bryde’s whale, the latter always in low bio-
masses. Finally, the highest peak of mysticete bio-
253
Fig. 6. Physical and biological context underlying variations in cetacean biomass in La Paz Bay. (a) Buoyancy frequency, with
the dashed white line representing the depth of the pycnocline (i.e. maximum buoyancy frequency at each profile). Gray dots
show the depth of the Niskin-bottle samples for (b) nutrients and (c) chl a. (d) The contributions of biogenic silica (SiO
2
·nH
2
O)
and calcium carbonate (CaCO
3
) to the total biogenic sinking matter, shown as 7 to 15 d absolute values. (e) Monthly cetacean
biomass
Mar Ecol Prog Ser 487: 245260, 2013
mass (1.87 t km
−2
) occurred during February 2008
and was also dominated by blue whales but in the
presence of fin and Bryde’s whales (Fig. 8). The first
peak of odontocetes biomass (0.30 to 0.27 t km
−2
, July
to August 2007) resulted from the increase in bottle-
nose dolphins and from the incursion of short-finned
pilot whales, whereas the second peak (0.35 t km
−2
)
was dominated by the common dolphins, with a mod-
erate increase of bottlenose dolphins, which domi-
nated the odontocete biomass during the rest of the
year (Fig. 8).
DISCUSSION
The strong mixing in winter and the isopycnal
shoaling in spring and early summer produced peaks
in surface and subsurface chl a concentrations,
respectively (Fig. 6c). The high proportion of opal in
the biogenic sinking matter (Fig. 6d) suggests that
these peaks were dominated by diatoms and sili-
coflagellates, whose blooms result from the input of
new nutrients into the euphotic zone (Egge & Aksnes
1992) and typically favor the aggregation of krill and
planktivorous fish (Kudela et al. 2008). Silicoflagel-
lates and diatoms have been previously found as
dominant among the micro- and nano-phytoplankton
within the bay (Verdugo-Díaz 2003). The former
have been associated with peaks of primary produc-
tion in winter and early summer (Villegas-Aguilera
2009, Martínez-López et al. 2012) and are abundant
in the siliceous fraction of the sediment trap samples
(Álvarez-Gómez 2010). These 2 chl a peaks observed
in the water column are in agreement with the
remotely sensed CHL peaks of the seasonal analysis
derived from the 11 yr least-squares regression
(Fig. 7). This constitutes evidence that the isopycnal
shoaling within the bay and its influence on phyto-
plankton is not a phenomenon particular only to the
sampled year cycle but a recurring intraseasonal
event of local nature. While the first CHL peak within
the bay corresponds to a general pattern of high CHL
values along the entire region of the southwestern
gulf (I in Fig. 2), especially near the coast, the second
corresponds to a local phenomenon, in which the bay
gets colder and CHL-richer than the surrounding
254
Fig. 7. Seasonal pattern of wind, sea surface temperature (SST), and chlorophyll concentration (CHL) in La Paz Bay. The black ar-
rows (a) and circles (b,c) represent the original monthly values. The seasonal fits of wind, SST, and CHL are drawn as blue arrows
and red and green lines, respectively. The seasonally adjusted maxima and minima of CHL are labeled as I, II, III, and IV, which
are the periods chosen to portrait the regional (i.e. the entire southwestern gulf) spatial-temporal fit of SST and CHL in Fig. 2
Pardo et al.: Role of environmental seasonality in a cetacean community
255
Fig. 8. Monthly estimates of cetacean biomass (±95% confidence intervals) for the dominant species and encounter rates (ER)
for the less-frequent species (bottom panel)
Mar Ecol Prog Ser 487: 245260, 2013
gulf (III in Fig. 2). Note that even when the cold water
is at subsurface during the period of isopycnal shoal-
ing (Fig. 5), its influence on SST is also noticeable,
with the surface remaining ~1.5°C cooler than the
surrounding gulf.
The blue whale specializes on krill and dominated
the cetacean biomass during these 2 periods of sur-
face and subsurface chl a peaks, suggesting those
were suitable conditions for low trophic level prey. It
is also the only migratory cetacean among all of the
species recorded that feeds actively during its win-
tering period in the gulf (Del-Ángel-Rodríguez 1997,
Gendron 2002, Bailey et al. 2009). Variations of its
seasonal migration may be responses to a larger
scale of interannual oceanic conditions in a manner
that is still unstudied. At the seasonal and intra -
seasonal scales, however, it seems that the distribu-
tion of the species within the gulf is guided by the
persistence of local pulses of biological production
(Pardo et al. 2011) that aggregate krill (Gendron
1992). In one of its major feeding grounds off Califor-
nia, the blue whale abundance also increases in
response to the aggregation of krill resulting from the
upwelling pulses of the California Current (Croll et
al. 2005). In contrast, the migratory humpback whale
has been recorded only sporadically feeding on krill
within the gulf (Gendron & Urbán 1993), and its
occurrence is more associated with breeding activi-
ties during winter. Although krill may also serve as
prey for fin and Bryde’s whales during the winter and
spring within the bay, these species can also exploit
juvenile stages of Pacific sardine that aggregate
along the western coast of the gulf during this period
(Hammann et al. 1988, Tershy 1992, Tershy et al.
1993, Gendron et al. 2001, Jaume-Schinkel 2004)
and thus reduce com petition with blue whales. Small
pelagic fish are also the most likely prey for common
dolphins (Gallo-Reynoso 1991, Niño-Torres et al.
2006), which exploit the bay in large numbers during
winter. The higher biomasses of bottlenose dolphins
over the entire iso pycnal shoaling period (May to
August) may reflect the availability of mesopelagic
fish and/or squid, which are likely prey for this op -
portunistic species (Pauly et al. 1998, Díaz-Gamboa
2009).
In contrast, the deep stratification of the late sum-
mer and autumn was not conducive to high near-
surface nutrient or chl a concentrations. The increase
in the proportion of calcareous content in the settling
biogenic particles (Fig. 6d) suggests the presence
of coccolithophorids, foraminifera, and/or pteropods
(Romero et al. 2002). Coccolithophorids are better
adapted than silicoflagellates and diatoms to growth
at limiting nutrient levels and tend to dominate under
oligotrophic conditions (Iglesias-Rodríguez et al. 2002).
Nevertheless, despite their dominance, the total flux
of coccolithophorids does not increase at all during
the late summer in the bay (Rochín-Bañaga 2012),
and values of primary production drop (Reyes-Salinas
et al. 2003, Cervantes-Duarte et al. 2005). How then
might one explain the high peaks of fin and Bryde’s
whales at this time? The period of high surface water
temperatures near the coast frequently marks the
spawning season for several pelagic fish species in
the southwestern gulf (Moser et al. 1973), including a
‘warm stock’ of Pacific sardine that enters the gulf
(Félix-Uraga et al. 2004). These are likely the main
prey for rorqual whales during the deep stratification
period, as has been suggested from the δ
15
N ratios
between fin whales and sardines (Jaume-Schinkel
2004). Similarly, the short-finned pilot whale, along
with the other teutophagous odontocetes, such as the
dwarf sperm whale and the sperm whale (Clarke
1996, Pauly et al. 1998), were in the bay predomi-
nately during summer. The maximum biomass peaks
of the short-finned pilot whale (August and October
2007; Fig. 8) occurred just when the surface temper-
ature within the bay was the warmest (Fig. 5). Squid
searching for warm waters near the coast to spawn
typically aggregate under such conditions (Staaf et
al. 2008). Thus, we surmise that spawning prey, at
least the squid and the Pacific sardine, could sustain
the biomass of teutophagous odontocetes and fin
whales, respectively, during the deeply stratified
summer conditions.
The physical origin of some of the observed water-
column conditions in the bay is still not fully under-
stood. During winter, Ekman upwelling occurs along
the eastern coast of the gulf (Lluch-Cota 2000, Lavín
& Marinone 2003), but most blue whale sightings
(Gendron 2002) and large krill aggregations (Brinton
& Townsend 1980) occur on the western side during
this period. It is not clear if the series of eddies that
form regularly along the gulf (Pegau et al. 2002)
could be responsible for cross-gulf transport of nutri-
ents and plankton from east to west, where the mate-
rial could be retained. Nevertheless, since blooms of
siliceous phytoplankton typically occur in response
to new nutrient input, it is more likely that the phyto-
plankton biomass of the southwestern gulf is gener-
ated locally due to the strong vertical mixing (Fig. 6a)
produced by the northwesterly winds blowing during
winter (Fig. 7a). The high surface salinity (>35) ob -
served during this period (Fig. 5) reinforces the
hypothesis that strong northwesterly winds lead to a
high rate of evaporation, which in turn enhances
256
Pardo et al.: Role of environmental seasonality in a cetacean community
vertical mixing. The causes of the intraseasonal iso -
pycnal shoaling, associated with the second peak in
CHL within the bay, are also poorly known. Previous
studies have described cyclonic circulation (Mon-
real-Gómez et al. 2001, Sánchez-Velasco et al. 2006)
and proposed that it could be related to the wind
curl and the overall seasonal circulation of the
gulf (Beier 1997). This CHL peak occurs at a time
when the southwesterly wind maximum takes place
(Fig. 7a), which could also force the cyclonic circula-
tion and resulting Ekman pumping, but the subject
has not been investigated in detail due to the lack of
high-resolution data.
Nevertheless, it is clear that the isopycnal shoaling
enhances subsurface phytoplankton aggregations
within the bay at a time when the rest of the south-
western gulf remains oligotrophic. Therefore, it may
also be responsible for the higher annual values of
CHL previously described for the bay (Santamaria-
del-Angel et al. 1994, Luch-Cota & Teniza-Guillén
2000, Kahru et al. 2004). This phenomenon extends
the period of phytoplankton blooms that normally
would be associated only with the winter mixing.
Recent results of a long-term analysis of blue whale
density, comparing La Paz Bay to Loreto Bay, showed
that blue whales leave Loreto in April, earlier than
their departure from La Paz, where they can be seen
as late as June (Pardo et al. 2011). This pattern sug-
gests the importance of the intraseasonal isopycnal
shoaling within the bay as a potential driver of
krill aggregation in the southwestern gulf at a time
when the surroundings are comparatively warmer
and oligotrophic (III in Fig. 2).
The presence of cetaceans with different require-
ments over the course of the year in the bay suggests
a sustained availability of prey, aggregated by high
biological production or suitable physical conditions.
Recent measurements of the proportion of particulate
organic carbon in the sinking matter and the monthly
fluxes (export production) in the bay (Silverberg
2009, Silverberg et al. 2009) show that these do not
vary much seasonally, indicating that biological pro-
duction extends throughout the year regardless of
the type of physical forcing. The export production of
the bay is more than double that of Guaymas Basin,
often considered a particularly high production area
in the gulf (García-Pámanes et al. 2011). All of these
characteristics lead us to propose that La Paz Bay
constitutes a biological hotspot in the southwestern
Gulf of California, driven by the seasonal evolution of
regional surface mixing conditions in winter, local
isopycnal shoaling in spring and early summer, and
deep stratification in late summer and autumn. This
physical contrast attracts a wide variety of cetaceans
foraging at different trophic levels at different times
of the year and probably also favors the incursion of
other species of marine megafauna.
Future work should focus on addressing the infer-
ences drawn in the present study regarding the phys-
ical and biological mechanisms that drive cetacean
occurrence in the bay. Such work would require a
sampling grid aimed at resolving spatial patterns in
environmental variables concurrently with measure-
ments of the low, mid, and high trophic levels. Test-
ing these mechanistic linkages would require a
numerical modeling approach. Two species that
would be particularly amenable for such work are
the blue whale and the short-finned pilot whale
because of their specialist diet and because they
showed the most evident relationships with the envi-
ronment, with blue whales using the bay during peri-
ods of cool temperature, high CHL, and a primary
producer community dominated by siliceous phyto-
plankton, while short-finned pilot whales occurred
during warm, oligotrophic periods dominated by
calcareous phytoplankton. The physical mechanisms
driving isopycnal shoaling in the bay during spring
and early summer, which make this area biologically
richer than the surrounding gulf, should be investi-
gated through a study of the effects of the wind
field in combination with the local physiography (as
shown by Wingfield et al. 2011). The role of the
northwesterly winds in the evaporation and subse-
quent mixing of the surface layer during winter
should be studied to understand the reasons for the
aggregation of krill and blue whales along the west-
ern coast of the gulf rather than along the upwelling-
influenced eastern coast.
Acknowledgements. The present study received financial
support from the Consejo Nacional de Ciencia y Tecnología
(CONACyT) through the projects Monitoreo ecológico con-
tínuo de la Bahía de La Paz: Serie de tiempo (47310-F;
PI: N.S.) and Investigaciones Oceanográficas del Sistema
Frontal de Baja California (SEP-2008-103898; PI: E.B.), as
well as MSc and PhD scholarships to M.A.P. The Instituto
Politécnico Nacional (IPN) funded part of the field work
through the projects Monitoreo Ecológico Continuo en
Bahía de La Paz (SIP 20040095, 2005-0274, 20060199,
20070664, 20080650, 20090523; PI: N.S.) and Estructura
poblacional y movimiento de algunos cetáceos del Golfo de
California (SIP 20070803; PI: D.G.). D.M.P. was supported by
funding from the NASA Applied Sciences Program, Earth
Science Division, through a grant provided by Research
Announcement NNH07ZDA001N, Research Opportunities
in Space and Earth Sciences (ROSES-2007), Program
Element A.20: Decision Support through Earth Science
Research Results. M.A.P. also received funding from the IPN
(PIFI grant), Centro Interdisciplinario de Ciencias Marinas
(CICIMAR-IPN; M.Sc. Recovery Funds), The Society for
257
Mar Ecol Prog Ser 487: 245260, 2013
Marine Mammalogy (Grants in Aid of Research 2009),
Cetacean Society International, American Cetacean Society
(Monterey Bay Grant 2008), and The Ocean Foundation. We
are grateful to all of the personnel from Laboratorio de
Ecología de Cetáceos y Quelonios and Departamento de
Oceanología at CICIMAR-IPN for their support during en -
vironmental and cetacean sampling. We also thank NOAA
CoastWatch Program, NASA’s Goddard Space Flight Center,
and GeoEye for making the satellite data products readily
available. Valuable comments during the study were pro-
vided by R. Palomares, G. De-La-Cruz-Agüero, O. Victori -
vich, R. Díaz-Gamboa, G. Busquets-Vass, and A. Martínez-
López from CICIMAR-IPN.
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260
Submitted: July 9, 2012; Accepted: December 6, 2012 Proofs received from author(s): April 1, 2013
... En la Bahía de La Paz (BLP) y sus aguas adyacentes los cetáceos han sido estudiados en diferentes escalas espaciotemporales, tanto en aproximaciones a nivel de especies individuales como de ensamblaje de especies (Chávez Rosales, 1995;Flores-Ramírez et al., 1996;Pardo, 2009;Salvadeo, 2008;Salvadeo et al., 2009Salvadeo et al., , 2011Urbán et al., 1997;Vázquez Morquecho, 1997). De las 27 especies de cetáceos documentadas para la BLP, tres se consideran residentes durante todo el año: el delfín común de rostro largo (Delphinus delphis bairdii), recientemente reclasificado en 2016 como (D. delphis bairdii); la tonina (Tursiops truncatus) y el rorcual tropical (Balaenoptera edeni), aunque sus abundancias cambian en el tiempo (Flores-Ramírez et al., 1996;Pardo et al., 2013;Urbán et al., 1997). La alta riqueza de especies observadas en la BLP se debe, al igual que lo observado para la porción suroeste del GC, al ingreso de especies con hábitos migratorios definidos, como por ejemplo las ballenas azules y las ballenas jorobadas, o bien a la presencia de especies que responden a cambios en la distribución de sus presas, como el rorcual común (Flores-Ramírez et al., 1996;Pardo, 2009;Urbán et al., 1997). ...
... Así, la heterogeneidad ambiental, incluida la diversidad de hábitats (p.ej. bentóniconeríticos, pelágico-oceánicos) y la variabilidad oceanográfica (cambios estacionales marcados en la productividad, la TSM, la termoclina, la haloclina y la picnoclina) (Lara-Lara et al., 2008;Obeso et al., 2002), parecen fundamentales en la modulación de la presencia o ausencia de diversas especies de este grupo en la BLP (Flores-Ramírez et al., 1996;Pardo et al., 2013;Urbán et al., 1997). ...
... La variación en la dominancia de las especies de misticetos relacionada con la temporalidad anual, parece ser independiente del esfuerzo de observación, ya que la falta de muestreo estandarizado aplica a todas las especies (figura 13) y coincide con lo documentado previamente para la zona, en donde tanto los rorcuales tropicales como las ballenas azules y los rorcuales comunes se alternaron como la especie más común (Flores Ramírez et al., 1996;Pardo et al., 2013). No obstante, lo anterior, el aumento en los registros de ballena jorobada a partir del 2010 indica que su recurrencia en los últimos años podría estar relacionada con la variabilidad climática de gran escala tal y como se ha reportado para otras especies (Salvadeo et al., 2010(Salvadeo et al., , 2011. ...
Chapter
Full-text available
Se compiló una base de datos de avistamientos de cetáceos realizados por el Programa de Investigación de Mamíferos Marinos de la UABCS (PRIMMA), para la zona de Bahía de La Paz y Canal de San José durante el período 2000-2015. Se registraron 17 especies distintas y por su permanencia en el tiempo las especies se dividieron en dos grupos: especies comunes (presencia en ≥ 60% del total de años) y especies raras (≤40 % de los años). Las especies comunes presentaron heterogeneidad en su habitualidad con respecto a la estacionalidad, mientras especies como la ballena azul y la ballena jorobada fueron más comunes en el período frío, otras como las toninas, los calderones y los rorcuales comunes fueron comunes durante todo el año. Las especies raras tuvieron presencia tanto en el período cálido como en el frío y fueron tanto de afinidad a aguas tropicales como templadas. Los patrones de distribución de utilización del hábitat, analizados con el método Local Convex Hull (LoCoH), indicaron que los cetáceos utilizaron todo el hábitat disponible, pero se concentraron principalmente cerca del archipiélago de Espíritu Santo, afuera de la punta de El Mechudo y en el Canal de San José; el patrón anual de utilización del hábitat está determinado principalmente por la distribución de las especies durante la estación fría; los cetáceos contraen su distribución y presencia durante la estación cálida; los odontocetos presentan distribuciones de utilización similares, aunque se encuentran más dispersos durante el período cálido; las distribuciones de utilización del hábitat son distintas entre las especies comunes y que las toninas son la única especie con una distribución de utilización más amplia durante la estación cálida. Diferencias marcadas entre el período cálido y frío fueron encontradas en los valores de temperatura superficial del mar (TSM) y "productividad" (considerada como una medida indirecta de la concentración superficial de clorofila). Finalmente, las especies comunes alternaron su dominancia interanualmente dentro del área de estudio, sugiriendo que la presencia o ausencia de éstas puede estar determinada por variaciones interanuales del hábitat, probablemente relacionadas con la disponibilidad de alimento o con variaciones en TSM, tal y como lo sugiere la dominancia de la ballena jorobada durante los años 2010 y 2014, que fueron años con valores de TSM mayores al promedio.
... Climate-driven changes to the physical-chemical properties of the water column alter biological productivity and food availability in marine ecosystems affecting the trophic web including top predators (Camphuysen et al., 2006). Thus, spatial and temporal distribution patterns of common bottlenose dolphin (Tursiops truncatus) populations are influenced by such environmental variations (Ballance, 1992;Defran and Weller, 1999;Baird et al., 2009;Bearzi et al., 2009;Pardo et al., 2013;Sprogis et al., 2018). The continuous presence of this species in highly productive areas, such as coastal lagoons and river mouths, has been well documented, in which individual home ranges are somewhat well defined and populations are well-structured (Ballance, 1992;Reza-García, 2001;Morteo et al., 2004;Rodríguez-Vázquez, 2008). ...
... The fastLSA time-series analyses on SST and Chl-a data highlighted the importance of local and regional oceanographic features that alter biomass production, resulting in a lagged response of 22 -29 days for the array of physical-chemical processes in the water column (Pardo et al., 2013;Kämpf and Chapman, 2016). This pattern has been documented in other neighboring areas in the Pacific such as Bahía de Banderas, Jalisco-Nayarit (Cerrillo-Espinosa and Barraza-Figueroa, 2007;Rodríguez-Vázquez, 2008), and the southwestern Atlantic (Moraes et al., 2012). ...
... It is also noteworthy that, although dolphin RA seemed to respond to these environmental changes within a week, higher RA could represent an influx of offshore bottlenose dolphins into the area, or that the same individuals remained in the study area and were encountered multiple times during a given survey. Even with these potential uncertainties, larger dolphin aggregations and higher residency times during upwelling periods are well-known in the GoC (Reza-García, 2001;Pardo et al., 2013;Guevara-Aguirre and Gallo-Reynoso, 2015), and the Mexican North Pacific (Morteo et al., 2004) Orozco et al., 2005;Kessler, 2006;López-Sandoval et al., 2009;Kono-Martínez et al., 2015) and the opposite occurred during El Niño conditions (especially in 2010), which has also been observed in other regions of the Pacific (e.g. Sprogis et al., 2018). ...
Article
Full-text available
Despite being one of the most common odontocetes off Sinaloa (Mexican Pacific coast), basic studies on the ecology of common bottlenose dolphins (Tursiops truncatus) are scarce in the region. This study aimed to describe changes in the relative abundance, group size, and behavior of this species during 2007 – 2012. We used boat-based surveys and satellite images of sea surface temperature (SST) and chlorophyll-a (Chl-a) to model changes in dolphin relative abundance (RA) over time, using correlations in time series analyses. Overall, mean RA was 3.6 dolphins h-1 (SD = 8.0), and significantly higher RA (6.4 –16.7 dolphins h-1) occurred in 2008, 2011, 2012, which was concurrent with hydrographic effects of La Niña oceanographic conditions, as well as during the upwelling season (February – April) (SST: 17.3 – 25.0 °C; Chl-a: 3.7 – 21.4 mg m-3). Conversely, significantly lower RA values (0.5 – 3.8 dolphins h-1) occurred in 2007 and 2010 that were likely associated with El Niño effects on the biological productivity of the area (Chl-a: 0.3 – 7.6 mg m-3). We found significant correlations between monthly Chl-a and SST average values with mean bottlenose dolphins RA, and lags (22 – 29 days) in the trophic response to variations of the hydrographic parameters. Significantly larger dolphin groups were recorded during La Niña years possibly because of the higher availability of their prey. This hypothesis is supported by higher feeding frequencies (35 – 73%) observed during the upwelling seasons, especially during La Niña conditions, whereas the most frequent behavior throughout all other years was traveling (28 – 69%). Our results show that RA and group size of bottlenose dolphins inhabiting the waters off Sinaloa, Mexico, are likely influenced by the changes in hydrographic parameters, especially during extreme climatic events.
... En la Bahía de La Paz (BLP) y sus aguas adyacentes los cetáceos han sido estudiados en diferentes escalas espaciotemporales, tanto en aproximaciones a nivel de especies individuales como de ensamblaje de especies (Chávez Rosales, 1995;Pardo, 2009;Salvadeo, 2008;Salvadeo et al., , 2011Vázquez Morquecho, 1997). De las 27 especies de cetáceos documentadas para la BLP, tres se consideran residentes durante todo el año: el delfín común de rostro largo (Delphinus delphis bairdii), recientemente reclasificado en 2016 como (D. delphis bairdii); la tonina (Tursiops truncatus) y el rorcual tropical (Balaenoptera edeni), aunque sus abundancias cambian en el tiempo Pardo et al., 2013;. La alta riqueza de especies observadas en la BLP se debe, al igual que lo observado para la porción suroeste del GC, al ingreso de especies con hábitos migratorios definidos, como por ejemplo las ballenas azules y las ballenas jorobadas, o bien a la presencia de especies que responden a cambios en la distribución de sus presas, como el rorcual común Pardo, 2009;. ...
... Así, la heterogeneidad ambiental, incluida la diversidad de hábitats (p.ej. bentóniconeríticos, pelágico-oceánicos) y la variabilidad oceanográfica (cambios estacionales marcados en la productividad, la TSM, la termoclina, la haloclina y la picnoclina) (Lara-Lara et al., 2008;Obeso et al., 2002), parecen fundamentales en la modulación de la presencia o ausencia de diversas especies de este grupo en la BLP Pardo et al., 2013;. ...
... La variación en la dominancia de las especies de misticetos relacionada con la temporalidad anual, parece ser independiente del esfuerzo de observación, ya que la falta de muestreo estandarizado aplica a todas las especies (figura 13) y coincide con lo documentado previamente para la zona, en donde tanto los rorcuales tropicales como las ballenas azules y los rorcuales comunes se alternaron como la especie más común Pardo et al., 2013). No obstante, lo anterior, el aumento en los registros de ballena jorobada a partir del 2010 indica que su recurrencia en los últimos años podría estar relacionada con la variabilidad climática de gran escala tal y como se ha reportado para otras especies (Salvadeo et al., 2010(Salvadeo et al., , 2011. ...
Book
Full-text available
En el Golfo de California, la Bahía de La Paz representa un importante capital natural. Su situación geográfica, la protección que ofrece contra fenómenos meteorológicos, la abundancia de sus recursos naturales y su innegable belleza, propiciaron que en este lugar se desarrollara la ciudad de La Paz hace más de 400 años. Desde entonces, sus aguas, costas e islas han sido fuente de sustento para sus habitantes. Esta obra comprende 26 capítulos agrupados en tres secciones, Oceanografía y Climatología, Zona Costera y Zona Marina. La Primera sección incluye trabajos sobre la variación batimétrica de la ensenada de La Paz, la distribución de nutrientes y clorofila en el estero El Conchalito, y las condiciones oceanográficas de la Bahía de La Paz durante El Niño. La sección de Zona Costera, presenta trabajos de aspectos biológicos, como el del camarón de salmuera en la salina de Pichilingue; de aspectos ecológicos como la florística del medio terrestre, los anfibios y reptiles terrestres de la bahía e islas; y de aspectos sociales y económicos como la caracterización antropogénica en el complejo insular de Espíritu Santo, los requerimientos de energía de la ciudad de La Paz, la vulnerabilidad y adaptación al cambio climático del turismo y la pesca, y la maricultura en la bahía. La sección de Zona Marina es la más grande y comprende trabajos de diferentes taxa enfocados a aspectos biológicos, ecológicos, de aprovechamiento y conservación. Los cuales incluyen toda la Bahía de La Paz, sobre corales escleractinios, copépodos, esponjas, tortugas y mamíferos marinos. Estudios en localidades específicas como el de comunidades ícticas en manglares, equinodermos en aguas someras de las islas, y moluscos de la facie rocosa de la isla Espíritu Santo. Trabajos sobre temas diversos como la reproducción de bivalvos, de los afloramientos de dinoflagelados, de los patrones espaciales de la biodiversidad animal marina en la bahía, y de varamientos de mamíferos marinos. Finalmente, dos estudios sobre taxonomía, conservación y aprovechamiento, uno sobre peces, y otro sobre tiburones y rayas.
... This subspecies shows a preference for upwelling and high productivity areas (Hui, 1979;Evans, 1994) like the GC, where there is an estimated population of 28,681 individuals (95% C.I. = 14,287-72,316) based on distance sampling (Gerrodette and Palacios, 1996). Regardless of the population size, records of this subspecies are scarce (Flores-Ramírez et al., 1996;Pardo et al., 2013;Salvadeo et al., 2009;Urbán et al., 2012) relative to those of longbeaked common dolphins (D. d. bairdii) in the GC (Heyning and Perrin, 1994;Vidal et al., 1993). It is possible that this rarity could explain the paucity of knowledge and publications about this subspecies in the GC. ...
... In the GC, SBCDs seem to be slightly more common during winter (Pardo et al., 2013;Salvadeo et al., 2009); they occasionally also have been observed close to shore or in shallower areas (Silber et al., 1994). This closeness to shore and additional factors, such as acoustic disturbance, have caused mass stranding events of this subspecies (e.g., Jepson et al., 2013). ...
... Stable isotope analysis was used to establish whether their signatures corresponded to the GC or outside of this region. Given the high mobility of the subspecies (e.g., Genov et al., 2012;Farías-Curtidor et al., 2017), its seasonal presence in the region (Pardo et al., 2013), and the location where the stranding occurred, close to the mouth of the gulf, we expect isotopic values in skin that do not correspond to the GC trophic web. This study provides important new data regarding biological aspects of a rarely studied subspecies by utilizing samples from dead individuals from a mass stranding to delve into its dispersal and habitat use in the GC. ...
Article
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The short-beaked common dolphin (Delphinus delphis delphis) (SBCD) is an odontocete inhabiting the Gulf of California (GC), Mexico; distributed widely in temperate to tropical waters, it is sometimes observed close to shore or in shallower areas. On 13 February 2018, a mass stranding of 54 SBCDs occurred in La Paz Bay in the southern GC; 21 individuals died, but the rest were released. Skin samples were removed from dead individuals for stable isotope analysis in order to infer their probable geographical provenance. Their mean S.D. were 17.0 ± 0.3‰ (N) and -18.3 ± 0.2 (C), with no significant differences between sexes or length. These isotope ratios do not correspond to the GC. Their low C and N values reflect offshore foraging grounds not under the 15N-enriching effects of the intense denitrification in the GC. Rather, these values were within the range of other piscivorous odontocetes that have been sampled in the GC and classified as outsiders (Pacific Ocean). Mean total length of these stranded SBCDs was similar to that reported for SBCDs off the Pacific coast of Baja California and southern California, but widely smaller than that reported for both eastern tropical Pacific SBCDs and long-beaked common dolphins (Delphinus delphis bairdii) in the northern and southern GC. These differences in isotopic niche and size between SBCDs stranded in La Paz Bay and long-beaked common dolphins from other regions of the GC highlight the need to be able to distinguish between the common dolphins (Delphinus) that inhabit the GC from those that visit the GC from off the Pacific coast of Baja California. Our findings confirm the ecological importance of the southern GC, where resident and visitor marine mammals congregate, leading to events like mass strandings that provide a unique source of information useful for research and conservation.
... This coincides with the pattern found in coastal waters of Panama, where the higher spotted dolphin sighting rate during the rainy season was associated with offshore movements and a scattered distribution that favored the availability of pelagic prey (García and Dawson 2003). Additionally, Pardo et al. (2013) indicated that odontocetes in the Gulf of California could be taking advantage of the coastal spawning season of pelagic fish species, which happens during periods of high SST, and this phenomenon is likely occurring in the MCP. In the Gulf of California the distribution and abundance patterns of bottlenose dolphins resemble those described here for spotted dolphins, with increased abundances being associated with warm and less productive periods and dolphin movements to deeper waters (Flores-Ramírez et al. 1996, Salvadeo et al. 2009). ...
... Esto coincide con el patrón encontrado en las aguas costeras de Panamá, donde una tasa de avistamiento mayor de delfines moteados durante la temporada de lluvias se asoció con los movimientos de los delfines en alta mar y una distribución dispersa que favoreció la disponibilidad de presas pelágicas (García y Dawson 2003). Además, Pardo et al. (2013) indicaron que, dentro del golfo de California, los odontocetos podrían estar aprovechando la temporada de desove costero de las especies de peces pelágicos, que ocurre durante los periodos de TSM alta, y es probable que este fenómeno esté ocurriendo dentro del PCM. En el golfo de California, los patrones de distribución y abundancia de los delfines nariz de botella se asemejan a los descritos aquí para los delfines moteados, con un aumento en la abundancia asociado con periodos cálidos y menos productivos y movimientos de delfines hacia aguas más profundas (Flores-Ramírez et al. 1996, Salvadeo et al. 2009). ...
Article
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Influence of environmental variability on the distribution and abundance of the pantropical spotted dolphin (Stenella attenuata) in the Mexican Central Pacific Influencia de la variabilidad ambiental sobre la distribución y la abundancia del delfín moteado pantropical (Stenella attenuata) en el Pacífico central mexicano Abstract. The Mexican Central Pacific (MCP) has complex oceanographic dynamics, with a well-defined seasonal pattern and influence of interannual sea surface temperature anomalies. The present study investigated the relationship between the distribution/abundance of spotted dolphins and the environmental conditions in the MCP. Dolphin sighting data were collected during January 2010-December 2015; distance to coast, sea surface temperature, and chlorophyll a (Chla) were obtained for each sighting location through georeferenced maps and satellite images. A total of 746 spotted dolphin sightings were obtained with a survey effort of 31,695 km. Spotted dolphins in the MCP showed a predominantly coastal distribution regardless of environmental conditions. Significant differences were detected concerning regional seasonal variation, with higher mean sighting rate during the stratified periods and higher density/abundance estimates during the mixed periods. Spotted dolphins showed preference for warmer conditions, with higher mean sighting rate, abundance, and distance to coast during the neutral and warm interannual periods. During the coolest conditions (cold periods), mean sighting rate and abundance decreased and spotted dolphins were located closer to the coast, where Chla concentration increased. These results suggest that changes in the distribution/abundance of these dolphins could be associated with habitat quality related to tropical (El Niño/Southern Oscillation) and extratropical (The Blob) oceanographic phenomena, which could be modulating their foraging activities.
... La Paz Bay (BLPZ) is located close to the western side of the gulf's mouth and is the largest coastal body within the southwestern Gulf of California (Fig. 1). This region is characterized by its high diversity and abundance of fish species, by its hosting of ecologically important marine mammal populations, such as whales, dolphins and sea lions (Pardo et al., 2013), and by being a natural refuge for feeding, breeding, and spawning for many ecological important species (Sánchez-Velasco et al., 2004b, 2006. The functioning of this ecosystem is possible thanks to the biological enrichment is particularly high associated with intense local upwelling generated by local seasonal wind pattern, the topographic configuration, and the water exchange between the bay and the GC (Salinas-González et al., 2003;Herrera-Cervantes, 2019). ...
... This study examines the interannual variability and the biophysical coupling of the satellite-derived chlorophyll pigment, SST, and wind intensity data during the 2003-2018 period in BLPZ, an area with high levels of biological productivity (Durán-Campos et al., 2020). For this region, climate variability associated with the presence of ENSO events has been the topic of few studies, and strong controversy remains, particularly regarding the ecological impacts of the ENSO signals at the BLPZ (Pardo et al., 2013;Coria-Monter et al., 2018;Guevara-Guillen et al., 2018). All these studies use a combination of observed hydrographic data and remote sensing images. ...
Article
The Bay of La Paz (BLPZ), located in the southwestern Gulf of California, is a region of significant biological productivity and strong environmental variability that remains to be understood. In particular, the spatial signature of different El Niño-Southern Oscillation (ENSO) conditions and their impact on the ecology of the bay remains puzzling because of contrasting field observations. In this context, the available satellite-derived data on surface chlorophyll (ChlSAT), sea surface temperature (SST) derived from the MODIS-Aqua sensor, together with surface wind intensity (WI) derived from reanalysis datasets from the period 2003–2018 were analyzed to identify the influence of local and remote forcing over the interannual variability of chlorophyll pigment concentration in BLPZ using WI, SST, and the Oceanic El Niño Index (ONI) as environmental factors. Significant relationships between environmental factors and ChlSAT were found using exponential models, with the higher slopes and correlations during La Niña events. Empirical orthogonal function (EOF) analysis was used to extract the principal mode of the ChlSAT and SST interannual variability. The first mode (EOF1) accounted for 64% and 89% of the ChlSAT and the SST interannual variance, respectively, showing a strong gradient along the coast of the bay. The corresponding amplitude of the ChlSAT mode was significantly correlated with the independent variables PC1SST, ONI, and WI anomalies (RSST = − 0.60, RONI = − 0.57, and RWI = 0.44, P < 0.001). Monthly time series of ChlSAT anomalies along the coast where examined as a function of each independent variable using a multiple linear regression analysis (LRA), which revealed that during the 2005 to 2013 period, high peaks of positive ChlSAT anomalies in the central and southern coast were related to cold ENSO events and favorable wind forcing, which smoothed the effect of weak and moderate El Niño episodes. This period of high biological productivity was only interrupted by the strong and long 2015–2016 El Niño event. Since late 2013, the ChlSAT and the WI anomalies showed a sharp drop to negative anomaly values, which may be the result of complex interactions of physical and biological processes observed during strong warm ENSO events, showing one peak of persistent warm SST anomalies via a weakening of the wind intensity (i.e., from atmospheric teleconnection) and a second one due to the oceanic connection. It is confirmed that BLPZ maintains high pigment concentration levels, comparable to those reported on the mainland coast of GC, where only strong warm ENSO episodes cause changes in chlorophyll concentration.
... From late spring to early autumn southeasterly winds, locally called "Coromuel", prevail with magnitudes of 4 msˉ¹. From winter to spring, strong persistent northwesterly winds reaching velocities of 12 msˉ¹ enhance evaporation and increase surface salinity (Obeso-Nieblas et al., 2007;Pardo et al., 2013). The local climate is semi-desertic with precipitations reaching an annual average of 187 mm. ...
... In our study area, nutrients are high, especially in the channel and transition zone due to shallowness and the presence of currents resuspending sunken organic matter in the water column (Jiménez-Illescas, 1996). The reduced nutrient availability in the surface and sub-surface of offshore waters during autumn could hence explain the observed decrease (Pardo et al., 2013), limiting the distribution of Navicula to the nutrient-rich Ensenada and channel. ...
Thesis
Phytoplankton plays a fundamental role in marine food webs, impacting the functioning of entire ecosystems. Environmental variables regulate the distribution of these microorganisms and even small changes can cause shifts in phytoplankton communities. Hence, this study aimed to describe the environmental preferences, spatiotemporal distribution and diversity of the main genera of phytoplankton in the Bay of La Paz using in situ measured phytoplankton abundance and Landsat 8 derived environmental factors through Generalized Additive Models (GAMs). While using the best-fitted models for 12 selected genera, spatiotemporal predictions were made for the study area. In total, 38 genera were identified with dinoflagellates being the most abundant taxa and Chaetoceros the best-represented genus. Shifts in phytoplankton communities occurred in response to the different environmental variables and phytoplankton abundance varied spatially and seasonally. The channel linking the Ensenada, a small coastal lagoon, to the bay was identified as the most important area in terms of phytoplankton diversity. The use of Species Distribution Models (SDM) in this study has been very effective for the environmental monitoring of phytoplankton and could be considered as a suitable tool for the development and application of management strategies in coastal areas.
... Fluctuations in cetacean distribution, abundance and habitat preferences may be observed across years [10,11], as well as across seasons [12][13][14][15], depending on the spatio-temporal scales of relevant processes [16]. There is some evidence of short time-scale movements [17], but poor understanding of the relationship between such infra-seasonal movements of populations and environmental conditions [18,19], inasmuch as the dynamics of spatial distributions at this temporal scale are at best poorly documented for most top predators. This is mostly due to the paucity of information about the whereabouts of top pedators, and in particular cetaceans, at fine scale, both spatially and temporally. ...
Article
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Cetaceans adjust their distribution and abundance to encountered conditions across years and seasons, but we poorly understand such small-scale changes for many species, especially in winter. Crucial challenges confront some populations during this season, such as the high levels of fisheries-induced mortality faced by the common dolphin (Delphinus delphis) in the Northeast Atlantic shelves. For such species, understanding the winter fine-scale dynamics is crucial. We aimed to identify the dolphin distribution drivers during the winters of 2020 and 2021, with a focus on determining the lag between changes in oceanographic conditions and dolphin distribution. The changes were related to temporal delays specific to the nature and cascading effects that oceanographic processes had on the trophic chain. By determining the most important conditions and lags to dolphin distributions, we shed light on the poorly understood intrusions of dolphins within coastal waters during winter: they displayed a strong preference for the coastal-shelf waters front and extensively followed its spatial variations, with their overall densities increasing over the period and peaking in March-April. The results presented here provide invaluable information on the winter distribution dynamics and should inform management decisions to help reduce the unsustainable mortalities of this species in the by-catch of fisheries.
... Temperature was the only variable related to seasonal productivity patterns, because Chl was excluded due to collinearity (Zuur, Ieno & Elphick, 2010;Dormann et al., 2013). Similarly to SST, Chl is not affecting marine mammals directly, but it is often considered in models as a productivity index of the ecosystem (Redfern et al., 2006;Salvadeo et al., 2009;Pardo et al., 2013;Lopes, 2017). Another indirect variable was the moon phase. ...
Article
Full-text available
Marine mammals in subtropical coastal habitats are sentinels of the health of the ecosystem and offer important ecosystem services. They rely on prey that pursues feeding opportunities, while both avoid unfavorable conditions. In many cases, these predator-prey dynamics fluctuate seasonally and are regulated by lunar, tidal, and/or diel cycles (hour). However, these rhythmical patterns may vary under different seasonal conditions. Bottlenose dolphins (Tursiops truncatus) in the Ensenada de La Paz in Baja California Sur, Mexico, were detected acoustically over the course of an annual cycle on 21 separate occasions, covering 640 h from June 2017 to May 2019. The presence of bottlenose dolphins was examined using Generalized Additive Models (GAM) including variables that are related directly to their habitat (direct variables: hour, distance, depth) and to their prey (indirect variables: SST, moon phase and tides). Seasonal differences in the presence of bottlenose dolphins were influenced more by indirect variables (explained deviance: 34.8% vs. 37.7%). Hourly acoustic detections occurred less frequently when SST exceeded 27.4 �C (Aug–End of Nov.) and more frequently at moderate temperatures (22.7 �C to 26.3 �C) in May through July. Moreover, bottlenose dolphins were detected more frequently during waning and new moon phases, at the onset of flood and ebb tides, and during day (04:00 to 20:00). The seasonal differences in acoustic detections rates were highlighted by the global GAM and hierarchical clustering. The strong seasonal pattern indicated possible interactions with rhythmic pattern of bottlenose dolphins. Four candidate variables (SST, moon, tide, and hour) were tested for plausible interaction terms additional to their individual consideration, out of which only hour changed significantly between seasons. The patterns of presence likely increase feeding opportunities or may favor other behaviors such as socializing, resting, or nursing. These might prove responsible for the distinct occurrence and hourly patterns of bottlenose dolphins.
... Indeed, the presence of mesoscale eddies induces vertical mixing and increases the supply of nutrients to the surface layers, which in turn induces high phytoplankton concentration and thereby supports the zooplankton biomass (Salas de León et al., 2011). The Bay of La Paz, the largest basin in the southern Gulf of California, provides a suitable habitat to many species, such as cetaceans, throughout the year, making it a hot spot for productivity (Pardo et al., 2013). The circulation pattern in the bay is dominated by the presence of a quasi-permanent mesoscale cyclonic eddy, which makes the region an interesting area in which to analyze the impact of physical forcing on planktonic organisms. ...
Article
Marine zooplankton play a pivotal position in the pelagic ecosystem and, as one of their principal members, copepods represent by far the most significant direct link between phytoplankton and the higher trophic levels. In this study, observational evidence of the effect of a mesoscale cyclonic eddy on the abundance of copepods (adult calanoids, adult cyclopoids and all copepodite stages) in the Bay of La Paz, southern Gulf of California, a region recognized as exhibiting high biodiversity, is presented. During a multidisciplinary research cruise in February 2006, high-resolution hydrographic profiles were obtained from a grid of stations throughout the bay, and oblique zooplankton hauls were made in its north-central portion. The results showed the presence of a mature mesoscale cyclonic eddy, which extended to 110 m depth and 30 km diameter, reaching a surface azimuthal velocity of 20 cm s–1 at its periphery. Below the mixed layer at 50 m depth this eddy induced a cold-core water dome (∼16.25 ºC) at its center. Within this cold water dome, nutrients concentrations were much higher than outside the eddy. Nitrate reached up to 15 μM, soluble reactive phosphorous (SRP) up to 1.6 μM, and soluble reactive silicate (SRSi) up to 30 μM within the eddy. The total abundance of the organisms analyzed varied according to group: the calanoid copepods reached values of 8112 ind 100 m–3, the cyclopoid copepods of 4281 ind 100 m–3, and the highest abundance was recorded by all copepodite stages, of 32,076 ind 100 m–3. The influence of the cyclonic structure on copepod abundance was to induce the formation of a circular shape around the eddy periphery, which could be called a ‘copepod belt shape’. We suggest that several mechanisms induce this aggregation, including: 1) the affinity of these organisms for waters with a favorable temperature, and 2) the availability of food (phytoplankton) generated by the presence of the cyclonic eddy and the sub-mesoscale processes there, as well as by the presence of a bathymetric sill. These results represent the first observational report on the influence of a mesoscale cyclonic eddy in the Bay of La Paz on zooplankton organisms in winter, which should contribute to a better understanding of the dynamics of this highly productive area.
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A large data set containing coincident in situ chlorophyll and remote sensing reflectance measurements was used to evaluate the accuracy, precision, and suitability of a wide variety of ocean color chlorophyll algorithms for use by SeaWiFS (Sea-viewing Wide Field-of-view Sensor). The radiance-chlorophyll data were assembled from various sources during the SeaWiFS Bio-optical Algorithm Mini-Workshop (SeaBAM) and is composed of 919 stations encompassing chlorophyll concentrations between 0.019 and 32.79 μg L-1. Most of the observations are from Case I nonpolar waters, and ~20 observations are from more turbid coastal waters. A variety of statistical and graphical criteria were used to evaluate the performances of 2 semianalytic and 15 empirical chlorophyll/pigment algorithms subjected to the SeaBAM data. The empirical algorithms generally performed better than the semianalytic. Cubic polynomial formulations were generally superior to other kinds of equations. Empirical algorithms with increasing complexity (number of coefficients and wavebands), were calibrated to the SeaBAM data, and evaluated to illustrate the relative merits of different formulations. The ocean chlorophyll 2 algorithm (OC2), a modified cubic polynomial (MCP) function which uses Rrs490/Rrs555, well simulates the sigmoidal pattern evident between log-transformed radiance ratios and chlorophyll, and has been chosen as the at-launch SeaWiFS operational chlorophyll a algorithm. Improved performance was obtained using the ocean chlorophyll 4 algorithm (OC4), a four-band (443, 490, 510, 555 nm), maximum band ratio formulation. This maximum band ratio (MBR) is a new approach in empirical ocean color algorithms and has the potential advantage of maintaining the highest possible satellite sensor signal: noise ratio over a 3-orders-of-magnitude range in chlorophyll concentration.
Article
Spotted and spinner dolphins (Stenella attenuata, S. longirostris) occurred primarily in tropical waters N of the Equator, but also in the seasonal tropical waters S of the Galapagos Islands. These dolphins were relatively infrequent along the Equator, off Costa Rica, and N South America. Common and striped dolphins (Delphinus delphis, S. coeruleoalba) tended to be more frequent in these same areas of less frequent spotted and spinner dolphins. Spotted and spinner dolphins are primarily in Tropical Surface water, where thermocline 'ridging' and relatively small annual variations in surface temperature are features. Common and striped dolphins prefer equatorial and subtropical waters with relatively large seasonal changes in surface temperature and thermocline depth and with seasonal upwelling.-from Authors
Article
The Bay of La Paz, a coastal and shallow basin of the Gulf of California (depth < 420 m), exchanges mixed-layer waters with the Gulf of California predominantly through Boca Grande (Big Mouth), in the northeastern part of the bay. Equatorial Surface Water (ESW) flows from the gulf to the bay; once there, due to evaporation processes, this water increases its salinity above 35.00, therefore becoming Gulf of California Water (GCW) Mass. During June 1998, the baroclinic circulation in the bay was dominated by a cyclonic gyre. The hydrographic dome, related to this gyre, is depicted until approximately 150-m depth. However, the vertical mixing of water between. the mixed-layer and the ones below practically does not occur because of the development of a sharp pycnocline. This fact and the isolation of the bottom of the bay by a bathymetric sill (approximately 250-m depth) at Boca Grande induce low oxygen content, particularly in the bottom-waters (O-2 < 0.1 ml/l. Chlorophyll values are higher and with larger vertical fluctuations within the Bay of La Paz than outside.
Article
At two different ecological zones (Canal de San Lorenzo and transitional), located at Bahía de La Paz, we analysed changes in the cetacean community (Shannon and Simpson indexes) in relation to annual and seasonal changes in sea surface temperature, water transparency and inferred and reported food availability. Six odontocete and six mysticete species were recorded during the study period. Due to their high relative abundance and persistent presence, the long beak common (Delphinus capensis) and bottlenose (Tursiops truncatus) dolphins, as well as Bryde's whales (Balaenoptera edeni) were, respectively, the most important odontocete and mysticete community members. The evidence indicates that sea surface temperature and water transparency, besides food availability, show predictable seasonal and annual changes related to the influence of tropical water masses and to large scale oceanographic phenomena, such as El Niño Southern Oscillation and La Niña. Nevertheless, cetacean diversity did not show consequent changes. The cetacean community was more diverse and stable at the transition between seasonal and annual oceanographic conditions. Cetacean diversity changed significantly at smaller time and space scales, defined by the interaction of particular annual, seasonal and local conditions. This and changes in species richness, mediated by the temporal residence of migrating species of temperate waters or more sporadic tropical visitors, indicate the existence of fast and important changes in species abundance and substitution rates, associated with the behavioural ecology of the species and fluctuations in the ecosystem's metabolism.
Article
Carbonates are the largest reservoirs of carbon on Earth. From mid-Mesozoic time, the biologically catalyzed precipitation of calcium carbonates by pelagic phytoplankton has been primarily due to the production of calcite by coccolithophorids. In this paper we address the physical and chemical processes that select for coccolithophorid blooms detected in Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color imagery. Our primary goal is to develop both diagnostic and prognostic models that represent the spatial and temporal dynamics of coccolithophorid blooms in order to improve our knowledge of the role of these organisms in mediating fluxes of carbon between the ocean, the atmosphere, and the lithosphere. On the basis of monthly composite images of classified coccolithophorid blooms and global climatological maps of physical variables and nutrient fields, we developed a probability density function that accounts for the physical chemical variables that predict the spatiotemporal distribution of coccolithophorids in the world oceans. Our analysis revealed that areas with sea surface temperatures (SST) between 3° and 15°C, a critical irradiance between 25 and 150 μmol quanta m -2 s -1, and decreasing nitrate concentrations (ΔN/Δt < 0) arc selective for upper ocean large-scale coccolithophorid blooms. While these conditions favor both Northern and Southern Hemisphere blooms of the most abundant coccolithophorid in the modern oceans, Emiliania huxleyi, the Northern and Southern Hemisphere populations of this organism are genetically distinct. Applying amplified fragment length polymorphism as a marker of genetic diversity, we identified two major taxonomic clades of E. huxleyi; one is associated with the Northern Hemisphere blooms, while the other is found in the Southern Hemisphere. We suggest a rule of "universal distribution and local selection": that is, coccolithophorids can be considered cosmopolitan taxa, but their genetic plasticity provides physiological accomodation to local environmental selection pressure. Sea surface temperature, critical irradiance, and ΔN/Δt were predicted for the years 2060-2070 using the NCAR Community Climate System Model to generate future monthly probability distributions of coccolithophorids based upon the relationships observed between the environmental variables and coccolithophorid blooms in modern oceans. Our projected probability distribution analysis suggests that in the North Atlantic, the largest habitat for coccolithophorids on Earth, the areal extent of blooms will decrease by up to 50% by the middle of this century. We discuss how the magnitude of carbon fluxes may be affected by the evolutionary success of coccolithophorids in future climate scenarios.