The Galapagos Islands, a volcanic archipelago straddling the equator, are recognized for their unique ecosystems. The extraordinary biodiversity, which inspired Charles Darwin, is fundamentally linked to the surrounding marine environment. This ecology is supported by a dynamic confluence of ocean currents that bring cold, nutrient-laden water to what would otherwise be a warm, nutrient-poor tropical sea. This unique oceanography creates a temperate-like environment, setting the stage for the islands’ distinctive life forms.
The Convergence of Major Ocean Currents
The archipelago’s waters are defined by the meeting of three distinct current systems. The cold, southward-flowing Humboldt Current (Peru Current) sweeps north from Antarctica along the coast of South America, carrying nutrient-dense water toward the equator. It then turns westward, joining the South Equatorial Current and influencing the southern islands.
The warm Panama Current flows from the north and northeast, bringing warmer water toward the islands. Its influence is most prominent between December and May, contributing to the region’s warm, wet season. The interaction between the Humboldt and Panama currents creates a seasonal oscillation in sea surface temperatures.
A third, highly influential force is the deep-sea Equatorial Undercurrent (EUC), often called the Cromwell Current. This current flows eastward beneath the surface, approximately 300 feet down. It transports vast quantities of deep, cold water directly toward the archipelago’s submerged platform.
The Mechanism of Deep-Water Upwelling
The uniqueness of the Galapagos marine environment stems from upwelling, the mechanism that forces deep, cold water to the surface. The Equatorial Undercurrent (Cromwell Current) plays a central role as it surges eastward. This current carries water rich in dissolved inorganic nutrients, such as nitrates and phosphates, accumulated from the decomposition of organic matter sinking to the deep ocean floor.
As the Cromwell Current continues its path, it encounters the submerged volcanic platform and islands. This physical obstruction forces the eastward-flowing water to abruptly change direction, pushing the cold, nutrient-dense water mass upward toward the surface layers. This process is known as topographic upwelling.
The forced vertical movement results in the Galapagos Cold Pool, a region of cold sea surface temperature west of the archipelago. The upwelling is especially strong around the western islands, such as Isabela and Fernandina, where the current’s collision is most direct. This constant infusion of deep, cold water into the sunlit surface layer maintains a cold-water ecosystem in a tropical latitude.
Ecological Consequences for Marine Life
The continuous supply of nutrients from upwelling forms the foundation of the archipelago’s prolific marine food web. The nutrient-rich water stimulates high primary productivity, resulting in substantial blooms of phytoplankton, the microscopic plants at the base of the food chain. Phytoplankton abundance supports large populations of zooplankton, which in turn feed dense populations of small fish and other marine life.
This high biological productivity allows for the survival of cold-water species unusual for an equatorial location. The Galapagos penguin, the only penguin species found north of the equator, thrives here due to the consistently cool water temperatures and readily available food supply. The flightless cormorant, another endemic species, also relies entirely on the productive, cool waters of the western islands for its fish diet.
The marine iguana, the world’s only sea-going lizard, is a direct beneficiary of the productive waters. These reptiles feed almost exclusively on the abundant green and red algae that grow along the cool shorelines, fueled by the upwelled nutrients. The entire ecosystem, from the smallest organisms to apex predators, is sustained by these complex, nutrient-driven currents, resulting in high biomass.
Variability Driven by Climate Cycles
The stability of the Galapagos current system is periodically disrupted by the El Niño-Southern Oscillation (ENSO) cycle. During an El Niño event, the normal pattern of trade winds weakens significantly, causing a temporary halt or deepening of the Equatorial Undercurrent and a reduction in upwelling. This results in an influx of warm, nutrient-poor surface water from the western Pacific, increasing sea surface temperatures by 3 to 4 degrees Celsius above the usual maximum.
The immediate consequence of this warming is a crash in primary productivity, as the vital supply of deep-sea nutrients is cut off. Phytoplankton populations decline rapidly, causing a domino effect throughout the marine food chain. Severe El Niño events caused widespread stress and mass starvation among marine species. During these periods, populations of Galapagos penguins and marine iguanas can decline severely due to the lack of nourishing algae.
The opposite phase of the cycle, La Niña, sees a strengthening of the trade winds and cold currents. This intensification leads to enhanced upwelling and a period of greater marine productivity. The oscillation between the nutrient-rich, cool conditions of La Niña and the nutrient-poor, warm conditions of El Niño is a highly impactful feature of the Galapagos marine environment.