The term “underwater snow” refers to a scientifically recognized deep-sea phenomenon known as marine snow. This continuous shower is not composed of frozen water crystals but consists of mostly organic particulate matter falling from the upper water column. This sinking detritus is a significant mechanism for transporting material, energy, and carbon from the sunlit surface to the deepest parts of the ocean, supporting life where sunlight cannot reach.
Marine Snow: The Ocean’s Organic Delivery System
Marine snow is a slow, continuous shower of organic detritus originating in the ocean’s surface waters that drifts downward through the water column. The composition of this “snow” is highly diverse, including:
- Dead or dying phytoplankton and zooplankton.
- Discarded shells.
- Fecal pellets.
- Sticky, mucus-like substances called extracellular polymeric substances (EPS) exuded by microbes and plankton.
These individual particles aggregate together, often held by the EPS, into larger, amorphous clumps known as flocs.
The formation of these aggregates increases the material’s sinking speed. As particles clump together, they become larger and denser, accelerating their descent from the sunlit euphotic zone to the aphotic zone below. This process ensures that organic carbon produced at the surface is exported deep into the ocean rather than being immediately recycled by surface microbes. The voyage to the deep seafloor can take weeks, depending on the aggregate’s size and density.
The prevalence and composition of marine snow fluctuate significantly with seasons and local ocean conditions. For instance, large blooms of photosynthetic phytoplankton often lead to a subsequent surge in sinking detritus. This fluctuation changes the intensity of the underwater snowfall, impacting the organisms that rely on it for sustenance.
Ecological and Global Significance of Marine Snow
Marine snow serves as the foundation of the deep-sea food web, acting as the main source of nutrients and energy for organisms in the dark abyss. Since sunlight does not penetrate the surface layer, marine snow provides the necessary organic carbon to sustain benthic (bottom-dwelling) and deep-water life. Filter-feeding organisms, such as sea lilies and certain jellies, actively capture these falling particles, while scavengers consume the material once it settles on the seafloor.
Beyond supporting deep-sea ecosystems, marine snow is an agent in one of Earth’s most significant biogeochemical processes, known as the biological carbon pump. This pump is the mechanism by which carbon dioxide from the atmosphere is sequestered into the deep ocean. Photosynthesis by phytoplankton converts dissolved carbon dioxide into organic matter in the surface ocean.
When this organic matter sinks as marine snow, it effectively transports the carbon to the deep sea, removing it from interaction with the atmosphere for hundreds to thousands of years. The process of sinking and subsequent burial in deep-sea sediments ensures the long-term storage of carbon. Changes in the efficiency of the biological carbon pump, which is highly dependent on the sinking rate and consumption of marine snow, directly influence the planet’s carbon cycle and climate regulation.
Other Forms of Underwater Precipitation
While marine snow is the most common and ecologically important form of underwater precipitation, other distinct phenomena also involve particles falling through the water column.
Hydrothermal Vents
One such process occurs around hydrothermal vents, which are fissures in the seafloor that release superheated, mineral-rich fluid. When this fluid, which can reach temperatures of 350 degrees Celsius, mixes with the cold surrounding seawater, the dissolved minerals immediately precipitate out of solution. This rapid precipitation creates dense plumes of tiny, solid mineral particles, often referred to as “black smoke” or “white smoke,” depending on the mineral composition. These particles, typically iron and zinc sulfides, fall back to the seafloor, contributing to the formation of massive sulfide deposits. This mineral precipitation is purely inorganic and is driven by temperature and chemical changes.
Polar Ice Phenomena
Ice phenomena in polar regions also offer unique forms of underwater precipitation, distinct from the detrital or mineral forms. Frazil ice consists of loose, randomly oriented ice crystals, a millimeter or less in size, that form in turbulent, supercooled water. This slush-like mix remains suspended in the water column and is a precursor to larger ice formations. Another dramatic example is a brinicle, an ice stalactite that grows downward from sea ice. It forms when extremely cold, dense, and saline water, known as brine, is expelled from growing sea ice and sinks, freezing the surrounding seawater into a hollow, downward-growing tube.