Differential cell sinking as a factor influencing diatom species competition for limiting nutrients

Abstract

The effect of differential cell sinking on competition between the diatoms Thalassiosira rotula Meunier (Clone 411) and Chaetoceros cf. vixvisibilis Schiller (Clone 847) for silicic acid and nitrate was examined in semicontinuous selective-loss culture in which the most rapidly sinking cells were removed preferentially. C. cf. vixvisibilis displaced T. rotula under silicic acid and nitrate limitation, regardless of whether the cultures were mixed immediately before the daily dilution (random loss) or fast-sinking cells were removed selectively (selective loss). However, the rate at which T. rotula was displaced increased in selective-loss cultures with either limiting nutrient. In addition, the daily removal of sunken cells selected against cells of both species with high C, N, and Si quotas. Differential cell sinking played only a minor rêle in the faster displacement of T. rotula in selective-loss culture, since both species had similar sinking rates in mixed-species culture. However, cell sinking did affect competition by increasing the supply rate of either limiting nutrient. Cell-removal rates exceeded the dilution rate in selective-loss culture, raising biomass-specific nutrient-supply rates above those in random-loss cultures operated at the same dilution rate. The resulting reduction in nutrient limitation led to higher steady-state growth rates, increased residual nutrient concentrations, and, in the case of N limitation, lower C : N ratios of cells from selective-loss cultures. These physiological responses enhanced the ability of C. cf. vixvisibilis to outcompete T. rotula for both Si and N. The results suggest that cell sinking may affect interspecific competition not only through the direct loss of cells from the euphotic zone, but also by altering the dynamics of nutrient competition among planktonic algae.