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Revista de Biología Tropical, ISSN: 2215-2075, Vol. 69(3): 773-796, July-September 2021 (Published Jul. 30, 2021)

SPECIAL ARTICLE

A fjord-like tropical ecosystem, Pacific coast of Costa Rica:

overview of research in Golfo Dulce

José A. Vargas-Zamora

1,2

*; https://orcid.org/0000-0003-3431-8802

Eddy Gómez-Ramírez

1,3

; https://orcid.org/0000-0002-8138-0222

Álvaro Morales-Ramírez

1,2

; 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.

ABSTRACT

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.

org/10.15517/rbt.v69i3.46406

https://doi.org/10.15517/rbt.v69i3.46406

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

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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

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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).

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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

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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

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at this depth range was 17.1

C and oxygen

concentration was 0.95 mg/l. Concentrations

of nitrate (16.6 µmol/l) and phosphate (2.7

µmol/l) at this depth range are similar to those

reported for equatorial subsurface waters (23

and 2.0 µmol/l, respectively) at the entrance

to the Gulf of Nicoya by Chaves and Birkicht

(1996). These studies, as well as previous ones,

illustrate the temporal variability of parameters

in Golfo Dulce and provide a unique frame-

work for future comparative studies focusing on

describing the seasonal and depth fluctuations of

environmental parameters.

Another survey by Rincón-Alejos and

Ballesteros-Sakson (2015) collected data dur-

ing the dry and rainy seasons of 2009-2010

using a CTD (Conductivity, Temperature,

Depth) recorder at 33 stations in the upper gulf.

The study also measured Secchi disk depth. In

marked contrast to the upper Gulf of Nicoya

that has maximum Secchi disk depth of 2 m

(Vargas-Zamora et al., 2018) the disk was vis-

ible in Golfo Dulce as deep as 21.5 m. A wide

surface salinity range was recorded during the

rainy season (18.0 to 31.4 psu). Temperature at

the surface ranged from 27.9 to 32.0

Oxygen stratification, H

S formation

and the anammox reaction

During the R/V Victor Hensen expedition

(Wolff & Vargas, 1994; Vargas & Wolff, 1996)

part of the time was devoted to biogeochemistry

studies. A transect from shelf waters to over the

sill and into the basin revealed that Golfo Dulce

had decreasing concentrations of dissolved oxy-

gen and nitrate at similar depths when compared

to those found offshore (Fig. 3).

As a result, the water column of the inner

region of Golfo Dulce was described as divided

Fig. 2. Occasional sampling (June 1993 to September 1995) at depth intervals from surface to 200 m depth at station T

(Upper Golfo Dulce, near Rincón Bay, Fig. 1). Top. Temperature (

C) oscillations. Bottom. Oxygen (mg/l) oscillations.

Oxygen concentrations of less than 2 mg/l are shaded in blue. Adapted from Acuña-González et al. (2006b).

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into four depth layers: the oxic layer between

the surface and the thermocline; the interval

between 40 to 100 m characterized by a steep

oxygen gradient; a third layer from about 100 to

200 m, which is sub-oxic but nitrate is present

and a fourth interval several meters above the

bottom sediments, with occasional nitrate deple-

tion and sulphide accumulation (Thamdrup et

al., 1996). Dissolved sulphide concentration

measured near station T (Fig. 1) on January

, 1994 (Fig. 3) increased from 1 µmol/l at

180 m to 7 µmol/l at 203 m, and decreased to

around 1 µmol/l after six days (Ferdelman et

al., 2006). H

S disappeared coinciding with

the presence of nitrate found during another

simultaneous study (Córdoba & Vargas, 1996).

Rates of oxygen consumption were higher in the

surface waters (up to 60 µmol/l/day), decreased

to below 10 µmol/ l /day in the pycnocline and

was non-detectable deeper than 60 m (Tham-

drup et al., 1996; Ferdelman et al., 2006).

Sulphide oxidizing bacteria of the genera Thio-

spira and Thiovulum, as well as the filamentous

bacterium Beggiatoa were identified in the

sediments (Kuever et al., 1996; Thamdrup et

al., 1996). Years later, Gallardo and Espinoza

(2007) identified large multicellular filamentous

bacteria (Beggiatoa spp, Thioploca spp), as

well as spherical Thiomargarita-like forms in

slightly sulphidic core samples taken at 195 m

depth in Golfo Dulce. These morphologically

diverse bacterial communities were also found

in organically rich sediments below the oxygen

minimum zone in coastal areas from central

Chile to Costa Rica.

An important line of research in Golfo

Dulce focused on the interactions at the water-

sediment interface of the basin. It was proposed

Fig. 3. Left: Golfo Dulce. R/V Victor Hensen, January 1994. Vertical profiles of dissolved oxygen and nitrate along a 40

km transect (Shelf edge - sill - basin). Note that no oxygen was found below 100 m in the basin and was near 25 µmol/l

in shelf waters at the same depth. Nitrate followed a similar pattern with values of 2.5 µmol/l at 150 m in the basin and 30

µmol/l in the shelf at the same depth. Right: Dissolved sulphide and suspended elemental sulfur near station T, upper Golfo

Dulce. Note change of sulphide at 200 m from 7 µmol/l to less than 1 µmol/l after 6 days. From Ferdelman et al. (2006).

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five decades ago that basin waters are renewed

on a more or less regular basis by coastal waters

flowing over the sill and removing sulphide

(Richards et al., 1971). Sulphide concentrations

of less than 3 µmol/l were found above 15 cm

of the sediment and decreased over a few days.

Organic matter debris degraded through bacteri-

al sulphate reduction may serve as patchy sourc-

es of sulphide (Stief et al., 2017). Other types

of micro-organisms studied in Golfo Dulce

are anaerobic ammonium oxidizing (Anam-

mox) bacteria that play a key role in the marine

nitrogen cycle through the anammox reaction.

Anammox activity has been reported in the sedi-

ment and water column of Golfo Dulce where it

accounted for up to 35 % of the total N

forma-

tion in the water column. This process makes

Golfo Dulce similar to other low oxygen zones

worldwide. The anammox activity quantified in

sediments ranged from 0.08 f mol/cell/day N

in Golfo Dulce,

to 0.98 f mol/cell/day N

in the

Gullmar fjord, Sweden (Schmid et al., 2007).

Other studies have focused on the bacteria and

related topics (Canfield, 1998; Dalsgaard et al.,

2003; Dalsgaard et al., 2005; Rush et al., 2014;

Stief et al., 2017).

The finding of the anammox reaction in

the deep waters of Golfo Dulce was an impor-

tant discovery facilitated by joint international

cooperation (Dalsgaard et al., 2003). This

reaction makes Golfo Dulce a nitrogen sink

(Acuña-González et al., 2006b). The anammox

reaction occurs in other ecosystems such as the

Black Sea (Dalsgaard et al., 2006) and com-

parisons of the reaction rates have been done

with those found in Golfo Dulce (Jensen et al.,

2008). More recently, studies on the diversity,

metabolic potential and activity of Marinimi-

crobia bacteria, microbial enzymes, and other

microbiological aspects have been conducted

in Golfo Dulce (García-Robledo et al., 2016;

Padilla et al., 2016; Bertagnolli et al., 2017;

Bertagnolli et al., 2020). The study by Padilla

et al. (2017) provided first field evidence for

methanotrophic partial denitrification under

anoxic conditions. However, more research

would certainly help to achieve a better under-

standing of these processes.

Oxygen producers

and primary production

Unlike the Gulf of Nicoya estuary (Vargas,

2016) there is no information on oxygen pro-

ducing phytoplankton species in Golfo Dulce

or primary productivity estimates based on the

release of oxygen by microalgae. The study by

Wolff et al. (1996) mentions that the phyto-

plankton of Golfo Dulce appears dominated by

diatoms and also by dinoflagellates that include

red tide species.

However, primary productivity data is

available from a station at the channel entrance

to a small secondary shallow bay on the North-

ern shores of Golfo Dulce where the port city

of Golfito is located (Fig. 1). At this site, net

primary productivity ranged from 695 to 1 371

gC/m

/year. Concentrations of chlorophyll-a

determined at this site ranged from 0.8 to 2.1

mg/m

, and dissolved oxygen was above satu-

ration (Vargas-Zamora et al., 2018). An unpub-

lished report by Lei (2002), based on remote

sensing techniques, estimated phytoplankton

primary production (PPC) of Golfo Dulce in

the range of 138 to 1 007 gC/m

/year. PPC

was found higher during the rainy season. As a

comparison, net primary productivity estimates

for the upper Gulf of Nicoya estuary ranged

from 191 to 1 601 gC/m

/year (Vargas-Zamora

et al., 2018).

The study by Rincón-Alejos and Balles-

teros-Sakson (2015) provided data on surface

chlorophyll concentrations at 22 stations along

Golfo Dulce over a period of one year (Fig.

4). A maximum chlorophyll-a concentration of

91.67 µmol/l was reported during the dry sea-

son (January, 2009) at the entrance to Golfito

Bay while a minimum concentration of 1.34

µmol/l was found near Rincon Bay. In marked

contrast to the results found in January 2009,

the maximum chlorophyll-a concentrations

found in August 2009 and January 2010 were

5.2 and 1.2 µmol/l, respectively (Fig. 4).

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The R/V Victor Hensen faunal survey

and other studies

Morales-Ramírez (2011) estimated 1 022

reported species of organisms (from bacteria

to mammals) for Golfo Dulce. This richness

represented close to 22 % of the total num-

ber of species reported for the Pacific coast

of Costa Rica. This percentage needs to be

updated based on the information made avail-

able after 2010.

A significant advance on the knowledge of

the biodiversity of Golfo Dulce was obtained

during the R/V Victor Hensen survey (Wolff

& Vargas, 1994). Plankton and trawl nets were

used to study the zooplankton and demersal

crustaceans. A total of 33 species of pelagic

copepods were found, including Paracalanus

aculeatus, Temoropia mayumbaensis, Lucicu-

tia ovalis, Candacia catula and Corycaeus

speciosus (Morales-Ramírez, 1996). The abun-

dance, biomass, and vertical distribution of

other micro-crustaceans, such as ostracods

were studied by Morales-Ramírez and Jakob

(2008). Four species of ostracods were iden-

tified belonging to the genera: Cypridina,

Euconchoecia and Porroecia. Their abun-

dance appears high when compared to offshore

waters. Another survey (Morales-Ramírez et

al., 2015) reported dominance (44 to 84 %)

of copepods such as Oncaea mediterranea,

Oithona plumifera, Acrocalanus gibber, Para-

calanus parvus, P. aculeatus and Centropages

furcatus. The gelatinous zooplankton was also

studied and 41 species were identified of which

33 were cnidarians, six were thalaceans and

two were ctenophores (Morales-Ramírez &

Nowaczyk, 2006). The biomass spectra and

species composition of the zooplankton com-

munities was studied by von Wangelin and

Wolff (1996). They found that at the inshore

and offshore stations high amounts of gelatinous

zooplankton (Siphonophores and salps) domi-

nated the largest size groups, and that ostracods

and brachyuran zoea appear as main crustacean

components inshore during the dry season.

Other group of organisms found during

the expedition were the arrow worms or chae-

tognaths (Hossfeld, 1996) with 10 species

recorded for Golfo Dulce of which nine were

of the genus Sagitta and one of the genus

Krohnitta. The ichthyoplankton was studied

by Molina-Ureña (1996) who reported larvae

of the families Bregmacerotidae, Myctophidae,

Engraulidae and Ophichthidae as the more

abundant among the 15 families identified for

Golfo Dulce.

Fig. 4. Chlorophyll-a concentrations (µmol/l) at the surface

of 22 stations along Golfo Dulce. January and August

2009 and January 2010. Adapted from Rincón-Alejos and

Ballesteros-Sakson (2015).

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The decapod and stomatopod crustacean

fauna were also evaluated in Golfo Dulce

with trawl nets during the R/V Victor Hensen

survey and the results summarized by Castro

and Vargas (1996), Jesse (1996), and Vargas et

al. (1996). No crustaceans were found in the

deeper low-oxygen waters. Golfo Dulce was

found to be a relatively species poor area, with

smaller biomass and diversity, when compared

to other sites on the Pacific coast of Costa

Rica such as the Gulf of Nicoya. The recent-

ly updated list of decapod and stomatopod

crustaceans collected during the R/V Victor

Hensen and those hand-collected intertidally

and by diving during other surveys includes

106 species, of which four were stomatopods

and 102 were decapods belonging to 32 fami-

lies (Vargas-Castillo & Vargas-Zamora, 2020).

More recently Salas-Moya et al. (2021) listed

the crustaceans associated with the soft corals

Eugorgia mutabilis and Leptogorgia cortezi, as

well as to the sea star Pentaceraster cunningi

and onuphid polychaete worms.

Fish diversity and fisheries

A trawl survey of the fish present in Golfo

Dulce was conducted as part of the R/V Victor

Hensen expedition and a total of 75 species

were collected there (Bussing & López, 1996).

The fish fauna was dominated by small species

of the genera Cynoscion and Porichthys, which

occurred in low densities, perhaps due to a low

carrying capacity of the embayment (Wolff,

1996). An updated list of pleuronectiform flat-

fish species collected in Golfo Dulce by the

R/V Victor Hensen was assembled by Vargas

et al. (2020) and included 19 species of which

Symphurus leei was the most frequent. Other

studies emphasized the critical role of Golfo

Dulce as a habitat and nursery area for the shark

Sphyrna lewini (Zanella & López-Garro, 2015;

Zanella et al., 2019). Feeding aggregations of

the whale shark Rhincodon typus were reported

for the inner Golfo Dulce by Pacheco-Polanco

(2015). Hanna (2016) reported 226 sightings

of R. typus for the period 2001-2007 which

included several juveniles (2-3.5 m long).

The fish fauna on the Eastern shore of

Golfo Dulce has been recently studied at the

mangrove swamp of Zancudo at the mouth of

the Coto River (Feutry et al., 2010). A total of

1 591 individuals belonging to 82 species in

30 families were found at Zancudo. Sciaenidae

was the most diverse family (8 spp) during the

rainy season and Carangidae (10 spp) was so

during the dry season. Species collected more

frequently were: Anchoa mundeola, Centropo-

mus armatus, Diapterus peruvianus and Mugil

curema. The fish fauna of an intertidal rocky

islet in Golfo Dulce was studied by Palacios-

Martínez (2015). He found a total of 28 species

of which the more abundant were Thalassoma

lucasanum (Labridae), and the pomacentrids

Chromis atrilobata and Abudefduf trocshelii.

Studies on the fisheries of Golfo Dulce

were initiated by Segura and Campos (1990)

who estimated in 5.5 % the post-harvest losses

at landing sites. A description of the artisanal

fishery was conducted by Guzmán-Mora and

Molina-Ureña (2007) who found that small

boats (4-10 m long) comprised the main fleet

of fishermen who employed hook and line,

long line and trammel nets. Four species (the

snappers, Lutjanus guttatus and L. peru, the

mackerel, Scomberomorus sierra, and the grunt

Pomadasys panamensis) accounted for 66.5 %

of the captures. The study by Guzmán-Mora

and Molina-Ureña (2007) concluded that the

fisheries in Golfo Dulce are mainly artisanal,

based at the port of Golfito, and are less devel-

oped than those in the Gulf of Nicoya, and only

four species account for up to 67 % of the cap-

tures. López-Garro and Zanella (2015a) evalu-

ated onboard operations of the artisanal fishing

fleet in Golfo Dulce and found that from 872

fish observed, 39.6 % were sharks, 26.1 % were

bait fish (Ariidae), 13.1 % were rays and 12.7

% were commercial fish of the families Lut-

janidae, Sciaenidae and Serranidae. They also

analysed 609 sharks representing 10 species

of which Sphyrna lewini accounted for 51.8 %

followed by Mustelus lunulatus (23.5 %). Most

of S. lewini were juveniles. Captures of the ray

Dasyatis longa were studied by López-Garro

and Zanella (2015b) who found that among 112

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rays captured 75.7 % were of this species. The

establishment of marine areas of responsible

fishing and co-management of fisheries has

been evaluated for Golfo Dulce by Fargier et

al. (2014).

Reptiles and cetaceans

There is an important base of information

on air breathing vertebrates that live on this

ecosystem. There are reports on the population

of the sea snake Pelamis platurus (Bessesen,

2012) and its colour variability (Solórzano,

2011), and on a recently described subspecies

(Bessesen & Galbreath, 2017). The population

and habitat use of the green turtle Chelonia

mydas has been described by Chacón-Chaver-

ri et al. (2015a), and Bessesen and Saborío

(2012). The trophic ecology of the hawksbill

sea turtle Eretmochelys imbricata was stud-

ied by Chacón-Chaverri et al. (2015b) and by

Méndez-Salgado et al. (2020).

Research on marine mammals has been

emphasized with studies on humpback whales

(Megaptera novaeangliae) by Bessesen (2015)

and Herra-Miranda et al. (2016), the false

killer whale Pseudorca crassidens (Sánchez-

Robledo et al., 2020), dolphins (mainly the

bottlenose dolphin Tursiops truncatus and the

spotted dolphin Stenella attenuata) and other

cetaceans (Acevedo, 1995; Acevedo-Gutiérrez,

1996; Acevedo & Burkhart,1998; Cubero-Par-

do, 1998; Cubero-Pardo, 2007a; Cubero-Pardo,

2007b; Oviedo, 2007; Herra-Miranda et al.,

2016; Moreno & Acevedo-Gutiérrez, 2016;

Oviedo et al., 2018). Based on 943 sightings

over a period of nine years the study by Oviedo

et al. (2015) identified 12 species of cetaceans

in Golfo Dulce. A gradient of cetacean diversi-

ty was observed from the inner Golfo to waters

near the shelf region. Bottlenose and spotted

dolphins dominated the inner area while bottle-

nose dolphins and migratory humpback whales

dominated the sill area.

Oviedo-Correa et al. (2009) evaluated the

potential impact of the proposed construc-

tion of tuna culture farms on the populations

of cetaceans of Golfo Dulce. The presence

of lacaziosis-like disease photographed in

T. truncatus has been reported by Bessesen

et al. (2014).

Sedimentary environments

The sedimentary environments of Golfo

Dulce are divided into two groups: those

exposed to oxic conditions to a depth of

around 30 m, and those exposed to hypoxic or

anoxic conditions at deeper waters. Sediments

at the inner basin consist mostly of turbidites,

while other areas include a wide range of soft

sediments, mainly silts and clays (Hebbeln &

Cortés, 2001). Black and very soft sediments

were reported in the inner basin (200 m), while

brown and soft ones were found near Rincón

Bay (50 m). Green and hard sediments were

found at 75 m near the sill (León-Morales

& Vargas, 1998). The first evaluation of the

benthic communities was conducted in 1969

aboard the R/V Thomas Thompson using a

van Veen grab at depths from 64 to 200 m in

the inner basin. The abundance and biomass of

invertebrates were found to be less than those

expected for a tropical environment. Most of

the fauna was found at depths around 70 m and

only a few to zero individuals at depths greater

than 100 m. The fauna included 25 species

of polychaete worms of which, Paraonis lyra

(Paraonidae) was the most abundant (Nichols-

Driscoll, 1976).

Another evaluation took place in 1993 dur-

ing the expedition of the R/V Victor Hensen.

A box corer was used to sample the bottom

sediments at depths from 43 to 200 m (Wolff &

Vargas, 1994) and the results were summarized

by León-Morales and Vargas (1998). Higher

numbers of individuals were found at depths

from 43 to 100 m in the upper region. The sta-

tion located near the sill (75 m) was the most

diverse (37 species), followed by those at the

mouth (34 and 29 species). A 50 m deep station

near Rincón Bay yielded 25 species. Sediments

deeper than 100 m were azoic. The fauna was

dominated by polychaetes (47 species), but

faunal composition was drastically different

from that reported by Nichols-Driscoll (1976).

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The polychaete P. lyra was not found. The most

abundant of the 27 species of polychaetes were

members of the genera Prionospio and Aricid-

ea, which accounted for 19.5 and 13.3 % of the

total number of 1 608 individuals of all groups

collected (Dean, 1996; León-Morales & Vargas,

1998). Other groups of benthic invertebrates

collected during the R/V Victor Hensen were

the mollusks (Cruz, 1996). The most abundant

gastropods in Golfo Dulce were Polystira

oxitropis and Strombina fusinoidea, while Tel-

lina ecuadoriana was a frequent bivalve. A

later inventory of intertidal and shallow-water

mollusks listed 252 species of gastropods,

mainly prosobranchs and pyramidelids (Hoi-

saeter, 1998). Information on other intertidal

soft-bottom fauna of Golfo Dulce is scarce and

work focused on sandy beaches at the entrance

to Golfo Dulce (Corrales-Ugalde & Sibaja-

Cordero, 2015). The upper intertidal was domi-

nated by the isopod Cirolana salvadoriensis

and the low intertidal by polychaete worms of

the families Nereididae and Pisionidae. The

intertidal sedimentary environments of the Bay

of Golfito yielded three new species of cirratu-

lid polychaete worms (Caulleriella dulcencis,

C. minuta and C. murilloi) described by Dean

and Blake (2007).

Models

Information on Golfo Dulce available up

to 1994 was used as input for the construction

of a steady-state trophic ecological model. To

facilitate comparisons the Golfo Dulce model

(Wolff et al., 1996) used the same analytical

approach applied later in the Gulf of Nicoya

model (Wolff et al.,1998). The model estimated

a Total System Throughput (sum of all flows) of

1 404 g/m

/year, which is low when compared

to other coastal ecosystems, such as the Gulf of

Nicoya estuary (3 049 g/m

/year) or the Caeté

estuary (10 559 g/m

/year) in Brazil (Wolff et

al.,1998; Wolff, 2006). The results indicate that

the ecosystem functions in a different manner

when compared to the Gulf of Nicoya model

and other tropical systems for several reasons.

The most important is that Golfo Dulce is

dominated by biomass and energy flow pro-

duced in the pelagic zone, thus resembling more

an open ocean system than an estuarine one.

Second, benthic pelagic coupling is minor as a

result of its deep bottom topography. The model

also predicted that, due to its reduced benthic

biomass and inferred low primary productivity,

an increase in the pelagic fishing pressure would

negatively affect resident populations of preda-

tors, such as fish and mammals. However, these

projections have not been tested yet. The trophic

model needs to be improved with quantitative

information on the biomass of the phytoplank-

ton and zooplankton fractions, the amount of

detritus entering the system, biomass of pelagic

and demersal fish, crustacean biomass, and

estimates of primary productivity and of other

imports and exports. The relative importance

of data on fish populations is crucial since the

model used 20 compartments of which 12 cor-

respond to fish groups.

A second modelling approach focused on

the circulation of water masses in Golfo Dulce.

A bidimensional non-linear numerical model

was developed by Quirós (2003). He concluded

that the bottom topography of Golfo Dulce

allows it to be considered a tropical fjord. How-

ever, the embayment is wider than other fjords

and allows the existence of temporal current

swirls in the interior region. The tidal wave is

the main force and the role of freshwater input

(mainly of the Coto River) is relatively minor.

Another physical oceanography modelling

effort in Golfo Dulce dealt with further under-

standing its water circulation patterns (Svend-

sen et al., 2006). The study conducted between

1999 and 2001 combined field measurements

(drifting buoys and current meter deployments)

with computer modelling. The results indicate

that along the longitudinal axis of Golfo Dulce

the gradients in salinity and temperature are

maintained by the freshwater input from the

three main rivers (Fig. 1). Local wind and topo-

graphic steering results in velocity gradients in

the upper layer. A three-layer current structure

is present at the sill area while in the inner part

a two-layer estuarine circulation occurs over

slowly moving deep water. The results also

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described the upper region of Golfo Dulce

as having surface waters of low salinities,

high temperatures, and low current speeds.

On November 1999, surface salinity was near

20 psu with a strong halocline in the upper 5

m. Temperature was 28

C at the surface and

decreased to 17

C at 60 m. On February 2001

the surface salinity was 33 psu and temperature

was 31

C. Temperature dropped to 15

C at

60 m. Current speed measurements (drifting

buoys) indicated lower velocities (range 3 to 39

cm/s) when compared to those (near 100 cm/s)

found in the Gulf of Nicoya estuary. The com-

puter simulations indicated that Golfo Dulce

has a less defined estuarine circulation pattern

than the Gulf of Nicoya (Voorhis et al., 1983;

Vargas, 2016), with an out-flow of low salinity

waters in the surface layer and a compensat-

ing inflow of coastal waters at intermediate

depths (5 to 30 m) in the sill area This pattern,

which needs to be explored further with the

deployment of anchor stations to measure cur-

rents speeds, appears to vary seasonally and

relatively strong currents are predicted by the

model at the sill most of the year (Fig. 5).

Pollution

The Golfo Dulce region is still relatively

underdeveloped as most of the Northern shore-

line is fringed by rain forests and the major

city (Port of Golfito, Golfito Bay, Fig. 1), has a

population of about 12 000. However, the con-

centrations of coliform bacteria in the waters of

Golfito Bay were found to be the highest when

compared to three sites along the Pacific coast of

Costa Rica (García et al., 2006). Concentrations

of petroleum hydrocarbons were evaluated at

Golfito Bay by Acuña-González et al. (2004),

and were found below the reference value of

10 µg/l considered as a borderline for polluted

conditions. The presence of trace metals in sedi-

ments of Golfito Bay were studied with atomic

Fig. 5. Simulated monthly (February, dry season – October, rainy season) current speeds (m/sec) in a vertical section along

the central Golfo Dulce. Small right upper boxes: note large scale differences between vertical and horizontal currents. The

sill area is located between positions 30 to 50 km. Adapted from Svendsen et al. (2006).

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absorption by García-Céspedes et al. (2004).

Maximum concentrations of Cu (128 µg/g),

Pb (8.2 µg/g), Zn (109 µg/g) and Fe (7.2 %)

may indicate point sources of metal pollution.

Concentrations of 13 elements, including metals

were analysed by X ray fluorescence in sediment

samples from several locations around Golfo

Dulce by Salazar et al. (2004). The results indi-

cate no evidence of anthropogenic pollution.

Agricultural activities focus around banana

and palm oil plantations. These activities prob-

ably have introduced some pollutants into

the system, however data is scarce. A study

conducted in 1995-1996 analysed residues of

organochlorinated pesticides in 47 silty to fine

sand sediment samples from several locations in

Golfo Dulce (Spongberg & Davies, 1998). The

results indicated that pesticide levels were below

those reported for contaminated areas elsewhere

and many compounds were near the detection

limits. The highest concentration found was of

pp-DDT, with a concentration of 76 µg/kg dw

of sediment. Concentrations of Polychlorinated

Biphenyls (PCBs) were also evaluated in Golfo

Dulce by Spongberg (2004). Results based on

29 sediment samples indicated that PCB con-

tamination was considered moderate despite the

relatively pristine conditions of the area. Maxi-

mum PCB concentrations were found at the port

of Golfito and reached 16 µg/g dw. PCBs were

also studied by Spongberg (2006) in speci-

mens of the sipunculan worm Phascolosoma

perlucens from rocky intertidal habitats near

the Rincón River (Fig. 1). A concentrations of

67.7 ng/g dw was the highest when compared

to those found in this species in other localities

along the Pacific coast of Costa Rica. How-

ever, PCBs concentrations in sediments and

organisms from Golfo Dulce were considered

low when compared to other sites worldwide

including those in temperate regions (Spong-

berg & Witter, 2008). A study of residues of

pharmaceutical and personal care products

(PPCP) in rivers draining into Golfo Dulce was

conducted by Spongberg et al. (2011). They

reported the presence of 28 compounds includ-

ing caffeine, salicylic acid and antibiotics such as

ciprofloxacin and doxycycline. A recent survey

of contaminants in rivers draining into Golfo

Dulce detected the presence of the herbicide

clomazone, the insecticide triazophos and the

fungicides isoprothiolane and propiconazole

(Fournier et al., 2019). Other sources of pollut-

ants may have originated far from Golfo Dulce.

An early report (Carpenter & Beasley, 1981)

studied transuranic radionuclides (Plutonium

and Americium) in 200 m deep surface sedi-

ments from Golfo Dulce. They found measur-

able activities of both. A new survey focusing

on the evaluation of these and other contami-

nants in Golfo Dulce is needed.

El Niño (ENSO) and La Niña

Among other external forces acting upon

the embayment are the El Niño (ENSO) and La

Niña. However, we found only three published

reports where this topic has been addressed for

Golfo Dulce: A study evaluated the impact of El

Niño 1997-1998 on water circulation (Quesada-

Alpízar & Morales-Ramírez, 2004) and found

this warming event enhanced vertical stratifica-

tion in Golfo Dulce. Lower temperatures and

higher concentrations of dissolved oxygen were

observed in deep waters perhaps indicating an

intrusion of offshore water over the sill. The

second report evaluated the impact of El Niño

1997-1998 on the non-gelatinous zooplankton

(Quesada-Alpízar & Morales-Ramírez, 2006).

Most of the groups were more abundant above

the thermocline. The copepods were the most

abundant (71 %), followed by the appendicu-

larians (8 %), ostracods (8 %), polychaete

larvae (1.8 %) and chaetognaths (1 %). During

El Niño the thermocline was deeper and the

abundance of the groups decreased. The third

study reported on the impact of El Niño 1997-

1998 on the corals of Golfo Dulce. The most

visible impact was coral bleaching, which was

observed in five species. Psammocora spp were

affected most (75 %), followed by Pocillopora

damicornis (61 %), and P. elegans (61 %). Mor-

tality was observed in the latter two species (12

and 5%, respectively). Branching colonies were

more affected by bleaching than massive species

(Jiménez et al., 2001).

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DISCUSSION

The published information available for

Golfo Dulce emphasizes the unique character-

istics of this deep tropical coastal ecosystem.

The middle basin allows the study of param-

eters along a 200 m vertical depth gradient,

from oxic to hypoxic and occasionally anoxic

conditions, leading to episodes of hydrogen

sulphide formation near the bottom. Golfo

Dulce is one of the few deep tropical sites

where anoxic conditions have been reported

and differs from similar environments in the

occasional renewal of its basin waters. How-

ever, the magnitude and frequency of this

renewal needs to be explored further. The sill

depth, the extent of the anoxic zone, the con-

centrations of oxygen and hydrogen sulphide

at basin depth and anammox rates have been

compared with other localities around the

world such as the Baltic and Black seas, the

Cariaco Trench in Venezuela, and several Euro-

pean fjords (Anderson & Devol; 1987; Jensen

et al., 2008). Both the Gulf of Nicoya and

Golfo Dulce are limited at the mouth by waters

characteristic of extensive zones on the Eastern

Tropical Pacific, particularly deep waters with

low oxygen and high nutrient concentrations

(Fiedler & Talley, 2006). Previous cooperative

research has focused on this feature particularly

on the Gulf of Nicoya (Voorhis et al., 1983;

Chaves & Birkicht, 1996), but several ques-

tions remain to be answered for Golfo Dulce

such as the magnitude and frequency of these

waters entering the basin.

The coral reefs of Golfo Dulce provide

opportunities for ecological studies which

include the impact of bioerosion by organ-

isms. The rate of destruction of these reefs

has been found to be higher than their growth

rate (Quesada-Alpízar & Cortés, 2006). Biodi-

versity studies need to be intensified in Golfo

Dulce and coral reefs and shallow sedimentary

environments probably host many yet unde-

scribed species.

Most of the available information on Golfo

Dulce comes from short surveys conducted

years ago. Also, a great deal of information

focuses on specific topics or groups of organ-

isms and a comprehensive multidisciplinary

exploration is urgently needed such as that

conducted during the R/V Victor Hensen expe-

dition. Thus, there is a need to update and

complement the existing information and carry

out new evaluations of environmental param-

eters using state of the art technology. Recent

advances in remote sensing, autonomous data

loggers, multiparameter electronic sensors, and

global positioning (GPS) facilitate data gath-

ering. These new information updates must

emphasize obtaining temporal data sets cov-

ering longer time and spatial frames. Ample

surveys in space and time may facilitate the

identification of the impact of local, regional

and global stressors acting at different scales.

The updates and new evaluations of environ-

mental parameters must be approached under

the perspective of increasing coastal develop-

ment and other threats such as eutrophication

(Cloern, 2001; Kennish, 2002; Cloern et al.,

2014). El Niño (ENSO) and La Niña tempera-

ture oscillations (Wang & Fiedler, 2006), and

the impact of climate change on estuarine nutri-

ents (Statham, 2012). The crucial role of ENSO

on the decline of Eastern Pacific coral reefs over

the past 2 500 years has been emphasized by

Toth et al. (2012). Latitudinal comparisons may

provide a unique opportunity for the under-

standing of the functioning of this ecosystem

and help to develop predictive models. Selec-

tion of target sites, with similar bathymetric

characteristics to Golfo Dulce are important in

these comparisons and international collabora-

tive efforts are important in this context.

Among the factors affecting coastal

embayments worldwide is the likely increase

of water temperature due to global warming,

that may lead to stronger water stratification,

increased microbial respiration, and conse-

quently increased water deoxygenation (Ni et

al., 2019). Climate change may intensify the

impacts of other stressors such as the dynamics

of pollutants (Delorenzo, 2016) or increases

in nutrient loads and eutrophication (Cloern,

2001). Comparative studies addressing these

and related topics in temperate and tropical

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systems such as the Gulf of Nicoya and Golfo

Dulce may hasten our understanding of these

processes and may also contribute to propose

on-time remedial measures.

Golfo Dulce has mangrove forests that

grow under the influence of semidiurnal tides

close to 3 m mean amplitude. Golfo Dulce also

has mud flats, sand flats and sandy beaches that

provide opportunities for the latitudinal com-

parative testing of several hypotheses proposed

years ago for tropical soft-bottom benthos

(Alongi, 1989). These include the diversity of

tropical vs temperate epifaunas and infaunas,

the study of vertical zonation patterns, the

role of opportunistic species, the intensity of

predation, competition, colonization, the role

of microbes, the estimates of secondary pro-

duction, and the response to pollution. Of

particular relevance is the impact of hypoxic

conditions (Wu, 2002). The relatively easy

access to intertidal soft-sediment environments

in Golfo Dulce facilitates studies on the season-

al population patterns of macrofaunal species,

such as that conducted in similar environments

in the Gulf of Nicoya by Vargas-Zamora et al.

(2015). The polychaete fauna is still relatively

unexplored in Golfo Dulce when compared to

the Gulf of Nicoya estuary. Moreover, the study

of meiofaunal and microfaunal groups is yet to

be initiated in Golfo Dulce. Subtidal sediments

provide the opportunity to study the influence

of grain size and low oxygen concentrations

on several groups of the demersal fauna like

macro-crustaceans, mollusks and demersal fish

such as flatfish, a group represented in Golfo

Dulce by 19 species (Vargas et al., 2020).

The extensive shallow region of the upper

Gulf of Nicoya and the smaller shallow areas

fringing Golfo Dulce lend themselves to the

study of the comparative importance of ben-

thic-pelagic coupling and the role of sedi-

ment-water oxygen and nutrient exchanges.

A recent survey of the literature by Boynton

et al. (2018) found 182 reports on this topic

worldwide. Most of the reports were from stud-

ies conducted in Europe and North America,

with only one report from Mexico. The first

report from Central America was published by

Gómez-Ramírez et al. (2019) and focused on

the Gulf of Nicoya estuary.

This literature survey indicates that infor-

mation on the Golfo Dulce ecosystem has

increased significantly since the construction

of the Golfo Dulce trophic model by Wolff et

al., (1996) which allowed scientific predictions

to be made of the functioning of the ecosystem.

The following research topics are suggested

as priorities for future studies in Golfo Dulce

aimed at providing data in support of an

updated trophic model or the development of

a new one:

• Estimate seasonal primary productivity of

the water column.

• Estimate detrital contribution to the

system.

• Identify the species and quantify the

biomass of the main planktonic autotro-

phic groups (diatoms, micro-flagellates,

dinoflagellates, others).

• Identify the species and quantify the bio-

mass of the main water column hetero-

trophic groups (protozoans, cnidarians,

micro-crustaceans, fish, cetaceans).

• Identity, quantify, and map the distribu-

tions of the main benthic groups (filter and

deposit feeding polychaete worms, crusta-

ceans, mollusks, demersal fish).

• Identify the frequency and magnitude of

the influx of offshore water entering the

basin over the sill.

• Estimate the frequency and concentrations

of H

S formation at the basin.

• Describe the temporal and spatial oscilla-

tions of the oxic-hypoxic-anoxic interfaces.

• Quantify the amounts of contaminants ente-

ring the system and their residence time.

• Perform paleo-oceanographic investiga-

tions based on sediment cores.

• Update the trophic model.

• Explore new modelling approaches to inte-

grate the available information.

In summary, Golfo Dulce is a unique and

accessible tropical ecosystem that provides

opportunities for cooperative and comparative

789

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 69(3): 773-796, July-September 2021 (Published Jul. 30, 2021)

research. In view of the increasing impact of

local, regional and global stressors there is a

need of studies which allow the estimation

of the value of the services provided by this

ecosystem as a whole and by certain com-

ponents such as fringing mangrove forests,

rocky shores and reefs, tidal flats, and fishery

resources as outlined by Barbier et al. (2011).

Ethical statement. The authors declare

that they all agree with this publication and

made significant contributions; that there is no

conflict of interests of any kind, and that we

followed all pertinent ethical and legal proce-

dures and requirements. A signed document has

been files in the Revista archives.

ACKNOWLEDGMENTS

We thank three anonymous reviewers and

Harlan K. Dean for comments on the manu-

script. We also thank Sergio Aguilar for prepar-

ing the figures.

RESUMEN

Un ecosistema tropical similar a un fiordo,

costa del Pacífico de Costa Rica: panorama de

la investigación en el Golfo Dulce

Introducción: Los ecosistemas costeros alrededor del

mundo están bajo la influencia de tensores locales, regio-

nales y globales, como la contaminación, la eutroficación

y el cambio climático. El Golfo Dulce es un profundo eco-

sistema tropical relativamente inalterado y accesible, que

provee oportunidades para la investigación comparativa y

colaborativa.

Objetivo: Resumir los informes publicados sobre inves-

tigaciones pasadas realizadas en el ecosistema, identificar

tópicos para estudios futuros y sugerir nuevas áreas de

investigación.

Métodos: Se hizo una búsqueda en la red de informes basa-

dos en investigaciones hechas en Golfo Dulce y publicadas

en revistas científicas. Fueron incluidos aquellos informes

sobre parámetros ambientales y la biota.

Resultados: Un total de 123 estudios que incluyen datos

sobre Golfo Dulce son citados. Los cuatro tópicos citados

con más frecuencia fueron: Los resultados de la expedición

del R/V Victor Hensen (1993-1994) y estudios de segui-

miento sobre microbiología, informes sobre parámetros

acuáticos, investigaciones sobre vertebrados y estudios

sobre zooplancton. Los informes sobre perfiles de oxígeno

y temperatura son presentados con mayor detalle, seguidos

por aquellos sobre la biota.

Conclusiones: Golfo Dulce tiene bajas concentraciones

de oxígeno por debajo de 50 m y es usualmente anóxico

a 200 m en el fondo, con formación ocasional de H

S. Sin

embargo, el ecosistema contiene una diversidad de organis-

mos identificados relativamente alta, desde bacterias hasta

ballenas. De relevancia particular para futuros estudios es,

entre otros, la conducción, de muestreos multidisciplinarios

orientados a obtener datos sobre productividad primaria,

la diversidad y biomasa de los principales grupos de orga-

nismos planctónicos, demersales y bénticos, así como la

frecuencia y magnitud del flujo de agua oceánica hacia el

interior. Estos datos, así como los obtenidos en el pasado,

son esenciales para actualizar el modelo trófico desarrolla-

do hace más de 25 años, o en apoyo de nuevos modelos

predictivos de funcionamiento del ecosistema.

Palabras clave: fondo anóxico; anammox; hipoxia; H

Beggiatoa; cetáceos; R/V Victor Hensen.

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