Marine snow

Marine snow includes fragments of dead plants and animals, fecal material and other particles, including mineral dust and soot. Tiny particles can clump into larger, often fluffy aggregates that resemble falling snow. Aggregate size, density and composition vary; the term describes a diverse particle flux rather than a single substance.

Small particles can collide in moving seawater and stick together, forming aggregates with different settling properties. Biological material, including phytoplankton, can contribute to these aggregates. Their sinking is influenced by their size, density, shape and interactions with water. Bacteria and other organisms act on the particles as they descend: microbes can solubilize and remineralize organic matter, while some dissolved material diffuses or is carried away from the aggregate. These processes mean that sinking does not simply preserve all of a particle's carbon unchanged.

Marine snow supplies food to organisms in the deep ocean, including animals that filter particles from the water or scavenge material from the seabed. The sinking of organic matter also contributes to the biological carbon pump: carbon fixed by photosynthesis near the surface can be transported downward. Most material is recycled in upper ocean layers or as it sinks; some reaches deeper water or the seafloor and can remain isolated from the atmosphere for varying periods. The amount and duration of storage depend on depth and subsequent ocean and biological processes.

Researchers study marine snow using observations, experiments and models to understand how particles form, sink and break down. Thomas Kiørboe's 2001 review emphasized that small-scale biological and physical interactions shape both aggregate formation and fate, and that aggregates can be sites of elevated microbial activity. These competing processes influence whether organic material is exported to depth or retained and recycled higher in the water column. Marine snow is therefore an important part, but not the whole, of ocean carbon transport.