Wiktionary
n. (context marine biology English) Sinking organic detritus in the ocean.
Wikipedia
In the deep ocean, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to the aphotic zone below. The term was first coined by the explorer William Beebe as he observed it from his bathysphere. As the origin of marine snow lies in activities within the productive photic zone, the prevalence of marine snow changes with seasonal fluctuations in photosynthetic activity and ocean currents. Marine snow can be an important food source for organisms living in the aphotic zone, particularly for organisms which live very deep in the water column.
Usage examples of "marine snow".
Outside, the water was becoming filled with falling marine snow, tiny light gray particles, consisting of dead organisms and waste produced by the sea creatures above.
The smaller figures often became blurred when covered by the detritus and marine snow that drifted into the chambers from the upwaters.
So continuous is this precipitation that it has earned itself the name of 'marine snow'.