Determination of biogenic silica in coastal waters: applicability and limits of the alkaline digestion method

The NaOH/HF digestion method, used for the determination of particulate biogenic silica (BSi) and of particulate lithogenic silica (LSi) in oceanic waters, has been applied to coastal waters. In the course of one year, 275 samples of suspended matter were collected at two stations located in the Bay of Brest and in the English Channel (Western Europe). In contrast to previous studies, significant leaching of silicic acid from lithogenic material during the NaOH attack is demonstrated, both for natural samples and for laboratory experiments with suspension of quartz, illite, and kaolinite (usual components of clay in suspended matter of those coastal waters); the interference of lithogenic material on biogenic silica determination averaged 15%. For samples collected during the period of low biological activity, plots of apparent biogenic silica concentrations vs. apparent lithogenic silica concentrations, show linear relationships for both study sites. These relationships are used to calculate corrected biogenic silica concentrations, BSi c. During the period of active diatom production, the precision for BSi c determination is better than 10%.