Silica

Silica is the name for compounds containing silicon in combination with oxygen. Silicon is the second most abundant element in the earth's crust, and silica is found in all waterbodies. Silica generally occurs in freshwaters in both dissolved and particulate forms. Silicon is represented in the Periodic Table of Elements as Si and the chemical formula for soluble silica is H₄SiO₄.

Although human activities (i.e., fluoridation of drinking water and some industrial processes), are sources of silica, even pristine waters contain silica compounds. Many freshwaters contain less than 5 mg/L of silica, while concentrations as high as 4000 mg/L have been measured in saline waterbodies.

The Role of Silica in Waterbodies
Silica is considered an essential micronutrient for microorganisms and diatoms (a type of algae). These organisms use silica to form shells and other protective structures. Diatoms are capable of using large amounts of silica, and diatom populations may be limited when silica is in short supply. Silica concentrations in water are affected by several mechanisms. For example:

As diatom populations increase, the rate at which they pull silica from the water also increases (usually in the spring). This can result in a decline of silica concentrations in the water.
Silica is removed from the water column altogether when diatoms die and sink to the bottom, forming silica-enriched sediments.
When pH is above 7, the amount of dissolved silica in the water column is affected by the presence of iron and aluminum; either one can reduce the amount of dissolved silica in the water column.
The amount of silica in the water column can increase when humic compounds (organic substances that make water tea-colored) are present.

In Florida
Waterbodies in the Florida LAKEWATCH database have silica levels ranging from 0 to about 14 mg/L. Over 75% of these waterbodies had silica concentrations less than 2.4 mg/L.

Health Concerns
Silica poses no known threat to human health at the concentrations found in waterbodies.

The information on this page was derived in part from the following publication of
Florida Lakewatch of the University of Florida Department of Fisheries and Aquatic Sciences.
This booklet may be downloaded in its entirety as a PDF file by clicking here.
Scroll down till you see the Circular 101 icon.

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A collaboration of the Center for Aquatic and Invasive Plants, University of Florida, and the Invasive Plant Management Section of the Florida Fish and Wildlife Conservation Commission.