How the effects of “El Niño” may be changing one of the most extraordinary marine ecosystems on the planet
Written and photographed by Giancarlo Velmarch
A Tropical Fjord Full of Unknowns
In one of the most biodiverse regions on the planet, where mountains covered in tropical rainforest descend into mangroves winding along the coastline, and coral reefs give way to extensive underwater meadows, a landscape that is less visible but equally extraordinary lies hidden: the tropical fjord of Golfo Dulce.
Located on Costa Rica’s southern Pacific coast, this gulf is one of the few tropical fjords in the world. Unlike classic fjords, carved out by the advance of ancient glaciers, the fjord at Golfo Dulce originated through tectonic processes that shaped a deep marine basin. Within it, the seafloor reaches depths of more than 200 meters, while the relatively shallow entrance to the gulf, at around 60 meters deep, limits the exchange of water with the open ocean.
These characteristics make this tropical trench an exceptional environment. Below depths of around 100 meters, its waters do not mix uniformly: differences in temperature and density create layers that influence water circulation. As a result, all the organic matter such as leaves, dead fish, and algae, that sinks to the bottom is decomposed by bacteria that consume the dissolved oxygen, resulting in anoxic conditions, meaning an absence of oxygen.
The Questions That Drove the Research
“At Conservación Osa, we decided to begin investigating this area to expand our knowledge of marine ecosystems as part of our focus on climate change and its consequences for our local environment,” says Gabriela Vinueza, ecologist and director of our Marine Program.
This research raised questions that no one had concrete answers to: How does water circulate between these layers? How often is deep water renewed? And how might these processes be affected by current phenomena such as climate change and El Niño? El Niño is a climate phenomenon that weakens or disrupts the trade winds that push warm water from the Pacific coast of the Americas toward Asia, causing ocean temperatures to rise and, consequently, influencing climate patterns and contributing to droughts.
Understanding what happens beneath the surface became one of the questions that prompted Gabriela Vinueza, an ecologist and our Marine Conservation Director, to seek new ways to study this ecosystem. In April 2025, we began our research by installing oceanographic platforms known as Spotter platforms.
These platforms record key variables for understanding the dynamics of the marine environment, including wave height, period, and direction, as well as swell generated by winds and storms that may occur far from the measurement site. They also measure wind speed and direction, changes in atmospheric pressure, and water temperature at three depths: the surface, 15 meters, and 30 meters. The platforms were installed at two strategic locations: one at the mouth of the gulf, where it connects directly with the open ocean, and another in its inner basin, where the fjord’s characteristics influence the exchange of water with the open sea. Temperature, water circulation, and oxygen availability all influence the conditions in which organisms live and how life is distributed throughout the gulf.
“These measurements throughout the water column allow us to better understand its fluctuations and establish reliable baselines that we can compare with historical data and with future data that we will continue to collect,” Gaby explains.
The First Discoveries
Initial data collected between April 2025 and March 2026 began to reveal differences between the two areas of the gulf. During this period, average wave height was approximately 20 centimeters in the inner basin and 71 centimeters in the outer basin, illustrating the differences in water movement caused by the wind. We also observed significant differences in average temperature between the two locations: the surface temperatures differed by approximately 1.03°C, while the difference at 30 meters was 2.23°C. Although these differences may seem small, they show that thermal conditions are not the same in both areas.
One of the most recent insights from this monitoring effort has been the opportunity to observe how water temperature varies during the phenomenon known as the “Super El Niño.” In October 2025, a year with normal conditions, the platform in the inner basin recorded an average temperature of 29.09°C at the surface and 24.93°C at a depth of 30 meters. In October 2026, temperatures reached 31.11°C at the surface and 29.39°C at 30 meters. This difference of approximately 2°C at the surface and 5°C at depth highlights a substantial variation between the two periods and raises new questions about the influence of climate phenomena such as El Niño on the gulf’s thermal dynamics. Understanding what changes may occur, and what impacts increasingly frequent climate phenomena could have, is particularly important in an ecosystem as distinctive as this tropical fjord, where temperature variations could influence the physical and biological conditions of the water. The more data we collect, the more accurately we will be able to assess these differences and better understand how this ecosystem responds to climate variability.
Scientists and local people are becoming worried since changes in ocean temperatures like these can have significant consequences for marine ecosystems, from altering the reproductive cycles and distribution of certain species to changing the conditions on which coastal communities depend. In Golfo Dulce, one question that remains to be investigated is how warming water might interact with stratification and oxygen availability in deep areas. In such a complex environment as this tropical fjord, rising temperatures could alter the conditions that keep these water layers separated. As a result, the anoxic zone could extend into shallower areas, affecting the species that depend on these waters. Furthermore, if the anoxic zone moves too close to the surface, certain weather events, such as strong storms or intense winds, could temporarily disrupt stratification and push oxygen-free water directly toward the coast or the surface. This could cause the sudden, mass death of thousands of fish, crustaceans, and mollusks unable to escape in time, leaving beaches covered with dead animals.
“Just a few years ago, fishing in the gulf was easy and rewarding, with abundant populations of highly sought-after fish such as groupers. Today, due to environmental changes and industrial fishing, it’s becoming harder every year to catch enough fish to feed our families.” – Michael, boat captain and local fishermen.
For now, the installed platforms do not allow us to directly determine how the anoxic zone, located at greater depths, is changing. In future stages, we will expand the necessary measurements to investigate these relationships and better understand the potential risks to the ecosystem.
before entering the water to install the platform.
What Do These Changes Tell Us About the Gulf’s Future?
Beyond the differences recorded, one of the main contributions of our monitoring effort is that it opens up new possibilities for understanding how Golfo Dulce responds to emerging environmental threats. Are these variations part of the gulf’s natural cycles, or do they reflect broader changes in how it functions? How might its oceanographic conditions evolve in response to increasingly intense climate phenomena? These questions still have no definitive answers, but continuous monitoring allows us to begin answering them. Over the years, this research will help us identify patterns, recognize recurring variations, and distinguish them from short-term changes that could be transforming the ecosystem’s functioning. It will also allow us to relate the information collected in the gulf to other environmental data and develop a more comprehensive understanding of its natural processes.
The challenge now is to turn these initial measurements into a database that allows us to track how Golfo Dulce evolves. Each new record adds another piece to our understanding of an ecosystem we still know very little about, whose conservation depends, to a significant extent, on our ability to recognize the changes taking place within it.
Looking Ahead
Our goal is to establish a marine observatory capable of continuously recording oceanographic conditions and progressively expanding our knowledge of the processes that govern this uniquely complex environment. To achieve this, one of the main challenges will be keeping the platforms operational and ensuring that measurements continue over the years.
However, an observatory’s value depends not only on the amount of data it collects, but also on its ability to turn that data into useful information for local communities and institutions. For this reason, our project seeks to strengthen collaboration with researchers, organizations, and government agencies by facilitating information sharing and the development of new initiatives. We aim to build partnerships that will help ensure the knowledge generated serves as a starting point for conservation projects, management strategies, and marine governance processes.
As the project continues and more data is collected, it will become possible to establish a reference baseline of Golfo Dulce’s oceanographic conditions against which future measurements can be compared. This baseline will be particularly valuable for assessing changes in the ecosystem and providing scientific evidence to inform conservation decisions. Current records do not yet allow us to establish long-term climate trends, but continuous monitoring will gradually help us develop a more accurate understanding of how the gulf is evolving and the threats it faces.
Ultimately, a better understanding of the Golfo Dulce will allow us to anticipate questions, identify changes, and develop evidence-based adaptation measures for different environmental scenarios. This effort requires time, continuity, and collaboration, but its value and importance grow with every new measurement. Protecting an ecosystem must go hand in hand with understanding the processes that keep it in balance.
