Granular, brownish material often accumulates in aquariums, ponds, or along shorelines, leading many to wonder if this sediment is dead algae. The answer depends entirely on the specific type of microscopic organism. While most decaying algae breaks down into soft organic matter, a unique group of single-celled organisms leaves behind a mineralized structure that can visually and chemically resemble actual sand. Identifying the difference requires understanding how these aquatic life forms decompose.
The General Appearance of Algal Detritus
Most species of algae do not form a hard, granular material when they die. Decomposition results in detritus, a fine particulate organic matter. This material is the decaying body of the organism, often mixed with bacteria and other microorganisms that feed on the dead cells.
This common form of dead algae appears as a fluffy, soft, and sometimes slimy layer. It is often green, brown, or black and settles in low-flow areas, easily clouding the water when disturbed. This organic sludge, sometimes called mulm, has a low density and consists primarily of carbon-based compounds that quickly break down. Lacking a rigid mineral structure, this detritus is soft to the touch and bears no resemblance to the hard, dense grains of mineral sand.
The Specific Case: Diatoms and Silica Structures
The confusion between dead algae and sand stems almost exclusively from diatoms, a specific class of microalgae. Diatoms are unique because they construct a rigid, porous cell wall, known as a frustule, made of hydrated silicon dioxide. This chemical compound, called biogenic silica, is the same base material found in quartz sand.
When a diatom dies, the organic matter inside the cell rapidly decomposes, but the intricate silica frustule is left behind. These microscopic, glass-like shells are mineral-based and extremely resistant to degradation in water. The sand-like appearance results from the mass accumulation of billions of these tiny, fossilized shells.
This accumulation of dead diatom frustules is known as diatomaceous earth. The particles are very small, ranging from 10 to 200 micrometers, and their volume creates a fine, granular sediment. Since the shells are essentially made of opal—a form of amorphous silica—the resulting deposit is mineral-based and chemically related to the quartz that makes up geological sand.
Key Differences Between True Sand and Algal Deposits
Despite the visual similarity, true mineral sand and deposits of dead diatoms possess distinct physical characteristics. True sand, such as quartz sand, is composed of dense, solid grains of rock that are much larger and more rounded by erosion. These grains are heavy, sink quickly, and feel hard and abrasive when rubbed between the fingers.
Diatom deposits, or diatomaceous earth, are much lighter due to the porous, hollow nature of the frustules. When dry, this material feels like a fine, light powder rather than heavy grains. The high porosity gives the deposit a lower density, causing it to be easily stirred up into the water column.
A simple physical test can distinguish the two: true sand is hard and cannot be easily crushed. A dry diatom deposit, though abrasive, is crushable into a finer, talc-like powder. Diatom deposits often form a thin, reddish-brown or yellowish layer that coats surfaces like glass or rocks before accumulating in thicker sediment. This fine, easily disturbed layer indicates recent diatom growth, whereas true sand is a heavier, more stable geological sediment.