Turbulence increases the average settling velocity of phytoplankton cells

Abstract

It is a well known fact that stirring keeps particles suspended in fluids. This is apparent, for instance, when shaking medicine flasks, when agitating tea deposits in a mug, or when heavy winds fill the air with dust particles. The commonplace nature of such observations makes it easy to accept that this feature will apply to any natural phenomenon as long as the flow is turbulent enough. This has been the case for phytoplankton in the surface mixed layers of lakes and oceans. The traditional view assumes that an increase in turbulence bears ecological advantages for nonmotile groups like diatoms that, otherwise, would settle in deep and unlit waters. However, this assumption has no theoretical ground, and the experimental results we present here point in the opposite direction. Phytoplankton settling velocity increases when turbulence intensifies from the low to the higher values recorded in the upper mixed layers of lakes and oceans. Consequently, turbulence does not favor phytoplankton remaining in lit waters but is rather an environmental stress that can only be avoided through morphological and͞or physiological adaptations. P hytoplankton in the upper mixed layers of lakes and oceans live in a turbulent flow regime. This circumstance may shape many features of their physiological and morphological functioning including the efficiency of nutrient assimilation (1), prey-predator interactions (2) and cell division (3). One of the key controls turbulence exerts on phytoplankton results from its effect on sedimentation rates. A compensatory role is usually assumed for turbulence, especially for nonmotile cells lacking other mechanisms to counteract their sinking from the surface to depth. However, this potential role cannot result from a direct effect of turbulence on the settling velocity of cells. The results presented in this paper clearly demonstrate that turbulence increases the settling of phytoplankton cells, regardless of the species considered and independent of turbulence-generation devices or velocity-measurement instruments. Our evidence questions the traditionally accepted notion that turbulence diminishes phytoplankton settling in the ocean. All experiments were made with particle concentrations Ͻ20 ppm by volume, with negligible effects on the flow. It is also a low concentration to expect significant deviations from Stokes terminal velocity due to interactions between the fields generated by two falling particles (4). These deviations scale as the ratio of particle diameter to distance between particles (4). The value of the latter can be estimated as the inverse cube root of the particle concentration (Ͻ20 ppm by volume) divided by individual volume (function of particle ESD). Ratios of ESD to distance between particle of the order of 0.01 are obtained. Deviations from Stokes terminal velocities observed in our experiments are much higher (see below). Therefore, interaction between particles cannot be the exclusive origin of the results presented below. On the other hand, temperature variations...

Causes of decoupling between larval supply and settlement and consequences for understanding recruitment and population connectivity

“…At these finer spatial 18 scales, flow velocities and substrate characteristics, which determine the boundary layer, 19 turbulent flows, and shear stress, will affect whether larvae attach or are re-suspended from a 20 substrate (Koehl, 2007). For example, turbulence increases the settling of negatively buoyant 21 phytoplankton cells (Ruiz et al, 2004), and some larvae use sinking behavior to promote 22 settlement in turbulent flows (Fuchs et al, 2004). In fast flows with high turbulence, contact rate 1 of larvae increases, resulting in higher settlement than in still water or in low flow conditions, 2 whereas above certain flow speeds larvae are not able to make contact or adhere (e.g., Eckman et 3 al., 1990;Pawlik and Butman, 1993;Qian et al, 2000;Pernet et al, 2003).…”

Settling velocity of quasi-neutrally-buoyant inertial particles

“…The vanishing of expressions (34) and (35) is due to (33), in the former case coupled with (1). Because of (29a) and (27b) (i.e.…”

“…The particle terminal velocity [29][30][31][32][33][34][35] is defined as a weighted average of the particle velocity, from the first equation in (2):…”

Intermittent turbulence and copepod dynamics: Increase in encounter rates through preferential concentration

“…Indeed, using DNS data Wang and Maxey (Wang and Maxey, 1993) have found an increase of the mean settling velocity of particles when their density is larger than the density of the fluid, due to a preferential sweeping effect. One may also find more recent studies on turbulence increasing the average settling velocity using numerical (Bosse et al, 2006) as well as experimental data (Aliseda et al, 2002;Ruiz et al, 2004). In their experimental study of the average settling velocity of phytoplankton cells, Ruiz et al (2004) have obtained a very small average settling velocity.…”

“…One may also find more recent studies on turbulence increasing the average settling velocity using numerical (Bosse et al, 2006) as well as experimental data (Aliseda et al, 2002;Ruiz et al, 2004). In their experimental study of the average settling velocity of phytoplankton cells, Ruiz et al (2004) have obtained a very small average settling velocity. Thus, this phenomenon is not expected to have an influence over small time scales, as considered in the experiment outlined in the present paper.…”