465 Marine and Fishery Sciences 37 (3): 465-513 (2024) https://doi.org/10.47193/mafis.37X2024010111 ABSTRACT. For conservation and sustainable fisheries, it is important to characterize the Oxygen Minimum Zones or OMZ in and around the methane seeps of the Eastern Tropical Pacific (ETP), Costa Rica, through the analysis of temperature, salinity, density, and oxygen profiles. The data used in this work were collected during several oceanographic research campaigns in the Pacific conti- nental margin and offshore of Costa Rica, between 2009 and 2019, using a CTDs, as the profiler of physical parameters of the water column. In general, it was observed that dissolved oxygen gradually decreases with depth to the thermocline, then its concentration decreases more rapidly and remains low, indicating the presence of the OMZ and tends to increase slightly at greater depths. Mean vertical extension of the OMZ near and around the seeps was 763 m and the mean depth for the minimum dissolved oxygen value was 393 m. Spatial differences of measurements taken at stations near the methane seeps were calculated with respect to the measurements at the station located above them. Overall, a greater variability of the oxygen anomalies was observed within the mixed layer, while under the thermocline their values remain stable and around zero. Key words: Anoxia, international collaboration, Central America, methane seeps, continental margin, CTD profiles. Caracterización de la Zona Mínima de Oxígeno en el Pacífico Tropical Oriental costarricense utilizando datos in situ de campañas de campo RESUMEN. Para la conservación y la pesca sostenible, es importante caracterizar las Zonas de Mínimo de Oxígeno o ZMO en y alrededor de las filtraciones de metano del Pacífico Tropical Oriental, Costa Rica, mediante el análisis de perfiles de temperatura, salinidad, densidad y oxígeno. Los datos utilizados en este trabajo fueron recolectados durante diferentes campañas de investigación oceano- gráfica en el margen continental del Pacífico de Costa Rica, entre 2009 y 2019, utilizando un CTD, como perfilador de parámetros físicos de la columna de agua. En general, se observó que el oxígeno disuelto disminuye gradualmente con la profundidad hasta la termoclina, luego su concentración disminuye más rápidamente y permanece baja, indicando la presencia de la OMZ y tiende a aumentar ligeramente a mayores profundidades. La extensión vertical media de la OMZ cerca y alrededor de las filtraciones fue de 763 m y la profundidad media del valor mínimo de oxígeno disuelto fue de 393 m. Se calcularon las diferencias espaciales de las mediciones realizadas en las estaciones cercanas a las filtraciones de metano con respecto a las mediciones en la estación ubicada sobre ellas. En términos ORIGINAL RESEARCH Characterizing the Oxygen Minimum Zone (OMZ) in the Costa Rican Eastern Tropical Pacific using in situ data from field campaigns Alejandro Rodríguez1, 2, 3, Eric J. Alfaro1, 2, 3, * and Jorge Cortés1, 4 1Centro de Investigación en Ciencias del Mar y Limnología (CIMAR), Ciudad de la Investigación, Universidad de Costa Rica (UCR), 11501-2060 - San José, Costa Rica. 2Escuela de Física, Sede Rodrigo Facio, Universidad de Costa Rica (UCR), 11501-2060 - San José, Costa Rica. 3Centro de Investigaciones Geofísicas (CIGEFI), Ciudad de la Investigación, Universidad de Costa Rica (UCR), 11501-2060 - San José, Costa Rica. 4Escuela de Biología, Sede Rodrigo Facio, Universidad de Costa Rica (UCR), 11501-2060 - San José, Costa Rica. ORCID Alejandro Rodríguez https://orcid.org/0000-0003-4618-6560, Eric J. Alfaro https://orcid.org/0000-0001-9278-5017, Jorge Cortés https://orcid.org/0000-0001-7004-8649 Marine and Fishery Sciences MAFIS *Correspondence: erick.alfaro@ucr.ac.cr Received: 2 June 2023 Accepted: 15 January 2024 ISSN 2683-7595 (print) ISSN 2683-7951 (online) https://ojs.inidep.edu.ar Journal of the Instituto Nacional de Investigación y Desarrollo Pesquero (INIDEP) This work is licensed under a Creative Commons Attribution- NonCommercial-ShareAlike 4.0 International License https://doi.org/10.47193/mafis.37X2024010111 https://orcid.org/0000-0003-4618-6560 https://orcid.org/0000-0001-9278-5017 https://orcid.org/0000-0001-7004-8649 https://ojs.inidep.edu.ar Marine and Fishery Sciences 37 (3): 465-513 (2024)466 INTRODUCTION The interaction between the atmosphere and the ocean allows oxygen transfer from the atmosphere to the surface layers of the ocean by diffusion and to the deeper layers by circulation of oxygenated surface water (Levin 2002). Water masses at in- termediate depths are classified as Oxygen Mini- mum Zones (OMZs) when oxygen concentrations are low (hypoxia) or null (anoxia) (Rixen et al. 2020; Kirchman 2021). OMZs are caused by high biological consumption and the effects of strong thermal stratification on ventilation (Fiedler and Talley 2006; Karstensen et al. 2008; Gooday et al. 2010; Cabré et al. 2015). Stratification restricts vertical mixing and upwelling as it separates wa- ter masses by age, where older water bodies have lower oxygen concentrations (Rixen et al. 2020). High primary production and the resultant increase in organic matter transport to intermediate depths affect the intensity of the OMZs since oxygen consumption is enhanced by decomposition of or- ganic matter (Levin 2002; Matear and Hirst 2003; Dale et al. 2015). Therefore, a decrease in primary productivity reduces the export of organic matter to intermediate depths, resulting in lower oxygen consumption (Sarma et al. 2020). Low oxygen con- centrations suppress the consumption of organic matter, preventing further oxygen depletion in the surface layers and allowing unconsumed organic matter to sink to the bottom (Rixen et al. 2020). Organic matter consumption is favored by slower sink rates that result in increased deoxygenation and thickening of the OMZ (Sarma et al. 2020). As a result, the respiration of the exported organic matter is favored at the bottom of the OMZ, in- tensifying deoxygenation and pushing the lower generales, se observó una mayor variabilidad de las anomalías de oxígeno dentro de la capa de mezcla, mientras que bajo la termoclina sus valores se mantienen estables y alrededor de cero. Palabras clave: Anoxia, colaboración internacional, América Central, filtraciones de metano, márgenes continentales, perfiles de CTD. boundary of the OMZ deeper (Rixen et al. 2020). The intensity of eddy activity could also influence oxygen levels, as oxygen injection through eddy pumping weakens the OMZs (Rixen et al. 2020; Sarma et al. 2020). There is no consensus on the definition of the limits of the OMZ despite its importance in im- proving the understanding of their location, char- acteristics, and biogeochemical effects (Truc- co-Pignata et al. 2019; Kirchman 2021). The OMZ definition/limits are usually defined as a function of O2 concentration among different classifications. For the present study, the OMZ was defined by a dissolved oxygen concentration of 0.5 ml l-1, equiv- alent to 22 µM or 20 µmol kg-1 (Levin 2003; Helly and Levin 2004; Karstensen et al. 2008; Gooday et al. 2010). In addition, Karstensen et al. (2008) specified three different thresholds: 0.1 ml l-1 or 4.5 µmol kg-1 for the suboxic level, a more rigid level defined as 45 µmol kg-1, and a more relaxed level at 90 µmol kg-1. According to sedimentary records, the OMZs can remain through millennia and extend for thou- sands of kilometers in open oceans with high pro- ductivity or in the eastern edges of ocean basins where the spatial and temporal variability is higher (Levin 2002; Gooday et al. 2010; Loescher et al. 2016; Rixen et al. 2020). Off the western conti- nental coasts, where wind-driven currents and the Coriolis effect replace surface waters with cold nu- trient-rich waters, primary productivity thrives and generates excessive organic matter that is decom- posed by bacteria with the consumption of oxygen giving rise to an OMZ when conditions persist over time (Levin 2002). The minimal oxygen and light conditions of an OMZ lead to accumulation of or- ganic matter due to the sluggish decomposition. The enhanced respiration of organic matter results in acidic conditions, pH values between 7.7 and 7.8 Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 467 in the core of the OMZ were observed in the East- ern Tropical Pacific (ETP) off Costa Rica (Gooday et al. 2010). The biodiversity in the OMZs depends on the oxygen and organic matter concentrations (Neira et al. 2018). OMZ waters are characterized by low diversity, with the lack of mobile microor- ganisms, but the waters in the lowest limit present an increase in biological productivity (Stramma et al. 2008; Gooday et al. 2010). In anoxic waters, hydrogen sulfide is usually observed due to the null concentrations of oxygen (Rixen et al. 2020). The ETP in Mesoamerica is a region with atmo- sphere and ocean features that interacts through its interface. Among those features are the Mid-Sum- mer Drought, a relative minimum observed in the precipitation between June and September (Ama- dor et al. 2016a, 2016b; Durán-Quesada et al. 2020; García-Franco et al. 2023), and two important upwelling systems. The first one is the seasonal coastal regions located in the Gulfs of Tehuante- pec, Papagayo and Panama, observed mainly in the boreal winter (Alfaro and Lizano 2001; Amador et al. 2016a, 2016b; Durán-Quesada et al. 2020; Escoto-Murillo and Alfaro 2021; Rodríguez et al. 2021) and the second one is the Costa Rica Ther- mal Dome (Alfaro and Lizano 2001; Amador et al. 2016a, 2016b; Lizano 2016; Ross-Salazar et al. 2019; Duran-Quesada et al. 2020). Those upwell- ing systems have some interactions mainly during the winter and during the Mid-Summer Drought occurrence in July-August. Comprehensive studies about the characteristics of a variety of atmospher- ic and oceanic systems dominated by multiscale interaction processes in the ETP, reviewing the climate and climate variability are presented in Amador et al. (2016a, 2016b). Additionally, two of the best studied, most in- tense, and largest OMZs in the world are located in the ETP, separated by the Equatorial Undercur- rent (EUC), which in addition to the Northern and Southern Equatorial Countercurrent (ECC), Sub- surface Countercurrent (SCC), and Intermediate Countercurrent (ICC) conform the eastward tropi- cal currents which transport relatively oxygen-rich water to the ETP (Stramma et al. 2008, 2010; Cabré et al. 2015). The upwelling systems men- tioned previously are important for the ETP OMZ since vertical displacement of water masses alters oxygen concentrations as deep-water upwelling brings nutrients to the surface, increasing biologi- cal productivity and the subsequent consumption of oxygen, while the descent of surface water masses counteracts productivity and injects oxygen into intermediate depths (Gruber et al. 2011; Rixen et al. 2020). Moreover, a faster circulation of the water allows the supply of oxygen to the deeper layers of the ocean, thus influencing the intensity of the OMZs (Sarma et al. 2020). There are OMZs also in the Atlantic, favored by the high productivity observed in both oceans because of the current up- welling systems in the eastern continental shelves and in the northern reaches of the tropical Indian Ocean, home to about a fifth of the ocean’s oxygen depleted areas, where the upwelling is associat- ed with the monsoon (Levin 2002; Stramma et al. 2008; Rixen et al. 2020). The expansion and intensification of the OMZ added to eutrophication can cause negative reper- cussions such as changes in benthic and pelagic ecosystems, decreases in marine biodiversity, dis- turbances in food chains, primary production, pop- ulations, fishery yield, biogeochemical processes, and more frequent sulfur events, as a consequence of the dependence of nutrient budgets, biological productivity, and carbon fixation on dissolved ox- ygen concentrations (Levin 2002; Stramma et al. 2008; Gooday et al. 2010; Fee 2012; Loescher et al. 2016; Breitburg et al. 2018; Rixen et al. 2020). The Costa Rica Thermal Dome (Lizano 2016; Ross-Salazar et al. 2019) corresponds to the north- ern OMZ and is characterized by a more intense deoxygenation and a larger and thicker extension than the southern OMZ, due to differences in oxy- gen income and a low oxygen core around 500 m (Stramma et al. 2010; Cabré et al. 2015). The ver- tical extents of the OMZs in the ETP range from around 100 m to 900 m (Karstensen et al. 2008). The OMZs in the ETP are caused by upwelling ef- Marine and Fishery Sciences 37 (3): 465-513 (2024)468 fects, sluggish horizontal transport, high primary productivity, and stratification driven primarily by temperature (Karstensen et al. 2008; Fiedler et al. 2013). In addition to this North-South pattern, the intensity and extent of the OMZs in the area tend to increase towards the east and are affected by the concentration of nutrients and oxygen, among other properties of the water source (Stramma et al. 2010). The OMZs in the ETP do not present strong sea- sonal variability (Paulmier and Ruiz‐Pino 2009). However, El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) are sources of interannual and decadal variability in the ETP (Durán-Quesada et al. 2020) influencing on the OMZs variability (Stramma et al. 2010; Czeschel et al. 2012). ENSO events can alter dis- solved oxygen concentrations in OMZs through physical and biogeochemical processes that affect biological productivity above the OMZ, ventila- tion, and thermocline and oxycline depths (Espi- noza-Morriberón et al. 2019). Equatorial Kelvin waves can deepen the thermocline and oxycline in the ETP during El Niño, the ENSO warm phase (Enfield 1987; Alfaro y Lizano 2001; Fuenzalida et al. 2009; Stramma et al. 2010; Escoto-Murillo and Alfaro 2021). As regards the PDO, it alters dissolved oxygen through isopycnal (‘heave’) in tropical waters and by subduction in subtropical waters (Ito et al. 2019). According to a study by Duteil et al. (2018), oxygen in the ETP decreases during a transition from the negative to the positive multi-decadal PDO. Water masses from different oxygen-rich sourc- es can be found in the ETP, as is the case of the Tropical Surface Water (TSW), located above the OMZs and characterized by warm temperatures and low salinities with values above 25 °C and below 34 respectively, the Subtropical Subsurface Water (SSW), also known as Subtropical Underwa- ter (STUW), a high salinity water identified by its subsuperficial salinity maximum with temperatures around 13 °C and salinities above 34.9, transported eastward by the EUC, and Antarctic Intermediate Water (AAIW), identified by its characteristic sa- linity minimum of 34.55 and cold temperatures under 5 °C, transported eastward by the Southern and Northern SCC and ICC (Wyrtki 1967; Brenes and Coen 1985; Fiedler and Talley 2006; Stramma et al. 2010). According to Mora-Escalante et al. (2020) TSW above 50 m depth and SSW below 60 m depth were observed in the ETP from CTD pro- files sampled in the region between 2008 and 2012. Previous paragraphs state that OMZs are often areas with depressed marine life due to the very low concentration of oxygen. However, specialized microorganisms can thrive in these ecosystems be- ing important regions for biogeochemical process in the ocean, mainly nitrogen and carbon. For this reason, the study of OMZ is important not only to the nutrient cycles, but also to understand the diver- sity and adaptation of life in this extreme environ- ment. OMZs can be altered by various environmen- tal factors, such as ocean temperature, circulation, upwelling and acidification among others, and so climate change could have significant implications on OMZs. This study has a significant dataset of oceanographic campaigns totaling around 10 years of data in the Eastern Tropical Pacific off Costa Rica (but not continuous measurements) which could be relevant to characterize variations of the OMZ of this region. Furthermore, it was based on the analysis of oceanographic data measured with CTD profilers at hydrographic stations in the ETP during five different scientific campaigns carried out between 2009 and 2019. Besides Brenes et al. (2016) and Mora-Escalante et al. (2020), this is one of the first studies to focus on the analysis of data obtained from in situ measurements of the study region. MATERIALS AND METHODS Study area and sampling campaigns An oceanographic database composed by the cast profiles of the water column of the hydro- Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 469 graphic stations sampled during five scientific campaigns carried out in the ETP of Costa Rica between 2009 and 2019 was compiled. The AT15-44 (from February 21 to March 8, 2009), AT15-59 (from January 6 to 13, 2010), and AT37- 13 (from May 20 to June 11, 2017) campaigns were carried out onboard the research vessel RV ‘Atlantis’. The JC112 campaign from December 5, 2014, to January 16, 2015, was carried out by the RRS ‘James Cook’. The FK190106 cam- paign from January 6 to 27, 2019, was devel- oped by the RV ‘Falkor’. In every campaign, the hydrographic stations were profiled with a Sea- Bird SBE 911+ CTD. In the campaigns of the RV ‘Atlantis’ a Sea-Bird SBE 19 CTD installed in the HOV ‘Alvin’ was also used. Samplings were carried out at several hydrographic stations distributed within the ETP of Costa Rica, with a special focus on stations located approximately above the methane seeps at 8.93° N and 84.31° W (Central stations) and in the surroundings of the seeps to the northeast (NE station) at 9.02° N and 84.20° W, to the southeast (SE station) at 8.85° N and 84.22° W, to the southwest (SW station) at 8.87° N and 84.43° W, and to the northwest (NW station) at 9.02° N and 84.41° W (Figure 1), with maximum depths between 391 m and 3,272 m. The latitude, longitude and date of all casts used in this work are included in Tables A1-A4 of the Appendix. This ecosystem found in the hydro- thermal or methane seeps is described in detail by Levin et al. (2012). Figure 1. Location of the CTD casts used. Red dots are AT15-44, blue for AT15-59, green for AT37-13, purple for FK190106 and orange for JC112 campaigns, respectively. Light blue area in the map represent the Pacific Costa Rican exclusive economic zone. Marine and Fishery Sciences 37 (3): 465-513 (2024)470 Data analysis Raw data were retrieved from online databas- es or provided by researchers associated with the campaigns. Data were processed with the SBE Data Processing software of Sea-Bird Electronics, which was used to convert the raw data to engi- neering units, resolve measurement inaccuracies due to sensors, remove inaccurate values, calculate the derived variables, average the data every meter of depth, export data in ASCII format, and plot preliminary profiles of the variables. The derived variables that were calculated are depth, seawa- ter density, and practical salinity (Sea-Bird Elec- tronics 2016). Dissolved oxygen concentrations were obtained from measurements of a sensor that counts the number of oxygen molecules per second through a polarographic membrane (Sea-Bird Elec- tronics 2013). Dissolved oxygen data from 47 casts were obtained only from the samplings carried out with Sea-Bird SBE 911+ CTD profilers, since these had the required sensor. Scripts for further data processing and visual- ization were programmed in Python with Jupyter Notebook (Van Rossum and Drake 2009). Maps with the locations of the hydrographic stations were plotted (e.g. Figure 1). For every campaign, all the casts done at the Central station, over the hydrothermal vents, were averaged. Spatial anom- alies in the vicinity of the hydrothermal or meth- ane seeps vents were obtained from the differences of the samplings of each neighboring station (NE, SE, SW, and NW) with respect to the averages of the Central station, by variable and campaign. Anomalies between the averages of the Central sta- tion, from data sampled by the HOV ‘Alvin’ CTD, and the averages of the Central station, from data measured by the RV ‘Atlantis’ CTD, by variable and campaign, were also obtained. Tables compile maximum and minimum values of each variable, maximum gradients, vertical extension and upper and lower boundaries of the water masses and of the OMZ, descriptive statistics, latitude, longitude and date of samplings. The oxygen values in the tables have four significant numbers so that their variability can be better appreciated. The profiles of the samplings, averages, and anomalies were plotted, by variable, station, and campaign. The T-S diagrams of the averages of the Central station over the methane seeps were plotted by CTD profiler and campaign. RESULTS Regarding the five hydrographic stations closest to the methane seeps, the AT15-59 campaign in- cluded samplings with the CTD of the RV ‘Atlantis’ at the Central, NE, SE, SW, and NW stations, the AT15-44 campaign at the Central, NE, SE, and SW stations, and the AT37-13 campaign at the Central and SE stations. During the three campaigns of the RV ‘Atlantis’, samplings were also carried out with the CTD of the HOV ‘Alvin’ at the Central station. As for sampling averages in the Central station for the three campaigns, it was found that the temperature decreases rapidly within the mixed layer and more gradually at greater depths (Figure 2). Salinity increases with depth in shallow wa- ters, peaks at the salinity subsurface maximum and then decreases slightly, stabilizing at greater depths. Density also surges with depth with a higher rate of change within the mixed layer. Dissolved oxygen decreases with depth up to the upper limit of the OMZ, decreases slower up to the core of the OMZ and then increases slightly with depth (Figure 2). With respect to the data measured with the HOV ‘Alvin’ CTD, the variables show a trend similar to that observed in the profiles obtained from the samplings with the RV ‘Atlantis’ CTD (Figure 3; Tables 1 and 2). Anomalies of data sampled in stations in the sur- roundings with respect to the averages of the data measured in the Central station, for the different campaigns, generally oscillate with greater ampli- tude in the mixed layer, while under the thermo- cline they remain stable and tend to zero. Particu- Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 471 larly, in the profiles corresponding to the AT15-44 campaign, within the mixed layer, positive anoma- lies of temperature and oxygen stand out in the SE station and negative anomalies of temperature in NE and SW, salinity in NE, SE, and SW, density in SE, and oxygen in NE and SW (Figure 4). Re- Figure 2. Averages of temperature, salinity, density, and oxygen, at the Central station, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-44 (2009), AT15-59 (2010), and AT37-13 (2017) campaigns. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Marine and Fishery Sciences 37 (3): 465-513 (2024)472 garding the AT15-59 campaign, positive anomalies of salinity within the mixed layer were observed in stations SE, SW, and NW, density in SE and NW, and oxygen in SW, while negative anomalies of temperature in NW, salinity in NE and oxygen in SE and NW (Figure 5). Under the thermocline the temperature anomaly in the NE and SW stations oscillates mainly within a negative range and in the SE station in a positive range. With respect to the AT37-13 campaign, in the profiles of the SE station it is observed that negative salinity, density, and oxygen anomalies prevail in the mixed layer and positive temperature anomalies stand out under the thermocline (see rigth column of Figure 4). Anomalies of averages of variables sampled in the Central station by the HOV ‘Alvin’ CTD with respect to the averages of the variables sampled in the Central station by the RV ‘Atlantis’ CTD follow a similar pattern, since profiles in Figure A1 show important oscillations within the mixing layer and tend to zero below it. In particular, profiles of the AT15-44 campaign exhibit the temperature anom- aly tends to the negative axis and the salinity and density anomalies to the positive axis, within the mixed layer, while below it anomalies of the three variables remain stable with values closer to zero. Regarding profiles corresponding to the AT15-59 campaign, the three variables were very similar Figure 3. Averages of temperature, salinity, and density, at the Central station, from data measured by the HOV Alvin CTD profiler during the AT15-44, AT15-59, and AT37-13 campaigns. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 473 and they keep the same trend along the water col- umn, and the profile of the salinity anomaly for the AT37-13 campaign remains more stable and nearer to zero under the mixed layer. There were three distinguishable water masses in the T-S diagrams of sampling averages over the hydrothermal vents at the Central station for CTD profilers used during the three campaigns of the RV Table 1. Maximum and minimum values of the averages of temperature and salinity, for the Central, NE, SE, SW, and NW stations, from data measured by the RV ‘Atlantis’ CTD and HOV ‘Alvin’ CTD profilers during the AT15-44, AT15-59, and AT37-13 campaigns, with their respective depth and station ID. Temperature (°C) Depth (m) ID Salinity Depth (m) ID Central Max. 29.98 5 AT15-59-1 34.98 116 AT37-13-2 Min. 4.47 1,025 AT15-59-8 30.79 1 AT15-59-3 NE Max. 29.91 4 AT15-59-2 34.92 72 AT15-44-2 Min. 8.15 437 AT15-44-2 29.99 2 AT15-59-2 SE Max. 29.37 2 AT15-59-4 34.97 124 AT37-13-5 Min. 9.23 393 AT15-59-4 31.07 1 AT15-59-4 SW Max. 29.34 1 AT15-59-6 34.93 142 AT15-59-6 Min. 2.49 1,811 AT15-59-6 31.33 1 AT15-59-6 NW Max. 29.71 2 AT15-59-9 34.93 157 AT15-59-9 Min. 4.02 1,155 AT15-59-9 31.09 2 AT15-59-9 ‘Alvin’ Max. 29.79 2 AT37-13-4909 40.71 27 AT15-44-4501 Min. 2.07 2,228 AT15-44-4507 30.82 3 AT15-59-4587 Table 2. Maximum and minimum values of the averages of density and oxygen, for the Central, NE, SE, SW, and NW stations, from data measured by the RV ‘Atlantis’ CTD and HOV ‘Alvin’ CTD profilers during the AT15-44, AT15-59, and AT37-13 campaigns, with their respective depth and station ID. Density (kg m-3) Depth (m) ID Oxygen (ml l-1) Depth (m) ID Central Max. 1,032.16 1,027 AT15-59-8 5.2268 15 AT37-13-1 Min. 1,018.62 1 AT15-59-1 0.0177 444 AT37-13-1 NE Max. 1,028.96 437 AT15-44-2 4.6523 8 AT15-59-2 Min. 1,018.01 2 AT15-59-2 0.0394 331 AT15-44-2 SE Max. 1,028.62 394 AT15-59-4 4.6777 26 AT15-44-14 Min. 1,019.00 1 AT15-59-4 0.0393 369 AT15-44-14 SW Max. 1,036.06 1,815 AT15-44-5 4.6064 9 AT15-59-6 Min. 1,019.21 1 AT15-59-6 0.0388 324 AT15-44-5 NW Max. 1,032.82 1,155 AT15-59-9 4.6216 6 AT15-59-9 Min. 1,018.90 2 AT15-59-9 0.0576 477 AT15-59-9 ‘Alvin’ Max. 1,038.05 2,225 AT15-44-4507 Min. 1,018.84 2 AT15-59-4587 Marine and Fishery Sciences 37 (3): 465-513 (2024)474 ‘Atlantis’ (Figure 6). Vertical extensions observed for water masses depended on the depth at which the CTD began or ended the sampling, so the full extension was not always recorded. The TSW was recorded during the AT15-59 campaign at depths extending from the surface to 58 m, correspond- ing to the maximum extension. The SSW showed subsurface salinity maximum between 19 m and Figure 4. Spatial anomalies of temperature, salinity, density, and oxygen, at stations NE, SE, and SW, with respect to the averages of the Central station, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-44 campaign, and at station SE, with respect to the averages of the Central station, from data measured by the RV ‘Atlantis’ CTD profiler during the AT37-13 campaign. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 475 344 m, with a maximum vertical extension of 210 m in the AT37-13 campaign. The AAIW was observed between 866 m and 1,061 m, with a max- imum vertical extension of 161 m in the AT15-59 campaign (Table 3). In general, profiles of the RV ‘Atlantis’ cam- paigns (Figures A2-A18) showed a decrease in dis- solved oxygen until it reaches its minimum values in the core of the OMZ, while at greater depths it returns to higher values. Oxygen concentrations Figure 5. Spatial anomalies of temperature, salinity, density, and oxygen, at stations NE, SE, SW, and NW, with respect to the averages of the Central station, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-59 campaign. Marine and Fishery Sciences 37 (3): 465-513 (2024)476 are highly variable above the OMZ, more stable within it, and slightly variable below it (for exam- ple, ranges in Appendix figures were described by horizontal dashed lines to represent the upper and lower boundaries of the OMZ). Oxygen profiles of samplings in the vicinity of the hydrothermal vents and in other stations in the ETP, from data meas- ured by the RV ‘Atlantis’ CTD, were dominated by Table 3. Depth of the upper and lower boundaries of the water masses in the averages at the Central station, from data measured by the RV ‘Atlantis’ CTD and HOV ‘Alvin’ CTD profilers during the AT15-44, AT15-59, and AT37-13 campaigns. TSW stands for Tropical Surface Water, SSW for Subtropical Subsurface Water, AAIW for Antarctic Intermediate Water, and VE for vertical extension. TSW SSW AAIW Upper (m) Lower (m) VE (m) Upper (m) Lower (m) VE (m) Upper (m) Lower (m) VE (m) ‘Atlantis’ AT15-44 0 13 13 65 131 66 AT15-59 0 55 55 129 207 78 866 1,027 161 AT37-13 4 14 14 79 289 210 990 1,000 10 ‘Alvin’ AT15-44 1 3 3 19 160 141 1,007 1,061 54 AT15-59 1 58 58 70 268 198 897 1,027 130 AT37-13 156 344 188 Figure 6. T-S diagrams of the averages at the Central station, from data measured by the RV ‘Atlantis’ CTD and HOV ‘Alvin’ CTD profilers during the AT15-44, AT15-59, and AT37-13 campaigns. Water masses are identified in red for Tropical Surface Water (TSW), orange for Subtropical Subsurface Water (SSW), and light blue for Antarctic Intermediate Water (AAIW). Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 477 the presence of the OMZ. The upper edge of the OMZ in the 33 samplings with the RV ‘Atlantis’ CTD, for the three campaigns, was located within a range of depths from 35 m to 241 m. For the 24 samplings including the full extension of the OMZ, the lower edge ranged between 758 m and 1,117 m and the vertical extension of the OMZ ranges be- tween 580 m and 1,082 m (Table A5). The mini- mum of oxygen observed in the core of the OMZ ranged from 0.0177 ml l-1 to 0.1034 ml l-1, and was located between 320 m and 521 m deep (Table A1). Regarding the seven casts of the FK190106 cam- paign (Figures 7 and 8), a remarkable OMZ can be observed in the oxygen profiles, which follow Figure 7. Temperature, salinity, density, and oxygen, at stations 1, 2, 3, and 4, from data measured by the RV ‘Falkor’ CTD profiler during the FK190106 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Marine and Fishery Sciences 37 (3): 465-513 (2024)478 the same trends seen in the case of the RV ‘Atlan- tis’ campaigns. The OMZ was defined by an upper edge within a range from 132 m to 298 m, a lower edge from 708 m to 971 m, and a full extension between 410 m and 839 m (Table 4). The oxygen minimum observed in the core of the OMZ ranged from 0.0599 ml l-1 to 0.2797 ml l-1, and was bound- ed by depths between 387 m and 534 m (Table A3). Figure 8. Temperature, salinity, density, and oxygen, at stations 5, 6, and 7, from data measured by the RV ‘Falkor’ CTD profiler during the FK190106 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 479 Oxygen profiles of the seven casts studied from the JC112 campaign (Figures 9 and 10) presented the same trends as the profiles obtained from other campaigns, with a OMZ extending from an upper edge located between 127 m and 206 m, and a lower edge from 719 m to 897 m, with a thickness between 520 m and 721 m (Table 5). The oxygen minimum observed in the core of the OMZ ranged from 0.0155 ml l-1 to 0.0202 ml l-1 and was located between 343 m and 466 m (Table A4). In profiles reaching depths greater than 2,000 m, oxygen concentrations tended to stabilize with in- creasing depth (Tables 6-8). With respect to temperature, salinity, and den- sity, obtained profiles from data sampled during the five campaigns in the study area (Figures A2- A18) followed the characteristics described above for the profiles of the averages at the Central sta- tion (Figure 2; Tables A4, A7-A9). In particular, profiles from data measured by the RV ‘Atlantis’ CTD were delimited by a temperature within a range from 2.05 °C observed at 2,227 m depth to 29.98 °C at 5 m, a salinity from 29.99 at 2 m to 35.02 at 143 m, and a density from 1,018.01 kg m-3 at 2 m to 1,038.04 kg m-3 at 2,227 m (Tables 9 and 10). Profiles from data sampled by the HOV ‘Alvin’ CTD covered temperatures from 2.07 °C at 2,228 m to 29.79 °C at 2 m, salinities from 30.82 at 3 m to 40.71 at 27 m, and densities from 1,018.84 kg m-3 at 2 m to 1,038.05 kg m-3 at 2,225 m (Tables 9 and 10). In the case of the RV ‘Falkor’ campaign, ranges for the profiles of variables corresponded to tem- peratures from 3.84 °C at 1,192 m to 29.38 °C at 12 m, salinity from 30.56 at 12 m to 34.98 at 105 m, and density from 1,018.66 kg m-3 at 12 m to 1,032.99 kg m-3 at 1,192 m (Figures 7 and 8; Tables 11 and 12). As with the RRS ‘James Cook’ campaign, ranges for the samplings were defined by a temperature from 1.81 °C at 2633 m to 29.46 °C at 6 m, a salin- ity from 29.06 at 8 m to 34.96 at 140 m, and a den- sity from 1,017.69 kg m-3 at 3 m to 1,042.77 kg m-3 at 3,272 m (Tables 13 and 14). DISCUSSION Our analysis has not only revealed the exist- ence of TSW, SSW and AAIW in the study region but also allowed the characterization of the OMZ, since it was observed that within the boundaries of an OMZ oxygen continuously decreases with depth, stabilizes at the lowest concentration in the core or microxic zone and then increases slightly to the seafloor (Levin 2002). Along with this, re- sults also showed spatial and time variability. Dis- turbances in the currents and upwelling systems mentioned above that supply oxygen to the ETP could be related to the temporal and spatial var- Table 4. Oxygen concentration and depth of the upper and lower boundaries, and vertical extension, of the OMZ, for every station, from data measured by the RV ‘Falkor’ CTD profiler during the FK190106 campaign. Upper (ml l-1) Depth (m) Lower (ml l-1) Depth (m) Vertical extension (m) CTD001 0.4876 267 CTD002 0.4975 243 CTD003 0.4976 192 0.4986 917 725 CTD004 0.4964 272 0.4990 817 545 CTD005 0.4259 273 0.4996 729 456 CTD006 0.4992 298 0.4992 708 410 CTD007 0.3987 132 0.4953 971 839 Marine and Fishery Sciences 37 (3): 465-513 (2024)480 iability of the OMZs (Stramma et al. 2010). The arrival of TSW at the ETP for example, causes stratification, in turn modifying the depth of the oxycline, as happened during El Niño 2015-2016 when oxygen-rich tropical water accessed deeper layers (Trucco-Pignata et al. 2019). Furthermore, higher stratification and warmer temperatures de- crease biological productivity reducing the supply Figure 9. Temperature, salinity, density, and oxygen, at stations 39, 40, 82, and 83, from data measured by the RV ‘James Cook’ CTD profiler during the JC112 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 481 of nutrients and the oxygen consumption, leading to a deeper, warmer, and more oxygen-rich ther- mocline as suggested by the correlation between heat content, respiration rates, and ENSO (Ito and Deutsch 2013; Trucco-Pignata et al. 2019). Nevertheless, Ito et al. (2019) affirm that during a positive PDO event the oxygen concentrations are high in the ETP, based on models and observations. Figure 10. Temperature, salinity, density, and oxygen, at stations 84, 85, and 86, from data measured by the RV ‘James Cook’ CTD profiler during the JC112 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Marine and Fishery Sciences 37 (3): 465-513 (2024)482 The strength of a PDO event is associated with the depth of the thermocline, whose influence on bio- logical productivity and the changes in circulation due to PDO events affect oxygen concentrations (Deutsch et al. 2011; Duteil et al. 2018). In the long term, observations in the Eastern Tropical North Pacific (ETNP) evidence that the vertical extent of the OMZ has increased, at both the upper and lower edges, and oxygen depletion is more severe (Fee 2012). The uncertainties of the global climate models are originated by the lack of a detailed study of the influence of global physi- cal changes over biogeochemical processes and ecosystems and the wrong representation of sub- mesoscale and mesoscale features due to a coarse resolution (Rixen et al. 2020). The influence of climate change on open ocean OMZs is uncertain, due to inconsistencies between predictions of different models and measurements. It is possible that a warmer ocean under a nutrient enrichment fosters the extension and strengthening of OMZs, in accordance with the results of climate and biogeochemical models (Levin 2002; Matear and Hirst 2003; Oschlies et al. 2008; Breitburg et al. 2018). However, numerical models of the tropical oceans showed increased deoxygenation Table 6. Descriptive statistics of oxygen concentration and depth of the upper and lower boundaries, and vertical extension, of the OMZ, and of maximum and minimum values of oxygen with their respective depth, for every station, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-44, AT15-59, and AT37-13 campaigns. Upper Depth Lower Depth Vertical Minimum Depth Maximum Depth (ml l-1) (m) (ml l-1) (m) extension (m) (ml l-1) (m) (ml l-1) (m) Count 33 33 24 24 24 33 33 33 33 Mean 0.4780 125.88 0.4960 892.04 762.67 0.0445 392.64 4.5143 12.15 Std 0.0264 61.25 0.0047 105.24 140.37 0.0175 58.86 0.2935 5.04 Min. 0.3742 35 0.4777 758 580 0.0177 320 3.7009 5 25% 0.4751 65 0.4945 782.5 639 0.0388 347 4.3204 8 50% 0.4850 142 0.4977 914 751 0.0401 371 4.6064 11 75% 0.4945 167 0.4986 959 838.75 0.0576 436 4.6559 16 Max. 0.4994 241 0.5000 1117 1,082 0.1034 521 5.2268 26 Table 5. Oxygen concentration and depth of the upper and lower boundaries, and vertical extension, of the OMZ, for every station, from data measured by the RV ‘James Cook’ CTD profiler during the JC112 campaign. Upper (ml l-1) Depth (m) Lower (ml l-1) Depth (m) Vertical extension (m) CTD039 0.4785 206 0.4907 769 563 CTD040 0.4997 192 0.4964 793 601 CTD082 0.4836 199 0.4993 719 520 CTD083 0.4879 205 0.4970 844 639 CTD084 0.4863 179 0.4999 897 718 CTD085 0.4988 133 0.4981 793 660 CTD086 0.4369 127 0.4940 848 721 Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 483 in the Pacific and Atlantic, while according to ob- servations in the ETP oxygen has been persistently declining and OMZs have expanded over the last few decades and in the Indian Ocean oxygen has been stable for the last 30 years (Stramma et al. 2008; Stramma et al. 2010; Czeschel et al. 2012; Breitburg et al. 2018). Ocean warming can reduce water solubility and weaken vertical mixing, decreasing oxygen supply to intermediate waters and intensifying deoxygena- tion in OMZs (Matear and Hirst 2003; Breitburg et al. 2018; Rixen et al. 2020; Sarma et al. 2020). In addition, deoxygenation favors the production of trace gasses associated with climate change, which could be released to the atmosphere by nearby coastal upwelling systems triggering feedback on global warming (Stramma et al. 2012; Loescher et al. 2016). Increased biological production due to eutrophication could lead to the accumulation of organic matter, such as zooplankton carcasses, Table 7. Descriptive statistics of oxygen concentration and depth of the upper and lower boundaries, and vertical extension, of the OMZ, and of maximum and minimum values of oxygen with their respective depth, for every station, from data measured by the RV ‘Falkor’ CTD profiler during the FK190106 campaign, with their respective depth. Upper Depth Lower Depth Vertical Minimum Depth Maximum Depth (ml l-1) (m) (ml l-1) (m) extension (m) (ml l-1) (m) (ml l-1) (m) Count 7 7 5 5 5 7 7 7 7 Mean 0.4718 239.57 0.4983 828.40 595 0.1331 442.86 3.5710 34 Std 0.0416 58.00 0.0017 114.77 181.99 0.0786 51.93 1.1632 19.35 Min. 0.3987 132 0.4953 708 410 0.0599 387 1.9130 17 25% 0.4568 217.5 0.4986 729 456 0.0764 409 2.8231 21 50% 0.4964 267 0.4990 817 545 0.1222 428 3.6038 27 75% 0.4976 272.5 0.4992 917 725 0.1587 466.5 4.5996 43.5 Max. 0.4992 298 0.4996 971 839 0.2797 534 4.6348 65 Table 8. Descriptive statistics of oxygen concentration and depth of the upper and lower boundaries, and vertical extension, of the OMZ, and of maximum and minimum values of oxygen with their respective depth, for every station, from data measured by the RV ‘James Cook’ CTD profiler during the JC112 campaign, with their respective depth. Upper Depth Lower Depth Vertical Minimum Depth Maximum Depth (ml l-1) (m) (ml l-1) (m) extension (m) (ml l-1) (m) (ml l-1) (m) Count 7 7 7 7 7 7 7 7 7 Mean 0.4817 177.29 0.4965 809.00 631.71 0.0176 408.57 4.4599 16.86 Std 0.0212 33.60 0.0032 58.74 75.74 0.0014 39.23 0.0303 5.15 Min. 0.4369 127 0.4907 719 520 0.0155 343 4.4068 11 25% 0.4810 156 0.4952 781 582 0.0170 390 4.4441 13.5 50% 0.4863 192 0.4970 793 639 0.0174 420 4.4743 17 75% 0.4933 202 0.4987 846 689 0.0180 425.5 4.4784 18.5 Max. 0.4997 206 0.4999 897 721 0.0202 466 4.4930 26 Marine and Fishery Sciences 37 (3): 465-513 (2024)484 Table 11. Maximum and minimum values of temperature and salinity, for all the stations, from data measured by the RV ‘Falkor’ CTD profiler during the FK190106 campaign, with their respective depth and station ID. Temperature (°C) Depth (m) ID Salinity Depth (m) ID Max. 29.38 12 CTD004 34.98 105 CTD006 Min. 3.84 1,192 CTD005 30.56 12 CTD004 Table 12. Maximum and minimum values of density and oxygen, for all the stations, from data measured by the RV ‘Falkor’ CTD profiler during the FK190106 campaign, with their respective depth and station ID. Density (kg m-3) Depth (m) ID Oxygen Depth (m) ID (ml l-1) Max. 1,032.99 1,192 CTD005 4.6348 23 CTD005 Min. 1,018.66 12 CTD004 0.0599 387 CTD007 Table 9. Maximum and minimum values of temperature and salinity, for all the stations, from data measured by the RV ‘Atlantis’ CTD and HOV ‘Alvin’ CTD profilers during the AT15-44, AT15-59, and AT37-13 campaigns, with their respective depth and station ID. Temperature (°C) Depth (m) ID Salinity Depth (m) ID ‘Atlantis’ Max. 29.98 5 AT15-59-1 35.02 143 AT37-13-3 Min. 2.05 2,227 AT15-44-12 29.99 2 AT15-59-2 ‘Alvin’ Max. 29.79 2 AT37-13-4909 40.71 27 AT15-44-4501 Min. 2.07 2,228 AT15-44-4507 30.82 3 AT15-59-4587 Table 10. Maximum and minimum values of density and oxygen, for all the stations, from data measured by the RV ‘Atlantis’ CTD and HOV ‘Alvin’ CTD profilers during the AT15-44, AT15-59, and AT37-13 campaigns, with their respective depth and station ID. Density (kg m-3) Depth (m) ID Oxygen Depth (m) ID (ml l-1) ‘Atlantis’ Max. 1,038.04 2,227 AT15-44-12 5.2268 15 AT37-13-1 Min. 1,018.01 2 AT15-59-2 0.0177 444 AT37-13-1 ‘Alvin’ Max. 1,038.05 2,225 AT15-44-4507 Min. 1,018.84 2 AT15-59-4587 Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 485 resulting in further deoxygenation and loss of ni- trogen through denitrification and anammox pro- cesses (Levin 2002; DeVries et al. 2013; Glud et al. 2015; Stief et al. 2017; Rixen et al. 2020; Sarma et al. 2020). Conversely, a decrease in the export of organic matter could counteract the loss of oxygen caused by lower solubility and decreased vertical mixing (Sarma et al. 2020). Works mentioned in this paragraph suggest that a thickening of the OMZ could be expected in the future. Therefore, long term monitoring or research based on numer- ical models are future lines of research, since exist- ing in situ measurements are insufficient to allow a time series approach, in particular, in a region with important natural climate variability. The analysis of in situ CTD observations also enables additional future research. For example, the estimation of other derived variables like geo- strophic currents (Brenes et al. 2016) and for the comparison of physical model outputs with numer- ical models results (Mora-Escalante et al. 2020). Additionally, Sarma et al. (2020) mentioned that results of numerical models could be improved considering biological variables like phytoplankton composition and including processes such as the transport of organic matter from the continental shelf and sinking carbon fluxes. Further studies of source waters, transport timescales, and export pro- duction are necessary to better understand the pro- cesses controlling oxygen levels within an OMZ and to improve the modeling of its evolution (Fu et al. 2018; Rixen et al. 2020). Also, more measure- ments are needed to fully understand the responses of the nitrogen cycle and other vital processes to low oxygen levels, especially under anoxic condi- tions (Rixen et al. 2020). ACKNOWLEDGMENTS This project was supported by several Vicerrec- toría de Investigación, Universidad de Costa Rica projects: B5298, C2103, A5037, B9454, B0810, A1715, C0610, C1403, C0074, B7286, A4906 and A5719. Authors thank to the UCR School of Phys- ics for giving us the research time to develop this study and to the UCR research centers CIMAR and CIGEFI for their logistic support during the data compilation and analysis. We are grateful to Lisa Table 13. Maximum and minimum values of temperature and salinity, for all the stations, from data measured by the RV ‘James Cook’ CTD profiler during the JC112 campaign, with their respective depth and station ID. Temperature (°C) Depth (m) ID Salinity Depth (m) ID Max. 29.46 6 CTD083 34.96 140 CTD039 Min. 1.81 2,633 CTD084 29.06 8 CTD082 Table 14. Maximum and minimum values of density and oxygen, for all the stations, from data measured by the RV ‘James Cook’ CTD profiler during the JC112 campaign, with their respective depth and station ID. Density (kg m-3) Depth (m) ID Oxygen Depth (m) ID (ml l-1) Max. 1,042.77 3,272 CTD082 4.4930 26 CTD040 Min. 1,017.69 3 CTD082 0.0155 380 CTD085 Marine and Fishery Sciences 37 (3): 465-513 (2024)486 Levin and Eric Cordes, for considering us as UCR scientist participants during the campaigns. Finally, we also thank to all the crew members of the ‘At- lantis’, ‘Alvin’, ‘Falkor’ and ‘James Cook’ vessels, for their collaboration during the data collection. 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Int J Oceanol Limnol. 1 (2): 117-147. ftp://mananui. soest.hawaii.edu/pub/rlukas/Klaus/papers/ Wyrtki1967%20IJOL.pdf. https://doi.org/10.1016/j.dsr.2020.103393 https://www.seabird.com https://www.seabird.com https://www.seabird.com https://www.seabird.com https://doi.org/10.3389/fmars.2017.00152 https://doi.org/10.3389/fmars.2017.00152 https://doi.org/10.1029/2009JC005976 https://doi.org/10.1126/science.1153847 https://doi.org/10.1126/science.1153847 https://doi.org/10.5194/bg-9-4045-2012 https://doi.org/10.3389/fmars.2019.00459 https://dl.acm.org/doi/book/10.5555/1593511 ftp://mananui.soest.hawaii.edu/pub/rlukas/Klaus/papers/Wyrtki1967%20IJOL.pdf ftp://mananui.soest.hawaii.edu/pub/rlukas/Klaus/papers/Wyrtki1967%20IJOL.pdf ftp://mananui.soest.hawaii.edu/pub/rlukas/Klaus/papers/Wyrtki1967%20IJOL.pdf Marine and Fishery Sciences 37 (3): 465-513 (2024)490 Table A1. Maximum and minimum values of oxygen with their respective depth, latitude, longitude, and date, for every station, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-44, AT15-59, and AT37-13 campaigns. Minimum Depth Maximum Depth (m) Latitude Longitude Date (ml l-1) (m) (ml l-1) (m) (°N) (°W) AT15-44-1 0.0387 320 4.2001 11 8.93 84.32 02/21/2009 AT15-44-13 0.0401 371 4.6349 19 8.93 84.31 03/05/2009 AT15-44-2 0.0394 331 4.2215 11 9.02 84.20 02/22/2009 AT15-44-14 0.0393 369 4.6777 26 8.85 84.22 03/06/2009 AT15-44-5 0.0388 324 4.2607 14 8.87 84.43 02/25/2009 AT15-44-3 0.0396 347 4.3204 14 9.02 84.24 02/23/2009 AT15-44-4 0.0387 329 4.2890 8 8.98 84.27 02/24/2009 AT15-44-6 0.0389 323 4.3332 10 8.97 84.62 02/26/2009 AT15-44-7 0.0395 359 4.2336 14 9.09 84.58 02/27/2009 AT15-44-8 0.0401 377 4.2357 7 9.21 84.64 02/28/2009 AT15-44-9 0.0403 376 3.7009 5 10.30 86.31 03/02/2009 AT15-44-10 0.0399 350 3.9668 5 10.30 86.30 03/02/2009 AT15-44-11 0.0401 368 4.9680 8 9.22 84.93 03/03/2009 AT15-44-12 0.0395 354 4.4763 5 9.09 84.85 03/04/2009 AT15-59-1 0.0560 452 4.6615 16 8.93 84.31 01/06/2010 AT15-59-3 0.0558 430 4.6138 16 8.93 84.31 01/07/2010 AT15-59-5 0.0588 462 4.6648 11 8.92 84.30 01/08/2010 AT15-59-7 0.0576 463 4.6510 10 8.93 84.31 01/09/2010 AT15-59-8 0.0585 347 4.5873 7 8.92 84.30 01/10/2010 AT15-59-2 0.0591 344 4.6523 8 9.02 84.20 01/07/2010 AT15-59-4 0.0559 392 4.6165 9 8.85 84.22 01/08/2010 AT15-59-6 0.0600 357 4.6064 9 8.87 84.43 01/09/2010 AT15-59-9 0.0576 477 4.6216 6 9.02 84.41 01/10/2010 AT15-59-10 0.0560 334 4.6603 18 9.12 84.84 01/10/2010 AT15-59-11 0.0585 418 4.6559 16 9.12 84.84 01/11/2010 AT15-59-12 0.0601 345 4.5976 19 9.17 84.80 01/11/2010 AT37-13-1 0.0177 444 5.2268 15 8.93 84.31 05/22/2017 AT37-13-2 0.0205 436 5.0329 15 8.93 84.31 05/25/2017 AT37-13-5 0.1034 407 4.4701 13 8.85 84.22 06/03/2017 AT37-13-3 0.0195 492 4.6449 16 9.09 84.83 05/28/2017 AT37-13-4 0.0179 507 4.6636 14 9.12 84.84 05/29/2017 AT37-13-6 0.0219 431 4.4676 19 8.97 84.63 06/09/2017 AT37-13-7 0.0192 521 4.3566 7 8.80 85.18 06/09/2017 APPENDIX Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 491 Table A2. Latitude, longitude, and date, for every station, from data measured by the HOV ‘Alvin’ CTD profiler during the AT15- 44, AT15-59, and AT37-13 campaigns. Latitude (°N) Longitude (°W) Date AT15-44-4501 8.93 84.31 02/22/2009 AT15-44-4502 8.93 84.31 02/23/2009 AT15-44-4503 8.93 84.31 02/24/2009 AT15-44-4504 8.92 84.30 02/25/2009 AT15-44-4505 8.92 84.30 02/26/2009 AT15-44-4511 8.93 84.31 03/05/2009 AT15-44-4506 8.96 84.64 02/27/2009 AT15-44-4507 8.94 84.64 02/28/2009 AT15-44-4508 9.03 84.62 03/01/2009 AT15-44-4509 9.12 84.84 03/03/2009 AT15-44-4510 9.17 84.80 03/04/2009 AT15-44-4512 9.02 84.50 03/06/2009 AT15-44-4513 9.12 84.84 03/07/2009 AT15-59-4586 8.93 84.31 01/07/2010 AT15-59-4587 8.93 84.31 01/08/2010 AT15-59-4588 8.93 84.31 01/09/2010 AT15-59-4589 8.93 84.31 01/10/2010 AT15-59-4590 9.12 84.84 01/11/2010 AT15-59-4591 9.12 84.84 01/12/2010 AT37-13-4909 8.93 84.31 05/24/2017 AT37-13-4911 9.12 84.84 05/26/2017 AT37-13-4912 9.12 84.84 05/27/2017 AT37-13-4913 9.12 84.84 05/28/2017 AT37-13-4914 9.12 84.84 05/29/2017 AT37-13-4915 9.12 84.84 05/30/2017 Marine and Fishery Sciences 37 (3): 465-513 (2024)492 Table A4. Maximum and minimum values of oxygen, latitude, longitude, and date, for every station, from data measured by the RV ‘James Cook’ CTD profiler during the JC112 campaign, with their respective depth. Minimum Depth Maximum Depth (m) Latitude Longitude Date (ml l-1) (m) (ml l-1) (m) (°N) (°W) CTD039 0.0174 400 4.4350 11 6.15 83.47 12/30/2014 CTD040 0.0168 343 4.4930 26 5.75 83.49 12/30/2014 CTD082 0.0202 421 4.4533 19 4.75 85.32 01/12/2015 CTD083 0.0178 420 4.4743 18 5.68 86.00 01/12/2015 CTD084 0.0172 466 4.4826 17 7.08 86.00 01/13/2015 CTD085 0.0155 380 4.4743 11 6.09 85.46 01/13/2015 CTD086 0.0182 430 4.4068 16 6.42 85.04 01/14/2015 Table A3. Maximum and minimum values of oxygen, latitude, longitude, and date, for every station, from data measured by the RV ‘Falkor’ CTD profiler during the FK190106 campaign, with their respective depth. Minimum Depth Maximum Depth (m) Latitude Longitude Date (ml l-1) (m) (ml l-1) (m) (°N) (°W) CTD001 0.1222 426 3.5304 29 8.87 84.24 01/10/2019 CTD002 0.1253 428 3.6038 17 8.84 84.23 01/11/2019 CTD003 0.2797 534 1.9130 65 8.05 85.77 01/16/2019 CTD004 0.1920 483 2.1157 58 6.91 85.88 01/17/2019 CTD005 0.0676 450 4.6348 23 5.42 87.20 01/20/2019 CTD006 0.0851 392 4.5714 27 5.04 87.44 01/21/2019 CTD007 0.0599 387 4.6278 19 9.70 85.92 01/26/2019 Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 493 Table A5. Oxygen concentration and depth of the upper and lower boundaries, and vertical extension, of the OMZ, for every station, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-44, AT15-59, and AT37-13 campaigns. Upper (ml l-1) Depth (m) Lower (ml l-1) Depth (m) Vertical extension (m) AT15-44-1 0.4850 54 0.5000 956 902 AT15-44-13 0.4601 89 0.4993 915 826 AT15-44-2 0.4991 59 AT15-44-14 0.4963 98 AT15-44-5 0.4918 80 0.4976 913 833 AT15-44-3 0.3742 65 AT15-44-4 0.4550 75 AT15-44-6 0.4465 59 0.4973 915 856 AT15-44-7 0.4579 74 0.4983 891 817 AT15-44-8 0.4869 57 AT15-44-9 0.4825 35 0.4948 1,117 1,082 AT15-44-10 0.4826 35 0.4945 1,099 1,064 AT15-44-11 0.4945 59 0.4978 972 913 AT15-44-12 0.4097 53 0.4933 926 873 AT15-59-1 0.4923 143 0.4980 787 644 AT15-59-3 0.4972 149 0.4978 816 667 AT15-59-5 0.4994 146 0.4988 786 640 AT15-59-7 0.4871 178 0.4987 758 580 AT15-59-8 0.4958 167 0.4937 772 605 AT15-59-2 0.4959 132 AT15-59-4 0.4812 174 AT15-59-6 0.4980 134 0.4997 770 636 AT15-59-9 0.4731 142 0.4999 772 630 AT15-59-10 0.4654 146 0.4944 771 625 AT15-59-11 0.4923 145 0.4986 769 624 AT15-59-12 0.4850 148 AT37-13-1 0.4818 213 0.4963 893 680 AT37-13-2 0.4757 225 0.4929 915 690 AT37-13-5 0.4929 241 AT37-13-3 0.4889 209 0.4985 980 771 AT37-13-4 0.4956 219 0.4902 950 731 AT37-13-6 0.4751 197 0.4777 998 801 AT37-13-7 0.4790 154 0.4962 968 814 Marine and Fishery Sciences 37 (3): 465-513 (2024)494 Table A6. Maximum gradients of temperature, salinity, density, and oxygen, for every station, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-44, AT15-59, and AT37-13 campaigns, with their respective depth. Temperature Depth Salinity Depth Density Depth Oxygen Depth (°C) (m) (m) (kg m-3) (m) (ml l-1) (m) AT15-44-1 24.22 25 34.44 28 1,022.97 25 3.8133 23 AT15-44-13 28.51 8 32.84 5 1,021.05 8 4.4658 22 AT15-44-2 25.49 18 33.25 1 1,021.07 1 3.3123 20 AT15-44-14 28.81 6 32.31 6 1,020.13 6 4.3233 29 AT15-44-5 24.50 20 33.86 20 1,022.71 20 4.0721 18 AT15-44-3 23.86 23 34.00 23 1,023.02 23 3.4548 22 AT15-44-4 23.91 25 34.09 25 1,023.09 25 2.8257 27 AT15-44-6 24.46 19 33.53 12 1,022.76 19 4.0863 17 AT15-44-7 23.97 23 34.08 23 1,023.05 23 3.8574 21 AT15-44-8 25.92 10 33.75 10 1,022.16 10 4.2107 8 AT15-44-9 19.46 17 34.58 17 1,024.66 17 2.7891 15 AT15-44-10 20.29 18 34.52 18 1,024.40 18 3.2440 15 AT15-44-11 28.12 5 33.24 5 1,021.06 5 4.6128 5 AT15-44-12 26.82 7 34.02 7 1,022.07 7 3.7103 16 AT15-59-1 29.89 9 30.97 9 1,018.78 9 4.5429 8 AT15-59-3 29.43 15 31.31 15 1,019.22 15 3.9474 54 AT15-59-5 25.73 51 31.03 6 1,018.91 6 4.3099 46 AT15-59-7 26.20 54 31.67 9 1,019.48 9 4.0530 53 AT15-59-8 29.58 6 31.03 6 1,018.92 6 4.4903 5 AT15-59-2 29.81 7 30.13 5 1,018.13 5 4.5272 3 AT15-59-4 29.36 4 31.10 4 1,019.04 4 4.4887 2 AT15-59-6 27.27 51 31.48 4 1,019.34 4 4.2699 52 AT15-59-9 29.68 3 31.13 6 1,019.05 6 4.4670 4 AT15-59-10 25.18 51 32.49 17 1,020.25 17 4.1476 43 AT15-59-11 29.16 14 32.40 14 1,020.12 14 4.3909 33 AT15-59-12 29.18 15 32.36 15 1,020.09 15 4.4628 29 AT37-13-1 28.20 9 33.49 9 1,021.23 9 4.6119 25 AT37-13-2 26.48 21 33.28 9 1,020.85 9 4.6160 23 AT37-13-5 28.95 10 32.91 10 1,020.55 10 3.6600 25 AT37-13-3 27.97 15 33.27 12 1,020.79 12 3.8292 29 AT37-13-4 27.07 16 33.38 6 1,022.12 16 4.3508 24 AT37-13-6 27.88 25 32.72 9 1,021.64 25 4.1880 28 AT37-13-7 28.32 15 33.74 15 1,021.41 15 4.2930 14 Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 495 Table A7. Maximum gradients of temperature, salinity, and density, for every station, from data measured by the HOV ‘Alvin’ CTD profiler during the AT15-44, AT15-59, and AT37-13 campaigns, with their respective depth. Temperature (°C) Depth (m) Salinity Depth (m) Density (kg m-3) Depth (m) AT15-44-4501 27.50 2 33.52 2 1,021.45 2 AT15-44-4502 23.53 20 40.25 29 1,026.42 25 AT15-44-4503 24.23 20 34.84 19 1,023.21 19 AT15-44-4504 22.37 21 36.22 21 1,025.13 21 AT15-44-4505 24.25 17 33.55 2 1,023.30 18 AT15-44-4511 27.14 2 33.95 2 1,021.89 2 AT15-44-4506 27.64 6 34.75 14 1,023.47 14 AT15-44-4507 27.44 6 33.58 6 1,021.53 6 AT15-44-4508 21.62 17 33.43 5 1,021.36 5 AT15-44-4509 27.68 3 33.86 3 1,021.66 3 AT15-44-4510 26.60 6 34.12 5 1,022.00 5 AT15-44-4512 28.14 9 32.74 9 1,020.69 9 AT15-44-4513 26.36 2 33.62 2 1,021.89 2 AT15-59-4586 28.95 18 32.09 7 1,020.27 18 AT15-59-4587 28.53 48 31.44 33 1,020.28 48 AT15-59-4588 27.24 61 33.47 61 1,021.75 61 AT15-59-4589 26.30 72 31.16 31 1,022.55 72 AT15-59-4590 24.74 68 35.79 82 1,024.99 82 AT15-59-4591 26.77 52 33.92 52 1,022.20 52 AT37-13-4909 29.79 2 33.34 9 AT37-13-4911 28.46 15 32.45 4 AT37-13-4912 27.53 14 32.62 1 AT37-13-4913 27.53 21 32.89 10 AT37-13-4914 26.00 27 33.00 3 Marine and Fishery Sciences 37 (3): 465-513 (2024)496 Table A8. Maximum gradients of temperature, salinity, density, and oxygen, for every station, from data measured by the RV ‘Falkor’ CTD profiler during the FK190106 campaign, with their respective depth. Temperature Depth Salinity Depth Density Depth Oxygen Depth (°C) (m) (m) (kg m-3) (m) (ml l-1) (m) CTD001 27.11 28 33.37 26 1021.60 27 3.2184 33 CTD002 26.33 29 32.90 14 1020.78 14 3.4887 30 CTD003 24.59 61 31.14 12 1019.16 12 1.6957 18 CTD004 22.89 63 30.56 12 1018.66 12 1.7320 14 CTD005 27.13 50 32.15 31 1021.39 49 4.3513 51 CTD006 26.20 53 32.00 44 1021.71 52 4.3495 53 CTD007 27.37 17 33.59 17 1021.61 17 4.4727 23 Table A9. Maximum gradients of temperature, salinity, density, and oxygen, for every station, from data measured by the RV ‘James Cook’ CTD profiler during the JC112 campaign, with their respective depth. Temperature Depth Salinity Depth Density Depth Oxygen Depth (°C) (m) (m) (kg m-3) (m) (ml l-1) (m) CTD039 26.35 46 30.47 30 1,021.69 46 4.1351 44 CTD040 28.06 35 31.30 34 1,019.78 34 3.7641 45 CTD082 24.86 27 29.18 15 1,017.84 15 3.9572 26 CTD083 28.65 39 29.67 10 1,017.96 9 2.4226 51 CTD084 27.72 43 30.43 17 1,018.60 16 4.3418 41 CTD085 26.78 49 29.49 9 1,017.96 9 3.5707 52 CTD086 29.04 12 30.55 15 1,018.48 12 4.1091 49 Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 497 Figure A1. Anomalies of temperature, salinity, and density, for the averages of the Central station, from data measured by the HOV ‘Alvin’ CTD profiler, with respect to the averages of the Central station, from data measured by the RV ‘Atlantis’ CTD profiler, during the AT15-44, AT15-59, and AT37-13 campaigns. Marine and Fishery Sciences 37 (3): 465-513 (2024)498 Figure A2. Temperature, salinity, density, and oxygen, at stations 1 and 13 (Central), and stations 3 and 4, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-44 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 499 Figure A3. Temperature, salinity, density, and oxygen, at stations 6, 7, 8, and 9, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-44 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Marine and Fishery Sciences 37 (3): 465-513 (2024)500 Figure A4. Temperature, salinity, density, and oxygen, at stations 10, 11, and 12, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-44 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 501 Figure A5. Temperature, salinity, density, and oxygen, at stations 2 (NE), 14 (SE), and 5 (SW), from data measured by the RV ‘At- lantis’ CTD profiler during the AT15-44 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Marine and Fishery Sciences 37 (3): 465-513 (2024)502 Figure A6. Temperature, salinity, and density, at stations 4501, 4502, and 4503 (Central), from data measured by the HOV ‘Alvin’ CTD profiler during the AT15-44 campaign. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 503 Figure A7. Temperature, salinity, and density, at stations 4504, 4505, and 4511 (Central), from data measured by the HOV ‘Alvin’ CTD profiler during the AT15-44 campaign. Marine and Fishery Sciences 37 (3): 465-513 (2024)504 Figure A8. Temperature, salinity, and density, at stations 4506, 4507, 4508, and 4509, from data measured by the HOV ‘Alvin’ CTD profiler during the AT15-44 campaign. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 505 Figure A9. Temperature, salinity, and density, at stations 4510, 4512, and 4513, from data measured by the HOV ‘Alvin’ CTD profiler during the AT15-44 campaign. Marine and Fishery Sciences 37 (3): 465-513 (2024)506 Figure A10. Temperature, salinity, density, and oxygen, at stations 1, 3, 5, and 7 (Central), from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-59 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 507 Figure A11. Temperature, salinity, density, and oxygen, at stations 8 (Central), and stations 10, 11, and 12, from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-59 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Marine and Fishery Sciences 37 (3): 465-513 (2024)508 Figure A12. Temperature, salinity, density, and oxygen, at stations 2 (NE), 4 (SE), 6 (SW), and 9 (NW), from data measured by the RV ‘Atlantis’ CTD profiler during the AT15-59 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 509 Figure A13. Temperature, salinity, and density, at stations 4586, 4587, 4588, and 4589 (Central), from data measured by the HOV ‘Alvin’ CTD profiler during the AT15-59 campaign. Marine and Fishery Sciences 37 (3): 465-513 (2024)510 Figure A14. Temperature, salinity, and density, at stations 4590 and 4591, from data measured by the HOV ‘Alvin’ CTD profiler during the AT15-59 campaign. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 511 Figure A15. Temperature, salinity, density, and oxygen, at stations 1 and 2 (Central), and station 5 (SE), from data measured by the RV ‘Atlantis’ CTD profiler during the AT37-13 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Marine and Fishery Sciences 37 (3): 465-513 (2024)512 Figure A16. Temperature, salinity, density, and oxygen, at stations 3, 4, 6, and 7, from data measured by the RV ‘Atlantis’ CTD profiler during the AT37-13 campaign. In the oxygen profiles, the horizontal dashed lines represent the upper and lower boundaries of the OMZ (dissolved oxygen < 0.5 ml l-1), and the horizontal dotted lines indicate the minimum oxygen. Rodríguez et al.: OMZ in the Costa Rican eastern tropical Pacific 513 Figure A17. Temperature and salinity, at station 4909 (Central), and stations 4911 and 4912, from data measured by the HOV ‘Alvin’ CTD profiler during the AT37-13 campaign. Temperature (°C) Temperature (°C) Temperature (°C) Salinity Salinity Salinity D e p th ( m ) D e p th ( m ) Figure A18. Temperature and salinity, at stations 4913, 4914, and 4915, from data measured by the HOV ‘Alvin’ CTD profiler during the AT37-13 campaign. Página en blanco