Spatiotemporal variability of oxygen concentration in coral reefs of Gorgona Island (Eastern Tropical Pacific) and its effect on the coral Pocillopora capitata

Abstract

Dissolved oxygen concentration (DO) is one of the main factors limiting benthic species distribution. Due to ocean warming and eutrophication, the ocean is deoxygenating. In the Eastern Tropical Pacific (ETP), deep waters with low DO (<1 mg L−1) may reach coral reefs, because upwelling will likely intensify due to climate change. To understand oxygen variability and its effects on corals, we characterize the Spatio-temporal changes of DO in coral reefs of Gorgona Island and calculate the critical oxygen tension (Pcrit) to identify the DO concentration that could represent a hypoxic condition for Pocillopora capitata, one of the main reef-building species in the ETP. The mean (±SD) DO concentration in the coral reefs of Gorgona Island was 4.6 ± 0.89 mg L−1. Low DO conditions were due to upwelling, but hypoxia (<3.71 mg L−1, defined as a DO value 1 SD lower than the Mean) down to 3.0 mg O2 L−1 sporadically occurred at 10 m depth. The Pcrit of P. capitata was 3.7 mg L−1 and lies close to the hypoxic condition recorded on coral reefs during the upwelling season at 10 m depth. At Gorgona Island oxygen conditions lower than 2.3 mg L−1 occur at >20 m depth and coincide with the deepest bathymetric distribution of scattered colonies of Pocillopora. Because DO concentrations in coral reefs of Gorgona Island were comparably low to other coral reefs in the Eastern Tropical Pacific, and the hypoxic threshold of P. capitata was close to the minimum DO record on reefs, hypoxic events could represent a threat if conditions that promote eutrophication (and consequently hypoxia) increase.

Physiological responses of Pocillopora corals to upwelling events in the Eastern Tropical Pacific

“…Despite the well-known negative effects of hypoxia and cool stress on scleractinian corals, and the occurrence of both conditions in the ETP during upwelling events, there is lack of research comparing the effects of hypoxia vs those of thermal stress (Rodrıǵuez-Troncoso et al, 2014;Rodrıǵuez-Troncoso et al, 2016;Glynn et al, 2017b;Tortolero-Langarica et al, 2017). To our knowledge, two reports have linked bleaching and mass mortality of Pocillopora corals with hypoxia after dinoflagellate blooms at Caño Island (Costa Rica) and Ensenada de Utrıá (Colombia) (Guzman et al, 1990;Vargas-Angel, 1996), and our previous work showed that the hypoxic threshold of Pocillopora capitata (Critical oxygen tension or Pcrit = 3.7 mg L -1 ) from Gorgona Island lies within the lower DO conditions reported on reefs in this island (3.0 -3.7 mg L -1 ) (Castrilloń-Cifuentes et al, 2023). The Pcrit, a metric for assessing the minimum DO level that maintains the metabolic rate (Negrete and Esbaugh, 2019;Regan et al, 2019), has been found to be closer to the lower DO levels recorded in the environment of different scleractinian corals, indicating their sensitivity to drops in DO below the ambient typical range (Dodds et al, 2007;Hughes et al, 2022).…”

“…The rest of the time (April -July), temperature was 27.6 ± 0.6°C, and salinity 34.1 ± 0.9 PSU (Supplementary Information File 1). All reefs of the island exhibited the same trend of lower DO during the upwelling season (3.5 ± 0.3 mg O 2 L -1 ) than during the non-upwelling season (4.7 ± 0.1 mg O 2 L -1 ) (Castrilloń-Cifuentes et al, 2023).…”

“…Additionally, as reported by Ceśpedes-Rodrıǵuez and Londoño-Cruz (2021), we found low growth rates at Playa Blanca reef, where higher sediment loads may account for depressed coral growth (Zapata, 2001). However, we discard the possibility that the low growth rate at Playa Blanca was the result of differences in temperature or DO between reefs, as both abiotic conditions respond to seasonal changes and there is no between-reef variability in such parameters (SM1; Castrilloń-Cifuentes et al, 2023). Although salinity was lower at Playa Blanca than at La Ventana during the non-upwelling season, conclusions cannot be made due to a lack of data for La Ventana during the upwelling season.…”

“…From January to April, there is low precipitation (350 mm month -1 ) and the thermocline is at its shallowest position (7.5 m), allowing for the lifting of cool, high-salinity, nutrient rich, and low-oxygen deep waters (25°C, 30.8 PSU, 0.6 mg L -1 of Chl-a, 4.0 mg O 2 L -1 ). For the rest of the year, precipitation increases (587 mm month -1 ), and the thermocline deepens (47 m), allowing for warmer and more oxygenated waters to occurs, but with lower salinity and chlorophyll-a conditions (27°C, 4.9 mg O 2 L -1 , 29.8 PSU, 0.1 mg L -1 of Chl-a) (Giraldo et al, 2008;Blanco, 2009;Giraldo et al, 2014;Sampson and Giraldo, 2014;Zapata, 2017;Castrilloń-Cifuentes et al, 2023). Changes in pH did not reveal a temporal pattern (Ramıŕez-Martıńez et al, 2022).…”

“…Interannual variability in oceanographic conditions occurs due to ENSO events. During El Niño, sea surface temperatures can increase by more than 1.7°C above the mean for several months and high oxygen conditions occur (5.1 mg O 2 L -1 ) due to less intense upwelling events (Ledesma et al, 2022;Castrilloń-Cifuentes et al, 2023). In contrast, cool temperatures as low as 18°C and hypoxic conditions (< 3.7 mg O 2 L -1 ) have been registered on the reef slope during La Niña events, when upwelling intensifies (Vargas-A ́ngel et al, 2001;Mora and Ospina, 2002;Castrilloń-Cifuentes et al, 2023).…”