Temporal variability in the chemical characteristics along the Rivière de l'Achigan: How many samples are necessary to describe stream chemistry?

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Abstract
We examined the temporal variability of several chemical characteristics at 12 different sites in a river system of the Laurentian region of Quebec. Mean–variance relationships (log–log) differed considerably among chemical variables with slopes ranging from about 1.3 to over 5. Guidelines and equations are given to help decide how many samples should be taken to obtain a mean value with an error of 20% or less for the different chemical variables. Relative temporal variabilities (coefficients of variation) were highest for nutrients and lowest for pH, alkalinity, and conductivity. Data from the same sites but for different years showed no systematic differences and similarly mean–variance relationships for TP and TDP obtained from a different region were not significantly different suggesting a wider applicability to our results. We also show that although the temporal variability of chlorophyll in streams is similar to that found in lakes, the same does not apply to phosphorus, which is about two to three times more variable in streams.