Revista de Biología Tropical, ISSN: 2215-2075, Vol. 69(3): 773-796, July-September 2021 (Published Jul. 30, 2021)
A fjord-like tropical ecosystem, Pacific coast of Costa Rica:
overview of research in Golfo Dulce
José A. Vargas-Zamora
*; https://orcid.org/0000-0003-3431-8802
Eddy Gómez-Ramírez
; https://orcid.org/0000-0002-8138-0222
Álvaro Morales-Ramírez
; https://orcid.org/000-0002-0883-5366
1. Centro de Investigación en Ciencias del Mar y Limnología (CIMAR), Universidad de Costa Rica, 11501-2060, San
José, Costa Rica; jose.vargas@ucr.ac.cr (Correspondence*)
2. Escuela de Biología, Universidad de Costa Rica, 11501-2060, San José, Costa Rica; alvaro.morales@ucr.ac.cr
3. Escuela de Química, Universidad de Costa Rica, 11501-2060, San José, Costa Rica; eddy.gomez@ucr.ac.cr
Received 25-III-2021. Corrected 28-VI-2021. Accepted 12-VII-2021.
Introduction: Coastal ecosystems worldwide are under the influence of local, regional and global stressors,
such as pollution, eutrophication and climate change. Golfo Dulce is a relatively pristine and accessible deep
tropical ecosystem that provides opportunities for comparative and collaborative research.
Objective: To summarize published reports on past research conducted in this ecosystem, identify topics for
further study, and suggest new research issues.
Methods: A search was made on the web for reports based on research conducted in Golfo Dulce and published
in scientific journals. Reports focusing on environmental parameters and on the biota were included.
Results: A total of 123 studies that include data from Golfo Dulce are cited. The four topics more frequently
addressed were reports based on the results of the R/V Victor Hensen expedition (1993-1994) and follow-up
work on microbiology, studies on water parameters, research on vertebrates, and zooplankton studies. The
reports focusing on vertical profiles of oxygen and temperature are discussed in detail, followed by those on
the biota.
Conclusions: Golfo Dulce has low oxygen concentrations below 50 m and is frequently anoxic at the 200 m
deep basin with occasional formation of H
S. However, the ecosystem contains a relatively high diversity of
identified organisms, from bacteria to whales. Of particular relevance for future studies are multidisciplinary
surveys aiming at obtaining data on primary productivity, the diversity and biomass of the main groups of
planktonic, demersal and benthic organisms, and the frequency and magnitude of the influx of deep offshore
waters over the sill into the basin. These data, as well as the information gathered in the past, are essential for
updating the trophic model developed more than 25 years ago and in support of new predictive models on the
functioning of the ecosystem.
Key words: anoxic basin; anammox; hypoxia; H
S; water parameters; Beggiatoa; cetaceans; R/V Victor Hensen.
Vargas-Zamora, J. A., Gómez-Ramírez, E., & Morales-Ramírez,
A. (2021). A fjord-like tropical ecosystem, Pacific coast
of Costa Rica: overview of research in Golfo Dulce.
Revista de Biología Tropical, 69(3), 773-796. https://doi.
As the world enters the third decade of
the XXI
century there is growing concern
for the increasing impacts of global stressors
such as costal alteration, pollution (Delorenzo,
2016) eutrophication (Cloern, 2001), and cli-
mate change (Ni et al., 2019). These impacts
become more noticeable at certain coastal
areas such as estuarine ecosystems. Most of
Revista de Biología Tropical, ISSN: 2215-2075 Vol. 69(3): 773-796, July-September 2021 (Published Jul. 30, 2021)
the information on coastal ecosystems and their
environmental dynamics comes from studies
conducted in the temperate regions. In spite of
this intensive research effort, a great deal of
questions formulated years ago to orient coastal
research remains yet to be answered (Nord-
strom & Roman, 1996). In this context, a better
understanding of the impact of global stress-
ors on these ecosystems might be reached if
research which focuses on comparative studies
in temperate and tropical sites is emphasized.
Comparative research may also help to better
identify the spatial and temporal variability of
these stressors and the feasibility of manage-
ment measures.
Golfo Dulce is a unique deep embayment
on the Pacific coast of Costa Rica and a coop-
erative multidisciplinary research effort involv-
ing local and foreign scientists was started 25
years ago with the R/V Victor Hensen expedi-
tion. The expedition found interesting simi-
larities between Golfo Dulce and temperate
fjords and prompted it to be called a fjord-like
embayment. The expedition produced pioneer-
ing biogeochemistry information and data on
the distributions of its benthic and planktonic
biota (Wolff & Vargas, 1994; Vargas & Wolff,
1996). This data base, complemented with
follow-up studies, remains as the main refer-
ence for many faunal groups and environmen-
tal parameters. However, it needs to be updated
and new research areas approached emphasiz-
ing comparative studies with similar environ-
ments worldwide. A first step towards reaching
this goal is to summarize the published litera-
ture and suggest priorities for further research.
Thus, the objectives of this review are to
introduce the reader to published research con-
ducted in Golfo Dulce, identify topics that need
to be explored further, and propose new issues
to be addressed in future surveys.
General description
Golfo Dulce (Fig. 1) is a gulf located on the
Southern Pacific coast of Costa Rica (8
30’N &
15’W). It has an area of about 750 km
a length of 50 km, and maximum widths
between 10 to 20 km. The coastline extends
for 195 km (Morales-Ramírez, 2011). Tidal
amplitude is around 3 m. In marked contrast to
the Gulf of Nicoya, a tectonic estuary also on
the Pacific coast of Costa Rica (Vargas, 2016),
the bathymetry of Golfo Dulce is characterized
(Hebbeln et al.,1996) by a deep (maximum
depth 210 m) basin in the upper region, a sill
(60 m) at the entrance, and a slope that deepens
to more than 200 m at the mouth (Fig. 1). In
Golfo Dulce a gradient from oxic to hypoxic,
and sometimes anoxic conditions, occurs.
This deep bathymetry and the presence of
a sill at the entrance makes Golfo Dulce unique
on the West coast of the American continent
and also makes it resemble a temperate fjord.
Also, in contrast to the Gulf of Nicoya, there
are no islands. Golfo Dulce is also a tectonic
gulf and the inner section is fringed by two
parallel faults along its Northern and South-
ern shores (Malzer & Fiebig, 2008). Narrow
shorelines that fall almost vertically down to
basin depths (Fig. 1) characterize this inner sec-
tion (Hebbeln et al.,1996). The Southern and
Eastern shores of Golfo Dulce include sandy
beaches and the latter has an extensive shallow
subtidal platform (Fig. 1). The tip of the Osa
Peninsula (Fig. 1) has a mixture of rocky shores
and sandy beaches.
Most of the Peninsula includes the Corco-
vado National Park (424 km
) with extensive
rain forests. The Northern and Western shores
also include rain forests which are protected
by the Golfo Dulce Forest Reserve (620 km
the Piedras Blancas National Park (140 km
and the Golfito Wildlife Refuge (13 km
). The
two main towns on the shores are the port of
Golfito on the Northern shore, and the port of
Jiménez on the Northern tip of the Osa Pen-
insula, with populations (2011) of 12 000 and
9 000, respectively.
Rivers, mangrove forests
and seagrass beds
The Rincón River on the West, the Coto
River on the East, and the Esquinas River on
the North shore are the main rivers discharging
Revista de Biología Tropical, ISSN: 2215-2075, Vol. 69(3): 773-796, July-September 2021 (Published Jul. 30, 2021)
into the embayment (Fig. 1). Information on
the latter two rivers has been provided by
Angulo et al. (2020), Michels et al. (2006) and
Tschelaut et al. (2008). In addition, several
small creeks reach the shores of Golfo Dulce
(Umaña, 1998). River flow varies seasonally
and ranges from 20 to 180 m
/sec (Svendsen et
al., 2006). Mangrove forests are found near the
mouths of these rivers and are dominated by
Rhizophora racemosa, R. mangle, Pelliciera
rhisophoreae, Laguncularia racemosa and Avi-
cennia germinans, among others. Species com-
position and dominance at these mangroves is
similar to that reported for the region (Samper-
Villarreal & Silva-Benavides, 2015). Recent
research on the mangroves of Golfo Dulce has
Fig. 1. Left: Map of Golfo Dulce, Pacific coast of Costa Rica. Stations B (basin edge), T (Rincón Bay), and G (Shelf edge)
are indicated. Light gray in the upper gulf encloses 200 m depth area. Dashed line across indicates R/V Victor Hensen
echograph transect. Right: Schematic depth profile along the central axis of Golfo Dulce. Arrows indicate water circulation.
Broken arrows represent occasional influx of shelf waters over the sill. Light gray represents deep Equatorial Subsurface
Waters. Bottom: R/V Victor Hensen echograph of a bottom topography transect across mid upper Golfo Dulce. Note that
the ecograph starts at the 50 m depth contour. The northern fault is evidenced by the steep wall at the right of the figure.
(Adapted from Hebbeln et al., 1996).
Revista de Biología Tropical, ISSN: 2215-2075 Vol. 69(3): 773-796, July-September 2021 (Published Jul. 30, 2021)
focused on isotopic evidence of nutrient enrich-
ment at nutrient loaded sites from mangrove
material. The macroalgae Bostrychia calliptera
was found growing on R. mangle prop roots
(Samper-Villareal et al., 2018). The mangroves
of Playa Blanca on the Southern shore and
those of two small estuaries in the inner Gol-
fito Bay include populations of the ark clam
Anadara tuberculosa, which sustain a small
fishery (Silva-Benavides & Bonilla, 2001; Sil-
va-Benavides & Bonilla, 2015). Stern-Pirlot and
Wolff (2006) have made recommendations for
the management of these A. tuberculosa popu-
lations. Information on physical and chemical
parameters of the small estuaries at Golfito
Bay is provided by Silva and Acuña-González
(2006). The amino acid and protein content of
the oyster Crassostrea gigas cultured at Rincón
Bay has been reported by Fonseca-Rodríguez
and Agüero-Pérez (2017).
Organisms recently recorded for Golfo
Dulce are the seagrasses Halophila baillonii
and Halodule beaudettei. On the Southern
shore seagrass meadows are located at Colibri
beach East of Rincón Bay and near Puerto
Jiménez. On the Northern shore the beds are
present at Golfito Bay and at the beach of
the wildlife refuge. Meadow extension ranged
from about 100 m
at the refuge and Golfito
to 900 m
at Puerto Jiménez and 900 000 m
at Colibri beach. Seagrasses in Golfo Dulce
are found within a narrow coastal belt at water
depths of 3 to 6 m in sediments containing
less than 6.5 % of silt+clay. The sediments
of Colibri included 26 species of polychaete
worms. Sea turtles are known to feed on sea-
grass beds in Golfo Dulce (Samper-Villarreal
et al., 2014; Samper-Villarreal & Cortés, 2020).
Coral reefs
Fringing, low diversity coral reefs are
found in the Northern shores of Golfo Dulce
and include the coral Porites lobata and sev-
eral other species. Sedimentation has degraded
these reefs considerably. Environmental condi-
tions promoted coral reef growth in the past
but are deteriorating mainly due to siltation
(Cortés, 1990; Cortés, 1992; Cortés et al.,
2010; Cortés, 2016). However, a Psammocora
stellata reef increased its coral coverage from
46 to 83 % since 1990 (Alvarado et al., 2015).
More healthy patches of coral are found in the
outer gulf and are characterized by a higher
richness of species. In the destruction of the
reefs is also important the impact of biological
eroders, such as sponges (Cliona sp), bivalves
(Lithophaga spp), sipunculans (Aspidosiphon
elegans), crustaceans (Upogebia sp), and others
(Fonseca et al., 2006). A recent “coral garden-
ing” experiment was conducted by Vargas-
Ugalde et al. (2020) in Golfo Dulce to promote
ecological restoration of damaged reefs. A new
species of soft coral (Leptogorgia cortezi) was
found by Breedy and Guzmán (2012) at Punta
Islotes and Punta Estrella on the Northern shore
reefs. A new species of cirratulid polychaete
worm (Aphelochaeta zebra) has been described
recently also from Punta Islotes reefs by Dean
and Blake (2016).
Temperature, oxygen
and salinity profiles
The Golfo Dulce region has less defined
dry-rainy season periods than the Gulf of
Nicoya (Cortés, 2016). Rainfall is common
during the whole year and may range from 100
mm to more than 600 mm/month. The drier
months are December through March, when
rainfall may be around 100 mm per month
while peak rainfall occurs in October (Svendsen
et al., 2006). Due to the enclosed nature of the
upper region which is protected from northerly
winds by forest covered mountains the waters
of the inner gulf are relatively calm most of
the time. An early survey of water parameters
was conducted in March 1969 during a cruise
of the R/V Thomas Thompson (Richards et al.,
1971). The results indicated that the embay-
ment was characterized by surface salinities of
31 to 33 psu and temperatures of up to 32.2
at the surface and as low as 14.3
C at 200 m
depth. The results of the cruise also provided
evidence for the hypothesis of the renewal of
Revista de Biología Tropical, ISSN: 2215-2075, Vol. 69(3): 773-796, July-September 2021 (Published Jul. 30, 2021)
basin water with offshore water flowing over
the sill (Fig. 1).
The deep bathymetry of Golfo Dulce
exerts an important control on the distribution
of oxygen and on the biota of the ecosystem.
The surface waters are well oxygenated most
of the time and oxygen concentrations decrease
rapidly with depth to hypoxic or even anoxic
conditions with occasional formation of hydro-
gen sulphide near the bottom. The hydrogen
sulphide is apparently removed by oceanic
water occasionally flowing across the sill and
is difficult to detect during sporadic visits (Cór-
doba & Vargas, 1996). The survey conducted
in March 1969 by the R/V Thomas Thompson
detected oxygen concentrations of less than
10 % below 30 to 40 m depths and hydrogen
sulphide was found a few meters above the
sediments of the basin. Bottom salinities were
near 35 psu (Richards et al., 1971). A study
of physical and chemical parameters was con-
ducted by Brenes-Rodríguez and León-Coto
(1988). They considered Golfo Dulce as a shal-
low sill estuary that could be divided into two
main layers separated by the 19
C isotherm. A
strong thermohalocline makes mixing difficult
between layers. The study also identified deni-
trification and nitrate reduction processes as
well as organic matter decomposition.
Another survey was conducted in Decem-
ber 1993 aboard the R/V Victor Hensen (Wolff
& Vargas, 1994; Vargas & Wolff, 1996). At sta-
tion G (200 m, Fig. 1) located at the entrance to
Golfo Dulce, oxygen concentrations (electronic
sensor, % saturation) were near 50 % at the
surface, increased to 60 % at around 10 m, and
dropped to 30 % at 50 m. Concentrations then
dropped sharply with depth and were near 2 %
at 200 m. Water temperature was 13
C at 200
m. These conditions are similar to those found
in offshore waters along the Pacific coast of
Costa Rica. At station B (200 m, Fig. 1) located
at the basin edge, the concentration of oxygen
at the surface was near 40 %, increased to 60 %
at 10 m below the surface, and then decreased
to near 2 % at depths of more than 150 m. Tem-
perature was 13
C at 200 m. Another profile
was obtained by Córdoba and Vargas (1996)
in January 1994 at station T (200 m, Fig. 1)
located in the North-West margin of the upper
Golfo Dulce. At this station temperature ranged
from 31.5
C at the surface to 17
C at 200 m. A
thermocline was evident between 20 and 50 m.
Salinity at the surface was 29.5 psu and close
to 35.0 psu at the bottom. Dissolved oxygen
concentrations (Winkler method, micromoles/
litre) varied between 425 µmol/l at 10 m to
non-detectable at 200 m. Nitrate was also non-
detectable at 200 m, but it was found several
meters above the bottom. Another survey by
Acuña-González et al. (2006b) at Station T
gathered data during occasional visits between
June 1993 and September 1995 (Fig. 2).
The survey at station T (Fig. 2) illus-
trated temporal oscillations of parameters at a
given depth: The highest surface temperature
observed (31
C) was recorded on January
1994. The minimum temperature (12
C) was
observed on July 1994 at a depth of more than
180 m. Maximum oxygen concentrations (mil-
ligrams/litre) near the surface were 6.5 mg/l
and observed in June 1993, January 1994, and
May 1995. The lowest oxygen concentrations
(0.15 mg/l) were seen at depths of more than
100 m during visits on February 1994 and June
1995 and lasted for short periods. Anoxic con-
ditions were not found. Hypoxia is defined as
oxygen concentrations lower than 2.8 mg O
(Wu, 2002). Thus, hypoxic conditions char-
acterize Golfo Dulce at depths or more than
30 m (Fig. 2).
A comprehensive study of water parameters
along Golfo Dulce was conducted between 2005
and 2007 by Morales-Ramírez et al. (2015) and
included an evaluation of rainfall, currents, and
nutrient concentrations. Rainfall ranged from
100 to near 700 mm/month. The study found
the presence of low speed currents (range 5.1
cm/s in the inner gulf to 17.8 cm/s at the mouth).
At a station on the Western end of Golfo Dulce
near Rincón Bay (Fig. 1) several surface water
parameters were recorded. A maximum Secchi
disk depth of 10 m was measured. At a depth
interval of 70 to 140 m the minimum tempera-
ture was 16.6
C and oxygen concentration was
0.5 mg/l. The maximum temperature found