Sinking rate dynamics of Cricosphaera carterae Braarud. I. Effects of growth rate, limiting substrate, and diurnal variation in steady-state populations

Abstract

The purpose of this study, aimed at evaluating relationships between buoyancy and nutritional status in actively growing populations, was to ascertain under a variety of nutrient regimes if sinking rates manifested systematic variation with the kinetics of population growth. Steady-state populations of Cricosphaera carterae Braarud showed no significant relationship between sinking rates and population growth rate, when limited by nitrate or ammonium. Under phosphate limitation, sinking rates were significantly greater at low growth rates than they were at high growth rates. The type of limiting substrate did not convey large sinking rate differences in these actively growing populations. Substrate-related differences, where discernible, were only apparent at low growth rates. Slow-growing, phosphate-limited populations sank somewhat more rapidly than slow-growing populations limited by nitrate or ammonium. There was no significant relationship between the sinking rates of C. carterae populations and the ratios C N , C cell volume , C cell , N cell volume or N cell under the various growth conditions examined. When sinking rate and C and N data for other cultured phytoplankton species were analyzed collectively for such relationships, sinking rates were significantly correlated with C cell and N cell . Since inclusion of the other species introduces a larger range of cell size, these relationships may be coincidental with the relationships of C cell and N cell to cell size. Marked diurnal sinking rate variations were common to all growth conditions investigated, and sinking rates varied more in response to the 12:12 L:D cycle than all other changes in growth conditions. Diurnal sinking rate patterns were similar under all growth conditions, and showed highest and lowest values at the ends of the dark and light periods, respectively. Sinking rates varied inversely with cell size during the diurnal cycle and illustrate the potential of physiological status to counteract the hydrodynamic considerations of particle size in influencing cell buoyancy.