The effect of sampling frequency on chemical parameters in acid-sensitive streams

We assessed the effects of different simulated sampling regimes (weekly, fortnightly, monthly and bimonthly) on parameters describing the water chemistry of 72 streams in acid-sensitive areas of Wales. For pH, sulphate, total hardness and aluminium, reduced sampling frequency had no discernible or systematic effect on the apparent annual mean chemistry relative to the values derived from weekly data. Standard deviations and coefficients of variation were either unaffected, or were reduced. However, sampling frequency had a moderate effect on mean pH when the data were separated into seasons: winter mean pH increased on average by 0.13 units and summer means decreased on average by 0.12 units, when using bimonthly data relative to weekly. Extreme values were detected less effectively at lower sampling frequencies, significantly altering the intercept and/or the slope of the strong relationships between the means and minimum pH or maximum Al. These effects almost certainly reflect the exclusion of extreme events (summer drought and winter floods) from low sample frequencies and reveal limitations in the use of mean values from periodic sampling programmes for summarising some aspects of site chemistry. Nevertheless, previously established relationships between mean stream chemistry, land use and stream biology were still strong at all sampling frequencies. Clear recommendations about the needs to fully parameterise episodic fluctuations depend on unanswered questions about: (i) whether biota respond to mean or episodic chemical conditions and (ii) whether baseflow chemistry, episodic fluctuations, or some combination of these, will best reflect trends in acidification.