Seasonal and Diurnal Fluctuations of Oomycete Propagule Numbers in the Free Water of a Freshwater Lake

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Abstract
Quantitative changes in the populations of oomycete fungi in the freshwater environment of a small freshwater lake were recorded throughout a period of 2 yr. A method of estimating the numbers of fungal propagules by continuous flow centrifugation, pour plating and scanning was tested and the probable losses during processing determined. For zoospores and zoospore cysts the loss of viable propagules was within the range 5-25%. This method was then used on samples from the lake. Zoospores and zoospore cysts were the predominant propagules recovered. There was a seasonal periodicity in propagule numbers, per unit volume, with 3, rather than 2, maxima: early spring, early summer and autumn. There was a diurnal periodicity in numbers, per unit volume of propagules with 2 maxima (early afternoon and late evening), the relative importance of which varied through the year. Similar diurnal and seasonal fluctuations were found in all the oomycete genera collected, with the possible exception of Phytophthora. Experiments showed that in lakewater zoospores had a short life-span. Thus the number, per unit volume, of zoospores recovered may be indicative of the activity of the fungi. No single factor accounted for the fluctuations observed.

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