The ocean’s twilight zone, also known as the mesopelagic zone, is a vast and mysterious part of our planet’s oceans. It stretches globally, positioned just beyond the reach of direct sunlight. The name “twilight zone” aptly describes its perpetually dim conditions, where only faint light penetrates from the surface waters. This immense area, which scientists are still actively exploring, holds many secrets about marine life and global processes.
The Environment
The ocean twilight zone lies between 200 and 1,000 meters (about 660 to 3,300 feet) below the ocean surface. Insufficient for photosynthesis, marine plants cannot grow here. Temperatures drop significantly from over 20 degrees Celsius at its upper boundary to around 4 degrees Celsius at deeper levels.
Water pressure intensifies considerably with depth, reaching up to 1,500 pounds per square inch, which organisms must withstand. With no sunlight for photosynthesis, the food web relies on organic matter sinking from the surface, often as “marine snow.” This marine snow, composed of dead plankton, bacteria, fecal pellets, and other carbon-rich particles, provides nutrients for deep-sea creatures.
Unique Inhabitants
The twilight zone is home to unique life forms adapted to its challenging conditions. Many species exhibit bioluminescence, producing their own light for communication, camouflage, or attracting prey. Animals here often have large eyes to capture minimal light or specialized mouths for feeding in the nutrient-scarce environment.
A prominent behavior among many twilight zone inhabitants is diel vertical migration. This involves animals ascending to shallower, food-rich surface waters at night to feed under cover of darkness, then descending back to the deeper, safer twilight zone at dawn to avoid predators. This daily migration is considered the largest animal migration on Earth, involving countless fish, squid, and crustaceans. Recent studies suggest the biomass of fish alone might be ten times greater than previously thought, potentially exceeding that of all other ocean fish combined. Examples include lanternfish, which may constitute 65 percent of the sea’s fish biomass, bristlemouths (the most abundant vertebrates on the planet), squid, and various jellyfish.
Global Importance
The ocean twilight zone plays a role in global ecosystems and climate regulation. It acts as a component of the “biological carbon pump,” transporting carbon from the surface ocean to the deep sea. Microscopic plant-like organisms called phytoplankton in surface waters absorb carbon dioxide through photosynthesis. When these phytoplankton die or are consumed by zooplankton, the carbon-rich material either sinks as marine snow or is carried deeper by migrating animals.
This downward movement sequesters carbon from the atmosphere, preventing it from returning to the air as a greenhouse gas. Estimates suggest that billions of metric tons of carbon are transported annually from the upper ocean into the deep sea through processes involving the twilight zone. The twilight zone also serves as a food source for deeper ocean creatures, connecting surface and deep-sea ecosystems. The daily vertical migration of its inhabitants links these zones, moving energy and nutrients throughout the water column.
Growing Concerns
Despite its remote nature, the ocean twilight zone faces threats from human activities and climate change. Ocean warming, deoxygenation (loss of oxygen), and acidification are altering its balance. Declining oxygen levels in the mesopelagic zone can reduce populations of species like lanternfish.
Interest in deep-sea fishing within the twilight zone is increasing, primarily for fishmeal and nutritional supplements rather than direct human consumption. Such large-scale extraction could have unknown consequences for marine ecosystems and the global carbon cycle. Other potential threats include waste plumes from seabed mining and processes aimed at carbon dioxide removal from the atmosphere. Given its importance in climate regulation and biodiversity, ongoing research and careful management are necessary to understand and protect this region.