The seasonal variation in silicon concentration in chalk-streams in relation to diatom growth

SUMMARY. Changes in silicon concentration were used to investigate the seasonal cycle of diatoms in chalk-streams. Bore-hole water shows no significant variation in silicon concentration. Five kilometres downstream there is a significant reduction between March and May. Further downstream the cumulative effect of biological silicon removal shows more complex patterns. There is a considerable variation in the seasonal pattern from year to year but an analysis of the seasonal pattern averaged over thirteen years on the River Frome confirms a later summer silicon reduction in August-September, in addition to a much larger reduction in the spring.
The negative correlation between suspended chlorophyll-α and silicon concentrations confirms that suspended chlorophyll-α concentrations respond more closely to changes in the loosely attached diatom flora than to other sources of chlorophyll-α. However, neither variable is always suitable for monitoring changes in benthic diatoms. In headwaters, for example, transitory changes in diatoms do not have a significant impact on silicon concentration, while further downstream the suspended solids are affected by many other factors.
Estimates of diatom production, based on changes in silicon concentration, gave rates of 1–2 g Si m_-2 day_-1 in a small headwater chalkstream during the spring.