OPTIMAL SAMPLING PERIOD FOR PREDICTING PHOSPHORUS AND CHLOROPHYLL CONCENTRATIONS IN NORTH-TEMPERATE LAKES

Because total phosphorus and chlorophyll a concentrations vary seasonally within north-temperate lakes, estimates of lake trophic status are usually based on several to many sampling visits. However, because such effort is not always logistically possible, the empirical relationships developed between these trophic status indicators may depend upon seasonal patterns of sampling. Clearly, it is desirable to know if some periods of the year are more likely to produce representative estimates of total phosphorus and chlorophyll a, or, alternatively, if sampling in other periods should be restricted or even avoided. Data from north-temperate lakes are used to examine the degree to which single monthly samples for total phosphorus and chlorophyll a represent seasonal means. Analyses indicate that the least precise and least accurate estimates of the means occur when single-visit synoptic surveys are conducted during April. Not until August to September do single sampling visits provide data that approximate those represented by the means garnered through more frequent sampling.