High-resolution monitoring of the pH under strong La Niña conditions in Gorgona Island, Colombian Pacific, Panama Bight

Abstract

Few studies have investigated the potential drivers of high-resolution (daily and 24-hour scales) on ocean acidification (OA) and the carbonate system in a coastal estuary during an intense La Niña event. Therefore, we conducted the first high-resolution total scale pH (pH_T) monitoring every three hours for 56 days (13 September to 7 November 2021) at the Colombian Pacific in El Muelle reef, Gorgona National Natural Park. Two moored autonomous submersible instruments (iSAMI-pH and CTD-Diver) were deployed at a depth of 2 m in an area influenced by extreme precipitation, river discharge, semi-diurnal tides, and southwest winds during La Niña 2020-2023. Total alkalinity was derived from salinity data and used alongside pH_T to calculate sea surface seawater partial pressure of CO₂ (pCO_2w; μatm), dissolved inorganic carbon (DIC; μmol kg^-1), and omega aragonite saturation (Ω_a). The findings suggest that the observed low pH (7.93) and aragonite saturation state (Ω_a = 2.22) values are likely attributed to increased precipitation. This enhanced precipitation resulted in higher river discharge, transporting naturally low-pH water to the island via mixing mechanisms (RiOMar type 2). Daily, decreasing solar radiation may reduce the seawater temperature, simultaneously elevating the pCO_2w levels and reducing pH_T. In contrast, elevated precipitation may reduce surface seawater salinity through freshwater dilution. Throughout the diurnal cycle, peak pH_T values were recorded during late afternoon hours, likely driven by photosynthetic activity, while minimum values coincided with early morning periods of maximal respiratory activity. These results underscore the dynamic nature of this area and emphasize the need for long-term evaluation.

1 Introduction

Ocean acidification (OA) driven by human-induced increases in atmospheric carbon dioxide (CO₂) (Sabine et al., 2004; Feely et al., 2009) has significantly altered carbonate chemistry, leading to increased concentration of hydrogen ions [H⁺] and reduced global pH levels (Kleypas et al., 1999; Caldeira and Wickett, 2003; Gattuso et al., 2015). The El Niño–Southern Oscillation (ENSO) modulates OA. During El Niño events in the Eastern Tropical Pacific, reduced trade winds weaken equatorial upwelling, limiting the transport of cold, nutrient- and carbon-rich waters from the deep ocean to the surface (Vaittinada Ayar et al., 2022). As a result, surface dissolved inorganic carbon (DIC) concentrations decrease, leading to relatively lower seawater pCO_2w and reduced CO₂ outgassing to the atmosphere (Feely et al., 2006; Ishii et al., 2014; Espinoza-Morriberón et al., 2019). In contrast, La Niña events strengthen the trade winds and enhance upwelling, bringing DIC and nutrient-rich waters to the surface (Vaittinada Ayar et al., 2022). This increase in upwelling elevates pCO_2w levels, thereby increasing CO₂ outgassing (Yasunaka et al., 2019) and can lead to lower pH levels in coastal waters, intensifying OA (Oliva-Méndez et al., 2018).

La Niña event also induces increased rainfall, which enhances river discharge in coastal areas (Restrepo and Kjerfve, 2000; Hernández et al., 2006; Blanco, 2009). This influx of freshwater significantly reduces the buffering capacity of estuaries to neutralize acids and maintain stable pH levels, because freshwater lacks the carbonate buffer system found in seawater. These conditions increase the solubility of CO₂ and reduce the dissociation of bicarbonates (HCO₃^-) into carbonate ions (CO₃^-2; Cai et al., 2021). Additionally, rivers contribute to pH reduction by transporting organic matter, which releases carbon dioxide as it oxidizes upon reaching the sea (Salisbury et al., 2008; Zhai et al., 2015; Vargas et al., 2016). Moreover, nutrients from land and rivers are transported to coastal waters via runoff, forming low-salt river plumes that extend to the continental shelf, driven by winds and tides (Dai et al., 2022a). The areas influenced by these plumes, associated with the 19 largest rivers globally, cover an average of 3.7 × 10⁶ km² annually (Kang et al., 2013), representing approximately 14% of the total global continental shelf area (Dai et al., 2022a). Thus, salinity variations in estuary systems are determined by the mixing process influenced by tidal movements, precipitation, river discharges, and seawater influx (Atekwana et al., 2022).

In the Colombian Caribbean, La Niña events have been observed to intensify precipitation, leading to increased freshwater inflow and a subsequent dilution that significantly reduces DIC and total alkalinity (TA) (Ricaurte-Villota et al., 2025). Furthermore, heavy rains in the tropical Pacific Ocean can modify salinity, with dilution effects persisting for over 10 days (Henocq et al., 2010). This phenomenon decreases two key parameters of the carbonate system: TA and DIC (Turk et al., 2010; Ashton et al., 2016; Ho and Schanze, 2020). The relationship between low salinity and variations in the carbonate system has also been documented in other coastal regions of the Eastern Tropical Pacific (10°N), where precipitation influences can extend over 40,000 km², due to surface ocean currents dispersing freshwater over larger areas (Ho and Schanze, 2020).

Estuaries and coastal waters present greater complexity compared to open ocean waters when estimating OA and the carbonate system, as they are highly variable environments due to the mixing of seawater and freshwater (Wang et al., 2019; Nehir et al., 2022). Additionally, these Tropical Pacific waters, often influenced by anthropogenic activities, are critical for acidification studies. However, their complexity and the time scales (ranging from hours to interannual changes) require high-resolution monitoring to accurately assess OA (Carstensen et al., 2018; Nehir et al., 2022). One of the key parameters to monitor in estuaries is the natural variation of water pH (Nehir et al., 2022), which is typically measured using the National Bureau of Standards scale (pH_NBS) and total pH scales (pH_T). The pH_T is preferred in oceanography due to its accuracy. It also considers other factors that affect proton activity in complex solutions such as seawater, including the presence of salts (Fassbender et al., 2021).

The equatorial Pacific waters are characterized by lower pH values (8.00; Zhong et al., 2025) compared to the global average (8.05; von Schuckmann et al., 2024), making this region a key area for analyzing OA and its ecological impacts. To address this, continuous monitoring efforts using in situ submersible spectrophotometric sensors, such as Lab-on-Chip (Yin et al., 2021; Nehir et al., 2022) and the iSAMI-pH (Valauri-Orton et al., 2025), have proven effective for small-scale autonomous monitoring of the total pH scale (pH_T). For example, in the Pacific Coastal Ocean, significant pH_T variation (7.93 to 8.37) has been associated with upwelling events (Monterey Bay, USA) which increase pCO_2w and reduce pH_T (Gray et al., 2011). In contrast, during periods without upwelling and in the rainy season, a more minor pH_T variation (ranging from 7.98 to 8.06) has been observed along the Pacific coast of Costa Rica (Bahia Culebra; Sánchez-Noguera et al., 2018). In this region, a distinct 24-hour cycle was detected, with the lowest pH_T occurring in the early morning hours, due to organic matter respiration at night, and the highest pH_T values recorded in the late afternoon, associated with photosynthesis (Sánchez-Noguera et al., 2018).

In the Colombian Pacific Ocean, high-resolution measurements of pH_T have not been conducted. Instead, discrete measurements related to carbonate chemistry and pH_NBS have primarily been performed in the Caribbean Sea and the Pacific Ocean, including sampling sites around Gorgona National Natural Park (GNNP) by Instituto de Investigaciones Marinas y Costeras José Benito Vives de Andréis (INVEMAR) as part of the program “Red de Vigilancia para la Conservación y Protección de las Aguas Marinas y Costeras de Colombia (REDCAM)”. In Tayrona National Natural Park, studies such as those by Ricaurte-Villota et al. (2025) have collected discrete samples of pH_NBS, TA, and DIC, revealing high variability strongly influenced by coastal upwelling, precipitation, and river runoff, which are further affected by ENSO events. The high variability of oceanographic processes in coastal regions underscores the need for high-resolution monitoring, as discrete measurements are limited in their ability to capture natural variability and achieve sufficient temporal resolution (Nehir et al., 2022). Gorgona National Natural Park, located on the Colombian Pacific continental shelf, is a potential hotspot for OA due to its exposure to frequent ENSO events, which drive significant climate variability through El Niño and La Niña events (Lavín et al., 2006; Emerton et al., 2017; Fiedler and Lavín, 2017; Berri et al., 2019). As part of the Eastern Tropical Pacific (ETP), it is influenced by the Choco Jet, which brings intense precipitation from September to November (Serna et al., 2018; Guzmán et al., 2014), further increasing the already high fluvial discharge from the Patia-Sanquianga deltaic complex, the largest in Colombia (Díaz, 2007). Additionally, the Intertropical Convergence Zone (ITCZ) intensifies cloudiness and precipitation during this period, reducing solar radiation and potentially impacting the carbonate system and OA by altering water temperature and CO₂ solubility, as observed in other regions (Webster, 2020; Cai et al., 2021).

Therefore, this study investigates fine-scale, high-frequency dynamics in coastal carbonate chemistry using continuous measurements of pH_T, temperature, salinity, and derivation of TA, DIC, pCO_2w, and omega aragonite saturation (Ω_a). Focusing on Gorgona National Natural Park (GNNP) in the Colombian Pacific Ocean (Panama Bight), it examines how freshwater pulses during strong rainy season La Niña events modulate coastal carbonate buffering capacity and influence pH stability and ecosystem resilience to ocean acidification under climatic stress. This study uses a fixed mooring equipped with autonomous sensors to characterize high-frequency variability (daily over 56 days and hourly over 24 hours) of pH_T and other carbon chemistry variables (TA, pCO_2w, and DIC). Additionally, it explores the influence of tidal dynamics (ebb vs. flood) on variations in pH_T, salinity, temperature, pCO_2w, DIC, and TA, synchronizing high-resolution measurements from the iSAMI-pH and CTD-Diver with tide height data to identify primary sources of pH_T variation (oceanic vs. riverine influences). Furthermore, the relationships, both in magnitude and direction, between pH_T, salinity, temperature, pCO_2w, DIC, and TA, with external drivers such as daily solar radiation and daily precipitation (influenced by the Intertropical Convergence Zone, ITCZ) were analyzed. Daily mean values of the dependent variables were calculated to assess their response to these environmental factors under extreme climatic and hydrological dynamics. Finally, the study evaluates whether pH_T, salinity, temperature, pCO_2w, DIC, and TA exhibit significant differences between early morning and late afternoon within a 24-hour daily cycle, providing insights into diurnal variability in the study area. By addressing the critical role of fine-scale processes in shaping coastal carbonate chemistry, this research fills a significant gap in understanding the dynamics of coastal systems under extreme conditions.

2 Methods

2.1 Study area

The Gorgona National Natural Park (GNNP; 2° 55’45” - 3° 00’ 55” N, 78° 09’ 00″ - 78° 14’ 30” W) is located on the continental shelf of the Colombian Pacific basin (Figure 1). This region is characterized by warm and low-salinity surface waters (Giraldo López, 2008; Giraldo et al., 2008, 2011, 2014), and constitutes one of the rainiest regions of the world, being also the rainiest area of Colombia with annual values ranging from 2500 mm to 8000 mm (Rangel and Rudas, 1990; Blanco, 2009). This condition is mainly associated with the latitudinal migration of the Intertropical Convergence Zone (ITCZ; Díaz Guevara et al., 2008) and the Choco Jet Stream, which increases cloudiness and precipitation due to the convergence of trade winds from the northern and southern hemispheres (Prahl et al., 1990; Díaz Guevara et al., 2008; Guzmán et al., 2014; Serna et al., 2018). During the second half of the year, the ITCZ moves north, altering the wind direction near 3°N latitude, which becomes dominant in the southwesterlies and forms the Choco Jet off western Colombia (Poveda and Mesa, 2000; Amador et al., 2006; Guerrero Gallego et al., 2012). During the study period (September 13 to November 7, 2021), the area was affected by the strong La Niña 2020–2023 event (thermal anomalies of -1.4 ± 0.1°C; Supplementary Table 1, IDEAM, 2021) that increased total precipitation in the Colombian Pacific.

Figure 1

Gorgona Park is in front of the Patia-Sanquianga delta complex (Figure 1B), the largest in the country, which contributes approximately 23% of the total freshwater discharged to the Colombian Pacific (2045 m³ s^-1; Díaz, 2007). This delta comprises several rivers, including the Guapi, Patia, Iscuande, Tapaje, and Sanquianga (Díaz, 2007, Figure 1B). Likewise, the tidal regime around GNNP is semi-diurnal, repeating twice in 24 hours, with two alternating high tides (5.82 m) and two low tides (-0.78 m; Flanders Marine Institute [VLIZ] and Intergovernmental Oceanographic Commission [IOC], 2025).

The Pacific coast has an average daily solar radiation of 3500 to 4000 Wh m^-2 and an annual availability of solar radiation that varies between 1,080,000 and 1,440,000 Wh m^-2 (UPME, 2005). During the year’s second half, solar radiation decreases until it reaches its minimum in December (Rangel and Rudas, 1990). Regarding its biodiversity, GNNP hosts one of the most critical coral reef areas in the Eastern Tropical Pacific (ETP) region in terms of structure and species diversity (Glynn et al., 1982; Zapata, 2001; Díaz et al., 2001) and connects the ETP with other Pacific regions (e.g., Hawaii, Polynesia) via larval potential dispersion (Romero-Torres et al., 2018). Remarkably, these reefs persist despite being regularly exposed to multiple environmental stressors. At the beginning of the year, they experience sub-aerial exposure during extreme low tides (Zapata, 2001; Zapata et al., 2010), low pH levels reaching as low as 7.4 (Giraldo and Valencia, 2013), cooler temperatures (16.69 °C; Giraldo et al., 2008), and hypoxic conditions (< 3.7 mg L^-1) associated with seasonal upwelling (Castrillón-Cifuentes et al., 2023a, b). During the second half of the year, these reefs are subjected to low salinity levels during the rainy season (as low as 25; Giraldo et al., 2008), warmer temperatures (27 °C; Giraldo et al., 2008), and increased sedimentation rates (23.30 ± 4.34 g m^-2 d^-1; Lozano-Cortés et al., 2014). Coral species such as Pocillopora spp. and Pavona spp. dominate the eastern (leeward) and southern areas of Gorgona Island, while occurring more sparsely in the western and northern areas (Figure 1A, Zapata, 2001). The sampling point (Figure 1A) was located at the north end of El Muelle reef (northward surface current), where the genus Pocillopora spp. predominantly dominates this and the major eastern coral patch of Azufrada (Zapata, 2001; Romero-Torres and Acosta, 2010).

2.2 In situ measurements

Continuous measurements of pH_T, conductivity (mS cm^-1), and temperature (°C) were made at a fixed depth of around 2 meters in El Muelle reef, GNNP (2° 57’ 39” N, 78° 10’ 25” W; Figure 1A) from 13 September to 07 November 2021, every three hours, using a buoy anchoring system designed to compensate the tidal changes and to keep the equipment at a similar depth. The iSAMI-pH equipment (programmed with SAMI Client v2.5 software) was used to record pH_T (accuracy: 0.0004 and precision: ± 0.0024) and temperature (accuracy: ± 0.14°C; at 24.55 °C Tris bottle number 10 Sunburst Sensors). The CTD Diver equipment (programmed with Diver Field software, Van Essen Instruments) was used to take conductivity measurements (accuracy: ± 1% mS cm^-1). Conductivity data were also downloaded using Diver Field software. The salinity was calculated using the practical scaling relationship proposed by Aminot and Kérouel (2004). The iSAMI-pH data were downloaded with the SAMI program (QC pH v4.4). Because the QC pH v4.4 program handles a constant salinity of 35 units by default, the pH_T data were corrected for salinity. To do the correction, the program was set to the corresponding average salinity (29.74 ± 0.94, 31.29; mean ± SD, maximum, n = 403) of the sampling point (2° 57’ 39” N, 78° 10’ 25” W). With the correction, differences in pH_T of ± 0.01 units were observed. We used the average salinity value to ensure consistency in salinity correction, as the CTD Diver recorded salinity 30 minutes after iSAMI-pH measurements from October 10 to November 7. When both devices recorded simultaneously between September 13 and October 10, the difference between pH corrected with average and measured salinity was insignificant (± 0.0001). iSAMI recorded 49 outliers out of 441 data points, which were excluded from the analysis as iSAMI-pH identified them as measurement anomalies (error codes 100, 1001, 1010, and 1000) associated with issues related to pumping, dye supply, or blank measurement. Additionally, 19 temperature and 38 salinity data points were identified as outliers when plotting variables and their relationship to total pH_T. These points were excluded from the analysis after applying a criterion based on standard deviation to consecutive measurements. Any change greater than 0.030 in pH, 0.30°C in temperature, or 0.94°C in salinity between measurements was considered a potential outlier. Flagged data points underwent contextual evaluation, considering environmental consistency (e.g., salinity-TA relationships, diel cycles, water mass characteristics), sensor artifacts, and physical plausibility. Only measurements inconsistent both statistically and contextually were excluded.

2.3 Carbonate system derivation

Total alkalinity (TA; μmol kg^-1) was calculated using a regression line between salinity (in situ data taken with CastAway-CTD equipment) and TA. Preciado (2023) built the regression (y = 57.479x+249.08; R2 = 0.94; n=268) using measured local discrete TA data obtained from 8 sampling stations around Gorgona, one of which included the iSAMI-pH deployment site. The data were collected over 13 sampling months, from September 2021 to October 2022, at depths ranging from 2 to 80 meters, following the SOP-3b protocol (Dickson et al., 2007). Discrete samples of TA (± 13 μmol kg^-1, Batch #182 2230.91μmol kg^-1) were collected monthly at the iSAMI-pH site from November 2021 to July 2022. Comparing the measured TA values ​​with those estimated from salinity, a mean difference of 11 ± 5 μmol kg^-1 and a regression coefficient of R² = 0.91 were observed (Supplementary Figure 1). Specifically, in November 2021, the difference was 10 ± 3 μmol kg^-1. This regression approach for estimating TA through salinity has been validated in previous studies (Lee et al., 2006; Takatani et al., 2014; Carter et al., 2016; Fassbender et al., 2017; Metzl et al., 2024).

For the derivation of the carbonate system pCO_2w (μatm), dissolved inorganic carbon (DIC; μmol kg^-1), and omega aragonite (Ω_a), we used the Carbonate System equation solution (CO2sys v3.0, Pierrot et al., 2021), with the pair pH_T (n = 392) from iSAMI-pH and TA derived from the former regression, as well as salinity from CTD (Aminot and Kérouel, 2004) and temperature (iSAMI-pH). Propagation error analysis using CO2sys v3.0 estimated uncertainties of ±52 µmol·kg^-1 for DIC, ± 33 μatm for pCO₂, and ±0.16 for Ω_a. In the carbonate system equation solution, the following constants were used: (i) Dissociation constants for K₁ and K₂ from Millero, 2010) for waters ranging from 0 to 40, given that the study area the salinities variation are between 27.03 to 31.29, (ii) KHSO₄ dissociation constant from Dickson (1990), (iii) KHF from Perez and Fraga (1987), (iv) Total pH scale (mol-kg SW), (v) [B]T value from Lee et al. (2010), and (vi) EOS-80 standard.

To evaluate the influence of temperature and salinity, pH_T was normalized using the mean temperature (27.39°C) and salinity (29.74), as recommended by Terlouw et al. (2019). Additionally, normalization was performed using constant values representing the minimum observed temperature (26.72°C) and salinity (27.03), to account for the most extreme conditions in the dataset (Supplementary Figure 2). This normalization process isolates the potential effect of temperature and salinity on pH_T, following the methodology outlined by Sarmiento and Gruber (2006). The influence of temperature and salinity corresponds to the difference between the normalized pH_T and the in situ measured pH_T.

2.4 Statistical analysis: evaluation of external forcing on carbon chemistry

2.4.1 Tidal movement data

Daily tide measurements (m) between 13 September and 07 November 2021 were obtained from the UNESCO Sea Level Monitoring Facility database (Flanders Marine Institute [VLIZ] and Intergovernmental Oceanographic Commission [IOC], 2025), specifically from the Buenaventura station (3° 53’ 26” N, 77° 4’ 51” W), which measures the tide using a radar sensor. These tide measurements were temporally synchronized with iSAMI-pH and CTD-Diver data and subsequently labeled according to the water movement (ebb or flood; Supplementary Figure 3). The dependent variables do not meet the assumptions of normality, so non-parametric tests were chosen (Supplementary Table 2). The Kruskal-Wallis nonparametric test was used to determine if there were significant differences (p < 0.05) in the dependent variables (pH_T, salinity, temperature, pCO_2W, DIC, and TA) when comparing tidal movements (ebb and flood tides).

2.4.2 Atmospheric variables, solar radiation, and precipitation, and correlation with carbon chemistry

For the study period, 56 days, daily solar radiation (Wh m^-2) was obtained from the “26015040 Arrayanales” fixed station (2° 26’ 53” N, 76° 26’ 9” W; DHIME, 2025), and daily total precipitation (mm) from the “57025020 Gorgona Guapi” fixed station (2° 57’ 47” N, 78° 10’ 28” W; DHIME, 2025).

Daily means of the dependent variables (pH_T, salinity, temperature, pCO_2W, DIC, and TA) were calculated. A Spearman correlation analysis was performed between the dependent physical and chemical parameters and atmospheric variables (solar radiation and total precipitation) to evaluate the importance and direction of their relationship (rho value).

2.4.3 Hour carbon chemistry cycle analysis

The hourly cycle analysis was performed by calculating the average of the variables (pH_T, salinity, temperature, pCO_2W, DIC, and TA) taken at the same hour every three hours between 13 September and 7 November 2021. Carbonate alkalinity was also calculated following the equation proposed by Munhoven, 2013:

The Kruskal-Wallis test executed in RStudio was used to determine if there were significant differences (p-value < 0.05) in the dependent variables (pH_T, salinity, temperature, pCO_2W, DIC, TA, and carbonate alkalinity) when comparing the early morning hours (5:00 - 6:00) with the late afternoon hours (17:00 - 18:00).

3 Results

The sampling period took place during a year with strong La Niña conditions displaying atmospheric thermal anomalies of -1.4 ± 0.1°C (Supplementary Table 1; IDEAM, 2021) and extreme precipitation values (959 ± 316 mm/monthly; DHIME, 2025). Throughout the study period, pH_T decreased from September until October 25 (Figure 2), after which it increased until November 7, while pCO_2w, DIC, and salinity increased until October 25 and then decreased until November 7. Throughout the study period, pH_T and pCO_2w exhibited opposite trends (Figure 2), with lower pH_T values corresponding to higher pCO_2w levels and vice versa (Table 1). pH_T averaged 8.01 ± 0.03, peaking on September 15 (8.09), then decreasing until October 25, reaching a minimum on October 16 (7.93), before increasing again until November 7. In contrast, pCO_2w averaged 397 ± 36 μatm, reaching its minimum on September 15 (302 μatm), increasing until October 25, reaching a maximum on October 16 (499 μatm), and then decreasing until November 7. Salinity averaged 29.74 ± 0.94, increasing until October 25, with a peak on October 20 (31.29), then decreasing until November 7, reaching its minimum on that date (27.03). Similarly, TA averaged 1959 ± 54 μmol kg^-1, increasing until October 25, with a maximum on October 20 (2048 μmol kg^-1), then decreasing until November 7, when it reached its minimum (1803 μmol kg^-1). DIC followed a similar pattern, averaging 1728 ± 49 μmol kg^-1, increasing until October 25, with a maximum on October 11 (1807 μmol kg^-1), and decreasing until November 7, with a minimum on November 5 (1574 μmol kg^-1). The temperature averaged 27.39 ± 0.30°C, with a peak on September 20 (28.50°C), decreasing until October 9 and remaining stable until October 25 (27.25 ± 0.16°C), before oscillating until November 7, when it reached a minimum on November 4 (26.72°C).

Figure 2

When pH_T was normalized using salinity and temperature, the variation attributed to mean salinity reached up to 0.001 pH units, and up to 0.047 pH units when using the minimum salinity. In contrast, minor temperature-related variations reached only 0.000028 pH units with the mean temperature and 0.017 pH units with the minimum temperature. These results suggest that salinity plays a more significant role than temperature in driving pH_T variability (Supplementary Figure 2).

3.1 Tidal movement

No significant differences were found between high and low tides for the dependent variables of pH_T, salinity, temperature, pCO_2w, DIC, and TA (Supplementary Table 3). However, pH_T during flood tide (8.010 ± 0.031) was slightly higher than during ebb tide (8.008 ± 0.029), with a lower positive slope on the regression line (Figure 3A). pCO_2w and DIC showed similar values during the flood tide (396 ± 38 μatm and 1726 ± 52 µmol kg^-¹, respectively) and the ebb tide (397 ± 35 μatm and 1730 ± 46 µmol kg^-¹, respectively), with a slight negative slope observed in the regression trend (Figures 3D, E, respectively). Variables such as temperature, salinity, and TA exhibited slopes close to zero, indicating that this area’s seawater and river water are well-mixed.

Figure 3

3.2 Physico-chemical seawater parameters and atmospheric variables (56 days’ time scale)

The independent atmospheric variables (solar radiation and total precipitation) were correlated with water type (sea surface temperature and salinity) and carbon chemistry data (pH_T, pCO_2w, DIC, and TA). Total daily solar radiation showed the strongest correlations with all dependent variables; suggesting that average radiation values of 3078 ± 1098 Wh m^-² (n= 56), as well as radiation values in a range of 1378 to 5968 Wh m^-² displayed positively correlations with mean daily temperature and pH_T, and negative correlations with salinity, pCO_2w, DIC, and TA (Table 2). In contrast, total daily precipitation (mean average of 31 ± 27 mm; n= 56) within the 0 to 115 mm range only correlated negatively with mean daily salinity (Table 2). No significant correlation was observed between daily precipitation and pH_T, temperature, pCO_2W, DIC, and TA (Table 2). In addition, rainwater samples collected during the high rainfall season (May 2022) in GNNP showed the lowest salinity values (0.26).

3.3 Diurnal, 24-hour cycle of the carbonate system

pH_T, temperature, pCO_2W, and DIC followed a pronounced diurnal cycle with significant differences (p < 0.05; Supplementary Table 4) between early morning and late evening hours (Figure 4). During early morning hours, after sunrise (5:00 - 6:00), at 8:00 the lowest values of pH_T (7.993 ± 0.040; Figure 4A) and temperature (27.23 ± 0.18; Figure 4B) were recorded, as well as the highest values of pCO_2W (413 ± 40 μatm; Figure 4C) and DIC (1739 ± 46 μmol kg^-¹; Figure D). In the late afternoon (17:00 - 18:00), during sunset, the highest values of pH_T (8.030 ± 0.025; Figure 4A) and temperature (27.61 ± 0.36; Figure 4B) were found, together with the lowest pCO_2W (374 ± 25 μatm; Figure 4C) and DIC (1718 ± 50 μmol kg^-¹; Figure 4D). In contrast, salinity (Figure 4E), TA (Figure 4F), and carbonate alkalinity did not show significant differences (p > 0.05, Supplementary Table 4) between early morning and late evening hours.

Figure 4

4 Discussion

4.1 Freshwater input, precipitation, and river discharges play a key role in the Pacific Colombian GNNP region

The high variability found in variables pH_T, salinity, temperature, pCO_2w, DIC, and TA at El Muelle reef highlights the influence of La Niña 2020–2023 event. La Niña event strengthened the Choco Jet (Hoyos et al., 2013; Arias et al., 2015; Serna et al., 2018), which caused an increase in precipitation in the Colombian Pacific basin and, consequently, an increase in the river flow into the sea (Restrepo and Kjerfve, 2000; Blanco, 2009; Serna et al., 2018). During the sampling period from September to November 2021, the station “57025020 Gorgona Guapi” recorded a total precipitation of 2878 mm, exceeding the total multiannual monthly average by more than 59% to 1815 mm.

Likewise, the station “53047010 Sangaral” recorded a monthly average flow of the Guapi River of 447 m³ s^-1, exceeding by 44% the total multiannual monthly average of 309 m³ s^-1. Thus, the sampling period was characterized by a considerable inflow of freshwater from river discharges and high rainfall that explained the reduced recorded salinity values and part of the pH_T variability (Figure 2). The salinity normalization and correlation (Table 2) suggest that salinity plays a significant role in pH_T variability. The interplay between low salinity and reduced alkalinity (directly correlated) and pH variability underscores its critical role in coastal biogeochemical cycles. Reduced salinity not only dilutes carbonate and bicarbonate ions but also disrupts the equilibrium of the CO₂ system, as observed in estuarine zones by Feely et al. (2010) and Cai et al. (2021), where freshwater influx exacerbates anthropogenic CO₂ uptake. These dynamics challenge the buffering capacity and resilience, particularly in reefs, where a range of water pH is essential for calcification.

Additionally, high rainfall and the consequent excess of freshwater (characteristic of a RioMar system) acted as a surface layer dilution factor (Zeebe and Wolf-Gladrow, 2001; Turk et al., 2010; Trujillo and Thurman, 2016; Ho and Schanze, 2020), contributing to reducing, even more, the salinity over time. For example, Ho and Schanze (2020) reported a rapid decrease in salinity of 4.4 at depths of 2 to 3 cm in response to heavy rainfall in the Eastern Equatorial Pacific, highlighting the impact of precipitation on coastal salinity dynamics. Salinity gradients and alkalinity depletion may act as synergistic stressors, which align with broader concerns about coastal acidification (Doney et al., 2009) and the ecological risks associated with climate change scenarios.

The tides exert a continuous physical influence, due considerable tidal variation, having maximum amplitudes of 5.82 m and minimum of -0.78 m in 2021 (3.39 ± 1.20 m; Flanders Marine Institute [VLIZ] and Intergovernmental Oceanographic Commission [IOC], 2025), implying millions of cubic meters of freshwater entering and leaving the coastal system, mixing oceanic water with freshwater coming from the Patia-Sanquianga complex plus the rainfall. The well-mixed RioMar water has minimal salinity variations throughout a full ebb and flow cycle (Figure 3). This high mixing condition persists up to 55 km downstream of the river mouth. As indicated by Palacios Moreno and Pinto Tovar (1992), due to the high tide, mixing begins at the river mouth, where seawater enters through the bottom of the Guapi River, resulting in waters with a salinity of 25.60 at 10 m, compared to 17 at 1 m. Furthermore, tides and the southern trade winds during La Niña events generate coastal currents and a strong swell (Osorio et al., 2014) that contribute to mixing these two water types, creating an estuary environment. Consequently, this coastal zone can be classified as a Type 2 RiOMar regime (Dai et al., 2022a), characterized by a high-energy tidal system and significant mixing near the river mouths (Lamarque et al., 2022).

In this scenario, the riverine influence reaching GNNP may explain the observed decrease in pH_T over the study period. This trend is likely driven by the input of CO₂ derived from riverine DIC, potentially due to the input and decomposition of organic matter, which increases H⁺ ion concentrations, as observed in other coastal regions (Vargas et al., 2016; Cai et al., 2021). In fact, in the Eastern Colombian Pacific, the observed decrease and low values of pH_T were strongly correlated with salinity, pCO_2w, and DIC (Table 2). Between 2018 and 2021, data collected at 13 river stations near the GNNP showed pH_NBS values ranging from 6.3 to 8.3. Suspended solids ranged from 11 to 407 mg L^-1, while nitrate (NO₃^-) concentrations ranged from 2 to 90.9 µg L^-1. Phosphate (P-PO₄^3-) levels fluctuated between 2 and 3.3 µg L^-1, and chlorophyll-a concentrations ranged from 0.07 to 9.8 µg L^-1. The highest values ​​for these parameters were recorded during the year’s second half, coinciding with the rainy season (INVEMAR, 2020b; INVEMAR, 2022), which could contribute to acidification. This phenomenon aligns with RiOMar systems, where high DIC and nutrient inputs from rivers reach nearshore waters, and organic matter is subsequently exported to the continental shelf (Gan et al., 2009; Dai et al., 2022a). Decreases in pH_T in other estuaries have been indirectly linked to fluvial discharges. For example, along the central-southern Chilean Pacific coast (Biobío River basin), the expansion of the river plume seaward led to reduced pH_T conditions (7.6) during periods of maximum higher river discharge, compared to more oceanic stations where pH_T ranged from 7.95 to 8.15 (Vargas et al., 2016). Likewise, in the South Pacific Ocean, in Coral Bay, fluvial discharges from low-pH rivers (7.739 ± 0.022) cause coastal areas with low pH_T (8.014 ± 0.015) (Aguilera et al., 2013), similar to low pH observed across the GNNP Colombian Pacific Region. Indeed, the surface values (2m) of pH_T (8.01 ± 0.03), salinity (29.74 ± 0.94), TA (1959 ± 54 µmol kg^-1), DIC (1728 ± 49 µmol kg^-1) and Ω_a (2.71 ± 0.17) at El Muelle reef (GNNP) are below the surface averages reported for the North Pacific (8. 105 ± 0.06, 34.05 ± 0.86, 2255 ± 34 µmol kg^-1, 1959 ± 42 µmol kg^-1 and 3.30 ± 0.7; respectively) and the South Pacific (8.079 ± 0.03, 35.29 ± 0.55, 2319 ± 35 µmol kg^-1, 2003 ± 39 µmol kg^-1 and 3.52 ± 0.4) (Feely et al., 2009).

On the contrary, oceanic areas without river input, such as North Pacific Monterey Bay, have recorded relatively high pH_T (Gray et al., 2011). However, at higher latitudes on the Pacific coast of Costa Rica, low pH_T levels were associated with the rainy season (Sánchez-Noguera et al., 2018). A drop in pH_T is also known to occur in the estuaries of Swartkops and Sundays in South Africa, where, due to strong inflows of freshwater, a high pH_T decrease of 0,46 pH units was produced (Edworthy et al., 2022). The relatively low pH_T of the Pacific Colombian Coast (El Muelle, GNNP, Table 3) is similar to studies in the East River China Sea, where the low pH_T (7.98 to 8.07) was attributed to the high river inputs from the Changjiang plume, known to carry organic carbon, nutrients, and CO₂ supersaturated water (Wu et al., 2021). We already know that freshwater inputs from rivers play a critical role in shaping carbonate chemistry in coastal systems, particularly in the continental shelf. As well as in our study site, Rérolle et al. (2018) have identified riverine discharges, characterized by high DIC and TA, as key drivers of reduced pH_T in the Irish Sea, southern North Sea, and Skagerrak region. Further studies on carbon chemistry in the region are needed, including direct measurements of DIC and dissolved organic matter in the Patia-Sanquianga complex during wet and dry seasons. These and other chemical parameters, such as nutrients, are crucial to evaluating rivers’ contribution to coastal carbonate chemistry, particularly near reef systems, and the impact on ocean acidification (Borges et al., 2005; Cai et al., 2021). The main knowledge gaps include the effects of remineralization on pH in tropical estuaries (Dai et al., 2022b; Vargas et al., 2016) to understand the vulnerability of ecosystems to OA.

4.2 Daily scale of carbon and physical seawater variability and atmospheric variables, solar radiation, and precipitation

During the study period, the ITCZ was located close to the equator in the Colombian Pacific Ocean (Díaz Guevara et al., 2008), which intensified cloudiness and precipitation due to the convergence of trade winds from the northern and southern hemispheres (Díaz Guevara et al., 2008). The increase in cloudiness likely contributed to the decline in solar radiation, which diminished from 3,709 ± 1,205 Wh m^-2 in September to 2,501 ± 789 Wh m^-2 in November, with a marked reduction between October 1 and 25 (2,488 ± 590 Wh m^-2; Figure 5A). Given the positive correlation between solar radiation and temperature (Trujillo and Thurman, 2016; Webster, 2020), the decrease in solar radiation may explain the low SST observed from September 13 to November 7. According to Millero (2007, 2013) lower temperatures increase CO₂ solubility in seawater, suggesting that the temperature decline likely improved oceanic CO₂ uptake by enabling its transfer from the atmosphere to the ocean during the study period. The resulting CO₂ increase could explain pCO_2w and DIC’s highest values (Figure 2), particularly between October 1 and 25, when solar radiation was at its lowest (Figure 5). Thus, the solar radiation and temperature reduction may partially explain the observed decline in pH_T during this period. The inverse correlation between pH_T and pCO_2w also suggests that fluctuations in pCO_2w (indirectly DIC) could help explain pH_T variations over September-November (Table 2; Zeebe and Wolf-Gladrow, 2001; Millero, 2013; Cai et al., 2021). After October 25, the rise in solar radiation and temperature may have led to a pH_T increment by approximately 0.028 units, along with a decrease in pCO_2w by 41 μatm and DIC by 100 μmol kg^-1 (Figures 2D, E).

Figure 5

Between October 3 and 16, total precipitation amounted to 22 mm (Figure 5B), with most days recording values below the mean daily precipitation of 31 mm. The observed reduction in precipitation likely diminished freshwater inflow relative to seawater, thereby enhancing salinity levels and elevating TA and DIC concentrations during this period. During the sampling period (September 13 to November 7), evaporation (Copernicus Climate Change Service, 2024) was highest in October (-62 mm), followed by September (-36 mm) and November (-15 mm). However, in all cases, the monthly accumulated evaporation remained well below the corresponding monthly precipitation totals of 680 mm, 727 mm, and 261 mm, respectively (DHIME, 2025).

In addition to previously discussed mechanisms, the elevated levels of pCO_2w, DIC, and salinity observed between October 1 and 25 may also be influenced by a coastal upwelling event. This hypothesis is supported by the positive values of zonal Ekman transport (ZET) during this period, which indicates eastward surface water movement, given the regional coastline orientation and prevailing wind patterns (Corredor-Acosta et al., 2020). The eastward Ekman transport suggests offshore water displacement and the potential for deeper, carbon- and nutrient-rich water upwelling (Supplementary Figure 4).

4.3 24-hour cycle of the carbonate system

During early morning hours (5:00 - 6:00 am), we registered the lowest pH_T and temperature values, along with the highest concentration of pCO_2w and DIC, suggesting the influence of night-time respiration processes before daylight (Millero, 2013; Albright et al., 2013; Sánchez-Noguera et al., 2018). On the other hand, the highest values of pH_T and temperature and the lowest values of pCO_2w and DIC recorded during the late afternoon (17:00 - 18:00) suggest photosynthetic processes during daylight driven by increased solar radiation (Cyronak et al., 2013). The mean diel change of 0.037 units in pH_T and 0.38 °C in temperature between early morning and afternoon hours fall within the expected diel variability for shallow environments (0.7–17 m), where biological and physical processes significantly influence pH and temperature dynamics (Cyronak et al., 2013).

The absence of a significant difference in TA and carbonate alkalinity between early morning and late afternoon hours may be due to the derivation of TA using a TA-salinity relationship, which primarily accounts for processes such as evaporation, precipitation, and mixing tides (Spaulding et al., 2014). In contrast, calcification and dissolution, more likely to exhibit variations over a 24-hour cycle, may not be adequately reflected (Zeebe and Wolf-Gladrow, 2001; Millero, 2013; Cai et al., 2021). However, the data indicate that the El Muelle reef, close to the sampling point, is composed predominantly of Pocillopora spp. (Figure 1; Zapata, 2001; Acosta et al., 2007), was exposed during the sampling period to low pH_T (8.01 ± 0.03), salinity (29.32 ± 1.01), and Ω_a (2.71 ± 0.17). These environmental conditions are considered challenging for Pocillopora spp., which are known to be highly sensitive to reduced pH and lowered salinity associated with river discharge (Alvarado et al., 2005; Lizcano-Sandoval et al., 2018; Sánchez-Noguera, 2019). Notably, the global, annually averaged tolerance limit for coral reefs is an Ω_a of 2.82, indicating that corals at this site live below the threshold commonly reported in the literature (Guan et al., 2015).

Despite these adverse conditions and frequent riverine influence, especially during the rainy season in the latter half of the year (Giraldo et al., 2008, 2011), Pocillopora spp. persist and even dominate the area (Zapata, 2001; Acosta et al., 2007; Lizcano-Sandoval et al., 2018). Previous studies have shown that Pocillopora spp. in GNNP can maintain growth rates (Lizcano-Sandoval et al., 2018) and tolerate hypoxia, low salinity, and temperature fluctuations by reducing reproductive output rather than growth (Castrillón-Cifuentes et al., 2023a, 2023). This suggests that these corals are either locally adapted or possess a degree of physiological tolerance to low pH, salinity, and Ω_a. Similar findings from the Pearl River Estuary in Southeast China indicate that long-term hypo-salinity acclimation can enhance the tolerance of Pocillopora spp. to low salinity by reducing energy consumption, slowing metabolism, improving the energy metabolism of their symbiotic algae (Symbiodiniaceae), and altering their symbiotic bacterial communities to avoid bleaching (Chen et al., 2024). Nevertheless, Pocillopora spp. corals at Gorgona Island exhibit lower calcification rates (3.16 g CaCO₃ cm^-2 yr^-1; Lizcano-Sandoval et al., 2018) compared to those reported in other reef systems, such as Panama (5–6 g CaCO₃ cm^-2 yr^-1; Manzello, 2010) and Mexico (2.99–6.02 g CaCO₃ cm^-2 yr^-1; Medellín-Maldonado et al., 2016; González-Pabón et al., 2021; Tortolero-Langarica et al., 2022). To establish a more robust correlation between the carbonate system and calcification rates in Pocillopora spp. at GNNP, future research efforts should prioritize in situ measurements of TA, dissolved oxygen, and direct calcification assessments (e.g., via buoyant weight or alkalinity anomaly techniques) on a 24-h scale during both rainy and dry seasons, to better quantify metabolic dynamics and clarify the adaptive capacity of this species under adverse environmental conditions.

In addition, future research should prioritize high-frequency measurements of pH, nutrient concentrations, and stable isotopes across diurnal, seasonal, and interannual timescales. These efforts should encompass both rainy and dry seasons and different ENSO phases, such as El Niño and neutral conditions, to better distinguish water sources in the region and assess the influence of these temporal scales on carbonate chemistry variability.

5 Conclusions

The natural variability of the Ocean Acidification on 24-hour and daily scales and during 56 days at El Muelle reef, Colombian Pacific, Gorgona Island, was assessed in a fixed station using high-resolution measurements of seawater pH_T, salinity, and temperature taken every three hours during a year with a strong La Niña event and extreme precipitation months. The prolonged 2020–2023 La Niña event amplified rainfall and freshwater discharges from the Guapi River, establishing a Type 2 RIOMar (river-marine) system dominated by high-energy tidal dynamics and intense river-sea mixing. This freshwater influx likely contributed to localized declines in seawater pH_T. Concurrently, the southward migration of the ITCZ triggered persistent cloud cover and strong southwesterly winds, attenuating solar radiation and lowering both sea surface temperatures and pH_T. Enhanced rainfall within the RIOMar system further diluted the surface layer, progressively reducing salinity and TA, thereby diminishing the system’s buffering capacity against carbonate chemistry fluctuations. Biological processes such as photosynthesis and respiration shaped pronounced diurnal patterns within the 24-hour cycles, driving decreased and increased pH, pCO_2w, and DIC. These results highlight the substantial and high-frequency variability of pH and acidification conditions formed by the heavy rainfall and runoff that characterize this Pacific Colombian region. Therefore, we strongly recommend implementing long-term monitoring programs (COCAS Ocean Decade Project) to support acidification impact studies and adaptive management and conservation efforts within this National Natural Park.

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