The ocean holds many undiscovered realms, and among them, the mesopelagic zone stands out as a vast and mysterious expanse. This global layer of water, often called the “twilight zone,” remains largely unexplored, yet it is teeming with life that has adapted to its unique conditions. Scientific expeditions are slowly unveiling the secrets of this deep-sea environment, revealing its immense biodiversity and its far-reaching influence on global processes. The ongoing efforts to understand the mesopelagic zone promise to deepen our appreciation for the interconnectedness of ocean ecosystems.
Defining the Mesopelagic Zone
The mesopelagic zone, deriving its name from the Greek word “meso” meaning “middle,” is a distinct layer of the open ocean. It extends from about 200 meters (660 feet) down to 1,000 meters (3,300 feet) below the ocean surface. This zone acts as a transitional area, bridging the sunlit epipelagic zone above, where photosynthesis occurs, and the aphotic bathypelagic zone below, which receives no sunlight.
Globally, the mesopelagic zone is an expansive habitat, covering about 60% of the planet’s surface and encompassing roughly 20% of the ocean’s total volume, making it a substantial part of the Earth’s biosphere. Conditions within this layer gradually change with increasing depth, creating a unique environment that supports a wide range of specialized marine life.
The Unique Environment
The mesopelagic zone presents a distinctive set of physical conditions. Light penetration is significantly reduced; only dim, filtered sunlight reaches these depths, less than 1% of the light found at the surface, and this light diminishes to nothing at the zone’s lower boundary.
Temperatures drop considerably compared to surface waters, ranging from over 20°C at its upper boundary to around 4°C at 1,000 meters. This temperature decrease with depth establishes a thermocline, a region of rapid temperature change. Hydrostatic pressure also increases steadily, adding about one atmosphere of pressure for every 10 meters of descent. At 200 meters, the pressure is about 21 atmospheres, increasing to roughly 101 atmospheres at 1,000 meters.
Many areas within the mesopelagic zone also feature an Oxygen Minimum Zone (OMZ). In these zones, oxygen levels are significantly depleted due to the respiration of sinking organic matter and limited gas exchange with the atmosphere. This makes aerobic respiration challenging for organisms.
Inhabitants and Their Adaptations
The challenging conditions of the mesopelagic zone have driven the evolution of remarkable adaptations in its diverse inhabitants. Fish such as bristlemouths and lanternfish are common, with bristlemouths being among the most abundant vertebrates on Earth. Crustaceans like krill and amphipods, along with various cephalopods and gelatinous zooplankton, also thrive here.
Bioluminescence, the ability to produce light through chemical reactions, is a widespread adaptation. Organisms use it for various purposes, including camouflage by counterillumination, where they match the faint overhead light to avoid being silhouetted by predators. It also serves to attract prey, deter predators, and facilitate communication and mating within species.
Many mesopelagic creatures undertake daily vertical migrations. They move up to shallower, food-rich waters at night to feed and then descend back to the deeper, darker mesopelagic zone during the day to avoid visual predators. This daily movement is considered the largest animal migration on Earth in terms of biomass.
Specialized Adaptations
Organisms exhibit specialized eyes adapted for low-light conditions, such as large eyes or tubular eyes that are often upward-looking to detect faint light from above. Some fish even possess yellow filters in their eyes to differentiate between natural light and bioluminescence. Additionally, species have evolved physiological adaptations to cope with high pressure and low oxygen levels, including low metabolic oxygen demand in colder waters.
Ecological Importance and Human Impact
The mesopelagic zone holds ecological importance, playing a role in global biogeochemical cycles. It is a major component of the ocean’s biological pump, a process that transfers carbon from the surface to the deep ocean. Organisms in this zone consume organic carbon from the surface and transport it downwards through their daily migrations, respiration, and the sinking of fecal pellets and dead organic matter. This process sequesters billions of tons of carbon dioxide from the atmosphere annually, a contribution often not fully accounted for in climate models.
The mesopelagic zone also serves as a link in the marine food web, supporting ecosystems both above and below it. Its abundant biomass provides food for a wide range of predators, including larger fish, marine mammals, and birds. However, human activities pose potential threats to this underexplored realm.
Threats from Human Activities
##### Overfishing
There is increasing interest in harvesting mesopelagic species for human consumption, fishmeal, or dietary supplements, with estimates suggesting a vast biomass of fish and squid. Any large-scale fishing operations could disrupt this ecosystem and its role in carbon cycling.
##### Climate Change
Rising ocean temperatures can reduce oxygen levels and increase ocean stratification, impacting mesopelagic communities. Studies indicate that the mesopelagic zone is vulnerable to deoxygenation, with some areas already experiencing oxygen loss.
##### Deep-Sea Mining
While not yet commercialized, deep-sea mining poses a future threat due to potential sediment plumes and heavy metal contamination, which could harm mesopelagic communities and disrupt visual communication among bioluminescent organisms. Continued research and exploration are underway to better understand this unique zone and inform policies for its protection.