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Spatial and temporal dynamics of the hydrology at Salinas Bay,
Costa Rica, Eastern Tropical Pacific
Alejandro Rodríguez
1,2
; https://orcid.org/0000-0003-4618-6560
Eric J. Alfaro
1,2,3
; https://orcid.org/0000-0001-9278-5017
Jorge Cortés
1,4
; https://orcid.org/0000-0001-7004-8649
1. Centro de Investigación en Ciencias del Mar y Limnología (CIMAR), Ciudad de la Investigación, Universidad de
Costa Rica, 11501-2060, San José, Costa Rica; alejandro.rodriguez_b@ucr.ac.cr, erick.alfaro@ucr.ac.cr,
jorge.cortes@ucr.ac.cr
2. Escuela de Física, Sede Rodrigo Facio, Universidad de Costa Rica, 11501-2060, San José, Costa Rica
3. Centro de Investigaciones Geofísicas (CIGEFI), Ciudad de la Investigación, Universidad de Costa Rica, 11501-2060,
San José, Costa Rica
4. Escuela de Biología, Sede Rodrigo Facio, Universidad de Costa Rica, 11501-2060, San José, Costa Rica
Received 08-I-2021. Corrected 20-III-2021. Accepted 10-V-2021.
ABSTRACT
Introduction: Salinas Bay is located in the warm pool of the Eastern Tropical Pacific (ETP), characterized by
warm, shallow surface waters, a strong and shallow thermocline, and an important biological diversity. The
primary productivity of the region is influenced by the coastal upwelling, which occurs during the boreal winter
as a result of the strengthening of trade winds.
Objective: To study the spatial and temporal dynamics of physical and chemical parameters at seven hydro-
graphic stations in Salinas Bay, Costa Rica, through the analysis of CTD data, and relate the warm and cold
events to the regional atmospheric conditions present when measuring the data.
Methods: Seven hydrographic stations, sampled at Salinas Bay between August 2008 and December 2014, were
selected. The variables processed for analysis are temperature, density, salinity, oxygen, chl-a and turbidity.
Once the data was processed, 42 Hovmöller kind diagrams were plotted.
Results: All variables, except turbidity, presented a seasonal periodicity associated with the upwelling. In
general, colder and denser waters, higher salinity and chl-a concentrations and lower dissolved oxygen values
were observed during the dry season, when the upwelling was active. Whereas, during the rainy season water
masses were warmer and less dense, salinity and chl-a concentrations decreased and dissolved oxygen values
tended to increase.
Conclusions: The spatial and temporal dynamics of the hydrology in Salinas Bay was influenced by the coastal
upwelling events. The region also presented an interannual variability associated with ENSO. Seasonal and inter-
annual variability can counteract their effects on the oceanographic parameters when they coincide temporally.
Key words: upwelling; transisthmian wind; CTD; Papagayo; synoptic conditions.
Rodríguez, A., Alfaro, E. J., & Cortés, J. (2021). Spatial
and temporal dynamics of the hydrology at Salinas
Bay, Costa Rica, Eastern Tropical Pacific. Revista de
Biología Tropical, 69(Suppl. 2), S105-S126. https://doi.
org/10.15517/rbt.v69iS2.48314
https://doi.org/10.15517/rbt.v69iS2.48314
The Eastern Tropical Pacific (ETP) has a
warm pool characterized by high temperature
and low salinity surface waters with a strong-
shallow pycno and thermocline (Fiedler &
Lavín, 2017; Fiedler & Talley, 2006). This hab-
itat supports a biological diversity that includes
reef-building corals, plankton communities,
fishes, marine mammals, and birds, among
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others. Salinas Bay is located a few kilometers
north of the Gulf of Papagayo. Climate drivers
and forcers are the same in both places, with
very similar seasonal cycles (Alfaro & Cortés,
2012; Alfaro et al., 2012; Amador et al., 2006;
Amador, Durán-Quesada, et al., 2016; Amador,
Rivera, et al., 2016). The dimensions of Salinas
Bay are small and being in a position such that
its axis is east-west, it is directly influenced by
the exit towards the Pacific of the Papagayo
jet, which flows in the lowlands south of
Nicaragua and north of Costa Rica with a
predominantly northeast direction (Amador et
al., 2006). Therefore, primary productivity is
affected by seasonal upwelling events in Sali-
nas Bay, as is the case in at least three other
coastal areas of the ETP: the gulfs of Tehu-
antepec, Papagayo and Panama (Amador et
al., 2006). The upwellings are caused by wind
forcing over the sea surface near the coast, at
gaps where transisthmian wind jets reach the
Pacific through topographic depressions of
the mountain range (Amador, Rivera, et al.,
2016; Fiedler & Lavín, 2017). Those wind
jets are generated by northeasterly trade winds
and easterlies winds that increase atmospheric
pressure in the Caribbean (Amador, Durán-
Quesada, et al., 2016; Kessler, 2006; Willet,
Leben, & Lavín, 2006). The upwellings take
place during boreal winter, which corresponds
to dry season in the Central American Pacific
slope (Amador et al., 2006), and they reach
their maximum intensity between February and
March (Brenes, Lavín, & Mascarenhas, 2008).
The cold fronts in the boreal winter intensify
trade winds, strengthening the Papagayo wind
jet, which forces the coastal upwelling (Alfaro
& Cortés, 2012; McCreary, Lee, & Enfield,
1989; Zárate-Hernández, 2013). Furthermore,
Intertropical Convergence Zone (ITCZ) migra-
tion southward favors the predominance of
northeasterly trade winds in Central America
during the dry season (Brenes & Gutiérrez,
1998). The ETP is also under the influence
of interannual variability sources, such as the
El Niño-Southern Oscillation (ENSO) warm/
cold cycle (Alexander, Seo, Xie, & Scott,
2012; Fiedler & Lavín, 2017). Interannual
variability impact on sea surface temperature
(SST) anomalies, in upwelling regions in the
Central American Pacific, is mainly attributed
to ENSO (Alfaro & Lizano, 2001). Alfaro et
al. (2012) found a relationship between warm
events in Culebra Bay, Gulf of Papagayo, and
anomalies in the Niño 3.4 index.
The development of an upwelling is evi-
denced by the spatial and seasonal dynamics
of the thermocline depth and the thermohaline
pattern in the water column (Brenes & Gutiér-
rez, 1998; Fiedler & Talley, 2006). D’Croz and
O’Dea (2009) consider that the upwelling is
“one of the most pervasive hydrological events
to influence the shelf waters of Pacific Central
America”. Intense winds displace sea surface
water offshore and set up vertical mixing in
the surface layers of the sea, allowing colder
thermocline and subthermocline water to rise
toward the surface, which results in lower SSTs
and higher salinity concentrations (Alfaro &
Cortés, 2012; Brenes, Hernández, & Gutiér-
rez, 1998; Kessler, 2006; Willet et al., 2006).
CTD data measured by a research cruise,
between February and March 1994, exposed
the upwellings in the Gulf of Papagayo and
Gulf of Panama, where high salinity cold
tongues surrounded by warm waters were
observed on the surface (Brenes et al., 2008).
According to Loza-Álvarez, Benavides-More-
ra, Brenes-Rodríguez, and Saxon Ballestero
(2018), temperatures were lower and salinity
concentrations were higher in a sampling car-
ried out in the Gulf of Papagayo during the dry
season than in another sampling during the wet
season. In hydrographic stations at the ETP
during March, there was a notable elevation
of the thermocline associated with the strong
Papagayos winds (Mora-Escalante, Lizano,
Alfaro, & Rodríguez, 2019). In the Gulf of
Papagayo, high concentrations of nutrients,
carbon dioxide (CO
2
) and phytoplankton from
the subsurface chlorophyll maximum emerge
into the euphotic zone during the upwelling,
increasing local chlorophyll production (Ball-
estero & Cohen, 2004; Willet et al., 2006;
Rixen, Jiménez, & Cortés, 2012). Loza-Álva-
rez et al. (2018) also found that chl-a maximum
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at 35 m in the rainy season was replaced by a
homogeneous distribution with higher chl-a
concentrations in shallow waters during the dry
season, as a consequence of the turbulent mix-
ture caused by the winds and the upwelling pro-
cesses. At Panama Bay, the upwelling displaces
nutrient-poor surface water with cooler, more
saline and nutrient rich waters (D’Croz & Rob-
ertson, 1997). In the Gulf of Panama, increases
in chl-a and dissolved oxygen concentrations
were observed at the surface, followed by
oxygen-poor waters below the thermocline and
higher turbidity, as a result of upwelling events
(D’Croz & O’Dea, 2007, 2009).
The alterations in the physico-chemical
parameters of the water caused by the upwell-
ings affect biological populations. Increases
in nutrient availability triggers phytoplankton
blooms (Stuhldreier, Sánchez-Noguera, Rixen,
et al., 2015); however, in the Gulf of Papagayo,
Stuhldreier, Sánchez-Noguera, Roth, Jiménez,
et al. (2015) observed that at the Matapalo reef
the upwelling did not influence the benthic
community. Wizemann et al. (2018) found
that carbonate bioerosion increased with the
upwelling in that same area. However, Rixen et
al. (2012) determined that in Culebra Bay, Gulf
of Papagayo, the negative impact of the upwell-
ing on coral growth might be counteracted by
high nutrient concentrations. Sanchéz-Noguera
et al. (2018) suggest that corals in Culebra Bay
are adapted to upwelling effects, in accordance
with Stuhldreier, Sánchez-Noguera, Roth, Cor-
tés, et al. (2015). In fact, Jiménez, Bassey,
Segura, and Cortés (2010) described coral
habitats in the Gulf of Papagayo, since most
of the coastal area is suitable for coral reef
growth despite the upwelling influence. Con-
versely, the conditions are not suitable for the
growth of coral reefs in Panama Bay due to
upwelling effects, according to D’Croz and
Robertson (1997). Increases in nutrient con-
centrations caused by upwelling events in ETP
are associated with the expansion of opportu-
nistic species in coral reefs (Roth, Stuhldreier,
Sánchez-Noguera, Carvalho, & Wild, 2017).
Regarding the study area of this research,
Sibaja-Cordero and Cortés (2008) detected a
seasonal change of algal assemblages in the
rocky intertidal zone related to the upwelling
in Salinas Bay. Moreover, Cortés, Samper-
Villarreal, and Bernecker (2014) discovered
that the growth rate of an algae, Sargassum
liebmannii, peaked during seasonal upwelling
in Salinas Bay.
Summing-up, within the ETP areas of
contrast are created between upwelling and
non-upwelling zones, as well as seasonal-
ity, which defines the marine ecology of the
Central American Pacific. As a result, a study
of the spatial and temporal dynamics of the
physical parameters in Salinas Bay is neces-
sary to understand the influence of upwell-
ing events on the hydrology and biology of
the bay. Accordingly, the objective of this
research is to produce the first report of the
spatial and temporal dynamics of temperature,
density, salinity, oxygen, chl-a and turbid-
ity in Salinas Bay, Costa Rica, based on the
analysis of CTD cast data measured through
33 one-day oceanographic campaigns, between
August 2008 and December 2014, and relate
the CTD warm and cold events to the observed
atmospheric conditions.
MATERIALS AND METHODS
Study area: Salinas Bay is located on the
northwest of Costa Rica and is shared with
Nicaragua. This region is exposed to a seasonal
upwelling that starts in December and ends in
April-May. There are small coral patches, con-
structed by the zooxanthellate corals. Filtering
organism are some of the main components of
the rocky bottoms, including several species of
ascideans, sea cucumbers, polychaets and octo-
corals (Cortés, 2016). Eight hydrographic sta-
tions were sampled along a latitudinal transect
11.8 km long, at 11.06 ºN latitude and between
85.67º W and 85.79º W longitude (Fig. 1).
Measurements and analyses: A CTD
database made up of 215 data-points was com-
piled from the results of 33 one-day oceano-
graphic campaigns, conducted in Salinas Bay
between August 2008 and December 2014
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(Table 1). The physical parameters of the water
were measured with a CTD model SBE 19plus
V2, which was calibrated under the manufac-
turer’s standards. In some campaigns, it was
not possible to obtain measurements for all
the stations and/or variables and certain data
measured were excluded because they were
not reliable. After the analysis of maximum
and minimum values listed in Table 2, Table
3, Table 4, Table 5, Table 6 and Table 7, data
from the closest station to the coast (VIII) were
discarded from this research, since it was too
shallow and sampling could not be done some
times or the data were unreliable. The maxi-
mum depth sampled in each station fluctuates
for the different campaigns due to variations in
Fig. 1. Location of Salinas Bay with respect to some geographic reference points in the North Pacific of Costa Rica and
location of CTD sampling stations within Salinas Bay.
TABLE 1
Dates of oceanographic campaigns at Salinas Bay, Costa Rica
Year
Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2008 20 25 4
2009 19 23 18 20 22
2010 7 6 24 19 21 10
2011 24 14 23 18 8
2012 15 25 14 23 18 5
2013 13 18 13 5
2014 20 12 18 4
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sea level that could be caused by environmental
conditions, such as the tide and the season. As a
result, an approximation of the maximum depth
was used for data plots (Fig. 2, Fig. 3, Fig. 4,
Fig. 5, Fig. 6, and Fig. 7). The approximate
maximum measurement depth at station I was
50 m, at station II 45 m, at station III 35 m, at
station IV 25 m, at station V 19 m, at station VI
18 m and at station VII 10 m.
The variables processed for analysis were
temperature, density, salinity, oxygen, chl-a
and turbidity. The raw data were downloaded
from the CTD and subjected to a series of
processes included in Sea-Bird Electronics
SBE Data Processing software that converted
the raw data to engineering units, subjected
to a low-pass filter some columns of the data,
aligned the data relative to pressure, performed
conductivity thermal mass correction, deleted
inaccurate values, calculated the derive vari-
ables, averaged the data every meter deep,
generated the data to ASCII files and plotted
preliminary profiles of the variables. Regard-
ing derived variables, the density of seawater
and practical salinity were computed with SBE
software from CTD measurements (Sea-Bird
Electronics, 2016). Dissolved oxygen concen-
tration computation was based on values mea-
sured by a polarographic membrane oxygen
sensor model SBE 43, which counts the num-
ber of oxygen molecules per second through
the membrane (Sea-Bird Electronics, 2013).
Chl-a and turbidity were measured with the
ECO-FL-NTU model fluorometer and scatter-
ing sensor, which determines the fluorescence
and the suspended particle concentration in the
water (Sea-Bird Electronics, 2011; Sea-Bird
Electronics, n.d.). For the oxygen, chl-a and
turbidity data, it was necessary an additional
visual quality control procedure in order to
manually delete unreliable data, since these
values were inconsistent or out of the nominal
ranges of measurement of the sensors.
After data processing, a Python code (Van
Rossum & Drake, 2009) was created to obtain
the maximum and minimum values of each
variable along with the corresponding date
and depth, and the results were stored in tables
separated by variable (Table 2, Table 3, Table 4,
Table 5, Table 6, and Table 7). A code was writ-
ten in MATLAB
®
(MATLAB
®
, 2018) to plot
Hovmöller diagrams (Fig. 2, Fig. 3, Fig. 4, Fig.
5, Fig. 6, Fig. 7), which reveal the temporal
distribution that each variable presented during
the studied interval. Maximum and minimum
values in Tables 2 to 7, retrieved with the
Python code, were used to define the respec-
tive color scales for each map. A Hovmöller
kind diagram was plotted for each of the seven
hydrographic stations and six variables, result-
ing in a total of 42 diagrams.
The annual cycle of the CTD variables
were then calculated as the average values for
all the stations and depth levels for a particular
month. Also, the climate values from a meteo-
rological station located in Santa Elena, La
Cruz, Guanacaste (10°54’59” N, 85°36’59”
W, altitude: 270 masl, Fig. 1, https://www.
imn.ac.cr/mapa) were considered to represent
the annual cycle of the air surface temperature
(maximum, mean, minimum), precipitation,
and wind magnitude. The climatology of the
sea subsurface temperature in a station located
in Salinas Bay (11º01.616’ N, 85º45.801’ W,
depth: 3.3–4.9 m) described in Alfaro and Cor-
tés (2021) was also included (see Fig. 8a in the
next section).
RESULTS
Temperature: The thermocline is located
around 30 m. Vertical temperature distribution
has a periodic variability, where cold cycles fit
with dry seasons and warm cycles with rainy
seasons (Fig. 2). Temperature variability for all
stations was similar, but with slight differences,
mainly due to depth near the coast. The mini-
mum temperature registered was 15.15 °C in
station I at approximately the maximum depth
in February 2011, and the maximum tempera-
ture, 29.93 °C, was observed in station IV at 7
m in June 2009 (Table 2).
Density: Density variability presents the
same periodicity as temperature, but with an
inverse relation (Fig. 3). Water masses were
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Fig. 2. Temperature vertical distribution at Salinas Bay
from August 2008 to December 2014. Stations ordered
from the furthest to the closest to the coast: I, II, III,
IV, V, VI, VII. Dots in vertical lines correspond to CTD
measurements (averaged every meter).
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denser during upwelling events and at greater
depths. Less dense water is observed mostly
at shallow layers and during the rainy season.
Sigma-t density ranged from 19.61kg m
-3
, in
station IV at 2 m in October 2010, to 26.00kg
m
-3
, in station I at approximately the maximum
depth in February 2011 (Table 3).
Salinity: Salinity had a seasonal variabil-
ity with higher values found in the dry season
(December-April) and lower during the rainy
months (May-November). The variation in
salinity was observed mainly within the first
25 m and at deeper layers tended to be stable
(Fig. 4). Salinity ranged from 31.48 PSU in
station IV at 2 m in October 2010, to 35.56
PSU in station V at 2 m in April 2009 (Table
4). Throughout the entire time series, salinity
at depths of 25 m and below was higher than
34 PSU, except between April and December
2012, when a warm episode took place.
Oxygen: Dissolved oxygen had high-
er values within the first 30 m depth with
TABLE 2
Maximum and minimum values of temperature for each station with their respective date and depth
Station
Maximum Minimum
Temperature (°C) Date Depth (m) Temperature (°C) Date Depth (m)
I 29.76 18/6/2009 -1 15.15 24/2/2011 -53
II 29.60 18/6/2009 -1 15.84 24/2/2011 -45
III 29.35 18/6/2009 -1 16.50 18/4/2013 -37
IV 29.93 18/6/2009 -7 17.45 13/2/2013 -25
V 29.49 20/8/2008 -1 17.21 13/2/2013 -18
VI 29.70 18/6/2009 -6 17.11 13/2/2013 -17
VII 29.86 18/6/2009 -4 17.38 13/2/2013 -10
TABLE 4
Maximum and minimum values of salinity for each station with their respective date and depth
Station
Maximum Minimum
Salinity (PSU) Date Depth (m) Salinity (PSU) Date Depth (m)
I 34.88 18/4/2013 -3 31.70 21/10/2010 -2
II 35.03 13/2/2013 -1 31.58 21/10/2010 -1
III 35.15 23/4/2009 -2 31.59 21/10/2010 -2
IV 34.99 13/2/2013 -3 31.48 21/10/2010 -2
V 35.56 23/4/2009 -2 31.53 21/10/2010 -1
VI 35.49 23/4/2009 -1 31.60 21/10/2010 -2
VII 35.15 13/2/2013 -1 31.60 21/10/2010 -1
TABLE 3
Maximum and minimum values of sigma-t density for each station with their respective date and depth
Station
Maximum Minimum
Density (kg m
-3
) Date Depth (m) Density (kg m
-3
) Date Depth (m)
I 26.00 24/2/2011 -53 19.83 21/10/2010 -2
II 25.83 18/4/2013 -46 19.67 23/8/2012 -8
III 25.64 18/4/2013 -37 19.73 21/10/2010 -2
IV 25.38 13/2/2013 -25 19.61 21/10/2010 -2
V 25.56 13/2/2013 -18 19.67 21/10/2010 -1
VI 25.47 13/2/2013 -17 19.74 21/10/2010 -2
VII 25.53 13/2/2013 -10 19.72 21/10/2010 -1
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Fig. 3. Sigma-t density vertical distribution at Salinas Bay
from August 2008 to December 2014. Stations ordered
from the furthest to the closest to the coast: I, II, III,
IV, V, VI, VII. Dots in vertical lines correspond to CTD
measurements (averaged every meter).
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Fig. 4. Salinity vertical distribution at Salinas Bay from
August 2008 to December 2014. Stations ordered from the
furthest to the closest to the coast: I, II, III, IV, V, VI, VII.
Dots in vertical lines correspond to CTD measurements
(averaged every meter).
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Fig. 5. Oxygen vertical distribution at Salinas Bay from
August 2008 to December 2014. Stations ordered from the
furthest to the closest to the coast: I, II, III, IV, V, VI, VII.
Dots in vertical lines correspond to CTD measurements
(averaged every meter).
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concentrations around 3–5 ml l
-1
(Fig. 5).
Annual variability associated with the upwell-
ing was not so clear for oxygen, however,
the tendency was for higher concentrations
of dissolved oxygen during the rainy season
when compared with the dry season. The low-
est value for dissolved oxygen was 0.17 ml
l
-1
in station I at approximately the maximum
depth in April 2011, and the highest was 5.61
ml l
-1
in station VII at 8 m in December 2013
(Table 5). Oxygen did not show a strong sea-
sonal variability and therefore the interannual
signal dominated.
Chl-a: High concentrations of chl-a were
predominant in the first 25 m and when the
upwelling was active (Fig. 6). Maximum chl-a
concentration, of 5.06 mg m
-3
was found in
station III at 6 m in December 2014, during
an upwelling event. On the other hand, chl-a
minimum took place in station I at 44 m, dur-
ing rainy season in June 2013 (Table 6). The
interannual signal was predominant for chl-a
dynamics, as a consequence of the weak sea-
sonal variability.
Turbidity: Turbidity did not have an evi-
dent periodic tendency associated with upwell-
ing, as is the case for other parameters, but
there were areas with higher turbidity, mainly
at the bottom of different stations during rainy
season (Fig. 7). Turbidity values ranged from
0.0184 NTU in station I at approximately the
maximum depth in April 2011, to 3.6706 NTU
in station VII at 9 m in June 2011 (Table 7).
Turbidity dynamics depended mainly on the
interannual signal, since its seasonal variability
was also weak.
Annual Cycle: Figure 8 shows the month-
ly mean values of the CTD variables described
above, plus the air surface temperature (maxi-
mum, mean, minimum), precipitation, wind
magnitude and sea subsurface temperature. Sea
subsurface temperature minimum was observed
in February-March, with a relative minimum in
TABLE 6
Maximum and minimum values of chl-a for each station with their respective date and depth
Station
Maximum Minimum
chl-a (mg m
-3
) Date Depth (m) chl-a (mg m
-3
) Date Depth (m)
I 4.57 4/12/2014 -6 0.01 13/6/2013 -44
II 4.88 4/12/2014 -7 0.01 23/8/2012 -3
III 5.06 4/12/2014 -6 0.02 18/8/2011 -3
IV 4.24 4/12/2014 -6 0.03 23/8/2012 -4
V 4.79 4/12/2014 -7 0.05 18/8/2011 -4
VI 4.84 4/12/2014 -7 0.07 13/2/2013 -17
VII 4.04 4/12/2014 -5 0.02 23/8/2012 -1
TABLE 5
Maximum and minimum values of oxygen for each station with their respective date and depth
Station
Maximum Minimum
Oxygen (ml l
-1
) Date Depth (m) Oxygen (ml l
-1
) Date Depth (m)
I 4.88 23/6/2011 -16 0.17 14/4/2011 -53
II 4.82 23/6/2011 -3 0.30 14/4/2011 -47
III 4.91 5/12/2013 -3 0.64 20/3/2014 -36
IV 5.02 5/12/2013 -1 1.81 20/3/2014 -24
V 5.10 14/4/2011 -15 1.78 20/3/2014 -17
VI 5.12 14/4/2011 -8 1.89 20/3/2014 -17
VII 5.61 5/12/2013 -8 2.13 23/6/2011 -10
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Fig. 6. Chl-a vertical distribution at Salinas Bay from
August 2008 to December 2014. Stations ordered from the
furthest to the closest to the coast: I, II, III, IV, V, VI, VII.
Dots in vertical lines correspond to CTD measurements
(averaged every meter).
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Fig. 7. Turbidity vertical distribution at Salinas Bay from
August 2008 to December 2014. Stations ordered from the
furthest to the closest to the coast: I, II, III, IV, V, VI, VII.
Dots in vertical lines correspond to CTD measurements
(averaged every meter).
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Fig. 8. Annual cycle of the: a) sea subsurface temperature in a station in Salinas Bay (11º 01.616’N, 85º 45.801’W, depth:
3.3-4.9m), b) air surface temperature (maximum-squares, average-solid and minimum-dots), c) precipitation, d) wind
magnitude, in Santa Elena, La Cruz, Guanacaste meteorological station (10° 54’ 59” N, 85° 36’ 59” W, altitude: 270 masl),
and e) density, f) oxygen, g) temperature and salinity, and h) chl-a and turbidity, for the CTD stations in Salinas Bay showed
in Figure 1.
TABLE 7
Maximum and minimum values of turbidity for each station with their respective date and depth
Station
Maximum Minimum
Turbidity (NTU) Date Depth (m) Turbidity (NTU) Date Depth (m)
I 3.13 18/9/2014 -53 0.02 14/4/2011 -52
II 2.45 5/12/2012 -46 0.04 14/4/2011 -36
III 1.89 5/12/2012 -37 0.04 14/4/2011 -34
IV 1.87 4/12/2014 -27 0.09 14/4/2011 -26
V 3.02 23/8/2012 -18 0.17 12/6/2014 -1
VI 2.87 23/8/2012 -17 0.21 13/2/2013 -1
VII 3.67 23/6/2011 -9 0.35 13/2/2013 -1
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July between the two maxima of May-June and
September-October (Fig. 8a). Maximum and
minimum air temperature presented their low-
est values in October-November and January-
February and the highest in March-April and
July, respectively. Mean air temperature is a
combination of the maximum and minimum
annual cycles, with maximum and minimum
in April and the quarter of October-November-
December (Fig. 8b). Precipitation presented
a typical bimodal distribution of the Central
American Pacific slope, with a dry season in
December-April and a rainy season with two
maxima in May-June and September-October,
with a reduction in July-August associated
with the Mid-Summer Drought (Fig. 8c). Wind
magnitude presented high values for the boreal
winter, from December to March and low
values in June, September and October. A
relative maximum is observed for July (Fig.
8d). The annual cycle of the temperature and
salinity from the CTD casts tend to covariate
in opposite way. From December to April,
high (low) salinity (temperature) values were
observed, meanwhile from May to October
respectively, high (low) temperature (salinity)
values were reported (Fig. 8g). Density annual
cycle responds to the salinity and temperature
ones, with high values from December to April
and low values from May to October (Fig. 8e).
Oxygen, chl-a and turbidity were minimum in
April. Chl-a and turbidity presented a peak in
June and the oxygen in December (Fig. 8f, Fig.
8g, Fig. 8h).
DISCUSSION
Temperature: Absolute maximum tem-
perature was observed within shallow waters
at 7 m, as is expected for warmer water
masses. Maximum for all stations, except for V,
occurred in June 2009, which is in accordance
with a warm event recorded by Alfaro et al.
(2012) at Culebra Bay, where a SST standard-
ized anomaly of 0.64 ºC was observed. June
corresponds to the rainy season when SSTs
in the ETP are expected to be warmer and the
upwelling is not active. According to the Costa
Rican National Meteorological Institute (IMN
in Spanish) weather bulletin, during June 2009
warm event atmospheric mean temperatures
and maximum absolute temperatures in the
Pacific region were higher than the historic
mean (Quirós, 2009). Maximum for station V
occurred in August 2008, under the influence
of a persistent warming of the SSTs in the equa-
torial ETP, and a positive anomaly of tempera-
ture offshore Costa Rican North Pacific coast,
according to IMN bulletin (Sánchez, 2008).
Minimum absolute and for stations I-II was
observed during a cold event in February 2011,
associated with a cold front and a high-pressure
system, which accelerated winds speed and
favored upwelling (Morera, 2011). Minimum
for station III took place in April 2013, a
month characterized by pressure anomalies
due to polar air masses and cold fronts that
increased winds intensity (Chinchilla, 2013).
Minimum for stations IV to VII was observed
in February 2013, under dry conditions, the
incidence of cold fronts and the predominance
of strong northeast winds (Morera, 2013b). The
Oceanic Niño Index (ONI) 3-month running
mean (3-MRM) anomalous SST for periods
that include the observation of the absolute
maximum temperature and of the maxima
temperatures at all stations (except V) were
neutral for April-May-June and May-June-July
but warm for June-July-August period (0.5
ºC 3-MRM). The ONI values for all periods
that include the maximum of station V were
neutral. The absolute maximum and stations
I and II maxima were observed during cold
events in accordance with the ONI index (-1.4
ºC for December-January-February, -1.2 ºC for
January-February-March and -0.9 ºC for Febru-
ary-March -April). ONI values were neutral for
all the periods that cover the observation of the
minima at stations III to VII (Climate Predic-
tion Center, n.d.).
Density: Absolute maximum density was
observed at Station I at approximately the
maximum depth, during a cold event caused
by a high-pressure in February 2011 (Morera,
2011), when temperature minima was observed.
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Stations II and III registered the maximum in
April 2013, influenced by pressure anomalies
due to polar air masses and cold fronts which
increased winds intensity (Chinchilla, 2013).
Maximum for stations IV-VII occurred in Feb-
ruary 2013, with dry conditions, the incidence
of cold fronts and the predominance of strong
northeast winds (Morera, 2013b). Absolute
minimum in October 2010 is associated with
shallow depths as it was found at 2m in station
IV, during the wet season when upwelling is not
present or is very weak. It should be noted that
according to Figure 8c, September and October
are the rainiest months in the region. Besides,
atmospheric temperature maxima were higher
than the historic mean (Solano, 2010). In
general, heat flux at ETP tends to be lower
during the wet season since the presence of the
cloud tops caused by the displacement of the
Inter-Tropical Convergence Zone reduces solar
radiation incidence over sea surface (Amador,
Rivera, et al., 2016). Nevertheless, during the
rainy season SSTs at Salinas Bay are warmer
given that the upwelling is inactive, resulting
in lower water densities locally. The other
stations registered density minimum the same
day that station IV at a depth between 1–2 m,
with the exception of station II where density
minimum was observed in August 2012 at 8 m.
This condition could be related to a warming
event, which started in April-May and peaked
in June-July, due to the increase in the air
temperatures and SST (Poleo & Stolz, 2012a).
In addition, the North Pacific of Costa Rica
presented a positive precipitation anomaly,
rain was greater than normal, and trade winds
weakened in August as a result of the weaken-
ing of the North Atlantic Anticyclone, resulting
in unfavorable conditions for an upwelling as
is expected during the rainy season (Poleo &
Stolz, 2012b).
Salinity: Higher salinities were observed
during upwelling events, associated with colder
and denser waters. Absolute maximum was
located in station V in shallow waters (2 m),
and is related to a high-pressure system, which
caused a cold front that accelerated trade winds
in April 2009, favoring the upwelling, which is
also the case for stations III and VI maximum
(Alfaro, 2009). Maximum for station I occurred
in April 2013, under the influence of pressure
anomalies due to polar air masses and cold
fronts that strengthened trade winds (Chinchil-
la, 2013). Maximum for stations II, IV, and VII
happened in February 2013, a month character-
ized by dry conditions, the incidence of cold
fronts and the predominance of strong north-
east winds (Morera, 2013b). Minima for all
stations were observed in October 2010, during
the wet season, as in the case of density. The
observed seasonal dynamics for salinity is con-
sistent with observations made in the Gulf of
Papagayo, where lower salinity concentrations
during the rainy season are associated with
increased rainfall (Loza-Álvarez et al., 2018).
Oxygen: Absolute maximum oxygen con-
centrations were observed at 8m, only a few
meters above the maximum chlorophyll layer,
in December 2013, a month characterized by
strong trade winds (Morera, 2013a). Stations
III-IV and VII maxima coincide temporally
with absolute maximum. In the case of sta-
tions I-II, maximum occurred in June 2011,
under fast winds on the Pacific coast and
short periods with dry conditions (Morera &
Stolz, 2011). For stations V-VI the maximum
was observed in April 2011, a windy month
with dry conditions, influenced by cold fronts
(Chinchilla, 2011a). Absolute minimum was
registered in April 2011, but it is related to
a high depth reached at station I rather than
synoptic conditions, since oxygen constantly
decreases with depth. The above applied for
station II, where the minimum was observed
at approximately the maximum depth the same
date. In stations III-VI the minimum was
located near the bottom in March 2014, a dry
month with high pressures and persistent trade
winds (Solano, 2014). Minimum for station VII
was also located near the seabed, in June 2011
during wet season, whereby it is more related
to depth than to weather conditions.
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Chl-a: Absolute maxima of chl-a were
recorded in December 2014, a month when
four cold fronts, which generated strong trade
winds and wind gust speed over 100km h
-1
,
were observed locally (Chinchilla, 2014).
However, this event was aborted by an easterly
wind burst in the western Pacific, which led
to an event of El Niño (Levine & McPhaden,
2016). Maxima for all stations was registered
in the same date. Chl-a concentrations tend
to increase during upwelling, associated with
an increase in biological productivity, and are
higher in the thermocline. Absolute minimum
was observed in station I, within deep waters,
at 44 m in June 2013, under typical wet season
conditions (Poleo & Stolz, 2013). In the case
of stations II, IV and VII, the minimum was
observed in August 2012, a month character-
ized by weak trade winds, a rainy pattern
above the historic records for the North Pacific
and atmospheric temperatures warmer than
the historic mean associated with an El Niño
(Poleo & Stolz, 2012b). Minimum for sta-
tions III and V occurred in August 2011, with
the incidence of wet conditions due to wind
from the west, which fostered rainy activity
in Guanacaste (Chinchilla, 2011b). Station VI
minimum was observed in February 2013
under dry conditions (Morera, 2013b), whereby
it is associated with depth (17 m) rather than
synoptic conditions.
Turbidity: In some stations a periodic-
ity in turbidity was observed, with remarkable
increases close to the seabed during wet sea-
sons. Absolute maximum was registered in sta-
tion VII in June 2011, during a short dry period
within the rainy season, with above normal
wind speed in the Pacific, weak trade winds
and atmospheric temperatures above the histor-
ic mean (Morera & Stolz, 2011). At station I the
maximum occurred in September 2014, when
trade winds weakened in Guanacaste favoring
an increase in humidity, and low-pressure sys-
tems resulted in extreme events, particularly on
September 13 and 18 (sampling day), fostering
intense precipitations in the region (Poleo &
Stolz, 2014). In addition, during that month
the influence of El Niño affected the weather
in the region (Alvarado, 2014b). For station II-
III, the maximum appeared in December 2012,
with predominance of trade winds and less cold
fronts than usual (Solano, 2012). For station
IV the maximum was registered in December
2014, a month characterized by indirect inci-
dence of cold fronts and strong trade winds,
particularly during an important event the first
ten days, which was present when the sam-
pling was carried out on the fourth day (Chin-
chilla, 2014). Also, El Niño was active during
December 2014 after intensifying in previous
months (Alvarado, 2014a). The maximum in
stations V-VI happened in August 2012, with
the presence of El Niño, weak trade winds and
positive anomalies of atmospheric tempera-
tures and precipitation (Poleo & Stolz, 2012b).
Absolute minimum was found in April 2011,
in the presence of a cold front, which caused
moderate wind (Chinchilla, 2011a). Minimum
for stations I-IV occurred the same month. For
station V the minimum was recorded in June
2014, when the North Pacific was experiencing
a drought associated with El Niño (Naranjo,
2014). The minimum for stations VI and VII
occurred in February 2013, a month affected
by windy conditions due to cold fronts (More-
ra, 2013b). The heterogeneity of atmospheric
conditions at the moment when maxima or
minima values were presented does not allow
to conclude a relation between turbidity, syn-
optic conditions and the upwelling, since the
values do not have a seasonal periodicity. Most
maxima values were observed when an event
of El Niño was active, which could indicate a
relation between them; however, a minimum
was also registered during an El Niño event.
All of the maxima were located at the seabed
of their respective stations. In the case of the
minima, only those presented in April 2011
are located in the bottom of the stations, and
turbidity levels remained low within the entire
water column.
Annual Cycle relationships: Densi-
ty annual cycle presented high values from
December to April, in agreement with the dry
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season in the Central American Pacific slope
and with the months in which the magnitude of
the wind is also high (Amador et al. 2006). This
season is associated with high incoming solar
radiation, important evaporation, low cloudi-
ness, sea water transport offshore, minimum
runoff and a deeper mixing layer (Amador,
Durán-Quesada, et al., 2016; Durán-Quesada
et al., 2020). During December-April, density
is consequently associated with high salinity
values and low sea temperatures of the water
column in Salinas Bay. Oxygen, chl-a and tur-
bidity were minimum in April, in which the air
surface temperature was maximum. From May
to October occurs the rainy season, increasing
the runoff-freshwater input to the bay and the
cloud cover. Additionally, the wind magnitude
decreases along with the evaporation, the sea
water transport offshore and the depth of the
mixing layer. These factors increased the Sali-
nas Bay sea water temperature and reduce its
salinity. Oxygen presented a positive trend
from the begin to the end of the rainy season,
when the wind magnitude and the air surface
temperature presented negative trends. Chl-a
and turbidity presented a peak in June, in agree-
ment with the first part of the rainy season.
Turbidity also presented a relative maximum in
October. These turbidity peaks could be related
with the sediment runoff transport during the
peaks of the rainy season. June chl-a peak
could be associated with the high incoming
solar radiation and low wind magnitude val-
ues observed during the mornings of the rainy
season (Alfaro et al., 2012), but the relative
maximum in December could be associated
with a decrease in the cloud coverture (Amador
et al., 2006).
Conclusions
Our results in Salinas Bay, should be
interpreted in the context of the Eastern Tropi-
cal Pacific seasonal and interannual variability
described in detail by previous works of Ama-
dor et al. (2006); Amador, Durán-Quesada, et
al. (2016), Amador, Rivera, et al., (2016) and
Durán-Quesada et al. (2020). The spatial and
temporal dynamics of the hydrology in Salinas
Bay was influenced by the coastal upwelling
events, as its effects include lower SSTs, higher
salinity and density, and the movement of dis-
solved inorganic nutrients toward the surface,
leading to favorable conditions for biological
blooming and increasing chlorophyll-a concen-
trations, as it was observed in February 2013.
The periodicity of oxygen, chl-a, and turbidity
is not as remarkable as with the other variables,
but it can be associated with the seasons. The
climatic conditions influenced the appearance
of maximum and minimum values for the
variables. For example, rainy patterns above
the historic records, as the June 2013, could
be associated with observed minima of chl-a
in some stations. The region also presented an
interannual variability, such as that associated
with ENSO, which can counteract the upwell-
ing during warm events such as the identified
cases of August 2008 and June 2009.
Cold fronts and high-pressure anomalies
associated with polar air masses are the main
synoptic conditions related to temperature and
turbidity minima (except in station V) and
density, salinity and chl-a maxima, given that
these conditions strengthen the winds that favor
the upwelling in the studied region. Moreover,
atmospheric and sea surface positive tempera-
ture anomalies, positive precipitation anoma-
lies, weak trade winds and the influence of El
Niño are the principal meteorological condi-
tions for temperature and most of turbidity
maxima and density, salinity, and chl-a minima
(except in station VI). Turbidity maxima at sta-
tions II, III and IV were recorded under strong
trade winds but with a lower incidence of cold
fronts and, in the case of station IV maximum,
during an event of El Niño. As a result, sea-
sonal and interannual variability can counteract
their effects on the oceanographic parameters
when they temporally coincide. Both oxygen
maximum and minimum were measured during
the upwelling but, oxygen minima were related
to depth rather than synoptic conditions.
Annual and interannual variation observed
in Salinas bay influences the local ecosystem
and that is reflected in the growth of algae
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(e.g., Sargassum), as reported by Cortés et al.
(2014). According to Morales-Ramírez, Till-
Pons, Alfaro, Corrales-Ugalde, & Sheridan-
Rodríguez. (2021), zooplankton in Salinas Bay
responds in accordance with changes in the
structure of the oceanographic conditions mod-
ulated through the seasonal upwelling in the
region. Compared to other coastal areas in the
Costa Rican North Pacific, Salinas Bay seems
to be a healthy local ecosystem.
Ethical statement: authors declare that
they all agree with this publication and made
significant contributions; that there is no con-
flict of interest of any kind; and that we fol-
lowed all pertinent ethical and legal procedures
and requirements. All financial sources are
fully and clearly stated in the acknowledge-
ments section. A signed document has been
filed in the journal archives.
ACKNOWLEDGMENTS
This project was initially supported by
the Vicerrectoría de Investigación of the Uni-
versidad de Costa Rica (project A5037), and
later partly support by UCR projects: B8766,
B9454, EC497and C0610. AR thanks the BSc
Physics Program at the UCR, for its support in
the FS0624 course. To Paula M. Pérez Briceño
for her assistance with Figure 1.
RESUMEN
Dinámica espacial y temporal de la hidrología en
Bahía Salinas, Costa Rica, Pacífico Tropical Oriental
Introducción: Bahía Salinas se encuentra embebida en la
piscina de agua cálida del Pacífico Tropical Oriental (PTO)
o del Este (PTE), caracterizada por aguas superficiales cáli-
das, poco salinas, una termoclina fuerte y poco profunda
y una importante diversidad biológica. La productividad
primaria de la región está influenciada por el afloramiento
oceánico, el cual se presenta durante el invierno boreal
como resultado del fortalecimiento de los vientos alisios.
Objetivo: Estudiar la dinámica espacial y temporal de
parámetros físico-químicos en siete estaciones hidrográ-
ficas en Bahía Salinas, Costa Rica, a través del análisis de
datos de CTD; y relacionar los eventos cálidos y fríos con
las condiciones atmosféricas presentes al medir los datos.
Métodos: Se muestrearon y seleccionaron siete estaciones
hidrográficas en Bahía Salinas entre agosto de 2008 y
diciembre de 2014. Las variables procesadas para su análi-
sis son temperatura, densidad, salinidad, oxígeno, clorofila-
a y turbidez. Una vez procesados los datos se graficaron 42
diagramas tipo Hovmöller.
Resultados: Todas las variables, excepto la turbidez,
presentaron una periodicidad estacional asociada al aflo-
ramiento. En general, se observaron aguas más frías y
densas, mayores concentraciones de salinidad y clorofila-a
y valores menores de oxígeno disuelto durante la estación
seca, cuando el afloramiento estaba activo. Por el contrario,
durante la estación lluviosa las masas de agua eran más
cálidas y menos densas, las concentraciones de salinidad y
clorofila-a disminuyeron y los valores de oxígeno disuelto
tendieron a aumentar.
Conclusiones: La dinámica espacial y temporal de la
hidrología en Bahía Salinas fue influenciada por los even-
tos de afloramiento costero. La región también presentó
una variabilidad interanual, como la asociada con el ENOS.
La variabilidad estacional y la interanual pueden contra-
rrestar sus efectos sobre los parámetros oceanográficos
cuando coinciden temporalmente.
Palabras clave: afloramiento; chorros transístmicos; CTD;
Papagayo; condiciones sinópticas.
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