Effect of Fluid Shear on Dinoflagellate Growth,
Physiology and Toxin Content

Andrew R. Juhl
Scripps Institution of Oceanography
University of California, San Diego, 2000

Dinoflagellate blooms often occur in calm, stratified
waters. One hypothesis is that the fluid shear of
turbulent flow directly affects dinoflagellate cell
physiology and inhibits dinoflagellate net population
growth. Previous studies provide quantitative support
for the hypothesis but have left unanswered questions.
This dissertation addresses 3 questions. Is the effect
of shear on net growth a constant function of the shear
exposure or do growth conditions influence the level of
growth inhibition? Does shear affect the growth and
toxin content of toxin-producing dinoflagellates? Finally,
is shear-induced inhibition of dinoflagellate net growth
due to reduced cell division or increased mortality?
The effects of turbulent fluid forces were simulated by
exposing cultured dinoflagellates to fluid shear
generated in Couette flow chambers and shaken
containers. The shear stress values used in
experiments were plausible for upper ocean turbulence
on a windy day.

Net population growth of the red-tide dinoflagellate,
Lingulodinium polyedrum, was inhibited by exposure to
a shear stress of 0.004 N/m^2 for as brief as 1 h/d.
Growth inhibition from a given shear exposure varied
consistently with growth conditions. Greater
depression of growth occurred from shear exposure
near dawn (the time of phased cell division), when
cultures were grown in low light, or when cultures were
in late exponential phase. If these results can be
applied to other species and to populations in the field,
night-time turbulence may have a greater effect than
daytime turbulence on dinoflagellate growth.
Turbulence may also have a greater effect on
dinoflagellate growth if other environmental conditions
are already suboptimal for growth. As suboptimal
conditions may be more common during later phases
of blooms, turbulence may be more important for
terminating dinoflagellate blooms than for inhibiting
their formation.

Net population growth of the saxitoxin-producing
dinoflagellate, Alexandrium fundyense, was inhibited by
>2 h/d of shear stress at a level of 0.003 N/m^2. As
growth declined, saxitoxin content/cell increased,
reaching 3 times control values. This is the first
suggestion that turbulence is an environmental
parameter that can increase cellular toxin content of
toxic dinoflagellates.

In early exponential phase cultures of L. polyedrum,
oceanic levels of shear stress (0.004 N/m^2) inhibited
net growth by reducing cell division, rather than
increasing mortality. Increased mortality and other
indicators of physiological stress were measured both
in late exponential phase cultures exposed to 0.004
N/m^2 and in early exponential phase cultures exposed
to higher shear stress levels (0.01 - 0.019 N/m^2).
Such information provides the first step towards a
physiological understanding of how dinoflagellate
growth is affected by shear. The dominant mechanism
of growth inhibition of field populations will depend on
the physiological state of the cells and the shear stress
level.