Effect of fluid shear and irradiance on population growth and cellular toxin content of the dinoflagellate Alexandrium fundyense

The potential for in situ turbulence to inhibit dinoflagellate population g rowth has been demonstrated by experimentally exposing dinoflagellate cultu res to quantified shear flow. However, despite interest in understanding en vironmental factors that affect the growth of toxic dinoflagellates, little is known of the effect of shear on the growth of toxin-producing dinoflage llate species. Cultures of the dinoflagellate, Alexandrium fundyense, a pro ducer of toxins responsible for paralytic shellfish poisoning, were exposed to quantified laminar shear generated in Couette flow for 1-23 h d(-1) ove r 6-8 d. Shear stress in all experiments was 0.003 N m(-2) similar to level s expected in near-surface waters on a windy day. Net population growth dec reased with shear exposures >1 h d(-1) and became negative with exposures > 12 h d(-1). Cellular toxin content at the end of each experiment was measu red by a receptor-binding assay that used [H-3]saxitoxin. Toxin cell(-1) of cultures sheared for >1 h d(-1) increased up to three times that of contro l cultures. Cellular toxin content increased significantly as growth rate o f sheared cultures decreased. However, varying culture growth rate using ir radiance had no significant effect on toxin cell(-1). Because shear stress levels used in this study were plausible for near-surface turbulent flows, oceanic turbulence may inhibit population growth and increase cellular toxi n content of A. fundyense. However, in natural populations it would be diff icult to distinguish the effect of turbulence on toxin content from other i nfluences on toxin variability, particularly if volume-or mass-specific, ra ther than cell-specific, measures of toxin are used.