Study of dissolved silicon and nitrate dynamics in a freshwater stream.

Dissolved reactive silicon and nitrate were measured at weekly intervals over a 3 year period (1991-94) on a 1.2km reach of a gauged Dorset Mill Stream. In addition, dissolved nitrite was measured over a 8 month period from the spring to late autumn in 1992. Two intensive studies with sampling at 2h intervals were also completed in low and high riverflow conditions. The results were analysed using a mass-balance approach with the loss and gains in nutrients dissolved in the water expressed in terms of areal rates. Losses of both nutrients occurred during periods of low streamflow in spring and summer. Losses of silicon are attributed to growth of epilithic diatoms whereas nitrate losses are consistent with a number of processes including the growth of aquatic plants, the development of epilithic biofilms and nitrogen transformations, such as denitrification, in bed-sediments. Stream water gained dissolved nitrite during its passage through the section. Silicon losses from the stream amounted to between 52 and 63 mmol m(-2) d(-1) (expressed per area of bed-sediment) for the spring periods in 1992-94. Nitrate losses were more variable with overall rates between 24 and 89 mmol m(-2) d(-1) for the summer periods in 1991-93.