ALGAL GROWTH-RATE FLUCTUATIONS OBSERVED UNDER UNIFORM AMBIENT TEST CONDITIONS USING STATIC AND SEMICONTINUOUS ASSAY TECHNIQUES

A simple, manually diluted, semicontinuous, 96-h algal growth assay te chnique was developed to measure changing rates of population growth f ollowing sublethal chemical exposure. Rates were estimated directly fr om changing cell counts in a fixed volume of cell suspension. Short-te rm rate fluctuations in exponential rate parameters were observed in S elenastrum capricornutum populations using this method and similar flu ctuations were also documented by reanalyzing conventional static cult ure assay data. Replicate cultures tended to fluctuate in unison, and patterns of population increase were similar in static assays initiate d suggested that nonuniform rates of S. capricornutum population incre ase were not due simply to environmental variation. All populations we re preacclimated to test conditions, but growth lags were consistently observed for 12-24 h following inoculation. Subsequent rare fluctuati ons probably resulted from a high degree of cell-cycle synchronization . Treating systematic rate fluctuations as random error lowered measur ement precision, especially with respect to estimates of rate changes over time. Systematic variance may be difficult to eliminate in practi ce, but repeated-measures regression methods can account for this effe ct and substantially reduce rate parameter confidence intervals. Findi ngs are expected to apply to endpoints such as dry weight, total cell volume, chlorophyll, or DNA.