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 growth has been demonstrated by experimentally exposing dinoflagellate cultures to quantified shear flow. However, despite interest in understanding environmental factors that affect the growth of toxic dinoflagellates, little is known of the effect of shear on the growth of toxin-producing dinoflagellate species. Cultures of the dinoflagellate, Alexandrium fundyense, a producer of toxins responsible for paralytic shellfish poisoning, were exposed to quantified laminar shear generated in Couette flow for 1–24 h d21 over 6–8 d. Shear stress in all experiments was 0.003 N m22, similar to levels expected in near-surface waters on a windy day. Net population growth decreased with shear exposures .1 h d21 and became negative with exposures .12 h d21. Cellular toxin content at the end of each experiment was measured by a receptorbinding assay that used [3H]saxitoxin. Toxin cell21 of cultures sheared for .1 h d21 increased up to three times that of control cultures. Cellular toxin content increased significantly as growth rate of sheared cultures decreased. However, varying culture growth rate using irradiance had no significant effect on toxin cell21. Because shear stress levels used in this study were plausible for near-surface turbulent flows, oceanic turbulence may inhibit population growth and increase cellular toxin content of A. fundyense. However, in natural populations it would be difficult to distinguish the effect of turbulence on toxin content from other influences on toxin variability, particularly if volumeor mass-specific, rather than cell-specific, measures of toxin are used. Field studies have found negative correlations between red-tide dinoflagellate abundance and physical parameters that relate to turbulence. Tynan (1993) and Rapoport and Latz (1998) both found that the abundance of red-tide dinoflagellates in La Jolla Bay, California, was negatively correlated with the significant wave height of previous days. In Lake Kinneret (Israel), yearly spring blooms of the dinoflagellate Peridinium gatunense (5cinctum) do not occur until after the yearly onset of stratification (Pollingher and Zemel 1981), and P. gatunense cell concentration within blooms is negatively correlated with «, the rate of turbulent energy dissipation (Berman and Schteinman 1998). Unusually high rates of energy dissipation in 1996 may explain why it was the only year of a 32-yr time series that lacked a spring P. gatunense bloom (Berman and Schteinman 1998). Such field observations are consistent with a number of hypotheses. Dinoflagellates are active swimmers and are 1 Corresponding author ([email protected]). Present address: US EPA Gulf Ecology Division, 1 Sabine Island Drive, Gulf Breeze, Florida 32561.