THE EFFECT OF SAMPLING FREQUENCY ON CHEMICAL-PARAMETERS IN ACID-SENSITIVE STREAMS

We assessed the effects of different simulated sampling regimes (weekl y, fortnightly, monthly and bimonthly) on parameters describing the wa ter chemistry of 72 streams in acid-sensitive areas of Wales. For pH, sulphate, total hardness and aluminium, reduced sampling frequency, ha d no discernible or systematic effect on the apparent annual mean chem istry relative to the values derived from weekly data. Standard deviat ions and coefficients of variation were either unaffected, or were red uced. However, sampling frequency had a moderate effect on mean pH whe n the data were separated into seasons: winter mean pH increased on av erage by 0.13 units and summer means decreased on average by 0.12 unit s, when using bimonthly data relative to weekly. Extreme values were d etected less effectively at lower sampling frequencies, significantly altering the intercept and/or the slope of the strong relationships be tween the means and minimum pH or maximum Al. These effects almost cer tainly reflect the exclusion of extreme events (summer drought and win ter floods) from low sample frequencies and reveal limitations in the use of mean values from periodic sampling programmes for summarising s ome aspects of site chemistry. Nevertheless, previously established re lationships 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 episo dic chemical conditions and (ii) whether baseflow chemistry, episodic fluctuations, or some combination of these, will best reflect trends i n acidification. Copyright (C) 1996 Elsevier Science Ltd