EXPERIMENTAL OBSERVATIONS ON THE FLOTATION OF MARINE DIATOMS. I. THALASSIOSIRA CF. NANA, THALASSIOSIRA ROTULA AND NITZSCHIA SERIATA

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Abstract
The influence of cell age, size, and shape and mode of colony formation on diatom sinking rates was investigated in 25 experiments with Thalassiosira cf. nana Lohmann, Thalassiosira rotula Meunier and Nitzschia seriata Cleve. The settling experiments were conducted in 45 ml of 32‰ seawater at 15C using a glass settling tube in which the water column height varied from 74 to 79 mm. The mean sinking rates of T. cf. nana and T. rotula increased with age of the culture: from 0.10 to 0.28, and from 0.41 to 2.10 m/day, respectively. The absolute minimum and maximum rates were 0.05 and 5.40 m/day for T. cf. nana and 0.18 and 44.93 m/day for T. rotula. An increase in cell size due to incompleted cell division accompanied increased culture age in T. cf. nana, and may partly account for the increased mean sinking rate accompanying senescence of this species. Regulation of both colony size and the distance between cells occurred in the chain‐forming T. rotula, but no persistent relationships with mean sinking rate were found.The mean sinking rate of N. seriata remained more or less constant with culture age and ranged from 0.26 to 0.50 m/day. The absolute minimum and maximum sinking rates were 0.18 and 2.96 m/day. A decrease in colony size accompanied increased culture age in N. seriata; there was a general tendency for the mean sinking rate to increase with colony size.At the absolute maximum sinking rates observed, from 2 to 34 days would be required by the species to descend 100 m, while from 556 to 2,000 days would be required at the absolute minimum rates. The data suggest that during vigorous growth an increased buoyancy adequate to ensure prolonged retention within the euphotic zone occurs and is independent of species morphological characteristics. Morphological changes in cell size and colony size appear to influence the increased sinking rate accompanying senescence, however. Changes in ratios of pigment absorbance at various wavelengths (352 : 474 mµ, 382 : 474 mµ, 404 : 474 mµ, 434 : 474 mµ, 414 : 352 mµ) were found to accompany changes in the rate of sinking, suggesting that the latter might also be influenced by changes in adaptive physiological mechanisms of flotation.

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