Net population growth of many red-tide dinoflagellates is reduced by exposure to fluid flow.In previous experiments using the red-tide species Lingulodinium polyedrum, net population growth was reduced by as much as 50% by exposure to laminar shear stress of 0.004 N m^-2 for 1-4 h d^-1. Flow cytometric cell cycle analysis demonstrated that reduced population growth was due to reduced division rate regardless of exposure duration. The reduction in division rate appeared to be due to a lengthening of the G1 phase of the cell cycle.

A similar lengthening of the G1 phase probably explains the flow-induced reduction of population growth of dinoflagellates in the closely related genus Alexandrium. Others have shown that the saxitoxin-producing dinoflagellate Alexandrium fundyense produces toxin only in the G1 phase. Therefore, extending the G1 phase by exposing A. fundyense to shear is hypothesized to increase cellular toxin content. In the present study, A. fundyense cultures grown on a 12:12 h LD cycle were exposed to constant laminar shear generated in Couette flow chambers for 1, 4 or 24 h d^-1 for 5-8 days. Shear stress during the daily exposure period was 0.004 or 0.01 N m^-2. Saxitoxin equivalents were measured with a receptor binding assay using rat brain synaptosome membrane fractions. Preliminary results demonstrate that population growth of A. fundyense is inhibited by shear and cellular toxin content is correlated to the level of growth inhibition. Extrapolating these results to field conditions, oceanic turbulence sufficient to inhibit the growth of toxic red-tide dinoflagellates may also increase cellular toxin content.