Sinking rates of planktonic diatoms in an unstratified lake: a comparison of field and laboratory observations

SUMMARY. 1. Natural population sinking rates were calculated by fitting an exponential regression to 6 years'observations on declining crops of Melosira italica subarctica O. Müll. and Stephanodiscus astraea (Ehr.) Grun.
2. Losses were described by an exponential model which yielded still water sinking rates of 0.86 md⁻¹ for M. italica and 0.45md⁻¹ for S. astraea.
3. Laboratory measurements of the sinking rate of natural populations showed that the rate increased abruptly from less than 0.2 m d⁻¹ in growing populations to 0.4 m d⁻¹ after silica depletion.
4. The measured sinking rate of S. astraea agreed well with that observed in the field. Where as populations of M. italica appeared to sink more quickly in the lake than could be accounted for by laboratory observations.